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The boiling point of CH3Cl is -24 degrees Celsius, whereas the boiling point of CH4 is -160 degrees Celsius.

The density of CH3Cl is 2.3 g/cm3, whereas the density of CH4 is 0.6 g/cm3.

CH3Cl is polar, hence reactive as well. Also, perchlorate salts found on Mars, which could be used to synthesize Cl2 to monochlorinate CH4 (produced from the Sabatier reaction: CO2 + H2O → CH4 + O2) in the presence of sunlight (free radical halogenation reaction of CH4).

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    $\begingroup$ Possibly because the Environmental Impact Assessment for a chlorine-gas-spewing death machine will be a bit more difficult to pass? Possibly because the fuel weight 3.15 times as much per molecule, yet releases only 85.8% as much energy, thus in total it is only 27% as good a fuel? $\endgroup$ Oct 21 at 5:25
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    $\begingroup$ I would also assume price could be a good reason. $\endgroup$ Oct 21 at 6:19
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    $\begingroup$ @BojanKogoj I think PcMan's covers it well. Even if CH3Cl was a tiny fraction of a percent of the cost of methane (which is not the case), a "chlorine-gas-spewing death machine" would still be a very bad idea. $\endgroup$ Oct 21 at 6:38
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    $\begingroup$ @jwenting hydrazine and other hypergolics produce exhaust with a mixture of CO2, H2O, and N2, not ~1/3 hydrochloric acid. $\endgroup$ Oct 21 at 10:59
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    $\begingroup$ You seem concerned about the boiling point of the fuel. Propane has a boiling point comparable to CH3Cl and is a much better fuel. Both propane and CH3Cl are more difficult to make on Mars than CH4 (which is formed by default when you react CO2 with an excess of H2, giving 3 substances that are easily separable due to very different boiling points: H2O, CH4 and unreacted H2.) But I don't understand the concern. 1 ton of CH4 needs 4 tons of O2 (of similar boiling point) for combustion so the problem of preventing O2 boiloff exists anyway. It's quite convenient that CH4 and O2 have similar b.p. $\endgroup$ Oct 21 at 22:24
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They chose $\rm CH_4 + O_2$ because that is a much, much better fuel.

$\rm CH_3Cl$ does burn with $\rm O_2$, producing: $$\rm 2CH_3Cl + 3O_2 \to 2CO_2 + 2H_2O + 2HCl + {\sim}1528\text{ kJ/mol energy}$$ This from a total atomic mass of $2(50.5) + 3(32) = 197\text{ g/mol}$. Your fuel, combusted with oxygen, thus delivers some $7.8\text{ kJ/g}$.

However, this combustion liberates hydrogen chloride gas. On reacting with water, say the moisture on your lungs or eyeballs, this promptly turns into hydrochloric acid. This tends to be bad for your health, and the EPA tends to frown on the release of multiple tons of hydrogen chloride gas in a sensitive environment (like anywhere on the surface of the Earth), so getting clearance on your Environmental Impact Assessment might be difficult.

Meanwhile, methane and oxygen burn with $$\rm CH_4 + 2O_2 \to CO_2 + 2H_2O + 890.8\text{ kJ/mol}.$$ This from a total atomic mass of $16 + 2(32) = 80\text{ g/mol}$. Methane, combusted with oxygen, thus delivers some $11.1\text{ kJ/g}$. This combustion generates $\rm CO_2$, water and heat. Much more benign to the environment.

Note: above combustion equations assume perfect combustion, not optimised for use in rockets, no consideration of incomplete combustion, etc. It's simply a mass vs energy checkup, with consideration of the waste products generated.

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    $\begingroup$ NASA did manage to get clearance for the shuttle SRBs, which each held 500 tons of APCP fuel containing (if I did my math right) about 20% chlorine by mass. So apparently spewing about 200 tons of hydrogen chloride into the atmosphere per launch is OK, at least as long as you're a government agency and don't do it too often. $\endgroup$ Oct 21 at 19:27
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    $\begingroup$ @IlmariKaronen Quite true. And the particulate Aluminium oxides from the SRBs caused problems of their own. NASA got a lot of flak for this, but at that time they were king, and mostly immune to critique. Still are, frankly. But a private company like SpaceX? Heh! Still, if the fuel was actually better, not 3 times worse than Methane, it might have been used. But it isn't, so it won't. $\endgroup$ Oct 21 at 19:35
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    $\begingroup$ Chloride is also pretty darn corrosive to most metals, seems like that might also play a role in something you want to keep around for a while $\endgroup$
    – Joel Keene
    Oct 21 at 20:56
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    $\begingroup$ @Ruslan: AIUI, from reading the same book I'm sure you're referring to, $\rm ClF_3$ was mainly considered for 1) military use, where strategic concerns tend to override environmental ones, and 2) as a deep-space propellant, where environmental concerns are negligible (assuming your rocket doesn't explode on its way to space). $\endgroup$ Oct 21 at 22:57
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    $\begingroup$ @Robyn Compared to making Methane; much more effort. The pathway you show requires CH4 be made as an intermediary step the make the CH3Cl. Yes it is easier to store, but you also need to make O2 and store that already, so you gain nothing. $\endgroup$ Oct 22 at 0:57
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"Why do we use ______ rocket fuel?" is complicated.

I recently read John Clark's Ignition!: An informal history of liquid rocket propellants, which I highly recommend. It's exactly what the title implies, and covers the history of fuel development, touching lightly on the pre-WW2 era, then hitting every major development through the sixties (i.e. it covers the time when most of the major development was done).

Before describing further, I'll mention that Elon Musk apparently puts a lot of stock in this book in particular. So, whatever Ignition! says is likely to have had a large influence on the fuels SpaceX uses. However, this influence is likely somewhat perverse: Clark spends about 180 pages gleefully recounting all the amusing ways people blew themselves up, or poisoned themselves, etc. with rocket fuels, while hunting for hypergolicity, temperature range, and an oxidizer that wouldn't spontaneously turn into green slime.

Instead, SpaceX seems to have read ignition as a cautionary tale, and went with liquid methane and LOX. This is most notable for being one of Tsiolkov's original suggested rocket fuels, way back at the dawn of rocketry. Liquid gas handling isn't exactly trivial, but it's way, way easier than most of the other options. In fact, Clark makes this exact point in his chapter on LOX and FLOX*, though he immediately traces the evolution of the most dangerous version of it (slushy liquid hydrogen and freon-ated liquid ozone!).

Overall, I suspect the folks at SpaceX wanted very much to avoid the excitement of modern rocket fuels, and methane was cheap, easy to work with, and had a minimum of exciting environmental impacts. Kerosene is the more usual default, but once you're handling cryonics anyway, you may as well go with a liquid-gas fuel.


* Clark says very little on Methane-LOX systems, but does mention someone's bright idea to make them into a monopropellant: "His idea was to set up a liquid oxygen plant alongside a natural gas well, tank up your ICBM on the spot, and push the button." Clark's horror at the idea of using a LOX-fuel mix as anything other than a bomb comes through clearly.

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Mere Speculation: The kinetics of that fuel's combustion may be too slow for a rocket motor. Halogens are known for their use as flame retardants.

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