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A methalox engine is fed from two cryogenic fuel tanks. Why can't the methane and oxygen be mixed as gases, in the desired proportions, and then chilled to a temp that liquefies both? From a single tank a single turbo pump could then feed this into the combustion chamber (part of flow going thru the nozzle cooling channels). Yes, a bit of same mix would be tapped off to power the turbo pump.

Saves the weight of tank bulkheads, separate plumbing, reduces turbo pump complexities. Must be a reason or the rocket scientists would already be doing this, but would like to know what it is.

Edit: Thank you to all who answered. Even the imperfect answers helped, as the comments helped me work through the whys and wherefores. I did know a methalox mixture, if it could exist, would be highly dangerous, but unsure how dangerous compared to a failure/fire of one tank causing the other tank to rupture, mixing the two. Even I can now see why the rocket scientists got it right.

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    $\begingroup$ But what if there is no temperature were both oxygen and methane are liquid? You may look for melting and boiling points of both methane and oxygen in wikipedia. $\endgroup$
    – Uwe
    Commented Mar 21, 2019 at 20:48
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    $\begingroup$ I've just asked Can a stoichiometric mixture of oxygen and methane exist as a liquid at standard pressure and some (low) temperature?; you may want to keep an eye on it. $\endgroup$
    – uhoh
    Commented Mar 22, 2019 at 1:51
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    $\begingroup$ If you use a cryogenic monopropellant mixture, how do you avoid that the ignition front in the combustion chamber is creeping back to turbo pump, pipes and finally the propellant tank? Ignition front may be much faster than creeping, it may be as fast as an explosion. $\endgroup$
    – Uwe
    Commented Mar 22, 2019 at 11:58
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    $\begingroup$ Why not premix them? One word: kaboom. A stray spark and you could have all the monoprpellant ignite at once. $\endgroup$
    – gwally
    Commented Mar 22, 2019 at 15:01
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    $\begingroup$ @BobJarvis explosive expansion is actually a well defined behaviour, just like isothermic expansion and isobaric expansion are. So explosions are indeed something "special" in the thermodynamic side of things. On the T-S curve an exposive expansion is a linear $\endgroup$
    – paul23
    Commented Mar 23, 2019 at 16:30

7 Answers 7

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Urged on at a similar question on Chemistry SE, it seems that the idea of mixing liquid oxygen and liquid methane is an old one. And one that, using some of the answers above, seems to be cloaked in at least some hyperbole.

Of relevance is R.L. Every and J.O. Thieme, Journal of Spacecraft and Rockets 2(5) 787-789 (1965) titled "Liquid oxygen and liquid methane mixtures as rocket monopropellants". In the introduction the authors note that previous work shows that the liquids are miscible in all proportions above 90K. Their tests show a specific impulse of almost 300 sec, and an exhaust velocity near 6000 fps.

In addition, since "shock sensitivities were reported" in the earlier work, they did some, well, vaguely disturbing experiments (hey, it was the 60's). "Tests were conducted to determine whether violent stirring or agitation, as found in an impeller-type pump, would detonate the mixture", although they didn't have any explosions there. They then went on to quantify the impact sensitivity, dropping a weight onto a stainless steel beaker of different liquid mixtures from increasing heights until an explosion occurred. Impact sensitivities of 20-60 ft-lb were discovered (these experiments were performed with open beakers of the cryogenic liquids, with light from the room shining on them - hence the hyperbole bit). They also looked at adiabatic compression as a cause of explosion and found the liquid oxygen/methane mixtures were safer than some other things like nitromethane (which really isn't that comforting).

So, it would appear that the idea of using the liquid mixture as a monopropellant isn't totally off the wall, but please do it somewhere far from me...

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    $\begingroup$ The Every & Thieme (1965) paper seems likely to be the "article seriously proposing an oxygen-methane monopropellant" mentioned by John D. Clark in the excerpt quoted by cjm. Nice find! $\endgroup$
    – Ilmari Karonen
    Commented Mar 22, 2019 at 15:52
  • $\begingroup$ This is the most useful answer yet. And - safer than nitromethane, which is used at many drag race tracks. Especial thanks for winnowing out some incorrect info in some replies. $\endgroup$
    – SpaceInMyHead
    Commented Mar 23, 2019 at 1:40
  • $\begingroup$ "vaguely disturbing" - almost poetic! $\endgroup$
    – Volker Siegel
    Commented Mar 23, 2019 at 23:43
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To quote John D. Clark's great book Ignition! (Chapter 11: The Hopeful Monoprops):

If Tannenbaum's mixtures were bad, that proposed at a monopropellant conference in October 1957 by an optimist from Air Products, Inc., was enough to raise the hair on the head of anybody in the propellant business. He suggested that a mixture of liquid oxygen and liquid methane would be an extra high-energy monopropellant, and had even worked out the phase diagrams of the system.* How he avoided suicide (the first rule in handling liquid oxygen is that you never, never let it come in contact with a potential fuel) is an interesting question, particularly as JPL later demonstrated that you could make the mixture detonate merely by shining a bright light on it. Nevertheless, ten years later I read an article seriously proposing an oxygen-methane monopropellant! Apparently junior engineers are allergic to the history of their own business.

