[net.columbia] Shuttle Main Engine stats request

lwe3207@acf4.UUCP (04/18/84)

Nf-From: acf4!lwe3207    Apr 17 19:43:00 1984


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Someone noted that the Shuttle doesn't use its main engines for
de-orbit burn.  I thought it would because I thought the RMS was
used primarily for direction-changing.  Which is used for "station-
keeping", i.e., in-orbit altitude changes?  How much fuel is actually
on-board for the main engines, and what does that translate to in
full-power burn time (or some other useful metric)?  What percentage
do the main engines contribute to the thrust required to achieve
orbit?  (I assumed that the solid rockets provided the majority.)

Someone told me that the Shuttle can't fly level, and that it doesn't
have enough lift to take off horizontally, even if it had the necessary
thrust.  Is this true?  If not, what are the actual "back-of-the-envelope"
estimates for the amount of main engine fuel required for the Shuttle
to take off horizontally under its own power?  Does it "break even"?:
i.e., would the poundage of fuel required to make it take off make
it too heavy to take off?

Just curious,

Lars Ericson
..cmcl2!acf4!lwe3207

henry@utzoo.UUCP (Henry Spencer) (04/19/84)

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

lwe3207@acf4.UUCP (04/20/84)

Nf-From: acf4!lwe3207    Apr 17 19:45:00 1984


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Oops...for "RMS" read "OMS".  ("Orbital Maneuvering System" vs.
"Cherry Picker".)

jerry@oliveb.UUCP (Jerry Aguirre) (04/23/84)

I don't have the article in front of me but I remember on reading
the thrust vs. weight figures that the shuttle engines can lift the
shuttle and its payload into orbit.  The solid fuel boosters lift
themselves and the strap on fuel tank.

					    Jerry Aguirre
    {hplabs|fortune|ios|tolerant|allegra|tymix}!oliveb!jerry