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This question, to my knowledge, is only applicable to Rocket Lab's Electron, pressure-fed engines that use non-hypergolic fuels, and other engine cycles that don't use a fuel/oxygen-rich preburner. My question is why can't someone like Rocket Lab store both LOX and RP-1 together in one common tank in a premixed ratio? I understand in something like a Falcon 9 it would at least be necessary to supply more fuel to the preburner but in something like the Electron that uses an electric turbopump that requires no preburner why couldn't both the oxidizer and fuel not be stored together? Intuitively, this would remove the hefty weight of tank bulkhead(s) that divide the oxidizer and fuel tanks and make your injector(s) much more simple.

YuccaWorks
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    Take a minute to look this up - Q: What is the freezing point of kerosene? And the boiling point of LOX? Then think about "cryogenic kerosene". – Organic Marble Apr 21 '20 at 21:00
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    It seems like a good idea to me unless you want your rocket to launch in a controlled fashion. Mixing an oxidizer and a fuel generally has spectacular results and not all of them are beneficial to a successful launch. – gwally Apr 21 '20 at 21:26
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    Oxygen should be stored without hydrocarbons. There have been explosions from fat or oil contaminations in oxygen tanks. Every tank, tube, valve, hose used for oxygen is cleaned from hydrocarbons very carefully before the first use. – Uwe Apr 22 '20 at 19:40
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    Putting some TNT under the rocket would further increase the possible lift off weight, at least theoretically. – Dmitry Grigoryev Apr 23 '20 at 06:54
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    Should be said that cryogenic O2 temperature is much lower then ignition temperature of kerosene - 220 degrees centigrade. So theoretically if the mix is in stable conditions - it will not autoignite/explode. But because kerosene freeses - it will be floating atop of liquid oxygen, because it is less dense. – Heopps Apr 23 '20 at 07:08
  • While I don't think the difference matters for this question, it's important to point out that the Rutherford engines in Rocket Lab's Electron are not pressure-fed; they are fed by electric pumps powered from batteries – Roel Schroeven Apr 23 '20 at 08:29
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    @Heopps Ignition temperature of kerosene is valid for air, it will be different for 100 % oxygen gas and invalid for a mix of LOX with kerosene. When an engine is ignited, temperature in the chamber is much lower than 220 °C. There is no chamber preheating before ignition. – Uwe Apr 24 '20 at 12:03
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    Up next on StackExchange Hot questions: Why don't we sell matches already lit, to save people the work of lighting them. – Tomáš Zato Apr 24 '20 at 12:29
  • @Uwe - I guess so. But I could not find any info about ignition temperature of frozen kerosene in liquid oxygen. My gut feeling is - the ignition temperature is still higher than LOX boiling point. But if somebody can prove the opposite, I'll delete my comment – Heopps Apr 24 '20 at 14:25

4 Answers4

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As Organic Marble hints, there is about 140 degrees Celsius between kerosene's freezing point and oxygen's boiling point; there's no temperature at which both are liquid.

Even if the propellants were more thermally compatible, putting your fuel and oxidizer in the same tank is a really dangerous idea. Typically, propellant tanks are pressurized with helium or nitrogen -- nonreactive gasses -- so that any spark in the tank won't start a fire. With both oxygen and kerosene readily available in the same space, the tank is a bomb waiting to go off.

While mass ratios are very important to overall rocket performance, safety is more important.

