The EZ-Rocket engine development was before my time there. Resolve an
ambiguity for me? Is that isopropyl/water or ethanol/water you mention
as being tried and rejected before y'all settled on straight isopropyl?
(Or, both?)
On a historical note, the V-2 engine ran OK on a 75/25 ethanol/water
mix, with a need for additional cooling often mentioned as one of the
reasons. You get more performance with less water, of course, as long
as the engine doesn't fry. (I just realized, XCOR experience is also
"on a historical note" now :-(
Though there's another thing about ethanol - it's so massively
hygroscopic that it's really hard NOT to have at least some water in the
mix.
Anyway, I expect any further details you recall about alcohol/water
mixes would be of interest here.
Henry
On 7/21/2021 2:04 PM, Doug Jones wrote:
Water alcohol mixes are much more viscous than either alone, and have poor mixing and energy release efficiency even at 90% alcohol. We tried it briefly at XCOR and went to straight IPA, never looked back.
On Wed, Jul 21, 2021, 1:19 PM Henry Vanderbilt <hvanderbilt@xxxxxxxxxxxxxx <mailto:hvanderbilt@xxxxxxxxxxxxxx>> wrote:
Stop me if you've heard this already, but one regen-cooled
incremental-testing trick available with ethanol fuel is to start
out tests with a water-ethanol mix, for extra cooling. Then once
you have the engine operating, you can back off the water in the
mix and work your way up toward your actual chamber cooling limits.
Turning the problem around, you might design your chamber and
cooling as best you can for your operating goals, then sneak up on
those goals incrementally by beginning testing with more water in
the fuel. It is almost inevitable you'll be off in your initial
cooling calculations, but this way you should get useful test
experience and data with your first design pass at cooling design
even if it turns out to be quite a bit off.
Henry
On 7/21/2021 12:44 PM, Charlie Jackson wrote:
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 <mailto:hspencer@xxxxxxxxxxxxx>
*Sent: *21 July 2021 19:32
*To: *Arocket List <mailto: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
