spike66 at att.net
Wed Dec 19 18:18:21 UTC 2012
>... On Behalf Of Keith Henson
>...Given communication satellites, I don't think so. But let me ask, how
much of a demonstration do you think is needed? Full scale, full power?
Reduced scale, reduced power? Exactly how much reduction?
Because at some point the demonstration costs as much as the full operating
Clearly Keith has learned the bitter lesson from the X-33 misadventure.
Lockheed tried to build a single stage to orbit Venture Star. I took
intense interest in that, because it is such a critical exercise in weight
engineering. Unlike a multistage rocket, where the lower stages are more
weight-forgiving (they don't go as high) in a SSTO, every gram of spacecraft
is payload. So there is intense pressure to trim out every bit of fat
everywhere, and keep trimming until it hurts, trim until you draw blood as
we used to say. The payload margin is so thin, it really does make sense to
do stuff like choose your astronaut based on her weight and how much she
eats. No kidding, do the math. One astronaut, preferably one without legs,
these being heavy useless structures for this application.
NASA wanted a half scale model as proof of concept. It didn't need to go
single stage to orbit, since in general a half scale model couldn't do that,
but they still wanted this half scale, and NASA spent a buuuuuuuttload of
money on this whole notion. But rockets don't really scale down. There are
all kinds of systems on a rocket that refuse to scale downward. All the
focus was on the weight of the fuel tank, but the problem is that (well, one
problem of many) if you scale down any pressure vessel, you need to scale
down the thickness of the tank as well. That results in a whole mess of
manufacturing problems, most prominently one we call scar weight, which is
the weight of the seams. Consider a pair of Levis jeans for instance, and
imagine making a scale version of that for a Ken doll. The weight of that
seam doesn't scale.
But wait, there's more. As you scale to half, the mass scales to one eighth
but the mass moment of inertia scales down by a factor of 32, which means
the flight control problem doesn't get easier, it gets harder. It's
feedback loop speed must increase by a factor of about 5.7 as near as I can
figure using only classical controls BOTECs, and all that has consequences
in structural strength needed to yank the vector hardware about at those
kinds of accelerations. So the thrust vector control problem gets way
harder, and it certainly does not scale to half size and eighth weight.
When you read accounts of why the X-33 design effort failed, it wasn't
because the aerospike engine was too complicated (it really wasn't too
complicated, (I love the name)) but rather the problem of how to vector the
thrust quickly enough was too complicated. We never did figure out how to
keep that bird flying pointy-end-first after it leaves the atmosphere.
That was in 2001. No one has really tried it since then.
The X-33 failure will mark the effective end of space development of SSTO,
and a grim victory for those who reluctantly concluded back in the early 90s
that this approach is not practical, me among them. I did those calcs 100
different ways, and kept concluding that anything you can do with one stage,
you can do better with two, and probably better still with two and a half
(using an air-breathing fly-back first half-stage, such as the Pegasus
Lesson: rocket's don't really scale down.
Lesson two: we wasted most of the money we needed for the Venture Star
development trying to build this useless and hopeless half scale model that
NASA insisted we needed as a proof of concept and process development, when
we already had a bunch of good controls engineers saying this approach
Good chance I have wandered far from the original thread. What were we
talking about? Oh yes, memory loss in senior citizens...
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