Aviation's Next Great Leap
Strapping rockets onto an aircraft creates a new breed of space plane. Look for cheaper satellite launch, ultrafast package delivery, and hypersonic passenger transport that would leave the Concorde in the dust.
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.
Getting Up To the Wild Black Yonder
Commercially successful rocketplanes will depend on developments in two key technologies: a reliable, reusable rocket engine and a robust thermal protection system to prevent damage during re-entry. In both cases, advanced solutions are at hand.
U.S. researchers have focused on improving component technologies and advanced materials, not on manufacturing actual rocket-engine products. In the former Soviet Union, however, researchers have continued to advance the development of rocket engine families. In particular, the Soviet space shuttle program led to a new generation of advanced reusable rocket engines fueled with kerosene (that is, conventional jet fuel), hydrogen, or a combination of the two. For example, the reusable RD-120, a kerosene-burning engine developed as an upper-stage engine for the Zenit booster, has been certified by its U.S. importer Pratt and Whitney as good for 10 flights-plus another 10 after a major overhaul.
Aerospace engineers also recognize that the thermal protection system on the space shuttle is not suitable for a truly robust vehicle. Although the shuttle is reusable, its heat shield is easily damaged. Moreover, ordinary weather conditions such as rain and wind-driven dust damage the heat shield. Following each landing, the shuttle must undergo a costly and time-consuming refurbishing involving toxic chemicals and special procedures, to replace the lost and damaged tiles so that the spacecraft can safely ascend to orbit again.
NASA’s efforts to correct these problems have yielded impressive results. Designers have a much broader selection of tiles, blankets, metal surfaces, and advanced composites and ceramics, all of which can make new-generation rocketplanes capable of withstanding wind and weather that would strip the heat shield off the shuttle in minutes. One of the new materials-the AETB-TUFI-C thermal protection tile developed by NASA’s Ames Research Center-has survived, undamaged, a test flight on an F-15. This result was all the more remarkable because the fighter flew through a rainstorm that scoured the paint off its surface.
Such advances have fortified the prospects of building a reusable rocket plane. But other design questions remain before such a craft becomes practical. First, to take advantage of the trillions of dollars of existing airport infrastructure, a rocketplane needs to be able to take off and land in a conventional, horizontal manner.
Moreover, a rocket engine works best in the vacuum of space; the denser the air, the more fuel the rocket must burn to develop the same amount of thrust. The atmosphere’s thick soup of air also imposes a drag penalty, forcing the rocket to squander huge amounts of fuel. Thus a rocket-powered aircraft needs some other means of propulsion to lift it from the ground to the upper reaches of the atmosphere; once reaching the fringes of the atmosphere, the rocket could ignite and propel the craft into space.
Aeronautical engineers have developed three principal schemes to accomplish this. In one, the rocketplane is attached to the belly of a jet aircraft, which takes off and flies to a high altitude. Then the rocketplane drops off to complete its flight. Chuck Yeager used this technique in 1947 to achieve the first human travel at supersonic speed.
In a modern variation of this approach, a jet aircraft tows a rocketplane to high altitude with a tether, much the way conventional airplanes launch gliders. This scheme is being developed at Kelly Space and Technology in San Bernardino, Calif. Kelly’s Eclipse aircraft is towed by a Boeing 747 to an altitude of about 14 kilometers. There, the Eclipse fires its rocket engine, disconnects from the tow line, and climbs to about 150 kilometers. The Eclipse then glides to an unpowered landing.
An advantage of these two techniques is that the rocketplane itself needs only one engine-the rocket. On the other hand, any vehicle that depends on another aircraft to launch it has a serious drawback. If the rocketplane lands in the wrong place, for example, it will have to await the arrival of a carrying or towing plane before it can get airborne again. Moreover, takeoff in such a tandem configuration would require longer and wider runways than those at existing airports. Moreover, if the rocket engine does not light after disconnecting from the carrier aircraft, the rocketplane would probably be lost.
Our company-Pioneer Rocketplane-favors a different launch-assist scheme. In the Pioneer approach, the Pathfinder aircraft would take off in a conventional manner and climb to nine kilometers under the power of conventional turbofan jet engines. There it would rendezvous with a large subsonic aircraft, such as a KC-135 transport or a Boeing 747, that would serve as a flying tanker. To prepare for the second phase of flight, the rocketplane would dock with this tanker and suck about 290,000 kilograms of liquid oxygen from it. Such transfers are common practice in military aviation, although the propellant being moved is jet fuel rather than liquid oxygen.
After disconnecting from the tanker, the aircraft would light its rocket engine and climb to 150 kilometers, reaching a speed of Mach 12. The rocketplane would then travel above the outermost fringes of the atmosphere, during which time a satellite attached to a small rocket upper stage could be released for transfer to orbit. The aircraft would then descend back into the atmosphere. After slowing to a subsonic speed, the turbofan engines would restart, propelling the aircraft to a landing field. Because it could take off from any medium-sized airfield, the Pioneer rocketplane would provide great flexibility in choice of launch site and abort options.
Riding a Rocketplane For Fun and Profit
The first thing that comes to many people’s mind in thinking about rocket planes is the potential for rapid personal travel. While that possibility exists, other applications promise a steadier revenue stream and will probably develop first.
satellite launch: Despite a healthy business from launching government and commercial satellites, the international space launch industry has for the past two to three decades suffered a period of almost complete technological stagnation. Most of the launch systems now in use-including the Delta, Atlas, Titan, Soyuz, Molniya, and Proton-were already flying in more or less their current forms in the mid-1960s. While a few additional systems, such as the European Ariane, have been introduced in the intervening decades, technological improvements have been so minor that older systems are still competitive. As a result, freight rates from the earth’s surface to orbit remain at about $10,000 to $20,000 per kilogram-the same as in the 1960s. This persistently high cost severely inhibits the commercial development of space.
Creating A New Space Industry
Since rocketplanes are a near-term technology with widespread commercial application, it should be possible to finance their development primarily with private investment. Nevertheless, development of novel flight systems always involves significant business risk, which could be mitigated by government participation.
Reference to our current era as the Space Age is a misnomer-like calling the 1910s the Air Age. Except for the military, the world did not really feel the impact of air travel until the technology became routine and commonplace and affordable to more than an elite few. Likewise, if a real Space Age is going to arrive, there needs to be a market for rocket vehicle technology that supports the manufacture of spacecraft components not in lots of ones or twos, but in hundreds or thousands.
Producers of these planes will have to start using the production methods common in commercial aviation rather than the costly small-lot production techniques that dominate the space industry today. Moreover, we will need a worldwide launch infrastructure that supports not hundreds of flights per year, but hundreds of flights per day. The only markets large enough to stimulate investment in such production capacity and launch infrastructure are long-distance package delivery and passenger transport.
For the same reason that military and then postal aircraft preceded passenger aircraft, satellite launch, military, and fast package-delivery rocketplanes will no doubt precede passenger rocketplanes. Nevertheless, the day will surely come when thousands of rocketplanes cross the globe daily, serving business and vacation travelers from New York to Tokyo-perhaps even into orbit.
Become an MIT Technology Review Insider for in-depth analysis and unparalleled perspective.Subscribe today