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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.

SpaceInMyHead
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7 Answers7

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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.

cjm
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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.

Tristan
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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...

Jon Custer
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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)

uhoh
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    There are explosives made by mixing liquid oxygen with a fuel see. A mixture of lampblack with liquid oxygen was even stronger than dynamite. – Uwe Mar 21 '19 at 23:12
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    @Uwe write an answer!! Just block-quote that source and it will be perfect! – uhoh Mar 21 '19 at 23:16
  • @Uwe if you are not going to write that answer let me know, and I'll include a block quote here. Thanks! – uhoh Mar 21 '19 at 23:48
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    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. – Bob Jacobsen Mar 22 '19 at 04:52
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    @BobJacobsen well I like the music on this one ;-) – uhoh Mar 22 '19 at 04:53
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    @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. – Uwe Mar 22 '19 at 11:41
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    @uhoh: Great mad-scientist hair, too. :-P – Vikki Jan 28 '23 at 19:38
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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

Bob Jacobsen
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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.

Community
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Dohn Joe
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  • Good answer! This is the engineering reason. "Because it won't work!" – Organic Marble Mar 21 '19 at 18:24
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    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. – SpaceInMyHead Mar 21 '19 at 18:56
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    This document shows that the densities of liquid methane and liquid oxygen are greatly different, so yes, the same applies. https://www.engineeringtoolbox.com/cryogenic-fluids-d_492.html Oxygen is more than twice as dense as methane. – Organic Marble Mar 21 '19 at 19:23
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    Furthermore at liquid oxygen temperatures, methane is solid. It might be possible to put them both in the liquid phase by using higher pressures, but still sounds like a very bad situation. – Blake Walsh Mar 21 '19 at 20:17
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    But liquid oxygen and liquid hydrogen could not exist at the same temperature any way. Either oxygen is solid and hydrogen is liquid or oxygen is liquid and hydrogen is gaseous. Hint read the links to wikipedia carefully before you use them in an answer. – Uwe Mar 21 '19 at 20:53
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    -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. – uhoh Mar 21 '19 at 22:22
  • @OrganicMarble If you are convinced, maybe you can help with these glaring problems? – uhoh Mar 21 '19 at 22:23
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    @uhoh I fear my ignorance of chemistry is on public display! – Organic Marble Mar 22 '19 at 00:01
  • @OrganicMarble I used to think that "thrust" was the stuff shooting out the back of a rocket – uhoh Mar 22 '19 at 00:09
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    It would stratify into CO2, H2O and debris – Ingolifs Mar 22 '19 at 02:02
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    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. – Bob Jacobsen Mar 22 '19 at 04:47
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    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? – David Richerby Mar 22 '19 at 12:09
  • The question is about methalox, not hydralox, and oxygen dissolves just fine in liquid methane. – Mark Mar 22 '19 at 21:01
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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.

PearsonArtPhoto
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