A ram accelerator has the same function as a gun; i.e., it is a device for accelerating projectiles; however, it is entirely different in that jet-engine-like propulsion cycles utilizing ramjet and/or scramjet combustion processes are used to accelerate a projectile to extremely high speeds. It is one possible means of non-rocket spacelaunch.
It is a long tube (barrel) filled with a mixture of combustible gasses with a frangible diaphragm at either end to contain the gases. The projectile, which is shaped like a ram jet core, is fired by another means (e.g., a light gas gun) supersonically through the first diaphragm into the end of the tube. It then burns the gases as fuel, accelerating down the tube under jet propulsion. Other physics come into play at higher velocities.
How it Works
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 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 having a sound speed of 1000 m/s (e.g. fuel-rich H2-O2 mixtures) are used, muzzle velocities in excess of 8000 m/s are possible.
To span a wide velocity range in a typical ram accelerator system, multiple stages with propellants having different sound speeds are used to maintain high performance. Thin membranes that are easily punctured by the ram accelerator projectile are used to isolate the propellant stages. 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 (Mach 6-8) 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. 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, thus typically incorporated into the projectiles are rockets, such as those designed in Project HARP, to achieve orbit. Tethers and space tugs are examples of other means for capturing projectiles at apogee that offer an alternative to carrying rockets. Using rockets for upper stages, it is believed that a launch cost of less than $500 (US dollars, 2004) per kilogram can be attained. Using a tether or space tug would further reduce launch costs.
The main competitors to a ram accelerator for direct space launch applications are two-stage gas guns (SHARP), multiple sidewall injection gas guns (JVL), 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.
As of July 2007, the Ballistic Flight Group has been promoting commercial investment into the ram accelerator technology.
Published in July 2009.
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