Though it is still in the stage of wind-tunnel studies and drawing-board proposals, the much-heralded U.S. supersonic transport, which is scheduled to carry 150 passengers at speeds up to 1,900 m.p.h. by 1974, is already old-hat to some aeronautical engineers. They are working on a new and swifter generation of jets that will streak into the still unexplored speed range between the Air Force X-15's record 4,104 m.p.h. and the 17,500-m.p.h. velocities of orbiting space capsules. Designed for the near future, these scramjets (supersonic combustion ramjets) will be powered by an engine out of the near past—an advanced version of the pulse jet that boosted Germany's V-l "buzz bombs" over Britain toward the end of World War II.
Lift from a Plane. Because the basic ramjet is just about the simplest power plant ever to be airborne, its promise has always excited aeronautical engineers. Unlike the conventional jet, it has neither a complex turbine nor a compressor; it is an open-ended cylinder, known as a "flying stovepipe," with only fuel injection and ignition systems inside.
To operate, it must first be accelerated to a speed of several hundred miles per hour by an auxiliary turbojet or rocket engine, or get a lift from a conventional plane. After that, enough air is rammed into the engine's front inlet to set up a pressure barrier that forces the burning gases to escape at the rear, thus providing thrust (see diagram).
Theoretically, an old-fashioned ramjet can fly through the atmosphere at almost unlimited velocities, but its top speed is limited to about 4,000 m.p.h. by practical considerations. The jet flame, burning conventional fuels, tends to blow out at supersonic flight speeds (above 720 m.p.h. at low altitudes). If it is to keep burning and providing thrust, the ramjet needs an inlet shape to generate its own shock wave, which will slow passage of air through the combustion chamber to a subsonic flow. Above 4,000 m.p.h., however such an inlet design could cause excessive temperatures and pressures in the combustion chamber, and thrust wouk be drastically reduced.
Hydrogen Cooling. These apparent limitations dampened interest in further ramjet development work until late last year, when Marquardt Corp. scientists convincingly demonstrated a practical method of maintaining combustion in a supersonic flow of air. Using hydrogen, which has a low ignition temperature, burns rapidly and provides high thrust, they kept an experimental scramjet burning in air moving as fast as 7,000 m.p.h. By redesigning their engine's inlet to allow it to gulp air at supersonic speeds, they were also able to eliminate the excessive temperatures and pressures. And they proved that useful thrust could be produced at flight speeds in excess of 17,000 m.p.h.
The hydrogen fuel also promises to pay an extra dividend. To be kept in liquid form, it must be stored in refrigerated tanks at a temperature of — 423 °F. And since a plane moving at scramjet speeds will be seared by the heat of friction as it moves through the atmosphere, the frigid hydrogen will make an ideal coolant to be pumped through the skin of wings and fuselage before it is burned.