text: The turtles struggle to contain an outbreak of mutations that occurs thanks to the leftover mutagen from the thwarted Kraang invasion. Kirby is among the victims of the outbreak, and a misunderstanding leads April to break off her friendship with the turtles. However, the turtles are able to earn her forgiveness when they save her from Karai, who has taken temporary command of the Foot while the Shredder is away in Japan. Along with April came her new friend Casey Jones, who helps repel an assault on the turtles' lair. Kirby is eventually restored to his human form when Donatello manages to concoct another Kraang chemical called retro-mutagen, which causes organic beings to reverse their major physical transformations.


title: Teenage Mutant Ninja Turtles (2012 TV series)

text: Meanwhile, Shredder returns from Japan with the mutated Japanese bounty hunter Tiger Claw as his new second-in-command. Tiger Claw is later sent through a portal to the 1987–1996 animated series reality, but later returns to the 2012 reality.


text: During a battle with the turtles, Karai is informed of her true nature by Leonardo (who has developed a crush on her since season 1), but she is too reluctant to accept it. When she is taken to the lair, she finally realizes the truth and disowns Shredder, who responds by imprisoning her. Repeated attempts to free Karai ultimately succeed, but Shredder captures her again and uses her as bait to kill the Hamato Clan. However, Karai ends up transforming into an albino horned viper, to everyone's horror.


text: In response to this, Shredder has the Foot Clan help the Kraang, who have just perfected the previously unstable mutagen, launch a second invasion of New York, starting by destroying the turtles' lair and forcing them to abandon it. The Kraang begin mutating New York's populace, including Kirby, despite the efforts of Earth's military and the turtles. Leonardo is gravely wounded when he is ambushed by the entire Foot, and Splinter is seemingly killed during a battle with Shredder. The turtles, April, and Casey are forced to flee to the O'Neil family's summer home in Upstate New York as the Kraang successfully conquers Manhattan.


text: The group takes refuge at the O'Neil family's summer home in North Hampton in order to recuperate from their loss at the hands of the Kraang and the Foot Clan. Leonardo takes an extended amount of time to recover, but guided by an apparition of Splinter, is able to overcome his physical and mental wounds. They eventually return to New York City with the goal of finding Splinter, Karai, Kirby, and their other missing allies and liberate the city from the Kraang. They are successful in finding Splinter and set up a temporary base in the abandoned Antonio's Pizza, and Donatello begins work on new samples of retro-mutagen as the search for Kirby and Karai continues.


text: Meanwhile, Shredder sends his new minions Bebop and Rocksteady to find Karai. The duo succeed, and Shredder promises to cure her. The turtles, run into a new team the Mighty Mutanimals, with they manage to banish the Kraang back to Dimension X and free New York. The effect of the invasion eventually earns Shredder cemented control of the city's criminal underworld for his efforts to create a mind-control serum meant for the turtles, Mutanimals and Karai.


text: The turtles are later offered by future time sorceress Renet to time travel to the past, where they meet up with a younger Hamato Yoshi and Oroku Saki, as well as Yoshi's late wife and Karai's mother, Teng Shen. During the battle between Yoshi and Saki inside the burning Hamato Clan dojo, Shen is accidentally killed by Saki, who meant to kill Yoshi instead. Saki leaves the dojo after his hair burns off from the fire and kidnaps Yoshi and Shen's baby daughter Miwa, vowing to raise her as Karai and leaving Yoshi to die. After his escape, the turtles save Yoshi, as they were "always destined to" before traveling back to the present with Renet.


text: When the Kraang return to Earth, the Turtles discover that the Kraang have another enemy from Dimension X in the form of the Triceraton Empire. Despite their best efforts and Shredder violating the Foot Clan's temporary truce with the Turtles by stabbing Splinter during the fight, the ruthless Triceratons activate the Heart of Darkness, a black hole-creating machine which annihilates both the Kraang, the Earth, and everyone on it. But just as it comes to the worst for the Turtles, April, and Casey, they are rescued by a friendly robot by the name of Professor Honeycutt (also known as the Fugitoid) who uses his spacecraft to take the Turtles, April and Casey to a journey into space.


text: After the Turtles, April, and Casey are saved from the destruction of Earth, the Fugitoid uses his spacecraft the Ulixes to travel six months back in time in order for him and the Turtles to prevent the Triceraton Empire led by Emperor Zanmoran from again assembling the three components of the Heart of Darkness that are scattered throughout the universe before everything on Earth is lost. Besides fighting the Triceratons, the Turtles also face new enemies in outer space like Lord Vringath Dregg of the planet Sectoid and the bounty hunter Armaggon, and even have an adventure with their interdimensional 1987 counterparts and their enemy Krang who is an exiled relative of Kraang Subprime. Despite the efforts of the Turtles, the Triceratons are still able to collect all three pieces of the Black Hole Generator, only for the Turtles to return to Earth and join forces with their past selves to warn Splinter before he is killed by the Shredder, stop the detonator of the Heart of Darkness, and defeat the Triceratons. The Fugitoid destroys the Black Hole Generator near the Triceraton spacefleet, causing to explode, presumably killing the Triceratons. In the aftermath of the fight against the Triceratons, the past versions of the Turtles, April, and Casey leave Earth with the past Fugitoid in the Ulixes while the present Fugitoid's head reactivates in Earth's orbit.


text: Weeks later following the Triceraton Invasion being thwarted and the Foot Clan's disappearance, April is promoted to kunoichi at the time when the witch Shinigami arrives and is revealed that she is Karai's friend as they plot to rebuild the Hamato Clan and dispose of Shredder who is still recuperating from his last fight with Splinter. While Karai and Shinigami got some ninjas on their side, the Foot Clan strengthens the Footbot army by creating the Elite Footbots. Furthermore, some other crime organizations have been plotting to take over the Foot Clan's territory and a crystal shard of unimaginable power which April received from the alien Aeons during their space adventure is beginning to exert a baleful influence on her. Using a special mutagen formula made by Stockman-Fly, Oroku Saki recuperates and becomes Super-Shredder in order to take back control of the Foot Clan. Once a stable formula is made, Super-Shredder attacks the Turtles, the Mighty Mutanimals, Karai, and Shinigami which ends with Splinter falling in battle against Super-Shredder and is buried at the O'Neil family's summer home in North Hampton. This leads up to the final battle at Super-Shredder's hidden mansion where Leonardo manages to slay Super-Shredder and throw his mutated Kuro Kabuto to the other Turtles in victory.


text: Following the death of the Super-Shredder, Tiger Claw has led the unspecified Foot Cultists into obtaining the Scroll of the Demodragon and a special control amulet called the "Seal of the Ancients" that would enable him to summon the demodragon Kavaxas. When Kavaxas is summoned from the Netherworld, Tiger Claw plans to use Kavaxas' wish-granting abilities to revive the Shredder. Upon demonstrating the revival ability on Rahzar who fell in an underwater battle against Leatherhead, Kavaxas states that he will need the Kuro Kabuto that contains Shredder's mental energies. Upon Rahzar and Tiger Claw reclaiming the Kuro Kabuto after Kavaxas drained the soul out of Shredder's former right-hand man Hattori Tatsu, Kavaxas states that they will need the Shredder's heart for the revival to occur. The Foot Clan obtains the Shredder's heart that was in the possession of Don Vizioso. Upon Shredder's heart being placed on his corpse, Kavaxas begins to work on reviving Shredder as the Zombie Shredder. This goes against what Tiger Claw eventually planned when Undead Shredder destroys the Seal of the Ancients which enables Kavaxas to raise the spirits of the Netherworld in order to rule both worlds. While Splinter's ghost helps those fighting on the surface, Michelangelo managed to use gum to stick the Seal of the Ancients back together enabling him to have Kavaxas undo his damages and open the portal to the Netherworld as the Zombie Shredder drags Kavaxas back to the Netherworld.


text: Afterwards, the Turtles face different challenges like the return of Lord Dregg when he collaborates with Newtralizer, being transported to Miyamoto Usagi's reality where they help to protect the pug Kintaro from the evil gray wolf Jei, an adventure featuring an alternate future ruled by mutant animals, and another adventure involving the return of Savanti Romero where he collaborates with Count Dracula to make Earth more habitable for monsters.


text: The Turtles later reunite with their 1987 counterparts when they face the return of Bebop and Rocksteady, who are recruited by the Shredder and the Kraang's counterparts. They plan to conquer the Earths of both the 1987 and 2012 realities with a fully powered Technodrome after their Bebop and Rocksteady were accidentally left behind. While the Turtles and their allies fight off a horde of Foot Soldiers from the 1987–1996 series reality and the 2012 Rock Soldiers led by Traag and Granitor, Bebop and Rocksteady discover Shredder and Krang's treacherous scheme to destroy the Earth and choose to become heroes in order to save the planet.


text: An alternate-continuity series finale, released as a made-for-TV film "Raphael: Mutant Apocalypse", features a flash-forward several decades into the future, following a major catastrophe as Raphael traverses a wasteland Earth, along with Donatello (whose consciousness is transferred into a robot). They eventually encounter Michaelangelo (who has become a hippie guru, "The Holy Chalupa") and finally Leonardo, who had lost his memory and sanity and mutated into the archvillain Maximus Kong, before Raphael is able to re-trigger Leonardo's consciousness, reuniting the four core characters for a happy ending.


text: On October 21, 2009, Viacom published a press release announcing that it had bought the complete rights of the "Teenage Mutant Ninja Turtles" franchise from Peter Laird for $60 million, and would be developing a CGI-animated TV series for its Nickelodeon family of channels for broadcast in 2012. The release also confirmed a feature film to be produced for Paramount Pictures (also a division of Viacom), set for an August 2014 release.


text: In a May 2011 issue of "The Licensing Book", Nickelodeon published subject-to-change concept art of all four turtles, along with artwork from the studio's revamped "Winx Club" series. Both shows were developed simultaneously at Nickelodeon Animation Studio as part of the company's strategy to reboot two established brands for new viewers. "TMNT" was geared to a core target demographic of boys aged 6 to 11, while "Winx Club" was intended to reach a similar age group of girls. For their version of "Teenage Mutant Ninja Turtles", Nickelodeon also took note of a secondary audience in teens and adults. In June, it was revealed that the show's creative team would be led by designer Ciro Nieli and writers J.R.Ventimilia and Joshua Sternin. The three crew members held an interview the following month; they mentioned that the turtles would be differentiated physically, but that their personalities would not be changed. Nieli stated that he had been influenced by the 1984 comics, while Ventimilia and Sternin were fans of the 1987 series and 1990s films in college.


