[AR] Re: What happened to the Space Shuttle?
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Mon, 22 Jul 2019 12:37:44 -0400 (EDT)
On Mon, 22 Jul 2019, David McMillan wrote:
... The *real* gems in the NASA archives are (IMO) the decades of
experiment and test results, which will generally include the critical
bits of "why we tried this," "what didn't work," "why it didn't work,"
and (hopefully) "this is what *did* work and why."
But alas, sometimes not. An example (chosen for the recent anniversary):
the Apollo LM descent engine ran fine at its original design thrust, but
"full" thrust in the actual flight LMs was set at 94% of maximum -- why?
Nobody knows.
And a search for the rationale, about 15 years ago, was unsuccessful.
That information might be *somewhere* in the archives. But good luck
finding it, among millions of pages of poorly-indexed documents on paper
and microfiche.
(There is a plausible guess. The individual engines varied slightly --
the design had no provision for trimming them all to the same thrust.
Moreover, any individual engine's thrust drifted upward slowly as it
accumulated burn time, because it was an ablative engine with no
refractory throat insert, so throat area grew as the ablator there
receded. The 94% setting *may* have been chosen to ensure that a
worst-case engine, in worst-case conditions, at the very end of its life,
would not exceed the design thrust. But nobody can be sure of that.)
And this is from a program that actually made an organized effort, near
the end, to preserve the details of its technology in case it was wanted
again in the future. When it became clear that Apollo production was
going to end and teams would be broken up and tooling scrapped, NASA spent
significant money trying to get as much as possible of the unwritten
knowledge written down. That was unusual. And even so, things have
gotten lost. It's rather worse in programs that don't budget for such an
effort.
There's a surprising amount of stuff that works in aerospace
engineering where no one really knows *why* it works -- someone with a
huge R&D budget decades ago just kept throwing spaghetti at the wall
until something stuck, worked out how to duplicate and productionize it,
and then everyone just kept following the recipe (mostly blindly) for
years or decades to come.
And hoping that, e.g., none of their materials suppliers would change
their products enough to mess things up. (It's happened.)
Henry
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