graphite as rocket fuel?

The JANAF heat of formation for C is 711.2 kJ/mol. That's a lot.

If I do the simple heat of formation at 0 K equation for a
hydrogen-oxygen rocket, I get:

H2 + 0.5*O2 - H20 + 238.9 kJ/mol (= 13.27 kJ/g)

If I do the same equation for a carbon monoxide rocket, I get:

C + 0.5*O2 - CO + 825.0 kJ/mol (= 29.45 kJ/g)

That's a lot better!

I haven't heard of this before, so I'm probably screwing up.
Can anyone suggest where I'm screwing up?

If not, here's my proposal: Rocket has LH2 and LOX tanks, and
a porous bed of graphite, basically, charcoal. The LH2 is used
to regeneratively cool the rocket, then passed through the charcoal
on the way to the combustion chamber. The hot, high pressure
hydrogen dissolves some amount of the carbon to methane, which
is then burned with the oxygen in the combustion chamber.

Obvious problems: how does one control the rate of carbon
dissolution? As the carbon dissolves, the surface area will
change, and you may end up with more hydrogen and less methane
in the resulting mix. The combustion chamber essentially switches
from a methane-oxygen rocket at takeoff to a hydrogen-oxygen
rocket once the carbon is gone, which naturally throttles the
thing down, as for the same amount of LOX and LH2 pumped, you
get less thrust.

In article .com,
wrote:
The JANAF heat of formation for C is 711.2 kJ/mol. That's a lot.

Uh, the heat of formation for an element in its room-temperature
state is *zero* by definition.

You're looking at the heat of formation for carbon *gas*. And it's
positive -- you have to put a bunch of energy *in* to get the gas,
starting from the solid. Which is not a surprise...

If I do the simple heat of formation at 0 K equation for a
hydrogen-oxygen rocket, I get:
H2 + 0.5*O2 - H20 + 238.9 kJ/mol (= 13.27 kJ/g)

Correct.

If I do the same equation for a carbon monoxide rocket, I get:
C + 0.5*O2 - CO + 825.0 kJ/mol (= 29.45 kJ/g)

No, sorry, you get about 111 kJ/mol, about 4 kJ/g. It's just the heat of
formation of CO. The two reactants are both elements in their normal
states, so their heats of formation are zero.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

Well, graphite is the low-energy form of carbon. My bad.

Diamond is a little higher, but nowhere near 700 kJ/mol.

Atomic carbon does have 700 kJ/mol, and would be an excellent
propellant, except... I have no idea what atomic carbon looks
like, nor have I read any reports of anyone getting any.

That said, atomic anything would be a pretty good propellant,
if you could stabilize the stuff. I read a report where some folks
at NASA were looking at atomic hydrogen, but this seemed
farfetched. If anyone could send pointers to papers reporting
the stable isolation of any lightweight atomic species (except
neon, helium, etc), I'd be fascinated.

Storing carbon seperately from the hydrogen, and combining
to get methane during flight does give a little more energy than
just storing methane. You could even up it a bit by preheating
the carbon to 1000 degrees C or so, which adds 1-2 kJ/mol.
But given that you'd have to solve the problem of handling LH2,
which would give a better Isp if you just burned that, the carbon
slug seems to have limited use.

wrote:

The JANAF heat of formation for C is 711.2 kJ/mol. That's a lot.

If I do the simple heat of formation at 0 K equation for a
hydrogen-oxygen rocket, I get:

H2 + 0.5*O2 - H20 + 238.9 kJ/mol (= 13.27 kJ/g)

If I do the same equation for a carbon monoxide rocket, I get:

C + 0.5*O2 - CO + 825.0 kJ/mol (= 29.45 kJ/g)

That's a lot better!

I haven't heard of this before, so I'm probably screwing up.
Can anyone suggest where I'm screwing up?

You're using the heat of formation of carbon *gas*, not graphite.
Graphite has a heat of formation of 0.

Jim Davis

There are several problems with this idea - the most important is that
the Isp of hydrogen/oxygen is much higher than the Isp of methane (or
carbon dioxide, for that matter). The reason is that the molecular
composition of the exhaust matters - a lot! Someone else can probably
explain this better, but essentially in carbon dioxide a lot of the
exhaust energy is put into making the molecule spin - and therefore is
unavailable to push the propellant out the back.

