Space Forum / Space Flight / October 2003

Generally not, some low performance LOx/kero may be worse than H2O2/kero.

H2O2 is hampered by all that water.

Much the same as any other engine.
The cube/square law means that cooling large engines is overall easier.
Kerosene works as a coolant, though you have to go to different grades
to avoid coking.
Conventional regenerative cooling, or perhaps dumping the coolant overboard
if there is too much pressure loss.
For large engines, wall cooling might be an option.
Run a very bad mixture at the very walls of the engine, and if done
right, you can significantly lower heat conduction, this is good in
large engines, as the fraction of the chamber area you need to cool
is quite small compared to big ones.

Radiation cooling is probably out until someone finds a large object
with some PGMs in it.

Some grades of kerosene (RP-1?) have low sulphur to greatly reduce this
problem.
Also, if you'r willing to mess with a third tank, you might consider H2O2
as an add-on.
H2O2 is a nice coolant, with plenty of heat absorbtion.
Run that tank at high enough pressure to overcome the pressure loss in
the regenerative cooling passages, and run it through an injector manifold
at the very edge of the chamber.

Some wall cooling.

However, while LOX may be a bad coolant, if you've got enough of it, it
may just about work.

No, it can't possibly match the Isp, all other things being equal.
The thermochemistry just isn't there, unless you have a much higher
pressure / expansion peroxide engine comparing against a lower
expansion LOX engine.  If Isp is your only figure of merit, you will
never choose peroxide.  On density-Isp the peroxide combinations look
far superior, but density-Isp as a metric overstates the value of
density, which really only factors into the tankage and turbopumps,
not the entire vehicle performance.  Storability and catalytic
autoignition are also potential benefits of peroxide.

In the long term, LOX will win because of pure economic factors, but I
still feel that peroxide has medium-term benefits in reducing system
complexity.  When we are worried about the $100 / lb to orbit delta
propellant cost, we will have made quite a bit of progress...

Regen cooling still works with pressure fed engines of any size, but
you pay a more significant chamber pressure penalty.  Expendables
would probably use an ablative, rather than throwing away that much
pressure drop from the already modest tank level.

Regen cooling a peroxide motor is a lot easier than a pressure fed lox
motor, because you can cool with the oxidizer, which you have more of,
and it has a very good specific heat.  You can cool with lox if you
have a high enough pressure to keep it above the critical point, but
pressure fed vehicles will not likely have tank pressures that high,
forcing you to use the fuel as coolant.

Regen cooling does get easier as the engine gets bigger, because the
chamber surface area to propellant flow ratio decreases by a square /
cube relationship (although the expansion nozzle surface area
doesn't).

John Carmack

Not quite.  Their oxygen essentially weighs twice as much as the LOX does,
because each O2 has a couple of H2Os along for the ride, and the stored
energy the peroxide brings to the party doesn't quite make up for that.

Fast answer:  with difficulty.  Pressure-fed systems have trouble with the
pressure drop of running one or both propellants through regenerative-
cooling passages.  Curtain cooling by itself won't be enough.  You might
be able to make it work with V-2-style rings of coolant-injection holes at
intervals down the length of the chamber, plus a radiation-cooled outer
nozzle.

You can do regenerative cooling with LOX just fine, but you need fairly
high pressures, and that essentially restricts this to pump-fed systems.
Kerosene is okay -- not great, but okay -- if you use rocket-grade stuff
like RP-1, but plain old jet fuel simply doesn't cut it for doing any
substantial amount of regenerative cooling.