This paper investigates the aerodynamic and stability characteristics of the aerodynamically controlled re-entry test vehicle HYPERION and two alternative vehicles. The baseline vehicle is a moderate lift-to-drag ratio (VD) re-entry capsule with a triangular cross-section and a blunt nose, and has originally been designed by Fokker Space (FS) and the Faculty of Aerospace Engineering of Delft University of Technology (DUT-FAE). The two alternatives are a capsule with a square cross-­section and a blunt nose, and a blunted sliced cone.
Vehicle dimensions are optimized for maximum lift-to-drag ratio (L/D), using a sequential quadratic programming algorithm. Maximum L/D yields maximum cross-and down-range, gives more flexibility regarding the choice of landing site and takes care of uncertainties in aerodynamic characteristics, entry conditions as well as the atmospheric conditions on the predictability of the landing site.
Subsequently, the vehicles are compared on basis of longitudinal stability in the hypersonic flight regime (M > 5). Stability characteristics that allow a generous travel of the centre of mass (c.o.m.) are favoured because they give flexibility in location of the c.o.m. and reduce the influence of uncertainties in vehicle aerodynamics and c.o.m. location on a stable and controllable re-entry.
The original method that has been used to design HYPERION is refined to allow calculation of the aerodynamics using the exact geometry. This is needed to distinguish between the quite similar shapes of the alternatives. Furthermore, a selection is made for a convective heat flux method, by comparing several methods available from literature. Finally, the heat flux constraint, based on the Chapman heat flux, has been replaced by a wall temperature constraint using an equilibrium wall temperature and the convective heat flux method of Tauber. Wall temperature has the advantage that it relates directly to the thermal protection system (TPS) materials.
Compared to the earlier design the lift-to-drag ratio has increased by 20 %, because of the refined method that uses the exact geometry and because of reduction of the vehicle mass. The new heat flux method, however, puts a more stringent constraint. The final result is that the three vehicles do not differ much in VD, but the sliced blunted cone offers a considerably larger c.o.m. travel.