Re: Foam and rescue possibilities

In article <1151241756.918806.205200@xxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Alex Terrell <alexterrell@xxxxxxxxx> wrote:
Anyone who understands a bit about structural engineering or materials
science knows that the strength of a metal (or metal alloy) generally
decreases with increasing temperature.

Though its brittleness decreases with falling temperature, and this
effectively reduces its performance.

I think you mean "increases with falling temperature". But whether that's
an issue depends on the metal. This problem has an exaggerated reputation
because our commonest structural material, carbon steel, is affected...
but in fact, a wide variety of structural metals *don't* get brittle when
they get cold.

The two phenomena are connected. Metals are generally ductile (as opposed
to brittle) because their ultimate strength considerably exceeds their
yield strength... so they yield, redistributing the load, before breaking.
Both strengths rise as temperature falls; the question is, which rises
faster? If yield strength goes up faster than ultimate strength, then
eventually they cross over, and the metal becomes brittle. For quite a
few metals, though, ultimate strength stays ahead of yield strength.

Metals tend to work very well at
room temperature. Go much below zero, and you can have a problem.

Just requires that you choose the metal properly. Aluminum alloys, in
particular, typically are ductile all the way down to absolute zero.

Given
the metal is inside the insulation, and next to the cryogenics, I
suspect being too cold is more of a problem than being too hot.

Jeff phrased it poorly; the (main) problem he's referring to is that if
you move the insulation inside, so the metal stays at room temperature,
then you lose the strengthening effect of low temperatures on the metal,
and must add more metal to compensate.

I still think with some redesign the foam could be removed just before
launch.

Maybe, and maybe not. Doing that *reliably* is not simple; for example,
you need helium purges to keep moisture from freezing in the gap. And
there is still a need for some protection against heating on the way up,
especially on the nose (air friction) and the base (radiant heat from
the SRB plumes).

Centaur used to jettison its insulation before ignition. Complexity went
down and payload went up when they switched to permanently-bonded foam.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. | henry@xxxxxxxxxxxxx
.

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