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    $\begingroup$ If you want to obey that first rule, an engine using liquid oxygen may never be started. The combustion chamber is the place were oxygen must be mixed with fuel. $\endgroup$
    – Uwe
    Commented Mar 21, 2019 at 20:41
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    $\begingroup$ @Uwe, obviously he meant "until you want a raging fire" but thought it unnecessary to say. $\endgroup$
    – cjm
    Commented Mar 21, 2019 at 20:42
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    $\begingroup$ That's a great quote! "...by shining a light on it..." Yowza. $\endgroup$
    – Organic Marble
    Commented Mar 21, 2019 at 20:53
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    $\begingroup$ @OrganicMarble, that's not even close to the best quote from that book. If you haven't read it, I highly recommend it. $\endgroup$
    – cjm
    Commented Mar 21, 2019 at 20:55
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    $\begingroup$ Thanks! I was also going to post an answer quoting that exact passage (again), but you saved me the effort. Have a +1. $\endgroup$
    – Ilmari Karonen
    Commented Mar 21, 2019 at 21:16
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In addition to what the other answer said, it would take very little provocation for such a situation to turn into a good way to test the blast resistance of nearby facilities.

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    $\begingroup$ Put another way: If there exists a tank of premixed liquid CH4 and LOX, I desire to be as far away as physically possible. $\endgroup$
    – Tristan
    Commented Mar 21, 2019 at 18:39
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    $\begingroup$ Good news! It's on its way to the moooon! $\endgroup$
    – David Richerby
    Commented Mar 22, 2019 at 12:06
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At STP:

This does not a priori prove that a solution of the two can not exist. However it does mean that they can not be handled as liquids at the same temperature, making mixing the two more difficult.

And so I've just asked Can a stoichiometric mixture of oxygen and methane exist as a liquid at standard pressure and some (low) temperature?

We know that liquid air exists which shows that LOX and LN2 can mix together. But methane is an organic molecules and we know that heavier $\text{C}_n \text{H}_{2n+2}$ hydrocarbons include oils and waxes don't like to dissolve in non-organic solvents.

The argument against premixing is the danger of ignition due to a spark or tiny localized generation of heat. As @Tristan and @PearsonArtPhoto both mention 1, 2 the SpaceX explosion "fast fire" occurred because of the presence of a combustible material in direct contact with LOX and a localized source of mechanically produced heat. See this answer and note that the situation is discussed at length in Scott Manley's video The Dumbest Mistakes In Space Exploration

Also watch the video below, discussed in more detail in Why doesn't carbon fiber overwrapping in LOX catch fire? (watch this video first)

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    $\begingroup$ There are explosives made by mixing liquid oxygen with a fuel see. A mixture of lampblack with liquid oxygen was even stronger than dynamite. $\endgroup$
    – Uwe
    Commented Mar 21, 2019 at 23:12
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    $\begingroup$ @Uwe write an answer!! Just block-quote that source and it will be perfect! $\endgroup$
    – uhoh
    Commented Mar 21, 2019 at 23:16
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    $\begingroup$ Liquid oxygen and charcoal briquettes was a really popular stupid YouTube trick for a while. So Mythbusters wanted to do a "LOX tanker spills load on asphalt road, boom" item but the small tests where so violent that they couldn't get insurance coverage for the full-scale. $\endgroup$
    – Bob Jacobsen
    Commented Mar 22, 2019 at 4:52
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    $\begingroup$ @BobJacobsen well I like the music on this one ;-) $\endgroup$
    – uhoh
    Commented Mar 22, 2019 at 4:53
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    $\begingroup$ @uhoh Just include the links from me into your excellent answer. Here is another one: NASA oxygen safety manual. See page 9-1 or 143: "liquid oxygen spills on pavements such as asphalt have resulted in impact-sensitive conditions that caused explosions from traffic or dropped items." The string explos is found 161 times in this document of 288 pages. $\endgroup$
    – Uwe
    Commented Mar 22, 2019 at 11:41
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On the chemical/physical question of whether such a mixture can exist: Yes it can.

There's a NASA report that looks into this: "ON THE SOLUBILITIES AND RATES OF SOLUTION OF GASES IN LIQUID METHANE", Hibbard and Evans, 1968 and concludes that such mixtures are possible.