Russell Borogove
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    Since solid rocket fuel has both the oxidant and the fuel together, what makes a liquid combination more dangerous? Is it that the amount of energy needed to start the combustion reaction is much less for a liquid mixture than a solid mixture? Or is it specific to the exact type of fuel and oxidants ? – aranedain Apr 21 '20 at 22:04
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    @aranedain solid rocket fuel is composed not to be explosive and not to be ignitable too easy just by accident. – Uwe Apr 21 '20 at 22:18
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    Actually, even if there was an overlap in temperature, you might not be able to keep the 'mixture' correct/even - think about oil floating on water. – MikeB Apr 22 '20 at 10:40
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    @MikeBrockington yeah, but both oxygen and hydrocarbons are unpolar, so that should be ok. Liquid oxygen and liquid methane would indeed be mixable at any ratio. https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19680020605.pdf https://doi.org/10.1007/978-1-4757-0540-9_2 – leftaroundabout Apr 22 '20 at 10:55
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    With both oxygen and kerosene readily available in the same space, the tank is a bomb waiting to go off - and connecting a fuel line from the tank to an engine makes a fuse, so starting the engine will result in the tank instantly exploding. It's not even a risk - it's a certainty. – J... Apr 22 '20 at 15:22
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    @J... Well... No... As long as you pump the fuel through the line faster than the heat propagates back up it it should be ok. Or have a flash suppressor of some kind (That's what those metal meshes in gasoline can nozzles are for. They prevent sufficient heat from traveling up the spout to explode the tank in the case of accidental ignition.) Still, mixing fuel and oxidizer is unnecessarily dangerous considering how little weight it takes to make it two separate tanks. – Perkins Apr 22 '20 at 19:38
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    @Perkins Gasoline in a can isn't stoichiometrically blended with oxidizer. Agreed, if you could pump it fast enough, you might be able to have the fuel outrun the reaction front, but it would be so unbelievably risky... tickling the dragon's tail would be an understatement. – J... Apr 22 '20 at 19:43
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    @J... once you pour half of it out you have air in there, and while it might not be a stoichiometric mix it'll still make enough of a bang to cause serious injury and/or start a major fire depending on the size of the can. Regardless, if you can keep the heat from building up to ignition temperature you could do something like this. It's just not worth the extra hazard in the case of a malfunction. – Perkins Apr 22 '20 at 19:47
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    @aranedain: Quite a bit of work goes into testing and formulating solid propellant compositions to ensure they only ignite when they're supposed to, and don't go into a an explosive runaway where increasing pressure increases burn rate. Further restricting yourself to miscible, stable liquids that don't react with each other makes this far more difficult, and we haven't been totally successful even with solids...they are treated as explosive devices, and handled with a great deal of caution. – Christopher James Huff Apr 22 '20 at 19:52
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    @Perkins: That's mostly nitrogen gas with a fraction of oxygen and some hydrocarbon vapor in something far from an ideal F/O ratio and an overall density in the area of 1 kg/m^3, not a mix of liquid fuel and undiluted oxidizer. A flame-stopper mechanism might still be possible, but will be far more difficult due to the higher energy density and reaction rates. – Christopher James Huff Apr 22 '20 at 19:56
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    @Perkins Flame arrestors for gasoline cans also only have to stop a weak deflagration of a poor mixture of fuel/air vapour at atmospheric pressure. I think with liquid fuel/oxidizer there would be no way to design a similar arrestor, especially when you can't afford to obstruct the fuel flow (whose delivery rate must remain extremely high). The only hope would be to flow the fuel fast enough to outpace the reaction front, which I'd expect to be a detonation, not a deflagration. We're talking fuel flows at thousands of m/s. – J... Apr 22 '20 at 19:56
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    @aranedain among other things, the fact that they're solid! It's only a slight oversimplification to say that solid things don't burn — the gases given off by solids when they're heated do. This means there's only a limited surface available for combustion to happen, and a fairly big energy barrier to overcome before it can start. Gases can go boom anywhere and with far less provocation. And liquid rocket propellants are usually (always?) in pressure equilibrium with their gaseous phase, so if you store them together, a gas mix will exist. – hobbs Apr 22 '20 at 23:05
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    I'd like to point out that when Russell writes "the tank is a bomb waiting to go off", that's not a figure of speech or a metaphor. The tank would literally be an actual bomb. – Tanner Swett Apr 23 '20 at 21:01
  • @TannerSwett and it's just waiting until it feels like it, not a signal from yourself :) I'm suddenly reminded of Dark Star for some reason. – Will Crawford Apr 24 '20 at 01:43
  • A great resource on rocket propellants, and notable explosions: Ignition, by John D Clark. http://www.sciencemadness.org/library/books/ignition.pdf – Thomas W Apr 24 '20 at 09:30
  • @J... now that I think about it, since this mix would be at least as volatile as ANFO you're probably correct about it detonating. So sinking sufficient heat to prevent that would be a challenge to say the least. It's probably technically possible, but no way would it be simpler or lighter than two separate tanks. Best strategy at that point would likely be some sort of pulse detonation engine. Those have theoretical advantages, but tend to be more prone to spectacular failures as well. – Perkins Apr 27 '20 at 22:06
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Because it will almost certainly go KABOOM.

Intimately mixed fuels and oxidizers are pretty much indistinguishable from explosives, and in particular, LOX intimately mixed with flammable hydrocarbons is wildly dangerous -- rather than being something you can handle, it tends to be set off by shock, vibration, or adiabatic compression that can be caused by closing or opening a valve.

Perhaps the most infamous LOX-hydrocarbon fuel mixture was LOX and liquid methane (which are perfectly miscible), discussed in Clarke's Ignition -- it was supposed to act as a monopropellant as you are describing, but was allegedly (though disputedly) so touchy it could be detonated with a bright light.

