Mach 20 or Bust

Weapons research may yet produce a true spaceplane.

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Skeptics might be forgiven their doubts. Achieving combustion in an air-breathing engine moving at thousands of miles per hour has been compared to keeping a match lit in a hurricane. Hyper-X protected the precious flame in its combustion chamber behind carefully focused shock waves, but only for seconds. The X-51A engine will have to run at least 30 times longer.

To cover their bets, DARPA and the Air Force have two companies, Pratt & Whitney Rocketdyne and ATK, developing two kinds of hypersonic engines. One major difference from Hyper-X is that the X-51A will burn conventional jet fuel instead of the liquid hydrogen that very-high-performance rocket and scramjet engines normally use. It won’t be the first scramjet to do so: In December 2005, a DARPA-Navy project called HyFly launched a missile perched on a booster rocket from Wallops Island in Virginia. The missile’s air-breathing engine, which ran on JP-10 aviation fuel, flew for more than 15 seconds under scramjet power.

Pratt & Whitney’s engine is called the X-1. When flying at hypersonic speeds, JP-7 aviation fuel rushes into the X-1’s three-foot-long combustion chamber at 3,300 feet per second. A closed-loop system cycles the fuel around the engine, using it as coolant to draw heat and pressure off the combustion chamber. In the process, the extreme heat—more than 3,000 degrees Fahrenheit—“cracks” the fuel’s molecular structure. The cracking shortens the molecules and allows the fuel to burn more quickly, which is imperative. If the fuel doesn’t ignite in the microsecond in which it flows through the chamber, it will spew out uselessly, producing zero thrust—and a very fast falling object.

Over the past year, the X-1 engine has worked as advertised in Langley’s test chamber, culminating in a 50-second-plus, simulated X-51A flight at more than Mach 5 last April.

In less than two years, the X-51A will have a chance to prove itself in the atmosphere. Each test flight will begin with a B-52 taking off from Point Mugu, California. The airplane will carry the 14-foot vehicle up to 49,500 feet over the Pacific, where it will be released attached to a booster derived from an Army missile. The booster will get the demonstrator to over Mach 4, whereupon the scramjet engine will fire to propel it to full speed.

With the X-51A attempting to prove that hydrocarbon scramjets can propel hypersonic missiles, it’s up to other projects to sort out how to achieve higher Mach numbers. For some of those answers, Lewis and the Air Force made a long flight down under to work with the Australians who came up with HyShot.

EVEN AT 500 MPH, it takes a long, long time to reach Australia. “Just eight movies and you’re there,” Australians joke. The country’s remoteness may account for its fascination with hypersonic flight; someday the travel time from London to Sydney may come down to one movie.

The Australian hypersonics program has been making steady progress for a decade, but it really took off in 2002, when HyShot fired the world’s first scramjet engine in flight. Building on that accomplishment, the Australian Department of Defense joined its U.S. counterpart, along with NASA, Boeing, and other partners, in an innovative international project called HiFire, for Hypersonic Flight International Research Experimentation. Funded with $54 million, HiFire includes a series of experiments and at least 10 test flights to be conducted over the next six years. Mark Lewis, who signed the agreement for the United States last November, says that the project complements the X-51A and other U.S. hypersonics efforts. “In HiFire, we’re looking at very fundamental science: all the problems we think we would anticipate in hypersonic flight.”
Next year the HyShot team will test a new free-flying vehicle as part of the HiFire program (earlier HyShots stayed attached to their booster rockets). One research goal is to try different shapes for scramjet engines in the search for greater efficiency, starting with the air inlet. Instead of a simple rectangular slot, shaped like the front of a Dustbuster vacuum cleaner, the inlet for the REST (rectangular-to-elliptical shape transition) engine is three-dimensional and more complex. The opening is still generally rectangular, but it includes faces that slant in toward the combustion chamber. Michael Smart, an associate professor in the HyShot group at the University of Queensland, explains: “The reason these 3-D inlets are more efficient is that the air is compressed by all surfaces of the inlet. A 2-D inlet only compresses the air in one plane: The side walls create drag, but don’t do any compression.”
The outer rectangular shape of the inlet offers an advantage: Stacking engines side by side is easier. But inside the vehicle, the inlet connects to an elliptical combustion chamber. Joined together, the pieces look like the different sections of a car’s exhaust system. This is a departure from the X-43A, which had a rectangular combustion chamber. Elliptical combustors are better, says Smart, because “round shapes are inherently stronger than rectangles. This leads to thinner walls and less weight. They have less surface area for the same amount of air flow through the engine. Less surface means less drag, less heating, and less weight.” And since energy tends to ebb in rectangular combustors’ corners, getting rid of the corners can increase overall thrust.

HiFire flights will launch from southern Australia’s Woomera test range, the largest testing grounds in the world. The size of the range, its isolation, and the chance to fly frequently are real benefits, says Lewis. “The costs are low enough that if the things break, if they don’t work, if they crash into the Australian Outback, we’ll keep the program going. We’re not going to give up because of one failure.” It’s a small-is-beautiful approach. “When you go to really, really expensive demonstrators, suddenly you’re so terrified of things not working or not flying that you paralyze your flight test program,” he says. “And that’s one of the things we’re trying to avoid.”
Of the three major hypersonic programs under way, the most ambitious is FALCON. HiFire’s short, up-and-down flights will reach Mach 10 or so. FALCON aims to fly up to Mach 20 over a distance of thousands of miles.

Led by DARPA, FALCON is short for Force Application and Launch from CONUS (continental United States). As the name implies, FALCON was conceived as both a potential weapons system with global reach and a capability to launch military space payloads as a quick response. The distant goal of the program is to develop, by 2025, an unmanned, reusable Hypersonic Cruise Vehicle (HCV) approximately the size and weight of a B-52. Taking off and landing like an airplane, the HCV would be able to deliver a 12,000-pound payload 9,000 miles from the continental United States within two hours. It’s the Orient Express turned into a bomber, without the pilot or passengers.

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