Starting on page 8:

Figure 5(a) presents the curves for oxygen, argon, carbon monoxide, and nitrogen. Also shown are the two experimental values for nitrogen. Agreement is excellent at 99.83K and good at 110.9K. The curves for these gases show that solubility should decrease with increasing temperature and the nitrogen data confirm this. This figure shows the mole fraction solubility of oxygen to be 1.0 at 90K. This means that oxygen, which has a normal boiling temperature of 90.1K would continuously condense in, and be miscible in all proportions, with liquid methane at 90K. This is confirmed by reference 11 where, in a study of the solubility of methane in liquid oxygen, it was concluded that these formed a near-ideal solution at -297 F (90K)

(emphasis added)

Figure 5 is reproduced below. Note how the solubility of oxygen rises rapidly as temperature drops.

Reference 11 mentioned in there is "Hydrocarbon-Oxygen Systems Solubility", McKinley and Wang, 1960 (unfortunately paywalled) which also has interesting discussion of the stability (i.e. presence or absence of a tendency to explode) of various mixtures. That covers, in delightfully calm terms, why such mixtures are not commonly used: "A composition as pictured at point A (n.b. mostly one component) is safe whereas the composition at point B (i.e. rocket fuel) can be exploded".

enter image description here

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They would stratify.

Liquid oxygen is much denser than liquid hydrogen, with 1.141 g/cm3 for LOX vs. 0.07099 g/cm3 for LH.

Thus, you need to install equipment to ensure proper mixing of the two liquids. This adds not only complexity to an already complex machinery, it also adds weight.

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    $\begingroup$ The question is about methalox. Would the same apply? Also, the mass cost/complexity of mixing needs to be balanced against the mass cost/complexity to two tanks and plumbing. $\endgroup$
    – SpaceInMyHead
    Commented Mar 21, 2019 at 18:56
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    $\begingroup$ -1 for several problems. 1) answer is based on the wrong gas to try to make a false point about different densities. 2) answer uses false analogy because oil and water do not mix due to molecular incompatibility (oil is hydrophobic), it's got nothing to do with density, 3) answer forwards false science that density difference is the reason liquids could not mix. $\endgroup$
    – uhoh
    Commented Mar 21, 2019 at 22:22
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    $\begingroup$ It would stratify into CO2, H2O and debris $\endgroup$
    – Ingolifs
    Commented Mar 22, 2019 at 2:02
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    $\begingroup$ Liquids only stratify if they're immiscible. LOX and methane dissolve in each other; at cold enough temperatures they've got a unity molar fraction so you can dissolve as much of one in the other as you'd like. $\endgroup$
    – Bob Jacobsen
    Commented Mar 22, 2019 at 4:47
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    $\begingroup$ Oh, right. So the significantly different densities of ethanol and water are why vodka stratifies into two layers. Now I understa... What was that? Vodka doesn't stratify, you say? $\endgroup$
    – David Richerby
    Commented Mar 22, 2019 at 12:09
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For the static fire test of AMOS-6 it is believed that the oxygen and RP1 fuel combined together due to a failed bulkhead. Even a smaller bit of fuel in oxidizer can cause enough of an explosion to start things moving, part of the reaction was with the carbon overwrap and oxygen, which had a small spark and started the larger explosion seen. In your proposed condition, only a small spark is required to cause a large explosion, as seen in the below video.

Mixing fuel and oxidizer is bad, even the slightest spark will cause it all to go up in flames. Plus there is a lot of things that can be done to optimize the flow for different conditions, flowing more oxidizer or fuel for certain conditions.

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    $\begingroup$ Yes, but it was only an anomaly. :-) $\endgroup$
    – SpaceInMyHead
    Commented Mar 21, 2019 at 18:52
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    $\begingroup$ Now imagine that happening orders of magnitude faster due to premixing. It’s essentially a fuel-air explosive enhanced by a factor of several thousand. $\endgroup$
    – Bob Jacobsen
    Commented Mar 21, 2019 at 19:31
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    $\begingroup$ Ignition happened between the carbon overwrap of the helium tanks inside the LOX tank, not between RP-1 and LOX. $\endgroup$
    – uhoh
    Commented Mar 21, 2019 at 22:25
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    $\begingroup$ Ignition happened between the carbon overwrap of the helium tanks inside the LOX tank, not between RP-1 and LOX. The bulkhead failed later, due to the violent reaction between the carbon overwrap + LOX. $\endgroup$
    – uhoh
    Commented Mar 21, 2019 at 22:52
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    $\begingroup$ Bob Jacobsen, this may be the key! Propose to the Pentagon it can be weaponized, that finding a way to mix and store methalox would produce "a fuel-air explosive enhanced by a factor several thousand." Mega-funding will ensue. :-) :-) $\endgroup$
    – SpaceInMyHead
    Commented Mar 21, 2019 at 23:08

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