1.1 Introduction

Hypersonic flight has special traits, some of which are seen in every hypersonic flight. Presence of these particular features during a flight is highly dependant on type of trajectory, configuration etc. In short it is the mission requirement which decides the nature of hypersonic atmosphere encountered by the flight vehicle. Some missions are designed for high deceleration in outer atmosphere during reentry. Hence, those flight vehicles experience longer flight duration at high angle of attacks due to which blunt nosed configuration are generally preferred for such aircrafts. On the contrary, some missions are centered on low flight duration with major deceleration closer to earth surface hence these vehicles have sharp nose and low angle of attack flights. Reentry flight path of hypersonic vehicle is thus governed by the parameters called as ballistic parameter and lifting parameter. These parameters are obtained by applying momentum conservation equation in the direction of the flight path and normal to it. Velocity-altitude map of the flight is thus made from the knowledge of these governing flight parameters, weight and surface area. Ballistic parameter is considered for non lifting reentry flights like flight path of Apollo capsule, however lifting parameter is considered for lifting reentry trajectories like that of space shuttle.
Therefore hypersonic flight vehicles are classified in four different types based on the design constraints imposed from mission specifications.

1. Reentry Vehicles (RV): These vehicles are typically launched using rocket propulsion system. Reentry of these vehicles is controlled by control surfaces. Large angle of attack flight of blunt nosed configurations is the need of these flights. Space shuttle ( US ), BURAN (Russian), HOPE ( Japan ) and HERMES (European) are some examples of these kind vehicles.
2. Cruise and Acceleration Vehicle (CAV): Slender configurations with low angle of attack flights are main features of these flights. These vehicles are prepared for high heating loads with ablative cooling system. Air breathing propulsion system of ram or scramjet type is generally preferred for these vehicles. Sanger, which is a two stage (TSTO) hypersonic vehicle, has first stage with air breathing propulsion and second stage is propelled with rocket. Hence first stage of Sanger falls in CAV category for which separation takes place at Mach 7.
3. Ascent and Reentry Vehicles (ARV): These vehicles have opposing requirements of their design due to dual duty of ascent, which is dominated by fuel requirements, and reentry by aero-braking. Rocket or air breathing propulsion systems can be preferred for these flights. NASP or National Aerospace Plane of US, Space Plane by Japan and HOTOL are some examples of these vehicles.
4. Aeroassisted Orbit Transfer Vehicle (AOTV): This is one more class in which hypersonic vehicles are classified. Ionisation and hence presence of plasma in the vicinity of the spacecraft is the major concern of these vehicles.

Each of these vehicles faces different flight challenges based on their missions and flight configurations. These challenges form the topic of research in the field of hypersonic aerodynamics.