text: At the American International Toy Fair in February 2012, the first line of tie-in figures was displayed. As previously announced, the line was produced by longtime "TMNT" licensee Playmates Toys. In March 2012, final artwork from the series was leaked on the Nickelodeon website before it was taken down. The images showed the designs of all four turtles, Shredder, Splinter, a teenage April O'Neil and the Kraang, an alien race that combines elements of both Krang and the Utroms. A trailer for the series was released on June 21, 2012, on Nickelodeon USA. In July, Playmates released the show's merchandise ahead of its television debut.


text: Ciro Nieli, who previously created "Super Robot Monkey Team Hyperforce Go!" and "" for Disney Television Animation, was put in charge as the showrunner and developer for the series. Jason Biggs originally voiced Leonardo and Rob Paulsen voices Donatello. In June 2011, it was confirmed that Sean Astin is playing Raphael and Greg Cipes is Michelangelo. In August 2011, it was revealed that Mae Whitman would be the voice for April O'Neil. In April 2012, it was announced that Phil LaMarr would be playing the role of Baxter Stockman and Nolan North would be playing a race of aliens known as the Kraang, while Roseanne Barr is confirmed to voice their leader, Kraang Prime. Actress Kelly Hu confirmed her role as Karai in May 2012. Corey Feldman, who previously voiced Donatello in the first and third TMNT films, was confirmed to play the role of Slash. Recurring TMNT character Casey Jones began appearing in the second season, and is voiced by former Nickelodeon star Josh Peck. In June 2014, it was announced that Seth Green would replace Jason Biggs and Dominic Catrambone as the voice of Leonardo in season 3.


text: On November 22, 2015, Nickelodeon aired a 2D animated special, "Half-Shell Heroes: Blast to the Past". In the special, the Ninja Turtles are accidentally transported back to the Cretaceous period by an ancient meteorite and must work together with Rocksteady and Bebop (along with some newly befriended dinosaur allies) to get back to their own time, while simultaneously fending off would-be predators and a faction of the Triceraton Army led by General Zera (voiced by Kate Mulgrew), coming to prehistoric Earth. Random House also released a book based on the special while Playmates released new dinosaur toys in the fall to coincide with the program. The special was seen by 1.41 million viewers. It was released on DVD, through Nickelodeon and Paramount Home Media Distribution, on March 15, 2016.


text: A series of short films were released during the course of the series, including: "Turtles in Time, Teenage Mecha Ninja Turtles, TMNT Team Up: The Short Series.


text: "Teenage Mecha Ninja Turtles" focuses on the eponymous group, four human teen mecha pilots trained by an aged Michelangelo (with Greg Cipes reprising his role from the series) set in a futuristic New York City (circa 2090) inhabited both by humans and anthropormorphic animals, including a Scruff McGruff-like police chief. The four children each pilot colored turtle like mechs, and consist of the following: team leader Frida (voiced by America Young) pilots the red, Raphael-like mech; the blue mech with cloaking abilities that resembles Leonardo is piloted by Frida's antagonistic teammate Jackson (Eric Artel); the battle happy Kusama (Tania Gunadi) pilots the Michelangelo-like orange mech; and the purple mech, which is heavily armored and wields a bo staff that can transform into a hammer, is piloted by the nerdy and somewhat anxious Basque (Khary Payton). The short features the Mecha Turtles subduing a group of anthropomorphic thieves resembling Hyenas and other canines, consisting of wolf-like leader Grimm, spotted hyena-like members Jester Joe (both voiced by Eric Bauza) and Jester Jim (voiced by David Kaye, who also voices the armored police officer Sgt. Swat), and several other silent members.


text: The series premiered on Canadian channel YTV on September 29, 2012. It also premiered on Nickelodeon in the United Kingdom and Ireland on October 1, 2012. It premiered on Nickelodeon in Australia and New Zealand on October 8, 2012. It premiered on Nickelodeon Canada on September 2, 2013. In India, it premiered in 2013 on Nickelodeon and Sonic and currently airs on Nick HD+. It also airs on Channel 5 in the UK.


text: IGN has given the show positive reviews, with many episodes reviewed being given the "Editor's Choice" title. The season 2 finale "The Invasion" has received the highest rating of 10/10. The series premiered in the U.S. to 3.9 million viewers.


text: "Variety" has praised the show, saying "...handsomely produced effort, with a strong vocal cast, considerable humor and scads of high-spirited action. If the goal was to introduce the Turtles to a new generation — amphibious mission accomplished."


text: Playmates Toys created a new line of "Teenage Mutant Ninja Turtles" consumer products to go along with the show.


text: In mid-2012, Playmates Toys released their first wave of basic Teenage Mutant Ninja Turtles action figures which consisted of "hero figures" Leonardo, Donatello, Raphael, Michelangelo, Splinter and April O'Neil as well as "villain characters" Shredder, Kraang, and a Foot Soldier. In early 2013, Playmates released series 2 which consisted of the new "hero" character Metalhead as well as new villains Dogpound and Fishface. In April 2013, the third wave appeared which included the "hero" character Leatherhead as well as villains Snakeweed and Baxter Stockman. Wave four was released in early August and it featured the villains Rat King and Spyroach as well as new "Stealth Tech" versions of the four turtles. Wave 5 was released in October with a 7 pack of Mousers as well as four baby versions of the Turtles. October saw the final release of 2013 with wave six which included two new villains: Spider Bytez and a newly sculpted Shredder figure featuring a removable helmet and cape. Wave seven first appeared in February 2014 with figures for Kirby Bat, Squirrelanoid, Casey Jones, and Mutagen Man.


text: In July 2014, Build-A-Bear Workshop released a series of plush versions of the Turtles, as well as other TMNT themed apparel including an outfit of Shredder, along with items such as the Turtles' signature weapons.


text: In April 2014, Diamond Select Toys unveiled the first series of Minimates mini-figures based on the Nickelodeon series. Two unpainted promotional figures were given away at Comic-Con International 2014, and Series 1 was released in fall 2014 as blind bags at Kmart and comic shops, and in 2-packs at Toys "R" Us. Each location had one or two exclusive characters in addition to their shared characters. Series 2 was released in comic shops in early-2015 and at Toys "R" Us in mid-2015, again with both shared and exclusive figures. Series 3 is scheduled for both locations for the fall of 2015. A Series 4 has been confirmed.


text: IDW Publishing released a spin-off comic series titled "'Teenage Mutant Ninja Turtles: New Animated Adventures" featuring original adventures set between episodes of the TV series starting July 2013. After 24 issues, the series was relaunched as "Teenage Mutant Ninja Turtles: Amazing Adventures" in August 2015. "Amazing Adventures" was published until September 2017, with a total of 14 regular issues, one special issue guest-starring Carmelo Anthony, a three-issue miniseries titled "Robotanimals", and the crossover miniseries "Batman/Teenage Mutant Ninja Turtles Adventures".


text: The series has also been released to home video, mainly DVD. Nickelodeon's typical Region 1 DVD release schedule is to release each season across three or four volumes, consisting of four DVDs total, with each disc containing about 6-7 sequential episodes.


text: In 2013, Nickelodeon published and distributed "" developed by Kung Fu Factory for iOS devices such as the iPhone, iPad, and iPod Touch.


text: On August 28, 2013, Activision released "", a downloadable 3D beat 'em up game, for Xbox 360 and Microsoft Windows. On April 15, 2014, a version was released for the PlayStation 3. The game features an online multiplayer co-op for up to four players.


text: Activision also released a "Teenage Mutant Ninja Turtles" video game based on the series for Wii, Xbox 360, and Nintendo 3DS on October 22, 2013. It was developed by Magic Pockets.


text: On September 4, 2014, Activision announced there being a second game based on the show called "". The game was released on October 28, 2014, for Nintendo 3DS, Xbox 360, and PlayStation 3. The game was developed by WayForward.


text: Another video game, "", was released for PlayStation 3, PlayStation 4, Xbox 360, Xbox One, and Microsoft Windows in 2016. It was developed by PlatinumGames.


text: Leonardo, Michelangelo, Donatello, and Raphael appear as playable characters in the racing video game "Nickelodeon Kart Racers", the first Nickelodeon crossover console game to feature "Teenage Mutant Ninja Turtles" characters. These same characters, along with Shredder, appear as playable characters in the sequel video game "".


title: General Educational Development

text: The General Educational Development (GED) tests are a group of four subject tests which, when passed, provide certification that the test taker has United States or Canadian high school-level academic skills. It is an alternative to the US high school diploma, as is HiSET. The GED Testing Service website currently does not refer to the test as anything but "GED".


title: General Educational Development

text: The American Council on Education (ACE), in Washington, D.C. (U.S.), which owns the GED trademark, coined the initialism to identify "tests of general equivalency development" that measure proficiency in science, mathematics, social studies, reading, and writing. Passing the GED test gives those who do not complete high school, or who do not meet requirements for high school diploma, the opportunity to earn their high school equivalency credential, also called a high school equivalency development or general equivalency diploma. It is called the GED in the majority of the United States, Canada, or internationally. In 2014, some states in the United States switched from GED to the HiSET and TASC.


title: General Educational Development

text: The GED Testing Service is a joint venture of the American Council on Education. Pearson is the sole developer for the GED test. The test is taken on a computer and in person. States and jurisdictions award a Certificate of High School Equivalency or similarly titled credential to persons who meet the passing score requirements.


title: General Educational Development

text: In addition to English, the GED tests are available in Spanish and in French in Canada, large print, audio, and braille. Tests and test preparation are also offered to persons incarcerated and on military bases in addition to more traditional settings. Individuals living outside the United States, Canada, or U.S. territories may be eligible to take the GED tests through Pearson Vue testing centers. Utah's Adult High School Completion program has been used as an alternative for individuals who opt to earn a diploma.


title: General Educational Development

text: In November 1942, the United States Armed Forces Institute asked the American Council on Education (ACE) to develop a battery of tests to measure high school-level academic skills. These tests gave military personnel and veterans who had enrolled in the military before completing high school a way to demonstrate their knowledge. Passing these tests gave returning soldiers and sailors the academic credentials they needed to get civilian jobs and gain access to post-secondary education or training.


title: General Educational Development

text: ACE revised the GED tests for a third time in 1988. The most noticeable change to the series was the addition of a writing sample, or essay. The new tests placed more emphasis on socially relevant topics and problem-solving skills. Surveys of test-takers found that more students (65%) reported taking the test with the intention of continuing their education beyond high school, rather than to get better employment (30%).


title: General Educational Development

text: A fourth revision was made in 2002 to make the test comply with more recent standards for high-school education.


