Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!unmvax!ncar!noao!asuvax!enuxha!kluksdah
From: kluksdah@enuxha.eas.asu.edu (Norman C. Kluksdahl)
Newsgroups: sci.space.shuttle
Subject: Re: 104% on shuttle launch
Summary: Rocket Thrust vs. Altitude
Message-ID: <127@enuxha.eas.asu.edu>
Date: 28 Apr 89 16:20:30 GMT
References: <883@sactoh0.UUCP> <9130002@hpwrce.HP.COM> <2516@mentor.cc.purdue.edu> <2859@crete.cs.glasgow.ac.uk>
Organization: Arizona State Univ, Tempe
Lines: 36

In article <2859@crete.cs.glasgow.ac.uk>, willisa@cs.glasgow.ac.uk (Mark Willis) writes:
> I always thought that the thrust level was measured at sea-level, so that
> as the orbiter ascended the engines were capable of higher thrust. Or does
> it work the other way round - thrust decreases with altitude? I am aware
> that the engines dont require an atmosphere to "push against", but it
> must have some effect.
> 
At a given altitude, the thrust of a rocket engine is maximized when the
pressure of the exhaust is exactly equal to the ambient air pressure.  This 
is achieved by tailoring the configuration of the nozzle to the design
altitude.  If the altitude is lower than the design, or optimum, altitude,
the ambient air pressure is higher than the exhaust, resulting in a net
reduction in thrust.  Shortening the nozzle will balance this condition.
At altitudes greater than the design altitude, the exhaust pressure is
greater than the ambient air pressure, and thrust is correspondingly
less than its theoretical maximum.  Lengthening the nozzle corrects this.

Putting it briefly, you want a short nozzle at low altitudes, where the 
air pressure is higher, and a long nozzle at high altitudes.  The design
altitude for a rocket engine is usually a compromise between the two.
(There also are double-geometry engines, which use a nozzle extension.
This extension, at launch, is retracted.  At a given altitude, the extension
is put into place, lengthening the nozzle and increasing the performance
of the rocket.)

[Oates, 'Aerothermodynamics of Rocket and Gas Engines', or something like
that--the reference isn't right in front of me.  Two chapters in the book
deal with rockets--the rest is boring jet engine stuff!]

**********************************************************************
Norman Kluksdahl              Arizona State University
            ..ncar!noao!asuvax!enuxha!kluksdah

standard disclaimer implied

Useful criticism always appreciated.  Senseless flames always discarded.