Methalox hot decomposition and reforming inside engine

[wtrix]

It is known that with the presence of specific catalyst, methane can be thermally decomposed to ethane and hydrogen or even deeper to methyl or methylene groups, different other hydrocarbons and hydrogen. Which made me think that as the fuel gets enriched this way with hydrogen, so the Isp should go up.

Thus the proposal - why not to use engine cooling as means to decompose fuel and thus achieve better specific impulse?

Am I just making things up or is it a plausible idea in case of methalox engines?

[beb]

Methane (CH4) does not "decompose" to ethane (C2H6). That's an endothermic reaction, one that absorbs energy. There would be and can not be an increase in energy in that reaction.

[wtrix]

Yes it does. Not 100%, but some minor part of it does in the presence of catalysts: http://www.nrcresearchpress.com/doi/pdf/10.1139/v75-516

There are other routes of decomposition as well (to C + 2H2): http://www.chem.tamu.edu/rgroup/goodman/pdf%20files/465_catal_19_06_164.pdf
However, in rocket engines the carbon soot is not desireable thing, thus the decomposition route to hydrocarbon gases+H2 should be favoured

Sure it is endothermic reaction, but doesn't cooling the engine involve removing a lot of heat from the inside wall of the engine and storing it in the coolant (i.e. fuel)? Thus, one gets a lot of "free energy" that can be used to mantain this endothermic reaction.

[wtrix]

One more try. Imagine a situation when fuel (methane) passes over engine for regenerative cooling. However, the cooling is designed in a way that the methane temp. rises to ~600-700C. In case the engine cooling shirt includes catalyst for methane decomposition, what arrives in combustion chamber is:
1. Mostly methane
2. Some other hydrocarbons wiht higher carbon percentage
3. Pure carbon powder
4. significant proportion of Hydrogen

This way some of the hydrogen is "liberated" from carbon bond and inside the chamber no energy has to be spent to do that (this H-C connection is what actually lowers the Isp of methane to levels only slightly above RP-1 - some 20-30s).
Thus, the Isp should be higher.

So. Am I making things up or is it plausible.

[Jim]

So. Am I making things up or is it plausible.

Yes, you are.  The system is closed.  You can't increase the energy.

[R7]

So. Am I making things up or is it plausible.

Yes, you are.  The system is closed.  You can't increase the energy.

[cautiously raises finger]

But you can tweak the mean molecular mass. Did a quick and dirty analysis of methane engine (my BE-4 guess) versus another with otherwise same parameters but burning fuel mixture where complete conversion of 2 CH4 -> C2H6 + H2 has happened. Result was a modest five second increase to both sea level and vacuum Isp.

The increase is explained by slightly higher combustion temperature and minor change in exhaust gas species from H2O and CO2 towards lighter H2 and CO.

However the reaction rates and selectivity in the conversion pdf seem way too low for this, requiring huge engine tweaks which would render the modest performance increase moot.

[gospacex]

So. Am I making things up or is it plausible.

Yes, you are.  The system is closed.  You can't increase the energy.

[cautiously raises finger]

But you can tweak the mean molecular mass. Did a quick and dirty analysis of methane engine (my BE-4 guess) versus another with otherwise same parameters but burning fuel mixture where complete conversion of 2 CH4 -> C2H6 + H2 has happened. Result was a modest five second increase to both sea level and vacuum Isp.

Not surprising.  2 CH4 -> C2H6 + H2 reaction consumes energy. If it happens in flight, then this energy is taken from e.g. hot combustion chamber.  No net overall win, as expected.

[wtrix]

So. Am I making things up or is it plausible.

Yes, you are.  The system is closed.  You can't increase the energy.

[cautiously raises finger]

But you can tweak the mean molecular mass. Did a quick and dirty analysis of methane engine (my BE-4 guess) versus another with otherwise same parameters but burning fuel mixture where complete conversion of 2 CH4 -> C2H6 + H2 has happened. Result was a modest five second increase to both sea level and vacuum Isp.