title: General Educational Development

text: A fifth revision was released on January 2, 2014, to be delivered on Pearson VUE, a proprietary computer-based testing platform. The new test applies to the United States and internationally, but not to Canada, which continues to use the 2002 version. It retained four content areas but with different content—language arts, mathematics, science, and social studies—that "measure a foundational core of knowledge and skills that are essential for career and college readiness."


title: General Educational Development

text: There are more than 3,200 Official GED Testing Centers in the United States and increasingly in Canada, as well as around the world. Testing centers are most often in adult-education centers, community colleges, and public schools. Students in metropolitan areas may be able to choose from several testing locations.


title: General Educational Development

text: Official GED Testing Centers are controlled environments. All testing sessions take place either in person or online according to very specific rules, and security measures are enforced. Breaks may be permitted between tests, depending on how many tests are being administered in a session. There may be restrictions on what test-takers may bring into the testing room.


title: General Educational Development

text: There are approximately three to six GED test forms in circulation at any time. This measure helps catch test-takers who may be cheating. As with any standardized test, the various test forms are calibrated to the same level of difficulty.


title: General Educational Development

text: Regulations governing eligibility to take the GED vary by state. According to GED Testing Service policy, students at least 16 years old and not enrolled in high school are eligible for the program. However, many states require the candidate to be 17 years of age and a resident of the state. Some states which allow students under 17 years of age to take the test require a letter of parental consent and a letter of consent from the student's school district. In South Africa and Namibia, students at least 17 years old are eligible for the program.


title: General Educational Development

text: The cost of the GED test for test-takers varies depending on the state. As of 2014, costs can be as low as $45 as is the case in Maryland, or free, as in New York, but the typical fees are $120 for all four tests, or $30 for each of the four subject tests. Testing online has an additional fee, typically $6 per test. The cost of each test outside of the US is $80 each.


title: General Educational Development

text: In Canada the testing is free in Nova Scotia, New Brunswick, Prince Edward Island, and the Northwest Territories. For the remaining provinces and territories the price varies between $6 per test in Newfoundland and Labrador to $40 per test in Alberta.


title: General Educational Development

text: Disabled persons who want to take the GED test may be entitled to receive reasonable testing accommodations. If a qualified professional has documented the disability, the candidate should get the appropriate form from the Testing Center:


title: General Educational Development

text: The candidate returns the completed form to the GED testing center. Each request is considered individually. If accommodations are approved, the local GED testing examiner will conduct the testing with the approved accommodations, which are provided at no extra charge. Accommodations may include, but are not limited to:


title: General Educational Development

text: Possible scores on an individual test within the GED battery range from a minimum of 100 to a maximum of 200. A score of 200 on an individual test puts the student in the top 1% of graduating high school seniors. ACE issues recommendations for what constitutes a minimum passing score for any given sub-test (currently 145) and for the test as a whole (currently 580—i.e., an average of 145 per test across all four sub-tests). Although most GED-issuing jurisdictions (for the most part, Boards of Education of U.S. states) adopt these minimum standards as their own, a jurisdiction may establish higher standards for issuance of the certificate if it chooses. Many jurisdictions award honors-level equivalency diplomas to students meeting certain criteria higher than those for a standard diploma in a given jurisdiction. Some districts hold graduation ceremonies for GED tests passers and/or award scholarships to the highest scorers.


title: General Educational Development

text: Colleges admit that grades based upon high school may require a minimum score on the GED test for admittance based upon the test. For example, Arizona State University requires an average sub-test score of 500 in addition to the certificate.


title: General Educational Development

text: If a student passes one or more, but not all four tests within the battery, he or she only needs to retake the test(s) not passed. Most places limit the number of times students may take each individual test within a year. A student may encounter a waiting period before being allowed to retake a failed test. Tests must be completed by the expiration date, which is generally every two years on the last day of the year.


title: General Educational Development

text: The GED test is available in many countries around the world. Since 2015, the GED test has become popular in African countries including South Africa and Namibia.


title: General Educational Development

text: Many government institutions and universities regard the GED test credential as the same as a high school diploma with respect to program eligibility and as a prerequisite for admissions. The U.S. military, however, has explicitly higher requirements in admissions for GED test takers to compensate for their lack of a traditional high school diploma. The test is administered to a representative sample of graduating high-school seniors each year, about 30% of whom fail the test. That only 70% of these students pass the test may show that it is harder than commonly believed.


title: General Educational Development

text: The GED certification itself (i.e., without further post-secondary education or training) does not create the same labor market opportunities available to traditional high school graduates. While people who have earned the GED test credential tend to earn more than dropouts and less than high school graduates, economist James Heckman has found that this is primarily due to existing differences in the characteristics and backgrounds of GED test graduates. When controlling for other influences, he finds no evidence that, for the average taker, the GED test credential improves an individual's economic opportunities above those for other dropouts.


title: General Educational Development

text: Some say overall, there is a certain level of stigma for GED certification holders that affects employability or pursuit of higher education.


title: General Educational Development

text: There are calls for the GED to be abolished. Those who support abolishing the GED say the program reduces high school graduation rates, is outmoded, and a financial burden for low-income participants.


title: Waves (film)

text: Waves is a 2019 American drama film written, co-produced and directed by Trey Edward Shults. Along with Shults, it was produced by Kevin Turen and Jessica Row. It stars Kelvin Harrison Jr., Lucas Hedges, Taylor Russell, Alexa Demie, Renée Elise Goldsberry, and Sterling K. Brown. It traces the emotional journey of a suburban American family as they navigate love, forgiveness and coming together in the wake of a tragic loss.


title: Waves (film)

text: Principal photography began on July 9, 2018 in Broward County, Florida and wrapped up on August 24, 2018. The cast was announced in July, with Demie joining in August.


title: Waves (film)

text: It had its world premiere at the Telluride Film Festival on August 30, 2019, and was released in the United States on November 15, 2019, by A24. It received positive reviews from critics, who praised the performances (particularly that of Harrison, Russell, and Brown), cinematography, and Shults' direction. It grossed over $2 million worldwide.


title: Waves (film)

text: Tyler Williams is a popular high school senior on the wrestling team. He parties with his friends and spends time with his girlfriend, Alexis Lopez, but is routinely pushed to be better by his domineering father, Ronald. Unbeknownst to anyone, Tyler suffers from a Level 5 SLAP tear, which he keeps secret from his family and team by stealing his father's painkillers. Against the recommendation of his doctor, Tyler continues to wrestle competitively until he is thrown onto his shoulder during a match, causing irreparable damage and ending his season, as well as his career. Tyler's life is further complicated when he receives a text from Alexis, who reveals that her period is late, indicating that she is pregnant. He takes her to get an abortion, but she has a last-minute change of heart. On the way home, the couple has a fight and Alexis walks home alone.


title: Waves (film)

text: Distraught, Tyler begins drinking heavily and abusing drugs. Eventually, Tyler texts Alexis, who is willing to mend their relationship. She tells Tyler that she has decided to keep the child with support from her family, but when Tyler demands to speak to her in person, Alexis breaks up with him and blocks his number, sending him into a rage as he destroys his bedroom. The night of the school's "Maverick Ball", a grounded Tyler goes on Instagram and sees a picture showing Alexis posing with another boy. He gets drunk and high and attempts to leave the house, but is confronted by his stepmom, Catherine. His father attempts to defuse the situation, but Tyler pushes him to the floor and leaves.


title: Waves (film)

text: He drives to a house party, where he sees Alexis go upstairs with the boy from the photo. Tyler's younger sister Emily notices his arrival from a distance but says nothing, and Tyler drinks heavily before following Alexis to the garage. An exchange of words leads to an argument, which turns physical when Alexis slaps Tyler and he hits her back, causing her head to hit the floor. She begins to bleed out, and, horrified, Tyler flees. Ronald arrives shortly after, finding Emily, who tearfully confirms Ronald's suspicions. Tyler returns home before attempting to run away on foot. However, he is quickly caught and arrested by the police. Paramedics attempt to resuscitate Alexis, but she dies from her injuries.


title: Waves (film)

text: Despite a guilty plea, Tyler is sentenced to life in prison for second-degree murder, with the eligibility of parole after thirty years. After school one day, Emily is approached by Luke, an awkward but kindhearted classmate who was also one of Tyler's wrestling teammates. He asks her out to lunch and the two begin dating. Emily begins to open up to Luke, participating in activities outside of school with him, such as swimming with manatees. During this time, Emily also overhears Ronald and Catherine arguing about Tyler, with Ronald blaming Catherine for her lack of presence in his and Emily's lives, and Catherine accusing Ronald of putting too much pressure on Tyler. During a conversation they have while fishing, Emily reveals to Ronald that she feels guilty, believing that she could've stopped Tyler and that she hates him for what he did. Ronald advises her not to hold a grudge against her brother and that no matter what, he loves them both. The two reconcile.


title: Waves (film)

text: Meanwhile, Emily and Luke become closer, eventually having sex for the first time. Emily learns from Luke that his estranged, abusive father is dying of cancer, and she urges him to make amends with him with the time he has left, as Emily and Tyler's mother died from a drug overdose when they were young. The two drive to Columbia, Missouri, where Luke's father is residing, and upon seeing his son, Luke's father's spirits are lifted, with Luke forgiving his father. He survives longer than expected, but passes away one night, which causes Luke to heavily sob. Emily consoles Luke on the drive home to Miami while Catherine visits Tyler in prison, Alexis' parents grieve, and Ronald reconnects with Catherine. Sometime after arriving home, Emily rides her bike down an empty street and lifts her arms from the handlebars.


title: Waves (film)

text: According to Adam White of "The Independent", initially the family of the film did not have a racial background specified in the plans.


title: Waves (film)

text: Demie suggested a scene where Alexis and Emily meet in a restroom, with the scene implying a change of focus from one woman to the other. Lauren McCarthy of "Nylon" stated that the film had a "collaborative nature" style.


title: Waves (film)

text: Demie stated that the filming of the initial driving scene made it seem like a film crew was not present. Demie also stated that Alexis, compared to Maddy Perez in "Euphoria", another character Demie portrayed, "is much stronger, she’s always fighting back and setting boundaries".


title: Waves (film)

text: In July 2018, it was announced Lucas Hedges, Sterling K. Brown, Kelvin Harrison Jr. and Taylor Russell joined the cast of the film. In August 2018, Alexa Demie joined the cast.


