For that matter, Diborane would be even better, considerably higher
ISP and reasonable boiling point, not too very toxic, not hard to make.
It is being developed, by ATK/XCOR, under a NASA contract (which will
probably be terminated to find funds for Ares/Orion).
My analysis, (very rusty chemistry skills probably flawed), starts by
assuming that the structural mass required for the tank depends on
both pressure at each liquid's and the mass ... however you are asking
about the volume of the tank which that total mass structure will
affect but probably not too significantly.
So, calculate the mass of each feul consumed by the fixed amount of
oxygen. And then look up of density of cryogenic hydrogen and
cryogenic methane. The density ration should give us the different
sizes required for tanks for the feuls.
Well a first cut. I guess we might count the different in volume of
insulation required if it is significant and I will not know if it is
until I look up the heat equations and discover how thick the
insulation required on each tank.
http://en.wikipedia.org/wiki/Hydrogen_storage has interesting energy
density and storage target tables but not the straight density (mass/
volume) at cryogenic equilibirum I am looking for. Article claims
21 degrees kelvin as temp and that should be good enough to get
estimated ratio you asked for ... moving on.
gives ratio of about 7 times tank volume required for LH2 as for
kerosene but do not give their analysis so we can check accuracy and
assumptions for ourselves. If we figure this answer out I will put an
abstract on the discussion page and link to Lunar Boom Town to help
create traffic there.
Interesting, Wade's Encyclopedia Astronautica has everything except
methane by any designation I can recognize. Perhaps someone can check
me. It seems unlikely he simply left out methane unless he is
conspiring with the academic grading mafia and methane is the current
training feul of choice in grading competitions. ?
provides specific gravity of .55491(Water=1) but no temperature
assoicated. Boiling point is -161.48 deg Centrigrade whle the
freezing point is -182.61 deg c.
I do not remember the precise definations of specifc gravity and the
version of density we have for hydrogen but a low level chemistry book
or CRC should have that.
I must leave this incomplete but I will check back. If want the
answer immediately you might take this partial assistance and cust and
paste it in (I give you permission to GPL it so you can put it on the
Wikiversity Help Desk. or just link to this thread from the question
hope this was helpful
There was a thread on the methane engine being developed
by Northrop at:
No one ever replied to that thread though.
>Am I correct in thinking that for a given amount of O2 that a fuel
>tank for CH4 would be 1/4 the size of the size of a LH2 tank?
Closer to one-half.
>Given the storability of CH4 vs LH2, I'd think a LOX/CH4 engine would
>be an extreme priority of NASA and am surprised it has not been developed.
Given the performance of CH4 vs LH2, it's an extremely low priority. Or
perhaps that should be phrased, given the storability of CH4 vs RP1, it's
a very low priority.
LOX/Methane has about the same performance as LOX/Kerosene, and Kerosene
is even more storable than Methane. There are arguments to be made that
we might put up with the extra hassle (especially if we're already using
LOX), for the slight improvement in performance, but the idea that Methane
offers Hydrogen-like performance in a storable package, is just plain
Methane is a hydrocarbon fuel, not hugely superior to other hydrocarbon
fuels, and we've already got perfectly good hydrocarbon-fuel engines.
Mind you, at this point most of them are *Russian* engines, but the
Russians have got capitalism pretty much down by now.
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LOX/CH4 is nice, if the carbon kept in the vehicle.
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