Sander Pool wrote:
> I'm no expert at this but I would guess that while it may be easier/cheaper
> to build lots of small engines the maintenance hassle must be humongous.
> It's tough enough to keep the shuttle space worthy with only 3 main engines.
> If it has 10 or more it must be even tougher. More turbo pumps more fuel
> lines to get cracks. More engines to gimbal.
It's because it only has three engines and they have to be massive and
operate at very high pressure, which means large turbo pumps and
because they use LOX/LH as propellants (which must be kept cryogenic
and have little mass) that the SSME's are so difficult to service. It
doesn't have to be that way. The DC-X vehicle, also using LOX/LH
propellants, could change an engine in two hours. NASA can't change a
shuttle engine in two weeks.
> I understand John's comment on how economy of scale already becomes relevant
> when designing Armadillo's craft. On the other hand as I remember the goal
> is to create a quickly re-usable vehicle. In that case the ability to
> quickly inspect and repair a vehicle may prove to be cheaper in the long
> term when the initial investment in fewer, larger engines is made.
Not likely. Small engines that can be quickly swapped out are more
efficient than large engines that take longer and require more
personnel. Multiple small engines provide engine-out recovery - a key
safety point. Multiple small engines provide much greater throttling
capability by their very nature - if you need less thrust, turn some
off. Small engines allow for differential throttling which reduced or
even eliminates the need for RCS engines, their plumbing and their
propellant tanks - thus reducing both weight and vehicle complexity.
In short, multiple engines provide significant advantages. Why aren't
they used today? Mostly because timing issues on ignition are complex
enough with LOX/LH (the shuttle has a 15 millisecond window on
ignition) that back in the 50's and 60's American engineers put more
faith in their ability to design big than their ability to coordinate
timing. With the HTP/Kero engines we're developing, timing is far less
of a critical issue (get the catpack exhaust up to temperature - squirt
in the Kero) and the aerospike potential of multiple engines has
benefits. The NUMBER of engines will be a big debate, but we're most
likely to develop multi-engine vehicles in the foreseeable future.
Michael
----------------------------------------------------------------------
Michael Wallis KF6SPF (408) 396-9037 [EMAIL PROTECTED]
President, Wallis International http://www.wallis.com
Coleman Orr:
"You either drift through life, or drive through it."
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