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.