On Wed, 31 Jul 2002 [EMAIL PROTECTED] wrote:
> There are many benefits, some of which are critical, for SSTO. Given
> multiple engines (16+, say), it allows differential throttling, which
> eliminates the need for any gimballing. Assuming your mass fraction
> is about 95%, upon landing you could shut down 12 of the engines, and
> throttle down the remaining 4 engines for landing/manuvering.
That last came out a bit garbled. :-) However, it touches on a very good
reason for an SSTO in particular to have a bunch of engines: some sort
of deep-throttling-without-Isp-loss capability is crucial for SSTOs,
because of their large mass ratios.
The thrust needed for takeoff with full tanks is vast overkill for orbit
insertion with nearly empty tanks. If acceleration loads on the
structure, not to mention the crew, are to remain reasonable throughout,
thrust has to be reduced by at least a factor of four, probably more.
(Note when calculating this that thrust goes up as the air thins out and
the back pressure on the engines is reduced -- Saturn V thrust at takeoff
was 7.6Mlb, but by T+2min, it was over 9Mlb. Just after that time, it got
reduced substantially by shutting down the center engine, to limit
acceleration loads on the second stage.)
Conventional throttling reduces chamber pressure, and this tends to
penalize Isp some, just at the time you need it most. (First-order
analysis says chamber pressure makes no difference except in atmosphere,
but there are second-order effects which do make a difference.) Also,
really deep throttling tends to cause stability problems unless you do
something tricky like a variable-geometry injector.
The best way to lose a lot of thrust is to shut down some engines.
Henry Spencer
[EMAIL PROTECTED]
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