Space Forum / Space Flight / June 2006

I'm trying to add a direct comparison of nuclear-thermal vs. chemical
engines to the article on the wikipedia. I'm hampered by a lack of even
simple data.

It seems that the nuclear program got closest to reality as a
replacement for the J-2 in the S-IVB stage. Wade's states the resulting
design was called S-N, although I'm not sure this was ever an official
name.

The J-2 runs on hydrogen and oxygen. In this case, the oxygen is
primarily there to produce energy, and in fact it lowers the
performance by increasing the mass ove the exhaust -- ie, if the same
amount of hydrogen was "magically" heated to the same
temperature/pressure without mixing it into a heavier compound, the
overall performance of the engine would increase.

However, this simplistic comparison ignores the fact that the engine
has to generate both energy AND some level of thrust. Both of the
"fuels" provide thrust in the end, so simply removing the oxygen and
heating the hydrogen directly would lower overall thrust levels --
albiet while dramatically increasing Isp.

So my question question is this: how much of the thrust of the engine
is due to the momentum contained in the oxygen? Is it what pops to mind
immediately, namely 1/8th? If this is the case, then it would seem that
the increase in Isp would more than make up for the loss in thrust. Is
this correct?

Moving on...

The S-IVB contained 39,000 lbs of hydrogen and 191,000 lbs of oxygen.
If one does remove the oxygen, then it would seem that as long as the
reactor replacing it weights less than 191,000 lbs, then the stage as a
whole will deliver more performance (all else being equalized).

Again, is this simplistic analysis essentially correct?

If so, then the final question is how much such an engine weighted.
I've poked around on the 'net, but to date the only figure I've found
puts the EX-prime at 5,600 lbs, which sounds REALLY low. I'm guessing
that this does not include the nuclear fuel.

Does anyone know an estimated fueled weight for the J-2 replacement
nuclear engine?

Thanks!

Maury