Newsgroups: sci.space.shuttle
From: henry@spsystems.net (Henry Spencer)
Subject: Re: SME Throttle mechanics
Date: Tue, 29 Jun 1999 20:20:14 GMT

In article <3778BC38.29BFA020@bigpond.com>,
Julian Bordas  <JBordas@bigpond.com> wrote:
>> If you mean the SSME, yes, assorted valves partly close, restricting the
>> flow of propellants somewhat.  (It's a complicated engine and there has
>> to be careful coordination of the sequence of various valve motions, but
>> that's a detail.)
>
>        I take it that there is no variable injector geometry ala LM descent
>engine?  Is this because the fuel is gaseous as it enters the combustion
>chamber?

Correct, the injector geometry is fixed.  No, this has nothing in
particular to do with the use of gaseous injection.  The reason why the LM
needed a variable-geometry injector was the very wide range of throttling
required.

In theory you could throttle a rocket engine as far back as you wanted by
reducing propellant flow, but there's a nasty problem in practice.  It's
essential to have an injector pressure drop of maybe 25% of chamber
pressure, because that makes propellant flow rates pretty much independent
of small variations in chamber pressure.  When you cut back on the flow
without changing the injector flow area, the pressure drop in the injector
falls off faster than the chamber pressure does... and when it gets too
low, coupling between the chamber and the feed system appears, and the
engine usually goes unstable.

You can throttle a typical engine down to 60-70% of nominal thrust without
serious stability problems.  The SSME originally was specified to throttle
down to something like 50%, and Rocketdyne was anticipating considerable
trouble making it stable at the low end, but NASA was able to relax the
requirement to 65%, which posed no great problem.

The LM descent engine, on the other hand, had a specified throttling range
of 10:1, to permit rapid braking early in the descent and fine control of
descent rate later.  Even with a variable-geometry injector, TRW had some
trouble making this work!  In the end, the maximum thrust was a bit lower
than intended (95%) because of stability problems at 100%, and thrusts
between maximum and about 65% could not be used at all because mixture
ratio was hard to control properly.

>What book(s) would provide this level of technical details?

You can find some of it in the usual references on rocket-engine design --
Sutton's "Rocket Propulsion Elements" and Huzel&Huang's "Modern Engineering
for Design of Liquid-Propellant Rocket Engines" -- but there is no single
book discussing the history of existing engines in any technical detail.
--
The good old days                   |  Henry Spencer   henry@spsystems.net
weren't.                            |      (aka henry@zoo.toronto.edu)