Secondly, the hydrogen coming out of the regenerative cooling ducts
isn't really hot, for most purposes it is quite cool. That's because
it must be far cooler than the engine (so that it will absorb heat),
which in turn must be cool enough not to melt. So I doubt that the
hydrogen would be hot enough to do anything.

I did look at a similar design a long time ago, where I pre-heated the
carbon to white hot, and used it as a hybrid by pumping oxygen through
small holes drilled in it. It would probably work, but it would be
hard to design the containment vessel to be simultaneously structurally
strong, an exellant insulator, and light-weight - and the fuel would
have to be used immediately after prep.
Interesting to think about, though.

-David

wrote:

If I do the simple heat of formation at 0 K equation for a
hydrogen-oxygen rocket, I get:

H2 + 0.5*O2 - H20 + 238.9 kJ/mol (= 13.27 kJ/g)

If I do the same equation for a carbon monoxide rocket, I get:

C + 0.5*O2 - CO + 825.0 kJ/mol (= 29.45 kJ/g)

Wrong ballpark. The standard enthalpy of formation at ntp of CO is -110.53
kJ/mol (water vapour is -241.82 kJ/mol).


--
Peter Fairbrother

wrote:
The JANAF heat of formation for C is 711.2 kJ/mol. That's a lot.

If I do the simple heat of formation at 0 K equation for a
hydrogen-oxygen rocket, I get:

H2 + 0.5*O2 - H20 + 238.9 kJ/mol (= 13.27 kJ/g)

If I do the same equation for a carbon monoxide rocket, I get:

C + 0.5*O2 - CO + 825.0 kJ/mol (= 29.45 kJ/g)

That's a lot better!

I haven't heard of this before, so I'm probably screwing up.
Can anyone suggest where I'm screwing up?

Monatomic C may be hard to find.

wrote:
The JANAF heat of formation for C is 711.2 kJ/mol. That's a lot.

If I do the simple heat of formation at 0 K equation for a
hydrogen-oxygen rocket, I get:

H2 + 0.5*O2 - H20 + 238.9 kJ/mol (= 13.27 kJ/g)

If I do the same equation for a carbon monoxide rocket, I get:

C + 0.5*O2 - CO + 825.0 kJ/mol (= 29.45 kJ/g)

That's a lot better!

I haven't heard of this before, so I'm probably screwing up.
Can anyone suggest where I'm screwing up?

First of all I think you're using the formation enthalpy of gaseous
Carbon. Solid carbon (graphite) has a formation enthalpy of 0.
Diamonds (crystaline carbon) are unstable, and will actually release a
little more energy compared to graphite when you oxidize them.

Next, carbon burns normally into CO2, but that will release more energy
than CO, so that's just better.
(CO is an incomplete combustion. Also it's very toxic and will cause
oxygen deprivation if inhaled.)

regards,
Christian

On Fri, 28 Jan 2005, in sci.space.tech,
said:

The JANAF heat of formation for C is 711.2 kJ/mol. That's a lot.

If I do the simple heat of formation at 0 K equation for a
hydrogen-oxygen rocket, I get:

H2 + 0.5*O2 - H20 + 238.9 kJ/mol (= 13.27 kJ/g)

If I do the same equation for a carbon monoxide rocket, I get:

C + 0.5*O2 - CO + 825.0 kJ/mol (= 29.45 kJ/g)

That's a lot better!

I haven't heard of this before, so I'm probably screwing up.
Can anyone suggest where I'm screwing up?

Have you taken the entropy of formation into account? Carbon monoxide
has a large positive entropy of formation. Carbon is unusual among
oxides, since most solid elements form solid oxides, not gaseous ones,
and thus have negative entropy of formation. But carbon dioxide makes
as many moles of gas as the oxygen it was formed from, giving it an
entropy close to zero, and carbon monoxide makes *two* moles of gas for
every mole of oxygen gas.

--
Del Cotter
Thanks to the recent increase in UBE, I will soon be ignoring email
sent to . Please send your email to del2 instead.

Ian Stirling wrote:

Monatomic C may be hard to find.

There has been some work on stabilizing high energy radicals
(like free atoms) in solid cryogens. At sufficiently low
temperature the atoms can't get together to join up.

This stuff would probably be extremely explosive at high
energy density.

Paul