Research and design for lifting reentry

Abstract

Compared with convex or flat-bottomed craft, a lifting reentry vehicle having concavity on the lower surfaces allows a stronger shock wave to be produced and the more highly compressed air can flow beneath the vehicle with much smaller losses in lifting pressure; this is associated with the fact that concavity reduces undersurface crossflows and spillage around the leading edges. The stronger shock and the preservation of lifting pressures result from and illustrate the principle of flow containment. Flow containment should reduce rates of heat transfer at stagnation points, since, at a given wing loading, flight speed and lift/drag ratio, deceleration will occur at higher altitudes and, hence, at given flightspeed, ambient air density will be reduced. It should also reduce heat transfer under the lower surfaces since, at given flight speed, local flow velocities will have been reduced by the stronger shock. Experiments initiated in 1970 have confirmed that at reentry Mach numbers (6 < M ∞ < 22) and reentry angles of attack (25° < α < 70°, say) a body of basically delta planform will produce significant increases in maximum lift coefficient, and in C L per unit L D if at least a part of the undersurface heatshield has anhedral. The present paper summarises the experimental data produced so far and demonstrates that existing analyses explain the trends and can indicate improvements in heatshield geometry. It also considers the design of realistic configurations having partly or wholly concave undersurfaces. Two families of vehicle are considered. The first is related to the current Space Shuttle orbiter, in which the specified payload size and shape has resulted in the use of a large fuselage mated to a low wing with slight dihedral and delta planform; it is shown that, when compared with the orbiter at reentry conditions, a high-wing orbiter with moderate anhedral and a partly concave undersurface can provide about 20 % higher CL at the same angle of attack, Mach number and lift/drag ratio. The second family is of aerodynamically more advanced vehicles, which can provide higher values of hypersonic L D and, thus, permit greater cross-range, while also offering possibly improved stability at reentry and in post-reentry conditions.