Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site decwrl.UUCP Path: utzoo!watmath!clyde!floyd!harpo!decvax!decwrl!dec-rhea!dec-dvinci!fisher From: fisher@dvinci.DEC (Burns Fisher, MRO3-1/E13, 231-4108) Newsgroups: net.space Subject: Re Rocket Propulsion Question Message-ID: <1631@decwrl.UUCP> Date: Sun, 17-Jun-84 21:37:01 EDT Article-I.D.: decwrl.1631 Posted: Sun Jun 17 21:37:01 1984 Date-Received: Thu, 21-Jun-84 03:15:58 EDT Organization: DEC Engineering Network Lines: 44 > Why is a rocket more efficient in space than in the atmosphere Let's try this two different ways, first a conventional conservation of momentum approach, and then using your intuitive argument of pressure in the combustion chamber. One explanation of rocket propulsion is the law conservation of momentum (which is actually derived from Newton's third, I think, but whatever...) which states that in a closed system, the total momentum never changes. This means that if some part of the system changes momentum (defined as mass times velocity), then something else also has to change to compensate. Thus, if a molecule of fuel suddenly finds itself bound to an oxidizer and shooting out the tail of a space shuttle, something else has to have moved forward (either less mass faster, or more mass slower). Notice that pressure anywhere has nothing to do with this. If the air interferes with the molecule of fuel coming out so it doesn't have as much velocity (and therefore momentum), then the shuttle must get less push forward in compensation. Now let's try it the other way. One intuitive notion is that the rocket gets pushed forward because of combustion chamber pressure which is greater in the front than in the back because of the exhaust hole. If air is "blocking up" the exhaust hole, then as you say, chamber pressure rises. But the premise of this whole explanation is that it is the differential in pressure from front to back that causes the thrust. If the exhaust hole is restricted, the differential is smaller, thus less thrust. The gun analogy: (1) The atmosphere is not a tube which forces the gases to apply their energy to anything, (2) there is nothing like a bullet to which the gases must transfer their energy, and (3) what happens if you make the gun barrel too long? The bullet speeding through it will build up the air pressure ahead of it (since the air can't escape the barrel fast enough) and will be slowed down. Thus there is an optimal barrel length. Similarly there is an optimal length for the rocket gas expansion. Hopefully this optimal length is similar to the length of the engine nozzle! Burns UUCP: ... {decvax|allegra|ucbvax}!decwrl!rhea!dvinci!fisher ARPA: decwrl!rhea!dvinci!fisher@{Berkeley | SU-Shasta}