Path: utzoo!utgpu!watmath!att!tut.cis.ohio-state.edu!cs.utexas.edu!uunet!zephyr.ens.tek.com!tektronix!psueea!parsely!bucket!loop!keithl
From: keithl@loop.UUCP (Keith Lofstrom)
Newsgroups: sci.space.shuttle
Subject: Why 3 SSME's?
Message-ID: <24@loop.UUCP>
Date: 19 Sep 89 18:13:55 GMT
Organization: Launch Loop; Portland, Or.
Lines: 36


Why three 2 MegaNewton SSME's rather than one 6MN engine?

Back when NASA was choosing contractors for the SSME, Pratt & Whitney
had a 2MN LH/LOX engine on the shelf;  NASA went with Rocketdyne for
political reasons.  As long as Rocketdyne was developing a new engine
from scratch, why did they choose three engines rather than one?

While there might be some cost savings early in the development of a
smaller engine, there seem to be significant operational advantages to
one large one:

1) Chamber pressure - There's room back there for a 6 meter nozzle,
rather than three 2.4 meter nozzles.  For the same thrust, you could
run at half the chamber pressure and pump pressure.  The turbopumps
are running at engineering limits as it is, they have to be changed
out after each mission, and there has been blade cracking (a failure
here could destroy the shuttle).  I'm told that these pumps are
running at 3 times the pressure of any previous effort.  This sounds
like a catastrophe waiting to happen.

2) Less things to fail - with one engine rather than three, even with
the same chamber pressure, there is a 3x improvement in reliability.

3) It is probably cheaper to change out one large engine than three
smaller ones.  Heavier lifting equipment is required, but it's easier
to use what you've got if you can come in from all sides.

Well, I could run on, but perhaps someone with more knowledge of rocket
engines, or the history of SSME development, can pick up from here.  
It's probably silly to reinvent the wheel, but it's good to know the
right way to do things when the time comes to do it again.

-- 
Keith Lofstrom    keithl@loop    tektronix!psueea!qiclab!loop!keithl
Launch Loop, P.O. Box 1538, Portland, Oregon 97207   (503)-628-3645