Not surprising.  2 CH4 -> C2H6 + H2 reaction consumes energy. If it happens in flight, then this energy is taken from e.g. hot combustion chamber.  No net overall win, as expected.

The main point was that the energy has to be taken from the combustion chamber anyway. Otherwise the chamber would melt. Gives proper name for the regenerative cooling :-)

However when it comes to reaction time then as R7 pointed out, this can be quite slow and thus requires a big reactor, which the combustion chamber's external wall isn't.

Only option would be to mix fuel with turbine exhaust to gasify the fuel and build catalytic converter already inside the tube leading from compressor to engine cooling shirt. Thus prolonging the reaction time possibly to "enough" given good catalyst. On the other hand, increased cost and weight of this system probably more than overweigh any potential gain.

[Jim]

The main point was that the energy has to be taken from the combustion chamber anyway. Otherwise the chamber would melt. Gives proper name for the regenerative cooling :-)

And it is still a benefit, because it increases propellant energy level vs cold propellant.  That is the regenerative part. 

[JasonAW3]

Methane (CH4) does not "decompose" to ethane (C2H6). That's an endothermic reaction, one that absorbs energy. There would be and can not be an increase in energy in that reaction.

If you started, initially, with straight Methlox and used the methane as a coolant for the engine, it should gain enought heat that when it's passed through the catalyst, it could decompose in the fashion suggested.

     Obviously it wouldn't be 100% effecient as som Methane would still get through, but if you got an 80% to 90% conversion, that should be enough to keep the engine pretty clear of carbon soot.

[wtrix]

Methane (CH4) does not "decompose" to ethane (C2H6). That's an endothermic reaction, one that absorbs energy. There would be and can not be an increase in energy in that reaction.

If you started, initially, with straight Methlox and used the methane as a coolant for the engine, it should gain enought heat that when it's passed through the catalyst, it could decompose in the fashion suggested.

     Obviously it wouldn't be 100% effecient as som Methane would still get through, but if you got an 80% to 90% conversion, that should be enough to keep the engine pretty clear of carbon soot.

I think that engine cooling channels are not long enough. Considering the speeds at which methane gets pumped into the engine, the reaction time would be very short. However, in researches described above the required decomposition times for any significant amount of CH4 to be composed are a lot longer.

My question was more in hypotetical manner like from the position "provided that we have catalyst that decomposes CH4 instantly, does the idea makes sense.

Second thing is that in practical rocket engines even H2 can be pumped around the cooling channels without the phase change. Rising the temperature to >500C and carrying endothermic reaction requires significant heat exchanger inside the engine, which proves Jim's point.

So it wouldn't possibly make practical sense due to low theoretical 5s gain in Isp. But it's nice that the idea got somehow clarified and quantified.

[JasonAW3]

Methane (CH4) does not "decompose" to ethane (C2H6). That's an endothermic reaction, one that absorbs energy. There would be and can not be an increase in energy in that reaction.

If you started, initially, with straight Methlox and used the methane as a coolant for the engine, it should gain enought heat that when it's passed through the catalyst, it could decompose in the fashion suggested.

     Obviously it wouldn't be 100% effecient as som Methane would still get through, but if you got an 80% to 90% conversion, that should be enough to keep the engine pretty clear of carbon soot.

I think that engine cooling channels are not long enough. Considering the speeds at which methane gets pumped into the engine, the reaction time would be very short. However, in researches described above the required decomposition times for any significant amount of CH4 to be composed are a lot longer.

My question was more in hypotetical manner like from the position "provided that we have catalyst that decomposes CH4 instantly, does the idea makes sense.

Second thing is that in practical rocket engines even H2 can be pumped around the cooling channels without the phase change. Rising the temperature to >500C and carrying endothermic reaction requires significant heat exchanger inside the engine, which proves Jim's point.

So it wouldn't possibly make practical sense due to low theoretical 5s gain in Isp. But it's nice that the idea got somehow clarified and quantified.

Yeah, you're probably right.  The expense alone to set up a system where that decomposition before being use just would not make economic sense for such a low increase in capibility.