title: Waves (film)

text: Trent Reznor and Atticus Ross composed the film's score, replacing Shults' frequent collaboration with Brian McOmber. In an interview, Shults stated, "I got one of the coolest e-mails of my life one day that Trent Reznor was a fan and wanted to meet. I got on a plane as soon as I could and went to L.A. and met him and Atticus for lunch. Instantly, the energy was great. They had seen and dug my other movies and just wanted to work together." The score was released on November 26, 2019, on Nine Inch Nails' online store.


title: Waves (film)

text: The film's soundtrack features a compilation of contemporary rap, R&B, alternative, and experimental pop music from artists including Animal Collective, Tame Impala, Frank Ocean, A$AP Rocky, Kendrick Lamar, The Shoes, H.E.R., Tyler, the Creator, Fuck Buttons, Amy Winehouse, Kanye West, THEY., Kid Cudi, Colin Stetson, SZA, Chance the Rapper, Radiohead, and Alabama Shakes. The full list of songs used in the film was made available on A24's website, in an article where Shults annotates the usage of the songs.


title: Waves (film)

text: Shults wrote specific music cues into the screenplay such as Lamar's "Backseat Freestyle" and embedded files into the scripts sent to actors so they could listen to the intended songs for each scene. He shot the film having not yet acquired the rights to certain songs and agreed to cut scenes if negotiations failed than reshoot. A letter and rough cut of the film was sent to Ocean, who cleared his songs for use after watching. Negotiations with West stretched over months before "I Am a God" was approved with a stipulation that a "clean" version would need to be featured due to West moving away from secular music.


title: Waves (film)

text: "Waves" premiered at the Telluride Film Festival on August 30, 2019. It screened at the Toronto International Film Festival on September 10, 2019. It was released on November 15, 2019 in New York City and Los Angeles, expanding to major U.S. cities the week after, with a wide release set for December 6. In September, Focus Features acquired the international distribution rights to the film, excluding Canada, China and Japan.


title: Waves (film)

text: "Waves" grossed a total of $1.7 million in the United States. In its limited opening weekend, the film made $134,333 from four theaters, a per-venue average of $33,583. Expanding to 21 theaters the following weekend the film made $168,760, and then $140,995 from 44 theaters in its third.


title: Waves (film)

text: On review aggregator Rotten Tomatoes, the film holds an approval rating of based on reviews, with an average rating of . The site's consensus reads: "An up-close look at one family's emotional ups and downs, "Waves" captures complicated dynamics with tenderness and grace." On Metacritic, the film has a weighted average score of 80 out of 100, based on 45 critics, indicating "generally favorable reviews".


title: Waves (film)

text: Paul Whitington of the "Irish Independent" gave the film five stars. Brian Tallerico, editor of RogerEbert.com, gave it four out of four stars and wrote "This is a deeper and more profound film than your average character drama, a masterpiece that’s hard to walk away from without checking your own grievances and grief."


title: Waves (film)

text: IndieWire's Eric Kohn gave the film a B+, stating "It’s a definitive statement on the present moment, evoking the sheer horror of every uncertain exchange, and the courage involved in moving ahead regardless of what it means for the future". Justin Chang of the "Los Angeles Times" called it "deeply rooted in its characters’ consciousness, alert to the feelings of dread, shame, rage and despair that threaten to bring these fast-moving lives to a standstill". It was chosen by the National Board of Review as one of the ten best films of the year.


title: Waves (film)

text: Sterling K. Brown stated, "I’ve never seen a middle to upper middle class black family dealing with the problems that [the family] has in this movie".


title: Waves (film)

text: Prince Shakur of "Teen Vogue" stated that Tyler's story shows the effects of "toxic masculinity". Harrison stated that due to the previously inferior position in society occupied by African-Americans, there is often "a pressure placed on young Black boys to excel".


title: Battle Hymn of the Republic

text: The "Battle Hymn of the Republic", also known as "Mine Eyes Have Seen the Glory" or "Glory, Glory Hallelujah" outside of the United States, is a popular American patriotic song written by the abolitionist writer Julia Ward Howe.


title: Battle Hymn of the Republic

text: Howe wrote her lyrics to the music of the song "John Brown's Body" in November 1861 and first published them in "The Atlantic Monthly" in February 1862. The song links the judgment of the wicked at the end of the age (through allusions to biblical passages such as and ) with the American Civil War.


title: Battle Hymn of the Republic

text: The "Glory, Hallelujah" tune was a folk hymn developed in the oral hymn tradition of camp meetings in the southern United States and first documented in the early 1800s. In the first known version, "Canaan's Happy Shore," the text includes the verse "Oh! Brothers will you meet me (3×)/On Canaan's happy shore?" and chorus "There we'll shout and give Him glory (3×)/For glory is His own." This developed into the familiar "Glory, glory, hallelujah" chorus by the 1850s. The tune and variants of these words spread across both the southern and northern United States.


title: Battle Hymn of the Republic

text: At a flag-raising ceremony at Fort Warren, near Boston, Massachusetts, on Sunday, May 12, 1861, the song "John Brown's Body", using the well known "Oh! Brothers" tune and the "Glory, Hallelujah" chorus, was publicly played "perhaps for the first time". The American Civil War had begun the previous month.


title: Battle Hymn of the Republic

text: In 1890, George Kimball wrote his account of how the 2nd Infantry Battalion of the Massachusetts militia, known as the "Tiger" Battalion, collectively worked out the lyrics to "John Brown's Body." Kimball wrote:


title: Battle Hymn of the Republic

text: According to Kimball, these sayings became by-words among the soldiers and, in a communal effort — similar in many ways to the spontaneous composition of camp meeting songs described above — were gradually put to the tune of "Say, Brothers":


title: Battle Hymn of the Republic

text: Some leaders of the battalion, feeling the words were coarse and irreverent, tried to urge the adoption of more fitting lyrics, but to no avail. The lyrics were soon prepared for publication by members of the battalion, together with publisher C. S. Hall. They selected and polished verses they felt appropriate, and may even have enlisted the services of a local poet to help polish and create verses.


title: Battle Hymn of the Republic

text: The official histories of the old First Artillery and of the 55th Artillery (1918) also record the Tiger Battalion's role in creating the John Brown Song, confirming the general thrust of Kimball's version with a few additional details.


title: Battle Hymn of the Republic

text: Kimball's battalion was dispatched to Murray, Kentucky, early in the Civil War, and Julia Ward Howe heard this song during a public review of the troops outside Washington, D.C., on Upton Hill, Virginia. Rufus R. Dawes, then in command of Company "K" of the 6th Wisconsin Volunteer Infantry, stated in his memoirs that the man who started the singing was Sergeant John Ticknor of his company. Howe's companion at the review, the Reverend James Freeman Clarke, suggested to Howe that she write new words for the fighting men's song. Staying at the Willard Hotel in Washington on the night of November 18, 1861, Howe wrote the verses to the "Battle Hymn of the Republic." Of the writing of the lyrics, Howe remembered:


title: Battle Hymn of the Republic

text: Howe's "Battle Hymn of the Republic" was first published on the front page of "The Atlantic Monthly" of February 1862. The sixth verse written by Howe, which is less commonly sung, was not published at that time. The song was also published as a broadside in 1863 by the Supervisory Committee for Recruiting Colored Regiments in Philadelphia.


title: Battle Hymn of the Republic

text: Both "John Brown" and "Battle Hymn of the Republic" were published in Father Kemp's "Old Folks Concert Tunes" in 1874 and reprinted in 1889. Both songs had the same Chorus with an additional "Glory" in the second line: "Glory! Glory! Glory! Hallelujah!"


title: Battle Hymn of the Republic

text: Julia Ward Howe was married to Samuel Gridley Howe, the famed scholar in education of the blind. Samuel and Julia were also active leaders in anti-slavery politics and strong supporters of the Union. Samuel Howe was a member of the Secret Six, the group who funded John Brown's work.


title: Battle Hymn of the Republic

text: "Canaan's Happy Shore" has a verse and chorus of equal metrical length and both verse and chorus share an identical melody and rhythm. "John Brown's Body" has more syllables in its verse and uses a more rhythmically active variation of the "Canaan" melody to accommodate the additional words in the verse. In Howe's lyrics, the words of the verse are packed into a yet longer line, with even more syllables than "John Brown's Body." The verse still uses the same underlying melody as the refrain, but the addition of many dotted rhythms to the underlying melody allows for the more complex verse to fit the same melody as the comparatively short refrain.


title: Battle Hymn of the Republic

text: <score sound=1>\relative c" { \partial 16 g16 g8. g16 g8. f16 e8. g16 c8. d16 e8. e16 e8. d16 c4 c8. c16 a8. a16 a8. b16 c8. c16 b8. a16 \partial 2. g8. a16 g8. e16 g4} \addlyrics {Mine eyes have seen the glo– ry of the com– ing of the Lord: He is tramp– ling out the vin– tage where the grapes of wrath are stored; } </score>


title: Battle Hymn of the Republic

text: Howe submitted the lyrics she wrote to "The Atlantic Monthly", and it was first published in the February 1862 issue of the magazine.


title: Battle Hymn of the Republic

text: <nowiki>*</nowiki> Many modern performances and recordings of the "Battle Hymn of the Republic" use the lyric "As He died to make men holy, let us "live" to make men free" as opposed to the wartime lyric originally written by Julia Ward Howe: "let us die to make men free."


title: Battle Hymn of the Republic

text: Howe's original manuscript differed slightly from the published version. Most significantly, it included a final verse:


title: Battle Hymn of the Republic

text: In the years since the Civil War, "The Battle Hymn of the Republic" has been used frequently as an American patriotic song.


title: Battle Hymn of the Republic

text: The lyrics of "Battle Hymn of the Republic" appear in Dr. Martin Luther King Jr.'s sermons and speeches, most notably in his speech "How Long, Not Long" from the steps of the Alabama State Capitol building on March 25, 1965, after the successful Selma to Montgomery march, and in his final sermon "I've Been to the Mountaintop", delivered in Memphis, Tennessee on the evening of April 3, 1968, the night before his assassination. In fact, the latter sermon, King's last public words, ends with the first lyrics of the "Battle Hymn": "Mine eyes have seen the glory of the coming of the Lord."


title: Battle Hymn of the Republic

text: Bishop Michael B. Curry of North Carolina, after his election as the first African American Presiding Bishop of The Episcopal Church, delivered a sermon to the Church's General Convention on July 3, 2015, in which the lyrics of the "Battle Hymn" framed the message of God's love. After proclaiming "Glory, glory, hallelujah, His truth is marching on", a letter from President Barack Obama was read, congratulating Bishop Curry on his historic election. Curry is known for quoting the "Battle Hymn" during his sermons.


