Science / Medicine : A Plane for Space : Technology Finally Catches Up With ‘50s Concept of Scramjet

When the aerospace industry’s most advanced plane streaks across the sky in the early 1990s, it will be propelled by an engine that has been waiting for decades to fly.

The yet-to-be-built plane, designated the X-30, will be an experimental aircraft designed to test the technology needed for a runway-to-orbit spaceplane. It will be powered by a supersonic combustion ramjet engine, better known as the scramjet. Proponents describe the engine as one with the power of a rocket and the fuel economy of a turbojet--and with no speed limit.

Although the concept of the scramjet was developed in the late 1950s, the engine has had to await technology advanced enough to make it practical.

Tokyo in 2 Hours

Its opportunity arrived with the advent of the National Aerospace Plane program in 1986. The program’s goal is to produce an airplane “that could, by the end of the next decade, take off from Dulles Airport and accelerate up to 25 times the speed of sound, attaining low-earth orbit or flying to Tokyo within two hours,” President Reagan said in his State of the Union address that year.

Such a plane, however, requires a new kind of engine. It cannot rely on turbojets because at around three times the speed of sound--Mach 3--they get hot from aerodynamic heating and their performance poops out. It cannot rely on rockets because, although they can readily reach orbit, at Mach 25--about 17,700 m.p.h.--they require a large amount of fuel and an on-board supply of oxygen to burn it.

The space shuttle, for instance, carries more than 650 tons of liquid oxygen in its big tank to burn its fuel; yet during much of its ascent, there is plenty of oxygen in the surrounding air. The scramjet will tap this resource, enabling designers to cut the aircraft’s weight while retaining rocket-like performance.

The payoff could be quite large. The aerospace plane program seeks to make possible launch vehicles that could cut the cost of spaceflight by at least 90%, while permitting flight to space on as little as a few hours’ notice. That would not only reduce costs in National Aeronautics and Space Administration and Defense Department satellite programs; it could also greatly ease the problem of launching spacecraft for the Strategic Defense Initiative.

The scramjet, a high-speed variation of the ramjets used in the 1950s and ‘60s to propel some U.S. missiles, is an important part of all this.

On its face, the engine appears simple. It is essentially a tube or duct with fuel injectors. Air rushes through at supersonic speed, due to the plane’s high velocity. Hydrogen fuel squirts into the flow and burns to produce thrust.

A scramjet differs from its predecessor, the ramjet, in the nature of its internal flow. In a ramjet, this flow slows to below the speed of sound, which makes it heat up. But in a scramjet, the internal airflow remains supersonic and thus experiences much less heating. By running cooler, the scramjet can deliver much better performance than either a ramjet or a rocket and has the promise of operating effectively at any flight speed.

Fuel is Cooling

It seems straightforward, but in high-speed flight, nothing is easy. Even with everything that can be done to alleviate aerodynamic heating, this heating is still intense and difficult to cope with. The basic method of cooling is to use the scramjet’s liquid hydrogen fuel to draw heat away from the engine. Then the resulting hot hydrogen is squirted through the fuel injectors and into the burner. That is like building a car that uses gasoline, rather than antifreeze, in the radiator.

Another problem posed by hypersonic flight is that planes must fly very high where the air density becomes thin. The reason lies in what is called dynamic pressure. When you stick your hand out the window while driving, you feel this pressure as the force of the wind. Dynamic pressure from a hurricane can blow down buildings, and scramjet-powered aircraft will fly far faster than the winds of any hurricane.

At low altitudes and high speeds, this pressure would tear apart even the best-designed airplane. Such aircraft, therefore, can find safety only in the low-density air at high altitudes.

At orbital velocity, scramjet-powered aircraft would probably fly at about 200,000 feet, an altitude of 38 miles. This is one-third of the distance to a low satellite orbit. And once having attained orbital velocity, the scramjet plane can coast the rest of the way into orbit.

After a series of studies on X-30 propulsion systems, National Aerospace Plane contracts of $85 million each were awarded in August to Rocketdyne in Canoga Park and Pratt & Whitney, which is being assisted by the smaller firm of Marquardt in Van Nuys, a longtime center of scramjet research. The funds will be used for engine design and development.

Fourth Major Engine

The large amount of federal funding means that the scramjet has a chance to emerge as a fourth major engine in aerospace, alongside the piston motor, turbojet and rocket.

The payoff could be a set of major advances in aviation. In a recent Pentagon study called Project Forecast II, the Air Force picked the X-30 as its top priority for research. It looks ahead to the possibility of fighters, interceptors and reconnaissance aircraft able to fly at virtually any speed the pilot chooses.

If the Soviets were to send conventional bombers westward from their bases near Scandinavia, scramjet-powered aircraft could scramble from bases in the United States and meet the attackers before they got as far as England.

And for travel-weary civilians, there is the eventual prospect of high-speed flight across the Pacific. Today that flight takes up to 15 hours, New York to Tokyo nonstop, and remains among the truly grueling travel experiences. High-speed airliners could cut this flight time to four hours, and perhaps as little as two. Recent technological advances have made such aircraft look promising. Thus, the airplane-building firms of Boeing and McDonnell Douglas, as well as their competitors in Europe, have been studying new supersonic designs with increasing seriousness.

If the scramjet proves successful, then Rocketdyne and Marquardt, as well as Pratt & Whitney and other engine builders, may have the chance to apply its technology to building engines for such transpacific speedsters.