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From: davidsen@crdos1.crd.ge.COM (Wm E Davidsen Jr)
Newsgroups: sci.space.shuttle,sci.space
Subject: Re: Exhaust velocity
Message-ID: <1437@crdos1.crd.ge.COM>
Date: 24 Oct 89 21:45:35 GMT
References: <538.252A3A3B@mamab.FIDONET.ORG> <34577@srcsip.UUCP> <1989Oct23.203306.24154@brutus.cs.uiuc.edu>
Reply-To: davidsen@crdos1.UUCP (bill davidsen)
Followup-To: sci.space.shuttle
Organization: GE Corp R&D Center
Lines: 41

In article <1989Oct23.203306.24154@brutus.cs.uiuc.edu>, zweig@brutus.cs.uiuc.edu (Johnny Zweig) writes:

|  I can't quite hack all the Isp, TC/m, and other mumbo-jumbo. I think it boils
|  down to the fact that the mass-flow (Kg/S) out the back of the motor gives
|  you part of your thrust, and the interaction between what comes out the back
|  "pressing against" the atmosphere is another. So if I have two rockets that
|  spit out the same mass of stuff per second, but one has higher nozzle
|  pressure, that one will give me somewhat more thrust off the launchpad (once
|  up in space there should be no difference since there's no atmosphere).

  Let me try to explain how this works. Picture a sphere filled with a
pressurized fluid. Call it a ballon full of hot air. If you bisect the
sphere with a plane (like, cut it in half), the pressure against each
half will be equal. No thrust.

  Now open a hole in the sphere and let pressure escape (let go of the
next of the balloon). If you think about the sphere pressure now, the
inner pressure is P (say 10 psi), and the diameter of the nozzle is D
(say 1/4 inch). That's an area of .785 square inches, which would have
had a pressure of 7.85 pounds against it. Therefore the "front" of the
balloon (or combustion chamber) has 7.85 lb more pressure on it than the
back.

  There are also action/reaction effects which are independent of the
pressure. 

  Note that atmospheric pressure didn't come into any of this, except
that it raises the pressure in the combustion chamber by a tiny amount
and therefore raises the thrust. Since lack of atmosphere will effect
either of the rockets you mentioned, there will still be more thrust for
the higher pressure. Since you said the samme mass per unit time, the
*velocity* may well be higher, as well, depending on the density of the
exhaust.

  Correct or expand on this as appropriate. The guy said he wasn't a
physicist so I didn't use metric.
-- 
bill davidsen	(davidsen@crdos1.crd.GE.COM -or- uunet!crdgw1!crdos1!davidsen)
"The world is filled with fools. They blindly follow their so-called
'reason' in the face of the church and common sense. Any fool can see
that the world is flat!" - anon