And Hypersonic Flight

supersonic velocities, the "ramjet" makes for a lightweight,more efficient propulsion than that of the ordinary turbojet.Beyond Mach 6, aerodynamic considerations dictate thathypersonic aircraft operate at altitudes higher than 100,000feet. The specific reason for this is that aerodynamic drag isa function of the density of the air. By going to higheraltitudes at which the air density decreases exponentially,drag is decreased to a minimum while sufficient aerodynamiclift is maintained. However, the low air density leadsto a substantial reduction in the rate at which oxygen canbe rammed through the engine.The solution to this aerodynamic-propulsion dichotomyof divergent requirements was provided by Antonio Ferriand his collaborators: the scramjet (Figure 1).In the scramjet design, most of the underside of the aircraftis utilized to scoop air into the engines. Early ramjetdesign concepts sought to keep the combustion process atsubsonic speeds. By disturbing the airflow in front of theengine, the incoming oxygen could be drastically slowed.But at greater than Mach 6 hypersonic speeds it becomesextremely difficult to slow the air inflow below the speedof sound. Even if this were possible, the resultant slowingprocess heats the air to 4,000 degrees F. At these temperaturesthe air molecules dissociate, making the combustionprocess far more inefficient. Additionally, there is the problemof the turbulent shock wave created when the air passesbelow the speed of sound.The scramjet therefore is predicated on achieving combustionwith air intakes at supersonic velocities. NASALangley scientists, in fact, succeeded in developing a dualmode scramjet engine. The innards of the scramjet are honeycombedwith fuel injection outlets. Half of them facetoward the rear and the other half are at a perpendicularangle to the airflow.At low Mach numbers, most of the fuel is injected parallelto the airflow to prevent the combustion process from occurringtoo far forward. As speeds increase, more and morefuel is injected perpendicular to the airflow in order tomaintain efficient burning.The Langley dual-mode scramjet module was tested upto speeds of Mach 7. The only reason it did not achieveeven higher speeds was that Mach 7 was the limit of thewind tunnel. NASA states that its current scramjet can attainMach 12 speeds, and in its recent report to Congress NASAreported, "The upper limit of speed for useful airbreathingFigure 3MCDONNELL DOUGLAS'S MACH 5 ORIENT EXPRESSThis methane-fueled "Orient Express " is based on state-of-the-art technology. It would carry 300 or more passengersto the Far East within a few hours, operate with minimal sonic boom, and would not disturb the ozone layer.FUSION January-February 1986 55