Inspired by this article, I've been looking at using metallic hydrogen as a rocket fuel using CEARUN. I'm looking at diluting the atomic hydrogen with liquid methane to reduce the chamber temperature to something that could be handled using known materials. The reaction that powers the rocket would be $\ce{H^. + H^. -> H2}$. I used a mixture ratio of 4.5 mass units of liquid methane to 1 mass unit of atomic hydrogen. I got an ISP of ~760-780 seconds at reasonable chamber temperatures (3650 kelvin). The main exhaust products in terms of 'molar fractions' are as follows: $\ce{H2} = \pu{79.011}\%,$ $ \ce{C (gr)} = 20.949\%$, with a remaining $0.04\%$ being mainly $\ce{CH4}$ (methane). The exhaust temperature was 1505.2 kelvin.

Here's my main question; am I right in assuming that $\ce{C (gr)}$ means atomic carbon? Would atomic carbon at a temperature around 1500 degrees kelvin cause any major problems for a rocket engine? I'm assuming that at such high temperatures the carbon wouldn't solidify and cause any serious problems, correct?

Finally; can anyone suggest any other substances that could be used as a dilutent in the aforementioned reaction that would yield a higher specific impulse (anything that isn't liquid hydrogen)?

EDIT: So does anyone have any educated guesses on what would happen to the engine in this case? Thanks for all the responses by the way :)

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    $\begingroup$ C(gr) is most likely graphite (chemistry.stackexchange.com/questions/81209/…) and is definitely not atomic carbon. $\endgroup$ – andselisk Jan 3 at 4:09
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    $\begingroup$ Also: you are not lucky: in fact, carbon is the element with the highest melting point of all the periodic table (that's why, for instance, graphite vessels are used in high temperature devices like ETAAS). The most common "form" (allotrope) of carbon, graphite, has a melting point around 3500°C $\endgroup$ – The_Vinz Jan 3 at 7:17
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    $\begingroup$ I think its worth noting that chamber temperature does not have to be reduced if you inject fuel along the wall of the combustion chamber to cool/insulate the materials. $\endgroup$ – A.K. Jan 3 at 7:21
  • $\begingroup$ Some known rocket fuels do produce a lot of carbon in their exhaust but theory is very poor at predicting their real performance. You should read the classic (and extremely well written) Ignition: an informal history of liquid rocket propellants by John D Clarke for some relevant evidence (and discussion of other extreme chemicals that have been tested like chlorine trifluoride.) $\endgroup$ – matt_black Jan 3 at 14:25
  • $\begingroup$ @A.K. : According to the authors even with active cooling measures such chamber temperatures could not be contained with any known materials. $\endgroup$ – chuckstables Jan 3 at 20:34

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