Hypersonic vehicles travel at least four times faster than the speed of sound or greater then Mach 4. An airplane, missile or spacecraft can be a hypersonic vehicle. Hypersonic air breathing systems offer unprecedented class of flight vehicle encompass endoatmospheric and space access vehicles which allow rapid response at long range, great maneuverability and better survivability. Rocket boosters were used in the past to propel hypersonic vehicles. Rockets carry fuel that is heavy making it pragmatic only for short and vertical flight. Therefore the development of lightweight air breathing hypersonic vehicle has become the need.
The temperature of the air rises when a fast moving vehicle travels through it as it compresses the air in front of it. This rise in temperature together with the heat of friction becomes terrific at very high speeds. Due to this extreme heat the air leading the fast moving vehicles makes a fire ball hotter than the surface of sun. To fly at the hypersonic speed and to reenter the atmosphere a vehicle must be designed to endure the shock waves and the searing heat due to friction. Metals and gases behave in a different way at hypersonic speeds so special precautions must be taken to protect it and insulate the instruments and people on board from the heat. At higher speed a jet engine’s assemblies can no longer maintain the subsonic velocities needed for combustion. Thus a totally different design from a turbo fan or turbojet engine is required. Scramjet - a special type of engine, supersonic combustion ramjet is being deployed in hypersonic vehicles. We must take a look on the physics involved to understand how a scramjet engine works. Jet engines and the rocket engines all work on the same principle as the balloon drawing at right.
If the neck of the balloon is tied off the forces due to pressurized gases in all directions cancel the effect of each other causes the balloon not to move.