On Wed, 31 Jul 2002, Sander Pool wrote:
> 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.
20 main engines, more if you count the vernier engines too. However,
there is some dispute about whether an engine is a chamber, a pump set, or
whatever the manufacturer says it is. E.g., the RD-170 is a four-engine
cluster if you count chambers, but Energomash says it's a single engine,
and it does have only one pump set.
(However, counting pump sets is tricky. Quick, how many pump sets in a
[pre-RD-180] Atlas? Any single answer you give is wrong! The pump layout
changed repeatedly, although there were always three main chambers.)
> 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?
However, engine-out tolerance is also highly desirable, and that tends to
push the number up. The Saturn V, with five engines, did not have complete
engine-out capability. The Saturn I, with eight in the first stage and
six in the second, did.
Also, if you want to use an aerospike -- certainly a direction that ERPS
has long been interested in -- then the chamber count goes up fast. You
need a bunch of engines if you want to keep an aerospike's central gas
bubble confined even after you've shut some of them down for acceleration
limiting.
> 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 ?
The Atlas III, with the two-engine RD-180 cluster, has throttlable engines
and is always pushing a heavy upper stage.
The earlier Atlases dropped the outer two engines midway up, and when
operating without upper stages, simply accepted quite high accelerations
near center-engine cutoff. The Mercury astronauts were hammered into
orbit at just under 8G.
Henry Spencer
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