title: Battle Hymn of the Republic

text: The inscription "Mine eyes have seen the glory of the coming of the Lord" is written at the feet of the sculpture of the fallen soldier at the American Cemetery in Normandy, France.


title: Battle Hymn of the Republic

text: The tune has played a role in many movies where patriotic music has been required, including the 1970 World War II war comedy "Kelly's Heroes", and the 1999 sci-fi western "Wild Wild West". Words from the first verse gave John Steinbeck's wife Carol Steinbeck the title of his 1939 masterpiece "The Grapes of Wrath". The title of John Updike's "In the Beauty of the Lilies" also came from this song, as did "Terrible Swift Sword" and "Never Call Retreat", two volumes in Bruce Catton's "Centennial History of the Civil War". "Terrible Swift Sword" is also the name of a board wargame simulating the Battle of Gettysburg. The song was used in the anime "Girls und Panzer" as the tune used when members of the American-inspired fictional Saunders University High School are seen moving in their various M4 Sherman variants.


title: Battle Hymn of the Republic

text: Words from the second last line of the last verse are paraphrased in Leonard Cohen's song "Steer Your Way". It was originally published as a poem in "The New Yorker" magazine. "As He died to make men holy, let us die to make men free" becomes "As He died to make men holy, let us die to make things cheap".


title: Battle Hymn of the Republic

text: The refrain "Glory, glory, hallelujah!" has been adopted by fans of a number of sporting teams, most notably in the English and Scottish Premier Leagues. The popular use of the tune by Tottenham Hotspur can be traced to September 1961 during the 1961–62 European Cup. Their first opponents in the competition were the Polish side Górnik Zabrze, and the Polish press described the Spurs team as "no angels" due to their rough tackling. In the return leg at White Hart Lane, some fans then wore angel costumes at the match holding placards with slogans such as "Glory be to shining White Hart Lane", and the crowded started singing the refrain "Glory, glory, hallelujah" as Spurs beat the Poles 8–1, starting the tradition at Tottenham. It was released as the B-side to "Ozzie's Dream" for the 1981 Cup Final.


title: Battle Hymn of the Republic

text: The theme was then picked up by Hibernian, with Hector Nicol's release of the track "Glory, glory to the Hibees" in 1963. "Glory, Glory Leeds United" was a popular chant during Leeds' 1970 FA Cup run. Manchester United fans picked it up as "Glory, Glory Man United" during the 1983 FA Cup Final. As a result of its popularity with these and other British teams, it has spread internationally and to other sporting codes. An example of its reach is its popularity with fans of the Australian Rugby League team, the South Sydney Rabbitohs (Glory, Glory to South Sydney) and to A-League team Perth Glory. Brighton fans celebrate their 1970s legend by singing "Mine eyes have seen the glory of the coming of the Lord, he played for Brighton and Hove Albion and his name is Peter Ward, etc."


title: Battle Hymn of the Republic

text: Some songs make use of both the melody and elements of the lyrics of "Battle Hymn of the Republic", either in tribute or as a parody:


title: Battle Hymn of the Republic

text: Other songs simply use the melody, i.e. the melody of "John Brown's Body", with no lyrical connection to "The Battle Hymn of the Republic":


title: Battle Hymn of the Republic

text: Irish composer Ina Boyle set the text for solo soprano, mixed choir and orchestra; she completed her version in 1918.


title: Battle Hymn of the Republic

text: The British Methodist Hymn Book used in the mid 20th century had Walford Davies's "Vision" as the first tune, and the "Battle Hymn" as the second tune.


title: Denatured alcohol

text: Denatured alcohol (also called methylated spirits in Australia, Canada, Ireland, New Zealand, South Africa, and the United Kingdom; wood spirit; and denatured rectified spirit) is ethanol that has additives to make it poisonous, bad-tasting, foul-smelling, or nauseating to discourage its recreational consumption. It is sometimes dyed so that it can be identified visually. Pyridine and methanol, each and together, make denatured alcohol poisonous; and denatonium makes it bitter.


title: Denatured alcohol

text: Denatured alcohol is used as a solvent and as fuel for alcohol burners and camping stoves. Because of the diversity of industrial uses for denatured alcohol, hundreds of additives and denaturing methods have been used. The main additive usually is 10% methanol (methyl alcohol), hence the name "methylated spirits". Other common additives include isopropyl alcohol, acetone, methyl ethyl ketone, and methyl isobutyl ketone.


title: Denatured alcohol

text: Denaturing alcohol does not alter the ethanol molecule (chemically or structurally), unlike denaturation in biochemistry. Rather, the ethanol is mixed with other chemicals to form a foul-tasting, often toxic, solution. For many of these solutions, it is intentionally difficult to separate the components.


title: Denatured alcohol

text: In many countries, sales of alcoholic beverages are heavily taxed for revenue and public health policy purposes (see Pigovian tax). In order to avoid paying beverage taxes on alcohol that is not meant to be consumed, the alcohol must be "denatured", or treated with added chemicals to make it unpalatable. Its composition is tightly defined by government regulations in countries that tax alcoholic beverages. Denatured alcohol is used identically to ethanol itself except for applications that involve fuel, surgical and laboratory stock. Pure ethanol is required for food and beverage applications and certain chemical reactions where the denaturant would interfere. In molecular biology, denatured ethanol should not be used for the precipitation of nucleic acids, since the additives may interfere with downstream applications.


title: Denatured alcohol

text: Denatured alcohol has no advantages for any purpose over normal ethanol; it is a public policy compromise. As denatured alcohol is sold without the often heavy taxes on alcohol suitable for consumption, it is a cheaper solution for most uses that do not involve drinking. If pure ethanol were made cheaply available for fuel, solvents, or medicinal purposes, it would likely be enjoyed as a drink by many people without payment of alcoholic beverage taxes.


title: Denatured alcohol

text: Despite its poisonous content, denatured alcohol is sometimes consumed as a surrogate alcohol. This can result in blindness or death if it contains methanol. For instance, during the Prohibition in the United States, federal law required methanol in domestically manufactured industrial alcohols. From 25–27 December 1926, which was roughly at the midpoint of the "Noble Experiment" of nationwide alcohol prohibition, 31 people in New York City alone died of methanol poisoning. To help prevent this, denatonium is often added to give the substance an extremely bitter flavour. Substances such as pyridine are added to give the mixture an unpleasant odour, and agents such as syrup of ipecac may also be included to induce vomiting.


title: Denatured alcohol

text: In the USSR, denatured alcohol was used as drinking alcohol surrogate, along with many other technical ethanol-containing products. This was especially common during various anti-alcohol campaigns initiated by the Soviet government. There is much evidence to that in both popular folklore and in literature and music. The word “denaturat” (Russian: денатурат) even gained a special symbolic meaning. Its consumption is mentioned in songs of Vladimir Vysotsky, as well as written works of Venedikt Yerofeev, Yuz Aleshkovsky, and Vyacheslav Shishkov.


title: Denatured alcohol

text: Diverse additives are used to make it difficult to use distillation or other simple processes to reverse the denaturation. Methanol is commonly used both because its boiling point is close to that of ethanol and because it is toxic. Another typical denaturant is pyridine. Often the denatured alcohol is dyed with methyl violet.


title: Denatured alcohol

text: There are several grades of denatured alcohol, but in general the denaturants used are similar. As an example, the formulation for "completely denatured alcohol", according to 2005 British regulations was as follows:


title: Denatured alcohol

text: The European Union agreed in February 2013 to the mutual procedures for the complete denaturing of alcohol:


title: Denatured alcohol

text: A specially denatured alcohol (SDA) is one of many types of denatured alcohol specified under the United States Title 27 of the Code of Federal Regulations Section 21.151. A specially denatured alcohol is a combination of ethanol and another chemical substance, e.g., ethyl acetate in SDA 29, 35, and 35A, added to render the mixture unsuitable for drinking. SDAs are often used in cosmetic products, and can also be used in chemical manufacturing, pharmaceuticals, and solvents. Another example is SDA 40-B, which contains tert-butyl alcohol and denatonium benzoate, N.F. In the United States and other countries, the use of denatured alcohol unsuitable for beverages avoids excise taxes on alcohol.


title: Turbofan

text: The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft propulsion. The word "turbofan" is a portmanteau of "turbine" and "fan": the "turbo" portion refers to a gas turbine engine which achieves mechanical energy from combustion, and the "fan", a ducted fan that uses the mechanical energy from the gas turbine to force air rearwards. Thus, whereas all the air taken in by a turbojet passes through the combustion chamber and turbines, in a turbofan some of that air bypasses these components. A turbofan thus can be thought of as a turbojet being used to drive a ducted fan, with both of these contributing to the thrust.


title: Turbofan

text: The ratio of the mass-flow of air bypassing the engine core to the mass-flow of air passing through the core is referred to as the bypass ratio. The engine produces thrust through a combination of these two portions working together; engines that use more jet thrust relative to fan thrust are known as "low-bypass turbofans", conversely those that have considerably more fan thrust than jet thrust are known as "high-bypass". Most commercial aviation jet engines in use today are of the high-bypass type, and most modern military fighter engines are low-bypass. Afterburners are used on low-bypass turbofan engines with bypass and core mixing before the afterburner.


title: Turbofan

text: Modern turbofans have either a large single-stage fan or a smaller fan with several stages. An early configuration combined a low-pressure turbine and fan in a single rear-mounted unit.


title: Turbofan

text: The turbofan was invented to improve the fuel consumption of the turbojet. It would do this by increasing the mass and lowering the speed of the propelling jet compared to that of the turbojet. This would be done mechanically by adding a ducted fan rather than using viscous forces by adding an ejector, as first envisaged by Whittle.


title: Turbofan

text: Frank Whittle envisioned flight speeds of 500 mph in writing his UK patent 471,368 "Improvements relating to the propulsion of aircraft", filed in March 1936, and in which he describes the principles behind the turbofan, although not called as such at that time. The turbojet uses the gas from its thermodynamic cycle as its propelling jet. There are two penalties for using the cycle gas for the propelling jet for aircraft speeds of 500 mph and they are addressed by the turbofan.


title: Turbofan

text: There is energy wasted because the propelling jet is going much faster rearwards than the aircraft is going forwards, leaving a very fast wake. The kinetic energy of the wake is a reflection of the fuel used to produce the wake rather than fuel used to move the aircraft forwards and, as such, is fuel wasted. However,it is a fundamental aspect of producing thrust in a fluid by accelerating some of it rearwards whether by a propeller or a combustor in a duct (ramjet) and, as such, can only be reduced and not eliminated. The turbofan reduces the speed of the propelling jet.


