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Written by: Paul Gargantiel
Written on: July 1st, 2001
Tags: aerospace engineering, space
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From Humble Beginnings

NASA Marshall Space Flight Center

This is Rocket Science: Mechanics of Space Flight

Before we probe into the future, we shall explore the science of rocketry and understand the mechanics behind getting spaceships airborne. As mentioned before, Newton's laws play a fundamental role in aeronautical science. It exists as the foundation of spacecraft propulsion. Newton's third law states, "To every action there is an equal and opposite reaction." Equivalently, "When two bodies exert mutual forces on one another, the two forces are always equal in magnitude and opposite in direction" [3]. In relation to rocketry, the action force is composed of the infinitesimal particles rapidly emitted from the spacecraft while the reaction force is the weight of the spacecraft. To further clarify, as the propellant of the spacecraft is burned, the rocket releases vast amounts of gas particles at extremely high velocities. As these particles leave in one direction, they exert a force on the spacecraft, pushing the spaceship into the opposite direction.
Also of great concern to rocket designers is spacecraft mass. Regardless of the velocity of the particles, if the spacecraft is too massive, the propulsion system will lose its effectiveness. Thus, scientists have to keep in mind Newton's second law:
F = m*a(1)
where F refers to a force, m is the mass, and a is the acceleration. Rewriting the above equation as a = F / m, we can see that the acceleration is inversely proportional to the mass of the body. The above equation suggests that less mass, assuming the force is constant, results to greater acceleration. This is the primary reasoning for releasing empty fuel tanks and engines that are no longer useful. With less weight, the rocket gains greater velocity.
Though the knowledge of these laws of physics improves performance, chemical propulsion systems have a relatively low maximum velocity. For the most part, chemical propellants are adequate, safe, and relatively reliable. However, the most advanced chemical propulsion system constrains the possibilities of space exploration and is an economic burden. Currently, according to a group at the Jet Propulsion Laboratory (JPL), the cost of low Earth orbit spacecrafts ranges between $10,000 and $20,000 per kilogram of net payload, while more sophisticated spacecrafts costs between $60,000 and $120,000 per kilogram of net payload. Les Johnson, a scientist from NASA, estimates an extra $7,000 per pound simply to mobilize the craft once its in space. In hopes to reduce these high figures, NASA created The Highly Reusable Space Transportation and Affordable In-Space Transportation systems, ultimately decreasing these numbers by a factor of 50. High production costs coupled with the fundamental limitations of chemical propulsion systems call for more sophisticated methods of propulsion [4].

Future Prospects

Currently several alternative methods of propulsion are being explored, some of which are highly promising. Wire tethers, for instance, is one method many favor since it uses no fuel, is recyclable and environmentally safe, and best of all, reduces the cost of spacecraft mobilization to only several hundreds of dollars per pound as opposed to thousands of dollars. Spacecrafts implemented with this system essentially utilize a simple 5-kilometer-long aluminum wire tether as a basis for propulsion. Les Johnson compares the concept of wire tethers with an electric generator: "As the tether moves across the Earth's magnetic field, a voltage is induced across its length. The upper end of the tether becomes positive, so electrons in the upper-atmosphere plasma, which carry negative charge, are attracted to the upper end. By using a device to emit the electrons back into space at the lower end of the tether, a current can be made to flow along the length of the wire." This movement of electrons gives thrust or in others words, acceleration, which in turn allows the spacecraft to move. In testing this propulsion system, NASA experimented with the Space Shuttle but failed when the wire broke. Though this mode of propulsion is expected to bring benefits, much work lies in wait for NASA engineers.
Other promising leads include electric and ion propulsion systems. Aeronautical engineers suspect this method of propulsion will be widely utilized in this century, especially for deep space missions. An ion propulsion system consists of three things: "a xenon propellant source, an electrical power processor, and a cylindrically shaped thruster" (Ashley). Essentially, each electric motor, which appears very much like a can with a dish-shaped screen on one end, produces an ion beam with a thrust of 18 milliNewtons. Though this figure is very small, this force suffices the tasks it performs. Moreover, this type of propulsion can generate speeds of 25,000 m/s, which chemical-based propulsion cannot reach. In addition, unlike chemical propulsion systems, ion rockets operate for months and even years. Thus, this propulsion technique is one of the most promising for use in deep space exploration [5].
Additional methods include solar and laser thermal propulsion: essentially chemical propulsion systems that use solar or laser light to heat a hydrogen fluid. In order to heat up the fluid, sunlight must be directed towards these metallic, balloon-like inflatable mirrors. These mirrors, in turn, are meticulously pointed through a small window to the engine, where the propellant is located.
Further, there are theoretical propulsion systems that would be revolutionary to say the least, if ever realized. For instance, scientists are investigating nuclear fission and fusion that offer 107 to 108 times more energy compared to ordinary chemical propulsion systems. Better yet, matter-antimatter annihilation is viewed to be the "ultimate source with a theoretical energy density of 1010 times of conventional chemical propellants" [4]. Such mind-boggling possibilities exist since the exhaust velocities of these theoretical systems are extremely high. Chemical propulsion systems are limited to a velocity of approximately 10 km/s. Those that utilize plasma as a source of heat can move within 20 km/s to 50 km/s. Ion propulsion systems, however, surpass both types, having a potential velocity of 200 km/s or more.
Theoretical propulsion concepts like matter-antimatter annihilation drastically transcend all types of propulsion methods currently being studied. Matter-antimatter annihilation, for instance, can theoretically achieve an exhaust velocity comparable to the speed of light, 300,000 km/s [4].

Endless Possibilities

Reviewing the past and contemplating the current rate of technological advance, we can conjecture that the boundaries of our potential are endless. Who would have thought that one day there would be space flights that would be open to the general public? There are a few companies that have invested in this commodity, offering to the public an opportunity to take a vacation outside Earth. One ambitious travel agency, Zegrahm Space Voyages, is presently taking reservations for the first tourist trips into space. For a cost of $98,000, one can spend some time in space and view the Earth from an entirely different perspective. It is only a matter of time [6].
The future holds even greater possibilities. In a recent panel discussion on the future of astronomical developments, nine leading astronomers made interesting and profound predictions for the future [7]. One item discussed was the idea of extraterrestrial worlds, in which several of the scientists optimistically predicted finding a planet that would be habitable by humans. They suspect that within the next 25 years, we will be able to locate a planet similar to Earth with liquid water on it. In addition, responding to the question regarding galaxy formation and evolution, one of the scientists predicted that we will eventually learn of the era when galaxies were formed, as well as the process that leads to merging galaxies. Moreover, according to Robert Naeye, an astronomy writer, we will soon answer the questions about the origins of the universe within the 21st century.


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