Liquid oxygen is cheap to obtain and available, but are there better oxidizers to achieve bigger thrust with JP1?
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2$\begingroup$ I think fluorine is the most energetic oxidizer, but it is rarely used because it is more expensive to produce and both the fluorine itself and the reaction products are toxic and corrosive, making it much more difficult and hazardous to handle. The benefits typically don't outweigh all the downsides. $\endgroup$– Anthony XMar 5, 2017 at 21:36
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1$\begingroup$ Define "better". $\endgroup$– Russell BorogoveMar 5, 2017 at 21:47
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2$\begingroup$ Related: Why are hydrogen-fluorine fuels not used for rockets more frequently? $\endgroup$– Russell BorogoveMar 5, 2017 at 21:47
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3$\begingroup$ There are better oxidizers. They're just a bit too good at oxidising things like fuel tanks, pipes and pumps. $\endgroup$– E.P.Mar 6, 2017 at 8:05
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3$\begingroup$ Well, there are better oxidizers that aren't excessively corrosive. They are incredibly explosive instead. Say, ozone. $\endgroup$– SF.Mar 6, 2017 at 8:20
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2 Answers
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This depends on what "better" is.
But let us talk about fluorine. The main point of using it is that turning your hydrogen into $HF$ gives more energy than getting $H_2O$ for pure hydrogen, that gives a small improvement of $I_{sp}$. However, for fuels containing carbon, like JP1, (or perhaps RP-1 or JP4. JP1 has some unfortunate properties for rocket engine use) the situation is made more complicated by the fact that too much fluorine causes a yield of $CF_4$. That will of course make the performance go down the drain. Also, fluorine is difficult to handle. From Ignition, an informal history of liquid rocket propellants, John D. Clark:
Not only did they get a good performance, but there were no ignition problems, liquid fluorine being hypergolic with almost anything that they tried as a fuel. Unfortunately, it was also hypergolic with just about everything else. Fluorine is not only extremely toxic; it is a super-oxidizer, and reacts, under the proper conditions with almost everything but nitrogen, the lighter of the noble gases, and things that have already been fluorinated to the limit. And the reaction is usually violent.
(chapter 6, 7, 8, 10 and 12 are highly relevant to this topic)
That oxygen has to be burned into carbon monoxide, $CO$. So a compound like oxygen difluoride is a good option. It even has an excellent density. Another approach would be to mix a little liquid fluorine in the LOX. The two compounds can perfectly mixed, in contrast to many other cryogenics.
Pro: Higher $Isp$ and better density. Con: More expensive, corrosive and toxic.
(Other compound to check out are $ClF3$, $ClF5$ and $O_2F_2$.)
Another option would be mixing some $O_3$ into the LOX, again giving slightly more energy and better density. It has the nasty property of sometimes violently decomposing for no apparent reasons.
For the other possible definition of "better", thrust, the solution is generally to use a denser oxidizer and not necessarily one with a good performance. For thrust alone, nitrogen tetroxide is an excellent choice.
answered Mar 6, 2017 at 6:33
SE - stop firing the good guysSE - stop firing the good guys
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1$\begingroup$ A working chemist's perspective on some of these reactants: blogs.sciencemag.org/pipeline/archives/2010/02/23/…, blogs.sciencemag.org/pipeline/archives/2008/02/26/…, blogs.sciencemag.org/pipeline/archives/2013/04/05/…, $\endgroup$– E.P.Mar 6, 2017 at 8:02
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7$\begingroup$ $ClF_3$ ...is also hypergolic with such things as cloth, wood, and test engineers, not to mention asbestos, sand, and water —with which it reacts explosively. -- Ignition. I believe there was one that reacted violently with fire extinguisher powder too. $\endgroup$– SF.Mar 6, 2017 at 8:34
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$\begingroup$ You touched on this briefly, but the note about NTO merits looking further into hypergolic propellants generally $\endgroup$ Mar 7, 2017 at 22:51
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Besides pure oxygen, several other oxidizers in theory have better performance with hydrocarbon fuels, but none of them are used because of stability, toxicity and price issues. But let mention some of them:
Ozone O3. It's allotrope of oxygen with enthalpy of formation 142,67 KJ/mol (2.97MJ/kg) which can bust combustion energy up to 30% depending on fuel used. But it's prone to detonation in concentrations greater than 10% in liquid oxygen. It's also very toxic. Risks involved prohibit it's use in rocketry,
Red oxygen, Tetraoxygen, oxozone (O4), meta-stable form of oxygen produced when solid oxygen is pressurized more than 10 GPa. It's stable under wide range of pressures therefore in theory can be used in rocketry. Being to expensive to fuel any rocket it's just a lab curiosity,
Oxygen difluoride OF2, gives superior specific ISP and density ISP comared to oxygen. It's capable of oxidizing carbon backbone to CO and CO2, instead to CF4 like fluorine would. In liquid form it has density of 1.9kg/l (at melting point - 224°C), one of the highest among all oxidizers. It's very reactive, reacts even with water. Combustion temperature is very high. Also it's very toxic, expensive and generally considered as unsafe gas. But it was seriously considered in past as rocket propellant especially in combination with boranes were it gives best performance.
Trioxygen difluoride (O3F2) - decomposes at temperatures higher than 115K but can be safely used in small quantities. It's pyroforic in contact to organic matter. It's consider as additive to liquid oxygen because even in low concentrations makes it hypergolic with most hydrocarbons.
Flurine nitrate FNO3 - Unstable, shock sensitive gas. It has similar specific ISP but superior density impulse compared to oxygen. It's liquid at wide range of temperatures. It's not used in rocketry. Even in chemistry it's avoided and chlorine nitrate is used instead.
