Just so we're all talking about the same thing, the early Soviet missiles
had what, 24 engines? I would consider this a multi-engine missile. Anything
less than 6 falls into the category of 'few' engines. Saturn 5, Titan, Space
Shuttle, Ariane all fall into this category.
I maintain that for an SSTO it's better to have five than to have 20 engines
for the reason I mentioned: inspection and maintenance effort. We are
talking about a ship that can be turned around quickly, right?
Having more than 1 engine sounds good for all the reasons mentioned here.
Thanks for that explanation.
I guess the question is, where is the sweet spot? I'd say it is as few
engines as possible while still being able to throttle back sufficiently by
shutting down engines. How does/did Atlas accomplish this with only 2
engines ?
Sander
----- Original Message -----
From: "Henry Spencer" <[EMAIL PROTECTED]>
To: "ERPS" <[EMAIL PROTECTED]>
Sent: Wednesday, July 31, 2002 10:28 AM
Subject: Re: [ERPS] Multiple engines
> 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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