A question on liquid propellant as it relates to stage size and fuel weight. . .

...this is beginning to look rediculous. I mean the second
stage is FAR larger than the first and the total ISP for the second
stage (H2/O2) is only about .85 that of the first...

One consideration you may have missed, if you're really starting from the
basics, is that LOX/LH2 stages generally do not run their engines at an
"optimum" mixture ratio. The LOX/LH2 mixture ratio for maximum Isp is
about 4.0, but even early LOX/LH2 stages ran at about 5.0, and modern ones
generally run at 6.0. You get optimum *stage* performance by accepting
some loss of Isp to reduce tank mass.



I was going off of SSME mix ratios.





...is it better to go with the higher ISP and
accept the larger structure or go with a lower ISP but a much more
compact structure assuming you have the same total lb/seconds?

The answer is, it depends. :-)


Dang. :-) I figured as much.



If you do not care about the total mass of the stage, typically you are
better off with LOX/kerosene. The mass ratio needs to be higher, but high
mass ratios are easier to achieve.

If you do care about the total mass, the first question to ask is why you
care. There is a long tradition in the field that gross liftoff mass is
the preferred figure of merit for a launcher. This is utter nonsense;
most of that mass is fuel, which is dirt cheap. There is a better case
for dry mass as the figure of merit, but even that is open to question,
because hardware cost is only vaguely related to hardware mass.


I don't really care about the mass that much.





The one case where total mass pretty clearly does matter is for an upper
stage being put on an existing launcher. There, the lower total mass of a
LOX/LH2 stage can be a win, because it permits a rather bigger upper stage
with rather higher payload.


Basically what I'm looking at is a two stage to orbit with ALL of the
second stage minus the propulsion module going into orbit and using
the things to make a REAL space station out of. It's just an exercise
for kicks. I was thinking of using the LOX/LH2 upper stage because it
seems like it would be useful for other things too. You could burn it
to get drinking water for example. (After you cooled it down a bit
LOL) Didn't want extra kerosene sitting around doing nothing and the
coking issue was a consideration also although the 1st stage would be
reusable and uses LOX/kerosene.