Next is a transition period where the spherical shock wave is converted into a sound wave. Finally is the acoustic decay period where and . It has been theorized that craters are related to the radius at which the transition occurs from supersonic to subsonic. One interesting application of shocks in a terrestrial application is that of ramjet engines and ram accelerators . In the case of a ramjet, the engine is shaped like a simple tube with a conical body slung inside.
Fuel is injected into the air stream ahead of the engine.
The conical body causes a series of shock waves which compresses and ignites the fuel. The thrust then pushes on the back of the cone pushing the engine forward. A ram accelerator works in a similar way except that it is the conical projectile which moves inside a tube of fuel and oxidizer. Ram accelerators are more efficient than conventional gas guns because the thrust is maintained at the projectile instead of at the beginning of the barrel. This technology presents a promising area of research for space launching.