Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!linus!decvax!ucbvax!ucbcad!tektronix!hplabs!sri-unix!jheimann@BBNCCY.ARPA From: jheimann@BBNCCY.ARPA Newsgroups: net.space Subject: Re: Rocket thrust Message-ID: <895@sri-arpa.UUCP> Date: Fri, 15-Jun-84 15:08:42 EDT Article-I.D.: sri-arpa.895 Posted: Fri Jun 15 15:08:42 1984 Date-Received: Tue, 19-Jun-84 01:07:39 EDT Lines: 23 From: John HeimannThe bullet/rocket analogy is specious because in fact the bullet is part of the material which is being ejected from the rifle; i.e. is analoous to the exhaust gasses, not the rocket. It is the rifle that is analogous to the rocket. If one were to have a rocket engine with a combustion chamber that was open at the front, analogous to an open breech rifle, then we would expect the rocket engine to have far less thrust (if any at all) than a closed chamber engine. Alternatively, as we would expect a bullet fired from a gun in space to have a higher velocity than one fired in the atmosphere since there would be no viscous drag on the bullet, and hence to impart a greater impulse to the rifle, so we would expect a rocket in space to eject its exhaust gasses at a higher rate than in the atmosphere and hence have greater thrust. Another way to look at it: the exhaust gasses escape rearward at a rate which is dependant on the difference in pressure between the engine throat and the end of the exhaust nozzle. The atmospheric "backpressure" that you refer to is just atmospheric pressure on the rear of the rocket, which is cancelled out by atmospheric pressure on the front of the rocket. At every moment I experience something like 5000 pounds of force on my back due to atmospheric pressure, yet I don't accelerate forward since there also happens to be the same force on my chest. John