Re: Air-fueled aircraft on Titan


"Wayne Throop" <throopw@xxxxxxxxx> wrote in message
news:1224751789@xxxxxxxxxxxx
: "Mike Dworetsky" <platinum198@xxxxxxxxxxxxxxxxxxxx>
: The lack of free atmospheric oxygen would seem to be a problem with your
: proposal. Essentially you could consider exactly the reverse of what you
do
: on Earth, where the oxidizer is free but you have to refine and carry
fuel;
: on Titan, the fuel is free as part of the air but you have to refine and
: carry the oxidizer (hence make oxygen by electrolysis from water ice,
for
: example).

That wasn't a "proposal". That was an observation that, if you are
carrying
the oxygen and getting the methane from the environment rather than the
other
way around, you will get much less energy per kilogram you carry.
Unless I've made an arithmetic error, something like one-quarter
the energy per kilogram you carry.

The reacion is CH4 + 2O2 -> CO2 + 2H2O. The CH4 masses 16.
The four oxygen atoms mass 64. So if you carry the oxygen and get the
methane for free out of the atmosphere, you have to carry four times
the mass for the same energy, as compared to carrying the methane and
getting the oxygen for free out of the atmosphere.

Somebody else pointed out the additional problem that you're going to
have a great deal of difficulty in igniting methane mixed with that much
nitrogen, so there's that difficulty to overcome also. And even when
you overcome it, you only get a quarter the energy per mass you carry.

And, looked at another way, if you are already forced to carry the
oxygen, you only pay an additional 25 percent to carry the methane
in concentrated form so you don't have to worry that it's diluted
by nitrogen.

Either way (using ambient CH4 or carrying it as fuel) you could do a bit
better using fluorine instead of oxygen as the oxidizer:
http://en.wikipedia.org/wiki/Liquid_rocket_propellants

The greater reactivity and energy of fluorine should improve the lean
flammability problem.

Of course you don't really want to use a pure rocket engine in the
atmosphere, you want to use the air as a working fluid to get more thrust
from the fuel that is burned. So a likely scheme would be to carry methane
and oxidizer, and inject both into a jet engine. The ambient CH4 might not
provide much of the effective fuel then, but would serve as a "floor" to
keep the fuel that is carried from being wasted.


.

Relevant Pages

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