I wonder whether there are any small amounts of additives in rocket fuels such as hydrogen and kerosene, or to the liquid oxygen, or if they are absolutely pure (barring any unintentional contamination). If not to improve combustion, then to facilitate cooling or handling of the liquids.

Are there any ideas about how additives might improve thermal conductivity to facilitate a more homogeneous cooling process or anti-catalysts to improve the resistance of carbon fiber tanks (which as I understand it get brittle at those very low temperatures in a pure oxygen environment) or something like that? Are there examples of additives in the use of solid fuel?

  • $\begingroup$ I don't have a definitive answer but I do know that great efforts are done to reduce the particle count in the fuel system. Especially the requirement for LOX is high as Oxygen likes to go boom. $\endgroup$
    – GittingGud
    Jun 25 '19 at 7:56
  • $\begingroup$ Answers to Does the Soyuz spacecraft chemically stabilize the hydrogen peroxide (H2O2) to remain ISS-dockable for 200+ days? suggest it will not have additives, and answers to Can LOX containing hydrogen peroxide be used as an oxidizer for a liquid fueled rocket? suggest thee's no LOX in this "liquid oxygen" to begin with! $\endgroup$
    – uhoh
    Jun 25 '19 at 10:35
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    $\begingroup$ @uhoh Let's call it CrOX then, and see if that catches on. $\endgroup$
    – LocalFluff
    Jun 25 '19 at 12:19
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    $\begingroup$ Are you including hypergolic propellants? Shuttle OMS oxidizer was nitrogen tetroxide with 3% nitric oxide additive. But you only mention kerosene, lh2, and lo2 in the question. $\endgroup$ Jun 25 '19 at 12:38
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    $\begingroup$ You may want to split the question: kerosene is a mix of various hydrocarbons, I don''t know if you can call that 'pure'. Solids are a mix too, using e.g. a binding agent to keep the fuel grains together. Both are very different from single-species like oxygen and hydrogen. $\endgroup$
    – Hobbes
    Jun 25 '19 at 12:39

Non-hypergolic liquid fuels i.e.: LH2, LOX, Methane etc are all kept as pure as possible. One of the major advantages of these is that they burn clean.

Any additions are potentially problematic: Complex combustion by-products, different density ratios leading to separation, and differential storage requirements etc seem to have outweighed any potential advantages to additives in modern rocket designs.

The slight exception is RP-1/kerosene. This is such a complex mixture that its hard to tell what should be considered an additive and what's just part of the mix. The specifications for it don't say how you make it, just what it should contain. I don't know the refining techniques, and getting the desired ratios may well entail adding in the fractions you don't have enough of. However the list of chemicals its allowed to contain (https://nvlpubs.nist.gov/nistpubs/Legacy/IR/nistir6646.pdf if you fancy the long read) are all petroleum derivatives in line with what any kerosene would be expected to contain. The party line seems to be minimising the 'extras' not adding them.

Hypergolics and solids are a different story. They already burn dirty and have additional problems that additives can help overcome. Consequently, both hypergolics and solid fuels very much do have additives.

In hypergolics there's a long list of things that are specifically added to combat some of the disadvantages of the base mixture. Most noticeably freezing temperature.

Solid fuel rockets are even more complex mixtures. It's common to add binders, catalysts and a host of other things to obtain desired physical, combustion, and handling properties.

  • $\begingroup$ I doubt that liquid fuels "are all kept as pure as possible". When separating oxygen from the other gases of air, a little impurities could not be avoided. A few argon and nitrogen within the oxygen would not harm as long as efficiency is not reduced substantially. There may be also some helium from pressurization. The Mercury-Redstone used a "a 75 percent ethyl alcohol 25 percent water solution" similar to the german V-2 rocket. Ethanol purified by destillation could not be more pure than 95.6 % anyway. $\endgroup$
    – Uwe
    Jun 26 '19 at 9:46
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    $\begingroup$ @Uwe: 'Possible' might be too a strong word, perhaps 'practical' would have been better. And certainly some contaminants are worse than others. However when specifically addressing deliberate contamination, I think as-is is accurate. Also note the caveat: this only applies to modern designs. Prior to the settling on LOX/LH2/RP-1/Methane as the non-hypergolic-liquid-propellants-of-choice: additives where all the rage. There may be an interesting question in itself there, but I assume OP is interested in current not historical practice. $\endgroup$
    – ANone
    Jun 26 '19 at 10:28

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