title: Turbofan

text: The other penalty is present because any action to reduce the fuel consumption of the engine by increasing its pressure ratio or turbine temperature causes a corresponding increase in pressure and temperature in the exhaust duct which in turn cause a higher gas speed from the propelling nozzle (and higher KE and wasted fuel). Although the engine uses less fuel to produce a pound of thrust more fuel is wasted in the faster propelling jet. In other words, the independence of thermal and propulsive efficiencies, as exists with the piston engine/propeller combination which preceded the turbojet, is lost. In contrast, Roth considers regaining this independence the single most important feature of the turbofan which allows specific thrust to be chosen independently of the gas generator cycle.


title: Turbofan

text: The working substance of the thermodynamic cycle is the only mass accelerated to produce thrust in a turbojet which is a serious limitation (high fuel consumption) for aircraft speeds below supersonic. For subsonic flight speeds the speed of the propelling jet has to be reduced because there is a price to be paid in producing the thrust. The energy required to accelerate the gas inside the engine (increase in kinetic energy) is expended in two ways, by producing a change in momentum ( ie a force), and a wake which is an unavoidable consequence of producing thrust by an airbreathing engine (or propeller). The wake velocity, and fuel burned to produce it, can be reduced and the required thrust still maintained by increasing the mass accelerated. A turbofan does this by transferring energy available inside the engine, from the gas generator, to a ducted fan which produces a second, additional mass of accelerated air.


title: Turbofan

text: The transfer of energy from the core to bypass air results in lower pressure and temperature gas entering the core nozzle (lower exhaust velocity) and fan-produced temperature and pressure entering the fan nozzle. The amount of energy transferred depends on how much pressure rise the fan is designed to produce (fan pressure ratio). The best energy exchange (lowest fuel consumption) between the two flows, and how the jet velocities compare, depends on how efficiently the transfer takes place which depends on the losses in the fan-turbine and fan.


title: Turbofan

text: The fan flow has lower exhaust velocity, giving much more thrust per unit energy (lower specific thrust). Both airstreams contribute to the gross thrust of the engine. The additional air for the bypass stream increases the ram drag in the air intake stream-tube, but there is still a significant increase in net thrust. The overall effective exhaust velocity of the two exhaust jets can be made closer to a normal subsonic aircraft's flight speed and gets closer to the ideal Froude efficiency. A turbofan accelerates a larger mass of air more slowly, compared to a turbojet which accelerates a smaller amount more quickly, which is a less efficient way to generate the same thrust (see the efficiency section below).


title: Turbofan

text: The ratio of the mass-flow of air bypassing the engine core compared to the mass-flow of air passing through the core is referred to as the bypass ratio. Engines with more jet thrust relative to fan thrust are known as "low-bypass turbofans", those that have considerably more fan thrust than jet thrust are known as "high-bypass". Most commercial aviation jet engines in use today are high-bypass, and most modern fighter engines are low-bypass. Afterburners are used on low-bypass turbofans on combat aircraft.


title: Turbofan

text: The "bypass ratio (BPR)" of a turbofan engine is the ratio between the mass flow rate of the bypass stream to the mass flow rate entering the core. A bypass ratio of 6, for example, means that 6 times more air passes through the bypass duct than the amount that passes through the combustion chamber.


title: Turbofan

text: Turbofan engines are usually described in terms of BPR, which together with overall pressure ratio, turbine inlet temperature and fan pressure ratio are important design parameters. In addition BPR is quoted for turboprop and unducted fan installations because their high propulsive efficiency gives them the overall efficiency characteristics of very high bypass turbofans. This allows them to be shown together with turbofans on plots which show trends of reducing specific fuel consumption (SFC) with increasing BPR. BPR can also be quoted for lift fan installations where the fan airflow is remote from the engine and doesn't flow past the engine core.


title: Turbofan

text: Considering a constant core (ie fixed pressure ratio and turbine inlet temperature), core and bypass jet velocities equal and a particular flight condition (ie Mach number and altitude) the fuel consumption per lb of thrust (sfc) decreases with increase in BPR. At the same time gross and net thrusts increase, but by different amounts. There is considerable potential for reducing fuel consumption for the same core cycle by increasing BPR.This is achieved because of the reduction in pounds of thrust per lb/sec of airflow (specific thrust) and the resultant reduction in lost kinetic energy in the jets (increase in propulsive efficiency).


title: Turbofan

text: If all the gas power from a gas turbine is converted to kinetic energy in a propelling nozzle, the aircraft is best suited to high supersonic speeds. If it is all transferred to a separate big mass of air with low kinetic energy, the aircraft is best suited to zero speed (hovering). For speeds in between, the gas power is shared between a separate airstream and the gas turbine's own nozzle flow in a proportion which gives the aircraft performance required. The trade off between mass flow and velocity is also seen with propellers and helicopter rotors by comparing disc loading and power loading. For example, the same helicopter weight can be supported by a high power engine and small diameter rotor or, for less fuel, a lower power engine and bigger rotor with lower velocity through the rotor.


title: Turbofan

text: Bypass usually refers to transferring gas power from a gas turbine to a bypass stream of air to reduce fuel consumption and jet noise. Alternatively, there may be a requirement for an afterburning engine where the sole requirement for bypass is to provide cooling air. This sets the lower limit for BPR and these engines have been called "leaky" or continuous bleed turbojets (General Electric YJ-101 BPR 0.25) and low BPR turbojets (Pratt & Whitney PW1120). Low BPR (0.2) has also been used to provide surge margin as well as afterburner cooling for the Pratt & Whitney J58.


title: Turbofan

text: Propeller engines are most efficient for low speeds, turbojet engines – for high speeds, and turbofan engines – between the two. Turbofans are the most efficient engines in the range of speeds from about , the speed at which most commercial aircraft operate.


title: Turbofan

text: In a turbojet (zero-bypass) engine, the high temperature and high pressure exhaust gas is accelerated when it undergoes expansion through a propelling nozzle and produces all the thrust. The compressor absorbs the mechanical power produced by the turbine. In a bypass design, extra turbines drive a ducted fan that accelerates air rearward from the front of the engine. In a high-bypass design, the ducted fan and nozzle produce most of the thrust. Turbofans are closely related to turboprops in principle because both transfer some of the gas turbine's gas power, using extra machinery, to a bypass stream leaving less for the hot nozzle to convert to kinetic energy. Turbofans represent an intermediate stage between turbojets, which derive all their thrust from exhaust gases, and turbo-props which derive minimal thrust from exhaust gases (typically 10% or less). Extracting shaft power and transferring it to a bypass stream introduces extra losses which are more than made up by the improved propulsive efficiency. The turboprop at its best flight speed gives significant fuel savings over a turbojet even though an extra turbine, a gearbox and a propeller were added to the turbojet's low-loss propelling nozzle. The turbofan has additional losses from its greater number of compressor stages/blades, fan and bypass duct.


title: Turbofan

text: While a turbojet engine uses all of the engine's output to produce thrust in the form of a hot high-velocity exhaust gas jet, a turbofan's cool low-velocity bypass air yields between 30% and 70% of the total thrust produced by a turbofan system.


title: Turbofan

text: The thrust (F) generated by a turbofan depends on the effective exhaust velocity of the total exhaust, as with any jet engine, but because two exhaust jets are present the thrust equation can be expanded as:


title: Turbofan

text: The cold duct and core duct's nozzle systems are relatively complex due to the use of two separate exhaust flows. In high bypass engines, the fan is situated in a short duct near the front of the engine and typically has a convergent cold nozzle, with the tail of the duct forming a low pressure ratio nozzle that under normal conditions will choke creating supersonic flow patterns around the core. The core nozzle is more conventional, but generates less of the thrust, and depending on design choices, such as noise considerations, may conceivably not choke. In low bypass engines the two flows may combine within the ducts, and share a common nozzle, which can be fitted with afterburner.


title: Turbofan

text: Most of the air flow through a high-bypass turbofan is lower-velocity bypass flow: even when combined with the much-higher-velocity engine exhaust, the average exhaust velocity is considerably lower than in a pure turbojet. Turbojet engine noise is predominately jet noise from the high exhaust velocity. Therefore, turbofan engines are significantly quieter than a pure-jet of the same thrust, and jet noise is no longer the predominant source. Turbofan engine noise propagates both upstream via the inlet and downstream via the primary nozzle and the by-pass duct. Other noise sources are the fan, compressor and turbine.


title: Turbofan

text: Modern commercial aircraft employ high-bypass-ratio (HBPR) engines with separate flow, non-mixing, short-duct exhaust systems. Their noise is due to the speed, temperature, and pressure of the exhaust jet, especially during high-thrust conditions, such as those required for takeoff. The primary source of jet noise is the turbulent mixing of shear layers in the engine's exhaust. These shear layers contain instabilities that lead to highly turbulent vortices that generate the pressure fluctuations responsible for sound. To reduce the noise associated with jet flow, the aerospace industry has sought to disrupt shear layer turbulence and reduce the overall noise produced.


title: Turbofan

text: Fan noise may come from the interaction of the fan-blade wakes with the pressure field of the downstream fan-exit stator vanes. It may be minimized by adequate axial spacing between blade trailing edge and stator entrance.


title: Turbofan

text: At high engine speeds, as at takeoff, shock waves from the supersonic fan tips, because of their unequal nature, produce noise of a discordant nature known as "buzz saw" noise.


title: Turbofan

text: All modern turbofan engines have acoustic liners in the nacelle to damp their noise. They extend as much as possible to cover the largest surface area. The acoustic performance of the engine can be experimentally evaluated by means of ground tests or in dedicated experimental test rigs.


title: Turbofan

text: In the aerospace industry, chevrons are the "saw-tooth" patterns on the trailing edges of some jet engine nozzles that are used for noise reduction. The shaped edges smooth the mixing of hot air from the engine core and cooler air flowing through the engine fan, which reduces noise-creating turbulence. Chevrons were developed by GE under a NASA contract. Some notable examples of such designs are Boeing 787 and Boeing 747-8 on the Rolls-Royce Trent 1000 and General Electric GEnx engines.


title: Turbofan

text: Early turbojet engines were not very fuel-efficient because their overall pressure ratio and turbine inlet temperature were severely limited by the technology and materials available at the time.


title: Turbofan

text: The first turbofan engine, which was only run on a test bed, was the German Daimler-Benz DB 670, designated the 109-007 by the Nazi Ministry of Aviation, with a first run date of 27 May 1943, after the testing of the turbomachinery using an electric motor, which had been undertaken on 1 April 1943. Development of the engine was abandoned with its problems unsolved, as the war situation worsened for Germany.


