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Path: utzoo!henry
From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: net.columbia
Subject: Re: Shuttle Main Engine stats request
Message-ID: <3764@utzoo.UUCP>
Date: Wed, 18-Apr-84 17:58:07 EST
Article-I.D.: utzoo.3764
Posted: Wed Apr 18 17:58:07 1984
Date-Received: Wed, 18-Apr-84 17:58:07 EST
References: <11700005@acf4.UUCP>
Organization: U of Toronto Zoology
Lines: 32

Once in orbit, the OMS is used for *all* maneuvering, orbit changes,
station-keeping, de-orbit burn, etc.  (Well, anything that's heavy
enough that the attitude-control engines can't handle it.)  The SSMEs
(Space Shuttle Main Engines, for anyone not in on the alphabet soup)
are just along for the ride.

There is *no* fuel on board for the SSMEs, and they are not restartable
in space.  Building a restartable engine isn't nearly as easy as
building an engine that gets, so to speak, "re-primed" between starts.
In general, the only engines that are restartable without manual
maintenance intervention are those that have to be, like the OMS.
The F-1 engines in the first stage of the Saturn V, for example, were
not restartable.

The main engines contribute quite a modest fraction of the thrust during
ascent; I don't recall the numbers, but the SRBs are definitely doing
most of the work during the early part of ascent.

Given that the SSMEs are non-restartable and that there is no fuel for
them on board, flying the Shuttle level is sort of academic.  Once upon
a time, the Shuttle was supposed to have jet engines for in-atmosphere
flight.  That was scrapped due to weight constraints, in favor of a
clip-on jet kit.  The latter would not be available during landings
from orbit, but would permit the orbiter to fly under its own power for
ferrying between landing and launch sites.  The clip-on kit was then
itself scrapped, a combination of complexity and budget problems I think.
The current Shuttle would need significant modifications for atmospheric
flight.  For one thing, the landing gear is not retractable in flight.
(Once you extend it for landing, it *stays* extended!)
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,linus,decvax}!utzoo!henry