The ram accelerator (sometimes, "ram cannon") is a projectile space launch concept based on exploiting the scramjet concept in a tube.1 Instead of reacting against a nozzle, the expanding gases react against a "boat tail", effectively turning the nozzle idea inside-out. In a scramjet engine, the oxidizer (oxygen) is in the atmosphere, the air it scoops up as the vehicle flies forward; the fuel is carried onboard. In a ram accelerator, the fuel isn't carried; both the fuel and the oxidizer are in the tube. The vehicle's fuel+oxidizer supply is always ahead of it, at least for the very short time it's in the tube.2
The vehicle is effectively "pinched" forward. Ram accelerator pioneer Carl Knowlen likens the mechanism to pinching a watermelon seed to make it fly -- the slimy surface of the seed reduces the frictional forces against thumb and fingertip; the seed speeds away much faster than the fingertips come together. A ram accelerator's internal detonation waves supply the "pinch". This means a vehicle can be accelerated to a velocity much greater than than the velocity of the detonation waves that strike the boat tail.3
References
1 "Gasdynamic Operation of Baffled Tube Ram Accelerator in Highly Energetic Mixtures", A.J. Higgins, C. Knowlen, C.B. Kiyanda, Submitted to: 20th International Colloquium on the Dynamics of Explosions and Reactive Systems. http://www.galcit.caltech.edu/~jeshep/icders/cd-rom/EXTABS/178_20TH.PDF ⇑
2 It's carrying fuel and oxidizer that make rockets to space so big and multi-staged. ⇑
3 Except for implosion-tube concepts, other gas-based projectile launch schemes have a speed limit related to the limit of the speed of sound in the gases used for propulsion. ⇑
Further reading
Extensive online bibliography on the RAMAC here: "The Ram Accelerator: Review of Experimental Research Activities in the U.S.", A Bruckner, C Knowlen, in Hypervelocity Launchers, Friedrich Seiler, Ozer Igra, eds., Springer pp. 79-109. ISBN 978-3-319-26016-7 (print), 978-3-319-26018-1 (ebook). https://doi.org/10.1007/978-3-319-26018-1
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