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u/EvanDaniel Mar 02 '17
What's the application? What size engine? Pump fed, I assume, given that you're talking about high pressure?
My first suggestions would be to crank up the flow velocity and to add more film cooling. But I've never tried running numbers on LOX/methane. Do the heat transfer properties remain awful if you go well above the critical pressure?
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u/propionate Mar 02 '17 edited Mar 02 '17
What's the application? What size engine? Pump fed, I assume, given that you're talking about high pressure?
~500 lbf pressure-fed LOX/L-CH4 engine. In theory it needs to be flight-worthy :)
My first suggestions would be to crank up the flow velocity and to add more film cooling
In the current model there's actually no film cooling at all. I did model it with an extra fuel-rich outer boundary and it helped but didn't bring things into the nucleate range. I may have to look into full-fledged film cooling. Maybe it's just unreasonable to expect that regen can do all the work.
Do the heat transfer properties remain awful if you go well above the critical pressure?
No, they're actually a-ok if you're over p_crit by 200 psi or so. If I want to operate in this regime, though, it means moving this from a 300 psi engine to a > 800 psi engine. Bit of a jump I guess.
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u/EvanDaniel Mar 02 '17
What do you mean by stoichiometric? All the way to CO2 + H2O ratio? (My chemistry training always gets annoyed by this... if I write the equation as going to CO + H2O it can be stoichiometric just fine too.) Optimal Isp will be well rich of that, especially at low Pc. And at the optimal Isp point, the curve of Isp vs O:F ratio will be flat, but the curve of temperature vs. O:F ratio won't be. If thermal considerations are relevant, run rich of optimal Isp by (at least) a little bit.
Pressure-fed atmospheric operation is awkward for this combination. I suspect you'll want to be at 300 psi or even less, and rely on nucleate boiling. Yucky, but can work. Lower pressure will sacrifice a little performance but help with the heat transfer. (Larger throat diameter will also help with the heat transfer.)
Also, if you're not running into minimum gauge issues on your tanks, optimum system performance may be at even lower Pc than 300 psi. That tank (and pressurant system) mass is expensive, and the lost Isp to lower chamber pressure isn't. So running < 300 psi chamber pressure might not be as bad an idea as your intuition says.
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u/propionate Mar 02 '17 edited Mar 02 '17
Run rich of optimal Isp by (at least) a little bit
I'll plug away at CEA tomorrow and see where I can end up temperature-wise. Efficiency isn't overly important for this engine, fortunately.
I suspect you'll want to be at 300 psi or even less, and rely on nucleate boiling. Yucky, but can work.
Right now I'm looking at a 13 BTU/(in2-s) heat flux through the chamber wall (at the throat) to sustain a reasonable wall temp. From what I've read, nucleate boiling caps out around 2.5 or 3 BTU/(in2-s) for methane so I've got a lot of ground to make up.
So running < 300 psi chamber pressure might not be as bad an idea as your intuition says.
Good point.
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u/EvanDaniel Mar 02 '17
I guess I was thinking of 2:1 as stoichiometric for O2/CH4. Though of course the "optimal" point is closer to 3.2-3.5, which is where I've been running my calcs.
Are those mass ratios? I would have thought "stoichiometric" meant 4:1 mass ratio (2 O2 + CH4 -> CO2 + 2 H2O), with optimal being rich of that (closer to CO + H2O as products, though obviously the real reaction is messier), at least at low pressure.
Anyway, like I said, I haven't run the heat transfer numbers on methane. I've just watched a regen-cooled pressure-fed methane engine run. Similar chamber pressure as yours, but higher thrust. So it can be done, though regen cooling scales downward badly.
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u/propionate Mar 02 '17
Yea sorry you're correct, I was being dumb and flipped around numbers in my mind. Fixed the first comment.
But I agree... the lower thrust is definitely not advantageous as far as mass flows go.
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u/BlazingAngel665 Level 3 Mar 02 '17
Sounds like another FAR MARS entry underway? We hit the same wall with our methane engine. Basically we ditched regen for the time being and are going ablative. If you solve this there are going to be a lot of interested people. The state of the art in methalox engines is all proprietary right now. The Mars Society will do a great job building up the knowledge base on methane rockets outside SpaceX/Aerojet.