[AR] Re: Steel And rockets?
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Wed, 21 Jul 2021 14:32:01 -0400 (EDT)
On Wed, 21 Jul 2021, Charlie Jackson wrote:
...I have noticed that they use steel for the combustion chamber
material...I understand that steel has a lower heat transfer rate
compared to copper but is it still viable?
If you're using ablative, film, etc. cooling rather than regenerative
cooling, then certainly -- and if you're trying for something small, bear
in mind that regen cooling scales down poorly. Even with regen cooling,
yes, steel is viable, although it will be harder to make the cooling
design work. It's been done many times, especially in the early days.
It has the advantage of being quite a bit stronger, which means you don't
need as much of it. The reason why the Big Boys seem to use rather weird
copper alloys a lot is that plain copper isn't very strong, but anything
added to make an alloy reduces conductivity. So they're willing to use
exotic alloying metals like zirconium, which give helpful strength
improvements even in quite small amounts. But those alloys aren't easily
available to amateurs or startups.
Remember that ordinary steel is brittle at LOX temperatures, and so isn't
safe for anything that's going to be exposed to LOX, even briefly. For a
LOX engine, you'd want to make the injector and LOX plumbing out of
something else, and think hard about transient exposures at startup or if
something goes wrong (it will).
A third option is aluminum -- light, highly conductive (not nearly as good
as copper, but a lot better than steel), fine for LOX, mostly not grossly
expensive. You have to cool the bejesus out of it, because it loses
strength very quickly as it warms up, but the high conductivity makes it
more forgiving than steel. It's an unusual choice but there are folks who
swear by it.
Particularly if you are trying for regen cooling, bear in mind that you
will have to make more than one chamber. You *won't* get it exactly right
the first time, or the second, and getting it wrong tends to mean ruining
the chamber. A practical development project, especially a low-budget
one, has to be able to tolerate failures: "if failure is not an option,
success can get expensive" (Peter Stibrany). If making one chamber, or
even two or three, is going to strain your budget, you're headed for
disaster and should rethink your plans. (And if *materials* look like a
big part of your costs, you're probably being too optimistic about a lot
of other costs.)
Henry
Other related posts:
