Scram cannon

The scram cannon is a hypothetical kinetic energy weapon based on ram accelerator technology. The scram cannon uses a projectile that uses supersonic combustion ramjet technology to increase the muzzle velocity to levels that are impossible with normal gunpowder based guns; firearms are limited by the speed of sound in the propellant gasses, which limits the velocities to about Mach 5 (see internal ballistics). A scram cannon uses a sealed barrel filled with a fuel/oxidizer mixture that undergoes supersonic combustion as the projectile passes by. The energy liberated by this combustion acts to propell the projectile through reaction, like a jet or rocket engine, rather than by pressure as in a gun. This allows the scram cannon to launch sizeable projectiles at velocities previously only attainable with railguns, light gas guns and rockets, but with significantly simpler techniques.

Scram cannons are frequently used in Science Fiction and various high tech role playing games. The earliest reference to a weapon resembling a scram cannon far predates the theory that made ram accelerators possible. In Robert A. Heinlein's 1940/1953 novella "If this goes on..." he describes a tank cannon that uses injectors that inject additional propellant in stages down the barrel, "to keep the pressure up" and provide higher velocities. While the approach as described wouldn't work for velocities in excess of about Mach 5 (any pressre based system will be limited by the speed of sound in the propellant), a system that used a scramjet projectile and injected high pressure fuel and oxidizer gasses into and unsealed barrel just before the projectile passed that point could be practical. Currently ram accelerators use plastic barriers at intervals down the barrel to hold the high pressure fuel/oxidizer mix in place until the projectile passes, but that leads to long down times between firings as the remains of the barriers are removed. Using injectors to inject the fuel/oxidizer just ahead of the projectile would eliminate the need for the plastic barriers.