Methylsilane CH6Si seems to have some useful properties for rocket fuel:

  • high hydrogen content, 6 atoms per molecule, 13% of overall mass;
  • high combustion energy, -2612 KJ/mol or 56.8 MJ/kg (better than methane 55 MJ/kg);
  • decent liquid density, 0,628 kg/l (better than liquid methane 0.422 kg/l); 11% higher hydrogen content per volume than liquid hydrogen itself;
  • non cryogenic, self pressurizing storage, 14 bar of pressure at 21°C, critical temperature 79.3°C;
  • low average molar mass of gaseous stoichiometric combustion products 21.7 g/mol (better than methane 23.2 g/mol). Fuel rich combustion has even lower average gas mass because CH6Si decomposes to SiC and 3H2.

But there are some drawbacks:

  • solid particles of SiO2 and traces of SiC in exhaust, but less solid residue in exhaust than in solid fuels;
  • possibly pyrophoric in high humidity atmosphere, but non pyrophoric in standard conditions;
  • non toxic, but harmful to inhalation and skin exposure.

Considering all of data, they show promise, but are there any works related to its use in rocketry?

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    $\begingroup$ All those benefits seems marginal compared with ethane while losing a molecule of gas (CO2 > SiO2) neutralizes all of them. If you can deal with this you might just go hydro-boron. $\endgroup$ May 15, 2020 at 14:58
  • $\begingroup$ I disagree, ethane has high negative enthalpy of formation - 84KJ/mol + silicon brings 911KJ of energy per mole of SiO2 compared to carbon 393.5KJ/mol for mol of CO2. It's more than good compensation for lost of molecule of gas. The same reason aluminum is added to composite fuels although it doesn't generate gasses but is packed with energy. Carbon dioxide is heavy gas and increases average molar mas while SiO2 doesn't. Plus ethane is cryogenic and with lower liquid density which means more dry mass for propellant tanks. Hydro-borons are all highly toxic and long time out of the game. $\endgroup$
    – WOW 6EQUJ5
    May 15, 2020 at 15:16
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    $\begingroup$ My immediate worry would be SiO2 depositing who knows where inside the rocket engine. Any flow disruption would seem to be very problematic. $\endgroup$
    – MaxW
    May 15, 2020 at 18:19
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    $\begingroup$ Actually silanes are already tested in rocket and scramjet engines and they showed excellent results, but are pyrophoric. SiO2 doesn't cause significant deposition problem even with much higher Si content like Si5H12. I already know this. For CH6Si i couldn't find any test results, but it's properties suggest it's even better than best silane SiH4. jstage.jst.go.jp/article/tstj/7/ists26/7_ists26_Pa_33/_pdf $\endgroup$
    – WOW 6EQUJ5
    May 15, 2020 at 23:42
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    $\begingroup$ Silica buildup can be helpful - According to Ignition, some experimenters deliberately added silanes or silicone to fuel to provide a self replenishing thermal barrier layer. But that much solid matter in the exhaust is probably bad news. $\endgroup$
    – ikrase
    May 16, 2020 at 0:57

1 Answer 1


tl;dr: I see that comments below the question by the OP argue against this being a deal-breaker problem but I'm going to point it out anyway as a partial answer. Any engine using "sand" as a reaction mass will have to avoid any significant production of silica nanoparticles so large that they are not accelerated in the nozzle.

Assuming a perfect, stoichiometric reaction with H, C, O and Si masses of 1, 12, 16, and 28, I get the following:

2CH6Si + 7O2 → 2CO2 + 6H2O + 2SiO2

with the exhaust mass fractions of 0.28, 0.34 and 0.38 for the CO2, H2O and SiO2 products, having masses of 44, 18 and 60.

The exhaust is potentially almost 40 percent sand!

The boiling point of silica (SiO2) is about 2950 °C, so when the exhaust cools below that temperature condensation of tiny nano- or even micro-particles of liquid glass is possible. Because they are much heavier than molecules, it is not guaranteed that they will be accelerated in the nozzle to the same exhaust velocity.

If a significant amount of reaction mass does not accelerate properly, the Isp will be lower than you'd expect for the amount of energy being produced.

This is not comparable to soot production in KeraLOX engines because this is the main reaction product, not a side-product from incomplete combustion. Nearly 40% of the reaction mass is SiO2!

As a side note, "glass soot" is what optical fibers and other high purity silica products (e.g. CMP slurries) are made from, though it's a different silicon precursor (sometimes SiCl4).

  • $\begingroup$ Your assumptions are actually wrong. Oxidation of Si in presence of elements with high affinity towards oxygen like hydrogen and carbon is done in two steps first to SiO and then to SiO2. First step is faster and SiO has boiling point around 1880°C. It's possible to make an engine which has all gaseous products till the end of nozzle. In reality it's allowed a part of SiO2 to be formed to get extra energy and improve ISP but much less than 40%. So in reality like in other cases fuel rich combustion is prefered. $\endgroup$
    – WOW 6EQUJ5
    May 16, 2020 at 10:22
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    $\begingroup$ @WOW6EQUJ5 Can you support your assertions by citing reports or studies somehow? Right now it looks like you're using the site to push your own ideas rather than asking a Stack Exchange question. It's perfectly fine to post an answer your own question. An answer will give you space to show how you know this is possible in a convincing way. Remember if your idea relies on incomplete combustion, then you can't take advantage of the combustion energy you advertise in your question, so again an Isp penalty! $\endgroup$
    – uhoh
    May 16, 2020 at 11:00
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    $\begingroup$ I have already attached, but once again. jstage.jst.go.jp/article/tstj/7/ists26/7_ists26_Pa_33/_pdf I don't promote my ideas, i am curious. Lot of work related to use silanes in rockets and scram-jets with conclusions that they are superior to hydrocarbons in fuel rich applications. But i couldn't find anything related to methylsilane which has better properties than SiH4. I posted question to find is there any evidence to this - is it really tested? If it's tested and the answer is no it sucks it's fine for me, but without any evidence it’s hard for me to believe. $\endgroup$
    – WOW 6EQUJ5
    May 16, 2020 at 11:12
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    $\begingroup$ Technically speaking, the question is asking if there are any references, not if it is a good idea, so this does not answer what was asked. But I agree the OP appears to not be open to input. $\endgroup$ May 16, 2020 at 11:15
  • $\begingroup$ @WOW6EQUJ5 Oh okay, I didn't read the comments thoroughly. I'll have a look at this and see what's there, thanks! Next time if you'd like something considered in answers you should highlight it in the question post itself, rather than several lines down in comments. $\endgroup$
    – uhoh
    May 16, 2020 at 11:17

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