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.
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21$\begingroup$ 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". $\endgroup$– Organic MarbleCommented Apr 21, 2020 at 21:00
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25$\begingroup$ 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. $\endgroup$– gwallyCommented Apr 21, 2020 at 21:26
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7$\begingroup$ 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. $\endgroup$– UweCommented Apr 22, 2020 at 19:40
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7$\begingroup$ Putting some TNT under the rocket would further increase the possible lift off weight, at least theoretically. $\endgroup$– Dmitry GrigoryevCommented Apr 23, 2020 at 6:54
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2$\begingroup$ 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. $\endgroup$– HeoppsCommented Apr 23, 2020 at 7:08
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4 Answers
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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.
answered Apr 21, 2020 at 21:06
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3$\begingroup$ 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 ? $\endgroup$ Commented Apr 21, 2020 at 22:04
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7$\begingroup$ @aranedain solid rocket fuel is composed not to be explosive and not to be ignitable too easy just by accident. $\endgroup$– UweCommented Apr 21, 2020 at 22:18
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8$\begingroup$ 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. $\endgroup$– MikeBCommented Apr 22, 2020 at 10:40
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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. $\endgroup$– J...Commented Apr 22, 2020 at 15:22 -
13$\begingroup$ @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. $\endgroup$– PerkinsCommented Apr 22, 2020 at 19:38
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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.
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2$\begingroup$ +2 internets for the Earth-shaking Kaboom $\endgroup$ Commented Apr 22, 2020 at 12:32
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3$\begingroup$ Boeing tried to do this under a DARPA development contract with notorious oxide mixed with acetylene. The result was KABOOM, to my lack of surprise. spacenews.com/darpa-airborne-launcher-effort-falters $\endgroup$– DMPalmerCommented Apr 22, 2020 at 17:46
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2$\begingroup$ @DMPalmer: Did you mean nitrous oxide? $\endgroup$– VikkiCommented Apr 22, 2020 at 19:21
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10$\begingroup$ @Sean See? Its reputation precedes it. $\endgroup$– TashusCommented Apr 22, 2020 at 20:13
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1$\begingroup$ Brings to mind Dave Barry's immortal column about an engineers' quest to light a charcoal grill the fastest. Pouring three gallons of LOX in the grill proved unbeatable. "Basically, the grill vaporized, " said Goble. "We were thinking of returning it to the store for a refund." $\endgroup$ Commented Apr 23, 2020 at 15:47
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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. ):
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3$\begingroup$ @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? $\endgroup$– TopcodeCommented Apr 22, 2020 at 13:14
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4$\begingroup$ @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. $\endgroup$– J...Commented Apr 22, 2020 at 15:17
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3$\begingroup$ Ah yes, the good old "It will explode violently"-Ignition, John D. Clark $\endgroup$ Commented Apr 22, 2020 at 17:41
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5$\begingroup$ @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. $\endgroup$– VikkiCommented Apr 22, 2020 at 19:24
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3$\begingroup$ @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. $\endgroup$– J...Commented Apr 23, 2020 at 11:58
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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.
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+1There's more about cryogenic RP-1 and LOX in Does the NK-33 engine require subcooled kerosene so cold that it turns to wax? $\endgroup$– uhohCommented Apr 23, 2020 at 11:22