Hydrogen is totally different and is an excellent coolant usually injected
as a gas after vaporizing in the cooling jacket.
The SSME ran at much higher pressures than the average rocket engine so you
can do things with LOx not possible with lower pressure engines.
Now other oxidizers like nitric acid is a different story and are preferred
as a coolant over their fuel counterparts.
Back to the original question, small engines are fundamentally more
difficult to cool than larger ones due to the cube/cube root law governing
the chamber volume to surface area ratio. But with clever control over the
boundary layer this lower thrust limit can get pretty low, I think Xcor's
concentric element injector does this well.
K
On Mon, Oct 19, 2020 at 6:31 PM Yucca Works <pyropickle1@xxxxxxxxx> wrote:
Makes sense, thanks for the answer and for digging into the archives. RPE
9th edition is very adamant about the holy cooling characteristics inherent
in Hydrogen.
On Mon, Oct 19, 2020 at 6:21 PM Troy Prideaux <troy@xxxxxxxxxxxxxxxxxxxxx>
wrote:
This is an answer from Henry Spencer on this list 10 years ago to a
similar suggestion:
*[quote]:*
*Assuming you mean *regenerative* LOX cooling, it's been done, but it's
quite rare. The problem is that it works reasonably well only in the
supercritical regime, which for LOX requires quite high pressures, maybe
1000psi to have some margin. (You don't want your coolant to pass close to
its critical point as it heats, because many properties change very sharply
just there and it's easy to get violent flow oscillations.)*
*Regen cooling with LOX at subcritical pressures just doesn't work very
well. The temperature range over which it is liquid is short, and the
latent heat of boiling is small, so it *will* boil. And unless you do
something tricky and unusual, you'll almost certainly get into the film
boiling regime, where heat transfer is greatly impeded and wall temperature
soars, usually to the point of self-destruction. Even if you somehow get
it past the boiling transition, the volume flow rate is orders of magnitude
higher as a gas, so pressure drop in the cooling passages will be large and
flow resistance will be high, and things get still more awkward if gas flow
starts to approach Mach 1... Basically, you really want a regenerative
coolant to stay (at least mostly) liquid.*
*There is also some long-standing superstition about oxidizer cooling
being unwise, but there have been a number of successful oxidizer-cooled
rockets (mostly using more cooperative oxidizers) and this *is* just
superstition.*
*The temperature is a very minor issue by comparison. Liquid hydrogen
makes a wonderful coolant despite being even colder, thanks to its very low
critical pressure and some other helpful properties. (People have
seriously proposed unbalanced-tripropellant systems which are mostly
LOX/kerosene, but also burn a small amount of LH2 so they can cool the
chamber with it.)*
*[end quote]*
Troy
*From:* arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx]
*On Behalf Of *Yucca Works
*Sent:* Tuesday, 20 October 2020 11:32 AM
*To:* arocket@xxxxxxxxxxxxx
*Subject:* [AR] Hypothetical Lox cooling
I want to get this group's thoughts, experience stories, and
possibly advice on regenerative cooling with liquid oxygen at the small
scale (<500lbf)? I have read the public NASA paper that concerns cracking
in the chamber and local hotspots and have looked into (what little has
been released to the public) Launcher's work with their small 3d printed
copper engines. On paper at least, it seems like one can get away with
cooling from the throat up (the entirety of the combustion chamber). This
is just curiosity-sparked research, nothing beyond that yet.