Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!watmath!clyde!floyd!cmcl2!seismo!hao!hplabs!sri-unix!David.Smith@CMU-CS-IUS.ARPA From: David.Smith@CMU-CS-IUS.ARPA Newsgroups: net.space Subject: Re: Rocket thrust Message-ID: <988@sri-arpa.UUCP> Date: Mon, 18-Jun-84 13:13:00 EDT Article-I.D.: sri-arpa.988 Posted: Mon Jun 18 13:13:00 1984 Date-Received: Thu, 21-Jun-84 01:06:09 EDT Lines: 37 Let's look at the impulse (momentum transfer) derived from burning a small parcel of fuel/oxidizer. The parcel has mass m and chemical energy E (which we assume is used with perfect efficiency). Equate the chemical energy to the kinetic energy of the parcel as it exits the engine (in the frame of the rocket). This gives us the exhaust velocity, v = sqrt( 2E/m ). The impulse to the engine is the same as the momentum imparted to the exhaust: I = mv = sqrt( 2mE ) >From these equations, it is clear that throwing extra inert mass into the engine (raising m without raising E) will lower the exhaust velocity, while increasing impulse (and thrust with it). This is the reason that turbofans (and turboprops) are more fuel-efficient than turbojets, at least up to the speed at which shock waves form on the blades. Suppose that m is raised not by dumping mass into the chamber, but by putting the rocket into the atmosphere and letting the exhaust entrain the air. More thrust, no? Of course, if the vehicle has to carry the extra mass to the point of use, it would be better to have it in the form of propellant, so that E is also raised. Carrying dead mass is pretty expensive. And in space, you have to carry your dead mass with you. David Smith P.S. As I stated in my original message, rockets really do generate more thrust in space than in the atmosphere. Nasa has stated this, and they ought to know, having operated engines in both places. I am still hoping that someone can either show the flaw in my reasoning or tell what other effects are operating. Perhaps energy is lost to sideways turbulence? DRS