Depends on how well your injector BLC design is, if the boundary is laminar
the heat flux will be minimized and a 1/4" wall could last 3-5 sec or if
there's a lean zone it could burn out in a sec. at the throat of course.
But copper is the best material for a cold wall TCA. If cost is a factor
then just hollow out a solid bar of cheap 1020 low carbon steel and the
throat will automatically be the thickest. But that won't guarantee a
longer burn duration since a high thermal gradient can develop with the
metal melting inside and barely hot on the outside, until heat soak changes
that.
Best to water cool your engine no matter what metal you use. If you're only
doing injector characterization like c^ efficiency then 1 sec burns are
enough with proper instrumentation.
Ken
On Fri, Jul 23, 2021 at 9:20 AM Charlie Jackson <
charliejackson8629@xxxxxxxxx> wrote:
If I were to use mild steel in parallel to a short burn duration (without
regen), how thick should the combustion chamber wall be? The engine is
running on an O/F ratio of 1.6 (GOX/95% ethanol) and a chamber pressure of
200 psi. And if so, what is your recommended burn duration for that
thickness?
Charlie
*From: *Troy Prideaux <troy@xxxxxxxxxxxxxxxxxxxxx>
*Sent: *22 July 2021 00:15
*To: *arocket@xxxxxxxxxxxxx
*Subject: *[AR] Re: Steel And rockets?
Another option getting started could be to build everything beefed up for
thermal capacity in addition to the regen and run short burn durations. You
can gather lots of data (for subsequent revisions) and produce lots of team
inspiring whoosh without burning stuff out. I assisted a couple of local
uni students static test their GOX methanol engine which was good to touch
(only felt warm) about 30 sec after firing. Big beefy engine (totally
impractical for flight) for the thrust it delivered and very basic
concentric sleeving for regen (using the methanol) but it served the
purpose nicely for the 1st try. We gathered flow and line pressure data.
https://www.propulsionlabs.com.au/Gox_Methanol_Test/Sunday_Hot_Test1.mp4
Troy
*From:* arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx]
*On Behalf Of *Charlie Jackson
*Sent:* Thursday, 22 July 2021 5:45 AM
*To:* arocket@xxxxxxxxxxxxx
*Subject:* [AR] Re: Steel And rockets?
Okay, so if I were to attempt to use aluminium would it be better to
increase the combustion chamber size to improve the effects of regenerative
cooling? I intend to try water-cooling at first so I’d assume that is going
to remove a lot of heat from the chamber in comparison to ethanol.
*From: *Henry Spencer <hspencer@xxxxxxxxxxxxx>
*Sent: *21 July 2021 19:32
*To: *Arocket List <arocket@xxxxxxxxxxxxx>
*Subject: *[AR] Re: Steel And rockets?
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