title: Turbofan

text: Later in 1943, the British ground tested the Metrovick F.3 turbofan, which used the Metrovick F.2 turbojet as a gas generator with the exhaust discharging into a close-coupled aft-fan module comprising a contra-rotating LP turbine system driving two co-axial contra-rotating fans.


title: Turbofan

text: Improved materials, and the introduction of twin compressors, such as in the Bristol Olympus, and Pratt & Whitney JT3C engines, increased the overall pressure ratio and thus the thermodynamic efficiency of engines. They also had poor propulsive efficiency, because pure turbojets have a high specific thrust/high velocity exhaust, which is better suited to supersonic flight.


title: Turbofan

text: The original low-bypass turbofan engines were designed to improve propulsive efficiency by reducing the exhaust velocity to a value closer to that of the aircraft. The Rolls-Royce Conway, the world's first production turbofan, had a bypass ratio of 0.3, similar to the modern General Electric F404 fighter engine. Civilian turbofan engines of the 1960s, such as the Pratt & Whitney JT8D and the Rolls-Royce Spey, had bypass ratios closer to 1 and were similar to their military equivalents.


title: Turbofan

text: The first Soviet airliner powered by turbofan engines was the Tupolev Tu-124 introduced in 1962. It used the Soloviev D-20. 164 aircraft were produced between 1960 and 1965 for Aeroflot and other Eastern Bloc airlines, with some operating until the early 1990s.


title: Turbofan

text: The first General Electric turbofan was the aft-fan CJ805-23, based on the CJ805-3 turbojet. It was followed by the aft-fan General Electric CF700 engine, with a 2.0 bypass ratio. This was derived from the General Electric J85/CJ610 turbojet to power the larger Rockwell Sabreliner 75/80 model aircraft, as well as the Dassault Falcon 20, with about a 50% increase in thrust to . The CF700 was the first small turbofan to be certified by the Federal Aviation Administration (FAA). There were at one time over 400 CF700 aircraft in operation around the world, with an experience base of over 10 million service hours. The CF700 turbofan engine was also used to train Moon-bound astronauts in Project Apollo as the powerplant for the Lunar Landing Research Vehicle.


title: Turbofan

text: A high-specific-thrust/low-bypass-ratio turbofan normally has a multi-stage fan behind inlet guide vanes, developing a relatively high pressure ratio and, thus, yielding a high (mixed or cold) exhaust velocity. The core airflow needs to be large enough to ensure there is sufficient core power to drive the fan. A smaller core flow/higher bypass ratio cycle can be achieved by raising the inlet temperature of the high-pressure (HP) turbine rotor.


title: Turbofan

text: To illustrate one aspect of how a turbofan differs from a turbojet, comparisons can be made at the same airflow (to keep a common intake for example) and the same net thrust (i.e. same specific thrust). A bypass flow can be added only if the turbine inlet temperature is not too high to compensate for the smaller core flow. Future improvements in turbine cooling/material technology can allow higher turbine inlet temperature, which is necessary because of increased cooling air temperature, resulting from an overall pressure ratio increase.


title: Turbofan

text: The resulting turbofan, with reasonable efficiencies and duct loss for the added components, would probably operate at a higher nozzle pressure ratio than the turbojet, but with a lower exhaust temperature to retain net thrust. Since the temperature rise across the whole engine (intake to nozzle) would be lower, the (dry power) fuel flow would also be reduced, resulting in a better specific fuel consumption (SFC).


title: Turbofan

text: Some low-bypass ratio military turbofans (e.g. F404, JT8D) have variable inlet guide vanes to direct air onto the first fan rotor stage. This improves the fan surge margin (see compressor map).


title: Turbofan

text: Since the 1970s, most jet fighter engines have been low/medium bypass turbofans with a mixed exhaust, afterburner and variable area exit nozzle. An afterburner is a combustor located downstream of the turbine blades and directly upstream of the nozzle, which burns fuel from afterburner-specific fuel injectors. When lit, large volumes of fuel are burnt in the afterburner, raising the temperature of exhaust gases by a significant degree, resulting in a higher exhaust velocity/engine specific thrust. The variable geometry nozzle must open to a larger throat area to accommodate the extra volume and increased flow rate when the afterburner is lit. Afterburning is often designed to give a significant thrust boost for take off, transonic acceleration and combat maneuvers, but is very fuel intensive. Consequently, afterburning can be used only for short portions of a mission.


title: Turbofan

text: Unlike in the main engine, where stoichiometric temperatures in the combustor have to be reduced before they reach the turbine, an afterburner at maximum fuelling is designed to produce stoichiometric temperatures at entry to the nozzle, about . At a fixed total applied fuel:air ratio, the total fuel flow for a given fan airflow will be the same, regardless of the dry specific thrust of the engine. However, a high specific thrust turbofan will, by definition, have a higher nozzle pressure ratio, resulting in a higher afterburning net thrust and, therefore, a lower afterburning specific fuel consumption (SFC). However, high specific thrust engines have a high dry SFC. The situation is reversed for a medium specific thrust afterburning turbofan: i.e., poor afterburning SFC/good dry SFC. The former engine is suitable for a combat aircraft which must remain in afterburning combat for a fairly long period, but has to fight only fairly close to the airfield (e.g. cross border skirmishes). The latter engine is better for an aircraft that has to fly some distance, or loiter for a long time, before going into combat. However, the pilot can afford to stay in afterburning only for a short period, before aircraft fuel reserves become dangerously low.


title: Turbofan

text: The first production afterburning turbofan engine was the Pratt & Whitney TF30, which initially powered the F-111 Aardvark and F-14 Tomcat. Current low-bypass military turbofans include the Pratt & Whitney F119, the Eurojet EJ200, the General Electric F110, the Klimov RD-33, and the Saturn AL-31, all of which feature a mixed exhaust, afterburner and variable area propelling nozzle.


title: Turbofan

text: To further improve fuel economy and reduce noise, almost all of today's jet airliners and most military transport aircraft (e.g., the C-17) are powered by low-specific-thrust/high-bypass-ratio turbofans. These engines evolved from the high-specific-thrust/low-bypass-ratio turbofans used in such aircraft in the 1960s. Modern combat aircraft tend to use low-bypass ratio turbofans, and some military transport aircraft use turboprops.


title: Turbofan

text: Low specific thrust is achieved by replacing the multi-stage fan with a single-stage unit. Unlike some military engines, modern civil turbofans lack stationary inlet guide vanes in front of the fan rotor. The fan is scaled to achieve the desired net thrust.


title: Turbofan

text: The core (or gas generator) of the engine must generate enough power to drive the fan at its rated mass flow and pressure ratio. Improvements in turbine cooling/material technology allow for a higher (HP) turbine rotor inlet temperature, which allows a smaller (and lighter) core, potentially improving the core thermal efficiency. Reducing the core mass flow tends to increase the load on the LP turbine, so this unit may require additional stages to reduce the average stage loading and to maintain LP turbine efficiency. Reducing core flow also increases bypass ratio. Bypass ratios greater than 5:1 are increasingly common; the Pratt & Whitney PW1000G, which entered commercial service in 2016, attains 12.5:1.


title: Turbofan

text: Further improvements in core thermal efficiency can be achieved by raising the overall pressure ratio of the core. Improvements in blade aerodynamics can reduce the number of extra compressor stages required, and variable geometry (i.e., stators) enable high-pressure-ratio compressors to work surge-free at all throttle settings.


title: Turbofan

text: The first (experimental) high-bypass turbofan engine was the AVCO-Lycoming PLF1A-2, a T55 turboshaft-derived engine that was first run in February 1962. The PLF1A-2 had a geared fan stage, produced a static thrust of , and had a bypass ratio of 6:1. The General Electric TF39 became the first production model, designed to power the Lockheed C-5 Galaxy military transport aircraft. The civil General Electric CF6 engine used a derived design. Other high-bypass turbofans are the Pratt & Whitney JT9D, the three-shaft Rolls-Royce RB211 and the CFM International CFM56; also the smaller TF34. More recent large high-bypass turbofans include the Pratt & Whitney PW4000, the three-shaft Rolls-Royce Trent, the General Electric GE90/GEnx and the GP7000, produced jointly by GE and P&W.


title: Turbofan

text: The lower the specific thrust of a turbofan, the lower the mean jet outlet velocity, which in turn translates into a high thrust lapse rate (i.e. decreasing thrust with increasing flight speed). See technical discussion below, item 2. Consequently, an engine sized to propel an aircraft at high subsonic flight speed (e.g., Mach 0.83) generates a relatively high thrust at low flight speed, thus enhancing runway performance. Low specific thrust engines tend to have a high bypass ratio, but this is also a function of the temperature of the turbine system.


title: Turbofan

text: The turbofans on twin-engined transport aircraft produce enough take-off thrust to continue a take-off on one engine if the other engine shuts down after a critical point in the take-off run. From that point on the aircraft has less than half the thrust compared to two operating engines because the non-functioning engine is a source of drag. Modern twin engined airliners normally climb very steeply immediately after take-off. If one engine shuts down, the climb-out is much shallower, but sufficient to clear obstacles in the flightpath.


title: Turbofan

text: The Soviet Union's engine technology was less advanced than the West's, and its first wide-body aircraft, the Ilyushin Il-86, was powered by low-bypass engines. The Yakovlev Yak-42, a medium-range, rear-engined aircraft seating up to 120 passengers, introduced in 1980, was the first Soviet aircraft to use high-bypass engines.


title: Turbofan

text: Turbofan engines come in a variety of engine configurations. For a given engine cycle (i.e., same airflow, bypass ratio, fan pressure ratio, overall pressure ratio and HP turbine rotor inlet temperature), the choice of turbofan configuration has little impact upon the design point performance (e.g., net thrust, SFC), as long as overall component performance is maintained. Off-design performance and stability is, however, affected by engine configuration.


title: Turbofan

text: The basic element of a turbofan is a spool, a single combination of fan/compressor, turbine and shaft rotating at a single speed. For a given pressure ratio, the surge margin can be increased by two different design paths:


title: Turbofan

text: Most modern western civil turbofans employ a relatively high-pressure-ratio high-pressure (HP) compressor, with many rows of variable stators to control surge margin at low rpm. In the three-spool RB211/Trent the core compression system is split into two, with the IP compressor, which supercharges the HP compressor, being on a different coaxial shaft and driven by a separate (IP) turbine. As the HP compressor has a modest pressure ratio its speed can be reduced surge-free, without employing variable geometry. However, because a shallow IP compressor working line is inevitable, the IPC has one stage of variable geometry on all variants except the −535, which has none.


title: Turbofan

text: Although far from common, the single-shaft turbofan is probably the simplest configuration, comprising a fan and high-pressure compressor driven by a single turbine unit, all on the same spool. The Snecma M53, which powers Dassault Mirage 2000 fighter aircraft, is an example of a single-shaft turbofan. Despite the simplicity of the turbomachinery configuration, the M53 requires a variable area mixer to facilitate part-throttle operation.


title: Turbofan

text: One of the earliest turbofans was a derivative of the General Electric J79 turbojet, known as the CJ805-23, which featured an integrated aft fan/low-pressure (LP) turbine unit located in the turbojet exhaust jetpipe. Hot gas from the turbojet turbine exhaust expanded through the LP turbine, the fan blades being a radial extension of the turbine blades. This arrangement introduces an additional gas leakage path compared to a front-fan configuration and was a problem with this engine with higher-pressure turbine gas leaking into the fan airflow. An aft-fan configuration was later used for the General Electric GE36 UDF (propfan) demonstrator of the early 1980s.


title: Turbofan

text: In 1971 a concept was put forward by the NASA Lewis Research Center for a supersonic transport engine which operated as an aft-fan turbofan at take-off and subsonic speeds and a turbojet at higher speeds. This would give the low noise and high thrust characteristics of a turbofan at take-off, together with turbofan high propulsive efficiency at subsonic flight speeds. It would have the high propulsive efficiency of a turbojet at supersonic cruise speeds.


title: Turbofan

text: Many turbofans have at least basic two-spool configuration where the fan is on a separate low pressure (LP) spool, running concentrically with the compressor or high pressure (HP) spool; the LP spool runs at a lower angular velocity, while the HP spool turns faster and its compressor further compresses part of the air for combustion. The BR710 is typical of this configuration. At the smaller thrust sizes, instead of all-axial blading, the HP compressor configuration may be axial-centrifugal (e.g., CFE CFE738), double-centrifugal or even diagonal/centrifugal (e.g. Pratt & Whitney Canada PW600).


title: Turbofan

text: Higher overall pressure ratios can be achieved by either raising the HP compressor pressure ratio or adding compressor (non-bypass) stages or T-stages to the LP spool, between the fan and the HP compressor, to boost the latter. All of the large American turbofans (e.g. General Electric CF6, GE90, GE9X and GEnx plus Pratt & Whitney JT9D and PW4000) feature T-stages. The Rolls-Royce BR715 is a non-American example of this. The high bypass ratios used in modern civil turbofans tend to reduce the relative diameter of the T-stages, reducing their mean tip speed. Consequently, more T-stages are required to develop the necessary pressure rise.


title: Turbofan

text: Rolls-Royce chose a three-spool configuration for their large civil turbofans (i.e. the RB211 and Trent families), where the T-stages of the boosted two-spool configuration are separated into a separate intermediate pressure (IP) spool, driven by its own turbine. The first three-spool engine was the earlier Rolls-Royce RB.203 Trent of 1967.


title: Turbofan

text: The Garrett ATF3, powering the Dassault Falcon 20 business jet, has an unusual three spool layout with an aft spool not concentric with the two others.


title: Turbofan

text: Ivchenko Design Bureau chose the same configuration as Rolls-Royce for their Lotarev D-36 engine, followed by Lotarev/Progress D-18T and Progress D-436.


title: Turbofan

text: The Turbo-Union RB199 military turbofan also has a three-spool configuration, as do the military Kuznetsov NK-25 and NK-321.


title: Turbofan

text: As bypass ratio increases, the fan blade tip speed increases relative to the LPT blade speed. This will reduce the LPT blade speed, requiring more turbine stages to extract enough energy to drive the fan. Introducing a (planetary) reduction gearbox, with a suitable gear ratio, between the LP shaft and the fan enables both the fan and LP turbine to operate at their optimum speeds. Examples of this configuration are the long-established Garrett TFE731, the Honeywell ALF 502/507, and the recent Pratt & Whitney PW1000G.


title: Turbofan

text: Most of the configurations discussed above are used in civilian turbofans, while modern military turbofans (e.g., Snecma M88) are usually basic two-spool.


title: Turbofan

text: Most civil turbofans use a high-efficiency, 2-stage HP turbine to drive the HP compressor. The CFM International CFM56 uses an alternative approach: a single-stage, high-work unit. While this approach is probably less efficient, there are savings on cooling air, weight and cost.


title: Turbofan

text: In the RB211 and Trent 3-spool engine series, the HP compressor pressure ratio is modest so only a single HP turbine stage is required. Modern military turbofans also tend to use a single HP turbine stage and a modest HP compressor.


title: Turbofan

text: Modern civil turbofans have multi-stage LP turbines (anywhere from 3 to 7). The number of stages required depends on the engine cycle bypass ratio and the boost (on boosted two-spools). A geared fan may reduce the number of required LPT stages in some applications. Because of the much lower bypass ratios employed, military turbofans require only one or two LP turbine stages.


title: Turbofan

text: Consider a mixed turbofan with a fixed bypass ratio and airflow. Increasing the overall pressure ratio of the compression system raises the combustor entry temperature. Therefore, at a fixed fuel flow there is an increase in (HP) turbine rotor inlet temperature. Although the higher temperature rise across the compression system implies a larger temperature drop over the turbine system, the mixed nozzle temperature is unaffected, because the same amount of heat is being added to the system. There is, however, a rise in nozzle pressure, because overall pressure ratio increases faster than the turbine expansion ratio, causing an increase in the hot mixer entry pressure. Consequently, net thrust increases, whilst specific fuel consumption (fuel flow/net thrust) decreases. A similar trend occurs with unmixed turbofans.


title: Turbofan

text: Turbofan engines can be made more fuel efficient by raising overall pressure ratio and turbine rotor inlet temperature in unison. However, better turbine materials or improved vane/blade cooling are required to cope with increases in both turbine rotor inlet temperature and compressor delivery temperature. Increasing the latter may require better compressor materials.


title: Turbofan

text: The overall pressure ratio can be increased by improving fan (or) LP compressor pressure ratio or HP compressor pressure ratio. If the latter is held constant, the increase in (HP) compressor delivery temperature (from raising overall pressure ratio) implies an increase in HP mechanical speed. However, stressing considerations might limit this parameter, implying, despite an increase in overall pressure ratio, a reduction in HP compressor pressure ratio.


title: Turbofan

text: According to simple theory, if the ratio of turbine rotor inlet temperature/(HP) compressor delivery temperature is maintained, the HP turbine throat area can be retained. However, this assumes that cycle improvements are obtained, while retaining the datum (HP) compressor exit flow function (non-dimensional flow). In practice, changes to the non-dimensional speed of the (HP) compressor and cooling bleed extraction would probably make this assumption invalid, making some adjustment to HP turbine throat area unavoidable. This means the HP turbine nozzle guide vanes would have to be different from the original. In all probability, the downstream LP turbine nozzle guide vanes would have to be changed anyway.


title: Turbofan

text: Both routes require an increase in the combustor fuel flow and, therefore, the heat energy added to the core stream.


title: Turbofan

text: The hot route may require changes in turbine blade/vane materials or better blade/vane cooling. The cold route can be obtained by one of the following:


title: Turbofan

text: Alternatively, the core size can be increased, to raise core airflow, without changing overall pressure ratio. This route is expensive, since a new (upflowed) turbine system (and possibly a larger IP compressor) is also required.


title: Turbofan

text: Changes must also be made to the fan to absorb the extra core power. On a civil engine, jet noise considerations mean that any significant increase in take-off thrust must be accompanied by a corresponding increase in fan mass flow (to maintain a T/O specific thrust of about 30 lbf/lb/s).


title: Turbofan

text: Aerodynamics is a mix of subsonic, transonic and supersonic airflow on a single fan/gas compressor blade in a modern turbofan. The airflow past the blades has to be maintained within close angular limits to keep the air flowing against an increasing pressure. Otherwise the air will come back out of the intake.


title: Turbofan

text: The Full Authority Digital Engine Control (FADEC) needs accurate data for controlling the engine. The critical turbine inlet temperature (TIT) is too harsh an environment, at and , for reliable sensors. Therefore, during development of a new engine type a relation is established between a more easily measured temperature like exhaust gas temperature and the TIT. Monitoring the exhaust gas temperature is then used to make sure the engine does not run too hot.


title: Turbofan

text: A turbine blade is subjected to , at and a centrifugal force of , well above the point of plastic deformation and even above the melting point.


title: Turbofan

text: Exotic alloys, sophisticated air cooling schemes and special mechanical design are needed to keep the physical stresses within the strength of the material.


title: Turbofan

text: Rotating seals must withstand harsh conditions for 10 years, 20,000 missions and rotating at 10 to 20,000 rpm.


title: Turbofan

text: Fan blades have been growing as jet engines have been getting bigger: each fan blade carries the equivalent of nine double-decker buses and swallows air the equivalent volume of a squash court every second.


title: Turbofan

text: Advances in computational fluid dynamics (CFD) modelling have permitted complex, 3D curved shapes with very wide chord, keeping the fan capabilities while minimizing the blade count to lower costs.


title: Turbofan

text: Coincidentally, the bypass ratio grew to achieve higher propulsive efficiency and the fan diameter increased.


title: Turbofan

text: Rolls-Royce pioneered the hollow, titanium wide-chord fan blade in the 1980s for aerodynamic efficiency and foreign object damage resistance in the RB211 then for the Trent.


title: Turbofan

text: GE Aviation introduced carbon fiber composite fan blades on the GE90 in 1995, manufactured today with a carbon-fiber tape-layer process.


title: Turbofan

text: GE partner Safran developed a 3D woven technology with Albany Composites for the CFM56 and CFM LEAP engines.


title: Turbofan

text: Engine cores are shrinking as they are operating at higher pressure ratios and becoming more efficient, and become smaller compared to the fan as bypass ratios increase.


title: Turbofan

text: Blade tip clearances are harder to maintain at the exit of the high-pressure compressor where blades are high or less; backbone bending further affects clearance control as the core is proportionately longer and thinner and the fan to low-pressure turbine driveshaft is in constrained space within the core.


title: Turbofan

text: For Pratt & Whitney VP technology and environment Alan Epstein "Over the history of commercial aviation, we have gone from 20% to 40% [cruise efficiency], and there is a consensus among the engine community that we can probably get to 60%".


title: Turbofan

text: Geared turbofans and further fan pressure ratio reductions will continue to improve propulsive efficiency.