Beyond Mach 6, it is necessary to use a supersonic-combustion ramjet, or scramjet. In the scramjet, the restriction is "very carefully designed so that it produces compression without significantly slowing the airflow," Jones said. "Theoretically, there is no limit to the speed that can be obtained with a scramjet."
But building a scramjet is not easy. "You have to mix the air with the fuel and have it burn in microseconds," Jones said. "That's barely enough time for combustion."
One way to reduce combustion time is to use as a fuel either methane or hydrogen, both of which burn much more rapidly than kerosene. And because both gases must be stored as low-temperature liquids, they also can be used as coolants before combustion to protect both the engine and the airframe.
Neither ramjets nor scramjets, however, function at subsonic speeds. Hence, the aircraft must also carry a conventional turbojet to allow it to reach supersonic speeds.
Such engines also do not operate at high altitudes, where there is too little oxygen to sustain combustion. The national aerospace plane will therefore also carry a rocket motor and oxygen as well as fuel.
One of the promising potential engines for the national aerospace plane is the air turbo ramjet developed by Aerojet TechSystems Co. of Sacramento. This engine illustrates how engineers are attempting to combine characteristics of the turbojet, the ramjet, and the rocket in one hybrid propulsion system.
The air turbo ramjet was first designed in the 1950s, said Ronald A. Samborsky of Aerojet. But it was shelved because no airframe, or body, of that era could absorb the engine's thrust without being torn apart. "New materials and designs," he added in a recent interview, "make use of the engine feasible."
Conversion to Gas
The key to the engine's design is the fact that the turbine that powers its compressor is turned by the engine's fuel before the fuel is burned. Liquid hydrogen, for example, must be converted into a gas before it is burned. The fuel's volume expands sharply during that conversion and the expanding gas can turn the turbine.
Because the turbine is no longer in the exhaust gas, it does not overheat at high speeds.
The lack of oxygen at higher altitudes, however, would require modification of the engine.
Aerojet is working with General Electric Co. to build a hybrid air turbo ramjet that would incorporate a rocket motor. That rocket would boost the plane into orbit and provide braking for reentering the atmosphere.
The Langley Research Center is also developing a hybrid scramjet. Most of the details are classified, but the overall configuration of the Langley engine is probably similar to that of the proposed Aerojet engine.
Wind Tunnel Tests
The NASA scientists have tested a model of the engine in a wind tunnel at speeds as high as Mach 7 and Jones is confident that the engine will surpass Mach 12.
Another engine that may be similar in concept was conceived by Alan Bond, a nuclear fusion researcher for Great Britain's Atomic Energy Authority. Bond designed the engine in his spare time and took it to British engine maker Rolls-Royce Ltd.
Both Rolls-Royce and the British government were apparently favorably impressed by the design: the company began work to develop the engine and the government stamped it "top secret."
About the only thing that is publicly known about Bond's engine is that it will burn liquid hydrogen, using oxygen from the air at low speeds and altitudes, and that it will switch over to its own liquid oxygen supply at higher altitudes.
Lockheed, McDonnell Douglas, Boeing, and British Aerospace are all working to develop airframes to be powered by these new engines. Not surprisingly, all of the airframes look much alike.
The chief feature that the designs all share is the integration of the engine into the fuselage. Engines hung from the wings, as is the case with most commercial airliners today, simply provide too much drag as the aircraft passes through the atmosphere.
Most of the designs, furthermore, are delta-shaped, somewhat like a cross between the space shuttle and the Concorde, because that configuration provides the most lift with the least resistance to passage through the air.
McDonnell Douglas envisions a hydrogen-fueled, 305-passenger, Mach-5.5 hypersonic transport that would be about the size of a DC-9 airliner. Such a craft could make a 6,500-nautical-mile trip in 2 hours and 16 minutes, Schaufele said, "allowing the crew to return to their home base during the same working day."
For the national aerospace plane, Lockheed envisions a craft 205 feet long and 60 feet high, with a wing span of 95 feet. The plane would be slightly smaller than Lockheed's C-5 Galaxy, the world's largest transport, the company says, "but almost twice as heavy since its high temperature metallic structure must stand up to the rigors of multiple flights to the threshold of space."
Heavy and Fast