[AR] Re: Vehicle static test this weekend
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Tue, 13 Mar 2018 22:39:51 -0400 (EDT)
On Tue, 13 Mar 2018, Robert Watzlavick wrote:
... However, it's not just the melting point of the material which is
1100 degF for aluminum. The max allowable for 6061-T6 in my design on
the inside hot wall is 700 degF where it has 10% of its strength left.
I'm not well versed in the high temperature steel alloys but I took a
quick peek at MIL-HDBK-5 and the highest temperature they show in any of
the tensile strength tables is 2000 degF...
Moreover, if you're thinking about seriously long life for stressed parts
that run very hot, you also have to consider creep resistance -- it's a
big issue for jet-engine turbine blades, even with extensive use of film
and regenerative cooling.
For those who haven't heard about this... A high temperatures, elastic
deformation (reversed when load is removed) and plastic deformation
(irreversible yielding, when load on a ductile material exceeds its yield
strength) are not the whole story. When absolute temperature exceeds
roughly half the material's melting point, many materials also show creep:
slow irreversible yielding at stresses well *below* the yield strength,
with rate strongly temperature-dependent. This is why the people making
turbine blades resort to exotica like directional solidification and
single-crystal casting, for better creep resistance.
(Creep is also how Earth's mantle, which responds to short-term loads like
tidal deformation as a strong solid, can flow on long time scales to drive
tectonic drift.)
Keeping the chamber walls cool is almost certainly easier and cheaper than
trying to make them highly creep-resistant.
...it seems like giving up almost
half the Isp would make a hard problem (vehicle mass fraction) even harder.
Keeping the same delta-V while cutting Isp in half requires *squaring* the
mass ratio -- if it was 8 (moderately aggressive), now it's 64 (almost
certainly impossible).
Henry
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