Ram accelerator

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A ram accelerator is a gun that utilizes ramjet or scramjet combustion modes to accelerate a projectile to extremely high speeds. In a normal ramjet, air is compressed for combustion between a spike-shaped ram and an outer casing. In a ram accelerator, a similar shaped ram 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 fuel-air mixture. As the ram compresses the mixture, it is ignited behind it. In a typical ram accelerator design, thin membranes designed to be easily punctured by the ram wall off sections of the barrel. Each section is filled with a different fuel-air mixture chosen so that later sections have higher speeds of sound. As such, the ram can be maintained at optimal speeds of mach 3–5 (relative to the mixture that it travels through) during its entire acceleration period. Ram accelerators optimized to use supersonic combustion modes can generate even higher velocities due to the ability to combust fuel that is still moving at supersonic speed.

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 initial charge detonation. The gun must be capable of withstanding the pressure of all of the gas from the reaction, compressed into a small location. As the projectile moves further down the barrel, the amount of acceleration upon the projectile decreases, 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 reaction of the propellant gasses burning, not the pressure in the barrel behind it. This leads to constant pressure being put both on the gun and the projectile itself. Consequently, far longer barrels are possible, while still delivering a strong constant acceleration to the projectile.

Ram accelerators have been proposed as a cheap method to get payloads into space. Due to wind resistance, the projectile still may need to utilize embedded rockets, such as those designed in Project HARP, to achieve orbit. Its main competitors are railguns and coilguns.

Ram accelerators are currently used primarily for research into supersonic combustion. The scram cannon science fiction weapon was inspired by ram accelerators.