[131] Hypersonic flight is arbitrarily defined as flight at speeds beyond Mach 5 although no drastic flow changes are evident to define this. To date, speeds of this magnitude have been achieved only by rockets and spacecraft and the NASA X-15 research airplane. Several formidable problems are encountered at these speeds. First, the shock waves generated by a body trail back at such a high angle that they may seriously interact with the boundary layers about the body. For the most part, these boundary layers are highly turbulent in nature. Secondly, across the strong shocks, the air undergoes a drastic temperature increase. Aerodynamic heating of the body is a major problem. For sustained hypersonic flight most normal metals used in today's airplanes would quickly melt; therefore new materials or methods that can withstand the high-temperature effects are required. The temperature of the leading edge of the airplane wing may be reduced by using a high degree of sweepback. Additionally, to obtain a good lift-drag ratio, a flat-plate design wing is used.
Control surfaces for hypersonic flight must be strategically placed so that they encounter sufficient dynamic pressure about them to operate. Otherwise, if shielded from the approaching flow by the fuselage, for example, they will be ineffective.
Figure 112 shows two proposed hypersonic airplanes that exhibited much of this design philosophy. The modified NASA X-15 research airplane shows the highly swept delta wing and the X-20 Dynasoar reentry craft has control surfaces out on the wing tips for effective control.
Although commercial hypersonic flight is a long way from being realized, studies are being conducted by NASA to obtain the basic knowledge necessary for design. Figure 113 shows a proposed hypersonic transport (HST).
Propulsion is another major problem at hypersonic speeds. Economically, the most promising prospect is the ramjet engine. The ramjet engine works on the principle that at high Mach numbers the shock waves compress the air for combustion in the engine. This does away with many moving parts and represents an efficient propulsion method. NASA research is also continuing in this field.
Because of the cost and safety, it has long been recognized that designs of reentering spacecraft must be found that would enable the crew to maneuver the craft to a landing from a great distance. Up to now spacecraft have reentered and followed near ballistic entries with little control over the landing site. Large recovery forces and...
....operations were usually necessary. Starting in the late 1950's, however, NASA has been involved in designing aircraft that produce more lift than drag and yet resemble spacecraft. They are called lifting bodies, for they have no wings but obtain lift because of their body shapes.
[133] Figure 114 shows four of the shapes being tested to evaluate the handling characteristics and flight qualities of this unusual concept. The M2 vehicle type developed at the NASA Ames Research Center is flat topped with a rounded belly and combines the advantages of stability at hypersonic speeds with high lift-drag ratios at subsonic speeds. The HL-10 lifting body developed by the NASA Langley Research Center is shaped to provide optimum trim at Mach 10+ and, in contrast to the M2 vehicle, it possesses a rounded top and a flat belly. The Martin Marietta X- 24A is very different in shape from the previous two since it is more rounded although it, like the HL- 10, has a flat bottom. Rebuilt as the X-24B, it now has a double-delta planform and a more pointed nose.
The lifting bodies being flight-tested are exploring the subsonic and low supersonic speed ranges to show how control over the lift-drag ratio may aid in the landing of more advanced vehicles. Representative of a new generation of vehicles primarily benefiting from this research is the Space Shuttle.
The Space Shuttle represents the United States' commitment to developing a lowcost method of delivering and returning payloads to and from orbit. The basic design settled upon is shown in figure 115(a). The booster stage consists of two recoverable solid-fuel rockets and a large nonrecoverable external fuel tank used by the orbiter stage engines to complete the boost into orbit. The orbiter stage shown in figure 115(b) is the actual part of the total vehicle to go into orbit and return to Earth to a controlled...
...landing. Aerodynamic interest is centered about the boost and landing stages of the mission when dynamic pressures are evident. The entire range of Mach numbers from subsonic to supersonic is covered. There are some unique problems associated with the boost phase such as the range of dynamic pressures acting on the vehicle, staging aerodynamics, the recovery of the solid-fuel boosters by parachutes, as well as stability and control considerations both at low and high Mach numbers.
The landing phase of the mission is an area of great concern. The orbiter vehicle must be able to deorbit and land like a conventional airplane. There are numerous aerodynamic research problems associated with this part of the mission.
The orbiter (fig. 115(b)) uses a double-delta wing configuration to optimize the hypersonic flight characteristics and still provide for a good lift-drag ratio in the landing phase. With this lift- drag capability, the orbiter has a side-to-side range capability of about 2000 km. The orbiter reenters the atmosphere at a high angle of attack-about 30°. This high angle of attack is used to concentrate the maximum [135] aerodynamic heating on the underside of the vehicle where the greatest thermal protection is provided. In the upper reaches of the atmosphere, attitude is controlled by a reaction control system, but as the dynamic pressure builds, the vertical tail (to control yaw) and elevons (combined elevators and ailerons to control pitch and roll) become effective. On landing, the rudder splits open to act as a speed brake and a parachute is deployed to slow the orbiter to a stop. The Space Shuttle represents a challenge to aerodynamic research for years to come and is a stimulus for probing further into the unknowns of high-speed flight.
