Space Forum / Space Flight / December 2003

The highest Isp I could find was 480 sec for the Advanced Expander
Cycle engine, a testbed; I recall some sort of development effort in
the 80's that was talking 490 sec and was having problems with the
turbopumps having to spin very, very fast to generate enough power
and pressure.

http://www.astronautix.com/engines/aec.htm    (aka RS-44, Rocketdyne)

I just found a discussion by one of the RL-10 developers that goes into
a lot of detail of the history of that engine, including tradeoffs
with mixture ratios and how it affected total stage mass.  It's a
bit lengthy, but very, very interesting:

http://fac14.cmps.subr.edu/Foustall.htm

My guess is that Isp in this case is limited by total available energy;
run too rich and there's not enough heat/energy to drive the
turbopumps to create the necessary high pressure.  The excess hydrogen
can't get any hotter, so exhaust velocity hits a wall.  Too hot and one
runs into practical materials problems and disassociation of the
combustion product which absorbs heat and you once again hit that
wall.

So far as I can tell from my limited understanding is that a simple
LH/LOX chemestry just doesn't have enough energy get even to 500 sec
with known materials.  I'm sure Henry will have more perspective on this
subject.  ;)

--Damon

To about 500, maybe, if you work really hard...  470-480 is the best I
recall anyone actually proposing to achieve with LOX/LH2, using
extremely-high-expansion nozzles (long enough that the limiting factor
on expansion is condensation of water in the exhaust...).

It helps, but not hugely.  Numbers depend on details, but the Saturn V's
upper-stage engines did change mixture ratios, for several reasons, and
going from 5.5 to 4.5 raised their Isp by only about 7 seconds.