It will be able to take off from an ordinary runway, accelerate to 25 times the speed of sound, climb into orbit and then, mission accomplished, land like any other airplane.
To President Reagan, it would be a new "Orient Express" that would carry passengers from Washington to Tokyo in less than two hours.
To NASA, it would replace the rocket-launched space shuttle.
To the Pentagon, it would reduce reliance on highly vulnerable launch pads and lift space weaponry into orbit.
When the President called for increased research funds for such a triple-threat aircraft in his State of the Union address Feb. 4--only a week after the Challenger explosion--visions of the futuristic vehicle understandably struck a responsive chord.
New Sense of Urgency
Plans for such a "trans-atmospheric vehicle" have been under consideration all during the 1980s. But they have taken on new significance and a new sense of urgency after the destruction of the space shuttle Challenger on Jan. 28.
Yet, knowledgeable engineers say the undertaking is an incredibly complex project that may have great difficulty simply getting off the ground--much less into space.
The problem, they say, is that such an aircraft would have to go through three distinct phases of flight. Therefore, such a plane would probably require three distinct propulsion systems.
Invoking an automotive analogy, engineers argue that developing such an aircraft is like trying to combine the four-wheel drive system of an off-road vehicle, the sustained power of an Indy 500 racer, and the high acceleration of a nitro-powered dragster in one chassis that is no larger than the family sedan--while still leaving room for the passengers.
Competition for Funds
But despite such potential difficulties, U.S. aerospace companies are vigorously competing for their share of the research funds, even though commercial airlines remain somewhat skeptical.
Those funds total $60 million for this fiscal year, and NASA and the Department of Defense have requested $212 million for next year. Space and defense officials estimate that the project will require $500 million over the next three years and as much as $3 billion before a prototype of the craft could be test-flown sometime in the 1990s.
The United States is not alone in undertaking to develop a new generation of aircraft to replace jet airliners and space shuttles.
The day after Reagan called for increased research funds in his speech, the British government authorized $4.1 million for a "proof of concept" study for a similar craft called the Hotol--for horizontal takeoff and landing.
In the United States, government officials appear to be lumping two distinct vehicles into one project under the rubric "National Aerospace Plane."
The first is a more or less conventional passenger plane, frequently called the hypersonic transport, or HST. It would be a successor to the Concorde supersonic transport that now ferries passengers from Europe to the East Coast in just 3 1/2 hours. The HST would attain speeds two to five times that of sound--Mach 2 to Mach 5, or 1,500 to 3,750 m.p.h.
Such an aircraft "could be built with existing technology," said Roger Schaufele, vice president for engineering at McDonnell Douglas Corp.
The second vehicle is a true spacecraft that could accelerate to escape velocity--Mach 25, or about 17,600 m.p.h.--to enter orbit. Such a craft would require considerable amounts of new technology, most engineers agree.
Range of Speeds
The key to both types of planes, however, is the development of appropriate propulsion systems that can power a plane over a range of speeds, from takeoff at about 100 m.p.h. to entering orbit at Mach 25.
Those new systems will probably be extensions of the turbojet engine that powers most modern planes. In the turbojet, propeller-like blades inside the engine suck in and compress air. Kerosene is added to the compressed air and ignited; the rapid expansion of the combustion gases forces them out the rear of the engine, providing thrust and turning a turbine that drives the compressor.
The turbojet is efficient at the 550-m.p.h. speeds of conventional airliners. But at much higher speeds, temperatures within the engine approach the melting point of most present-day materials.
So a more efficient device above Mach 2 is the ramjet, which is little more than a hollow tube with a restriction in it. "Air flowing into the front of the ramjet compresses itself at the restriction and is slowed below the speed of sound," said Robert A. Jones of NASA's Langley Research Center in Hampton, Va.
After the compression, combustion is the same as in a conventional turbojet.
"But as you go to higher speeds," Jones said in a telephone interview, "the temperature of the air increases because of the slowing. When you get near the temperature of the flame--at about Mach 6, or about 4,500 m.p.h.--the air will no longer support combustion" because at that temperature molecules break apart and combustion cannot be sustained.