At least one of the Bell X-1 rocket planes was destroyed when LOX in contact with a oil-impregnated leather gasket detonated due to vibration. A bulk mixture of LOX and kerosene fuel is far worse -- it surely could not survive passing through a turbopump, and there would be little to stop the detonation in the combustion chamber racing up through the fuel lines and setting off the whole tank. (Detonation arrestors exist, but they don't save your engine and fuel lines from being rapidly disassembled, nor do they protect your tanks from shrapnel, as Clarke learned).

Additionally, RP-1 is going to freeze at LOX temperature. The interaction of small particles of hydrocarbon probably won't help matters.

ikrase
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to put it in 3 words "it will explode." you see fuel burns with oxidiser and mixing the two in the same storage will well let me make it in steps:

you ignite the engine... the flame flows into the tank... that burns too... in a closed space... rapidly... which causes the tank to burst... and then you have no more rocket. ):

Topcode
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    @mike-brockington how? last i checked mixing hypergolic fuels in a tank will explode as you are filling it because thats how hypergolic fuels work, please explain how that makes them impossible. its not even that complex, fuel burns with oxygen, can you not understand that? – Topcode Apr 22 '20 at 13:14
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    @MikeBrockington "Hypergolic fuels are fuels that react spontaneously upon contact with an oxidizer and do not require an outside ignition source such as spark plugs." – DKNguyen Apr 22 '20 at 13:49
  • @Topcode I commented the wrong person – DKNguyen Apr 22 '20 at 13:51
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    Sorry, total brain-fade there - I meant monopropellant fuels. – MikeB Apr 22 '20 at 14:02
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    @mike-brockington still, how does it invalidate my answer? – Topcode Apr 22 '20 at 14:11
  • A mono-propellant is essentially exactly what the OP described: fuel and oxidiser in the same mixture, in the correct ratio. You are also not mentioning the third part of the 'fire triangle' - not all fuel mixtures combust spontaneously. – MikeB Apr 22 '20 at 14:21
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    @MikeBrockington Monoprop needs a catalyst to initiate the decomposition. In storage the propellant is stable and the decomposition can't back propagate into the tank (where there is no catalyst). That's very different from intimately mixed fuel/oxidizer where it would be almost impossible to isolate the fuel line from the active combustion in the chamber. – J... Apr 22 '20 at 15:17
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    @MikeBrockington examples of monopropellants containing fuel and oxidizer mixed together please? – Organic Marble Apr 22 '20 at 15:58
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    Ah yes, the good old "It will explode violently"-Ignition, John D. Clark – YuccaWorks Apr 22 '20 at 17:41
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    @MikeBrockington: Yes, and the monopropellants that are safe enough to actually use provide absolutely crap performance compared to typical bipropellants - as that very (relative) stability that allows them to be handled (relatively) safely also means that they don't release enough energy upon decomposition to provide good performance. – Vikki Apr 22 '20 at 19:24
  • @OrganicMarble Fuel and oxidiser pre-mixed is pretty much the definition of a mono-propellant. Not intuitively a good idea, and as Sean notes, not particularly good in practice either, but they ARE possible, and do exist. – MikeB Apr 23 '20 at 08:44
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    @MikeBrockington That's not the definition of a monopropellant. Probably the most common monoprop is hydrazine, which isn't a mixture of anything - it's just hydrazine (N2H4). Exothermic decomposition is initiatied in the presence of an iridium catalyst which decomposes the hydrazine into nitrogen, ammonia, and hydrogen gas, releasing energy. There is no oxidizer. – J... Apr 23 '20 at 11:58
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    @MikeBrockington An example of a monopropellant containing fuel and oxidizer mixed together please? you might want to ponder the meaning of "mono" – Organic Marble Apr 23 '20 at 12:11
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The history of rocket fuel development has been dominated by getting them to work at the range of temperatures we want. For instance finding a fuel that is liquid enough to work in the Arctic, and not so volatile that it can't be stored at reasonable pressure in the desert.

Unfortunately, kerosene freezes well above the boiling point of oxygen. It might be possible to turn it into a slush in oxygen, with development, and admixtures to stabilise it. After all, various powdered metals, boron, aluminium, beryllium (nasty exhaust!), have been added to fuels to increase the heat of burning, with various gels and black-magic to keep the mixture liquid. However, the history of monopropellants has been very much what you'd expect from trying to store what is basically a liquid bomb.

Read Ignition for one man's on-the-spot story of the development of rocket propellants.

Neil_UK
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