39.1. Design Considerations of Hypersonic Vehicles
39.1.1. Major concerns of hypersonic flight
The design of a typical hypersonic vehicle can be under taken by a successful experimental test in a ground based facility. However, design criteria of any flight vehicle are governed by the regime of the flow, which the vehicle is going to encounter. ‘Streamlined body’ is the criterion for design of a subsonic vehicle while ‘reduction of wave drag’ is the criterion for design of a supersonic vehicle. Earlier civil or fighter aircraft and missiles designed all over the globe were based on these themes. Excessive heating is the greatest concern in the design of ballistic missiles and spacecraft, since it could melt their surface. Temperature at the nose of the hypersonic vehicles, flying with the Apollo reentry speed, will be around 11,000 K. Hence, design of hypersonic vehicle is dominated by aerodynamic surface heating where ‘reduction of heat transfer rate’ plays an important role. Allen Julian showed that stagnation point aerodynamic heating varies inversely to the square root of the nose radius. This means the aerodynamic drag coefficient is inversely proportional to the heat load. Therefore, the blunt body configuration, which has a detached shock wave, experiences less heating than the traditional shape with its attached shock wave, is the choice of the hypersonic vehicle. Spacecrafts for the Mercury, Gemini, and Apollo programs were designed using this concept. However the maximum temperature that a space vehicle experiences in its hypersonic flight is far above the maximum sustainable temperature of any material. Hence, a proper heat shield should be designed to withstand the heating loads. Consequently nose bluntness increase the aerodynamic drag experienced by the body.  Increase in wave drag is useful during reentry of the spacecraft for aero breaking. Moreover, it is disadvantageous during the ascent stage for a vehicle since increased wave drag demands for more fuel. Therefore, different heat transfer and drag reduction techniques are devised for safer and cheaper hypersonic flight.

39.2 Cooling techniques
The cooling techniques to remove thermal energy from the surface of spacecraft are broadly classified into two categories such as active cooling and passive cooling techniques. ‘Radiative shielding’ (e.g. Molybdenum and Zirconium) and ‘Insulation’ (e.g. Dynaquartz) cooling are the most widely used techniques in the area of passive cooling. A schematic of these techniques is shown in Fig. 39.1 Fig. 39.2 respectively. These techniques are used to avoid the oncoming heat to the vehicle and therefore they are called passive cooling techniques. ‘Convective cooling’, ‘ablative cooling’, ‘transpiration cooling’ and ‘film cooling’ are classified as ‘active cooling techniques’.  These techniques are directly used to cool the vehicle surface.