ALL EXISTING space vehicles, including the most advanced SDI launch systems and the renovated Space Shuttle, rely on a principle more than 800 years old. Whereas aviation has progressed over the centuries from hot-air balloons to propeller- and jet-driven engines, and now stands on the verge of further breakthroughs, spacecraft are all driven by rocketry, embodying techniques that, although refined, date from the invention by the Chinese of powder-propelled fire arrows, in the twelfth century.
Although the principle of rocketry—essentially Newton's third law, on action and reaction—is elegant in its simplicity, it sets an upper limit on the efficiency and performance of rocket engines. A rocket can neither derive its motive power from the medium it moves through, like a clipper ship, nor refuel during its journey, like a bomber. The fuel that a rocket-powered-spacecraft requires for the final part of its ascent must be carried throughout the journey. The Space Shuttle is more than 90 percent propellant by weight as it sits on the launch pad; the percentage for vehicles capable of reaching higher orbits or other planets is even greater.
Although propellant is not a major factor in the cost of the space program—the four million pounds burned in a typical shuttle mission account for less than two percent of the launch's cost—its enormous consumption demonstrates that the physics of rocketry relies on brute force. Aircraft employ the earth's atmosphere, using it for lift and taking advantage of its winds; rockets simply punch through it. Greater efficiency comes from more dangerous propellants or higher burn temperatures, which require highly advanced engines with relatively short working lifetimes.