Ram accelerator

A ram accelerator is a device for accelerating projectiles or just a single projectile to extremely high speeds using jet-engine-like propulsion cycles based on ramjet or scramjet combustion processes. It is thought to be possible to achieve non-rocket spacelaunch with this technology.In a normal ramjet, air is compressed between a spike-shaped centerbody and an outer cowling, fuel is added and burned, and high speed exhaust gases are expanded supersonically out of the nozzle to generate thrust. In a ram accelerator, a projectile having a shape similar to the ramjet centerbody is fired, (often from a conventional gun), into the accelerator barrel, causing compression between the projectile and the barrel's walls. The barrel contains a pre-mixed gaseous fuel-air mixture. As the ram accelerator projectile compresses the fuel-air mixture, it is ignited and the combustion is stabilized at the base of the projectile. The resulting pressure differential generates a prodigious amount of thrust that can accelerate projectiles to in-tube Mach numbers greater than 8. Thus, if propellant mixtures with a speed of sound of 1000 m/s (e.g. fuel-rich H2-O2 mixtures) are used, muzzle velocities in excess of 8000 m/s are possible.Scramjet projectiles were tested in the HARP project in the 1960s.The chief advantage of a ram accelerator over a conventional gun is its scalability. In a normal gun, maximum pressure is exerted at the time of the charge ignition. As the projectile moves further down the barrel, the amount of acceleration upon the projectile decreases as the gas behind it expands, eventually reaching amounts trivial enough that a longer barrel is no longer justified (see internal ballistics). With a ram accelerator, the projectile is propelled primarily by the pressure generated by the reaction of the propellant gases burning just behind the projectile. This leads to constant pressure being put both on the gun and the projectile itself. Consequently, far longer barrels are possible than conventional guns, while still delivering a strong constant acceleration to the projectile.Ram accelerators have been proposed as a cheap method to get payloads into space. Impulsive launched projectiles need some means to circularize their trajectory for orbit insertion, so rockets, such as those designed in the 1960s in Project HARP, are typically incorporated into the projectiles. With multi-stage rocket projectiles the launch cost was estimated at US$500 per kilogram in 2004.