Despite its aura as a cutting-edge industry, aerospace has stagnated for decades. The same types of jet airplanes that went into service in the 1960s still prevail in commercial and military flight. In the United States, there hasn’t been a significant new rocket-engine program since the space shuttle main engine was developed 20 years ago. Nothing about the Boeing 777 would perplex Eisenhower-era aircraft designers.
Today, however, the availability of reliable, reusable rocket engines could make possible the next major step in aerospace transportation: the rocketplane. Rocketplanes combine rocket propulsion with aviation, allowing aircraft that take off and land from conventional airports to fly up and out of the atmosphere. Rocketplanes will lower the cost of satellite launch, accelerate the delivery of packages, and, ultimately, provide a way for people to zip from one side of the world to the other in an hour or so. Farfetched though this vision may seem, the technology is at hand.
The idea of a rocket plane is not new. The first such aircraft-the German Heinkel He-176-flew in 1939. It was in the rocket-powered X-1 that Chuck Yeager first broke the sound barrier, 50 years ago last October. During the 1980s and early 1990s, NASA and the U.S. Department of Defense cooperated on the National Aerospace Plane Project-an effort, since cancelled, to develop technologies that would make possible a vehicle that would take off like an ordinary aircraft, accelerate into orbit around earth, then return through the atmosphere for a runway landing.
But recent advances in technology-from more efficient rockets to more reliable and robust thermal shields-have pushed the rocketplane closer to practical reality. At the same time, the market for the services such a vehicle could offer is growing. The need to launch satellites economically may provide the first stimulus for developing a rocketplane. In the future, though, the main day-to-day use of these hypersonic vehicles may well lie in delivering passengers and valuable packages around the world.
It’s no mystery why aircraft designers have taken so long to embrace rocket engine technology. First, rockets are inefficient, consuming fuel seven times as fast as turbojet at full power. And while a jet engine “breathes” air from the atmosphere to burn its fuel, rockets are designed to work in the vacuum of space and so must carry not only fuel but also oxidant-usually in the form of liquid oxygen. This requirement imposes a greater weight burden on a rocket than a jet.
Second, rockets have generally not proved to be as reliable as gas-turbine engines. This unreliability stems in part from the fact that these engines operate at extremely high temperatures. In addition, aircraft designers and engineers have relatively little experience with rockets, compared with their billions of hours of experience with jet engines.
But rockets have some countervailing advantage. Although they guzzle fuel, they weigh only a fraction of what gas turbines do. The best jet engines now in development generate about 9 times as much thrust as the weight of the engine. By contrast, even a very heavy rocket engine produces a thrust-to-weight ratio of 50. Moreover, with present technology only a rocket can attain the Mach 25 speed needed to overcome the tug of gravity and enter earth’s orbit. (Mach 1 is the speed of sound in air-roughly 740 mph, or 1,200 kilometers per hour.) Even the fastest air-breathing jet engine slogs along at only about Mach 4.