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This question is in regard to rocket propellants that are liquid at room temperature & pressure (not solid fuel, and not liquid hydrogen liquid oxygen, both of which would be gasses at STP).

It's well documented (by SpaceX and others) that the colder the temperature of the liquid fuel, the more thrust is potentially able to be produced, either increasing payload capability, or reducing launch weights. Are there any currently operating launch systems that do not make an attempt to chill the fuel component of the fuel/oxidizer components before liftoff? I would imagine that most do, but am curious if this is universal nowadays?

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  • $\begingroup$ There is no rocket fuel that is non toxic, liquid at room temperature and non explosive that delivers enough energy for space exploration. $\endgroup$
    – Uwe
    Commented Feb 13, 2018 at 9:37
  • $\begingroup$ Slightly related: Is there a stable and non-toxic hydro-nitrogen-oxygen compound that's liquid in room temperature? $\endgroup$
    – uhoh
    Commented Feb 6, 2019 at 0:05
  • $\begingroup$ @Uwe Is MXP-351 (space.stackexchange.com/questions/43363/…) not a counterexample? 322 ISP is not great compared to cryogens, but is it really too little for space exploration? $\endgroup$
    – ikrase
    Commented Apr 19, 2020 at 19:28
  • $\begingroup$ @ikrase If MXP-351 is hypergolic, could it be non explosive? $\endgroup$
    – Uwe
    Commented Apr 21, 2020 at 14:31
  • $\begingroup$ H2O2 + RP1 would fit the bill. But nobody actually uses the stuff because it requires about as much care in handling and storage as LOX but has quite a bit less kick, and cryogenic O2 is not that big a deal at all. $\endgroup$ Commented Oct 23, 2021 at 10:49

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Using liquid oxygen (which is "naturally" extremely cold) is the standard for almost all professional civilian launchers. There are some odd exceptions - launchers like Lambda 4S using solid propellant. A lot of systems use RP-1 or other non-cryogenic fuels, but for oxidizer in commercial, planned, civilian launch systems that were developer as civilian since moment one, LOX is the king.

("Small" civilian rocketry - amateur, sounding rockets etc often utilize nitrous oxide $N_2 O$ for oxidizer - but it's a gas at STP too)

The situation is quite different with the military. The rockets either use solid propellant, or hypergolic fuels which aren't especially cooled. This also applies to civilian launchers that are derived from military - like Proton (as mentioned by Organic Marble) which was designed as a super-heavy ICBM launcher, or Start-1 which is a modified mobile ICBM launcher repurposed for civilian use.

The reason is fairly simple: boil-off. Rocket's LOX tanks aren't made to withstand high pressures - the liquid oxygen slowly boils off. It means the rocket needs to be fueled several hours before the launch. This takes time - which is not something the military systems can afford. The rockets need to be ready to launch at moment's notice and sit in the launch silo, fueled, for months or years. This requires propellants that don't boil in room temperature.

By the way: what SpaceX does is additional cooling way below boil-off or ambient temperature. Such propellants in fact have worse performance per mole or kilogram of propellant - but decrease of temperature decreases volume - increases density; you're getting a better performance out of liter of the propellant - essentially you can fit more propellant into the same volume of a rocket tank.

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  • $\begingroup$ If you call liquid oxygen extremely cold, you need to invent another word for the much colder liquid hydrogen. $\endgroup$
    – Uwe
    Commented Feb 13, 2018 at 9:29
  • $\begingroup$ @Uwe: I don't need to invent anything, LH2 is mere 60K colder. That's less of a difference than between normal (water) ice and dry ice. $\endgroup$
    – SF.
    Commented Feb 13, 2018 at 9:41
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The Russian Proton booster uses storable propellants (nitrogen tetroxide and hydrazine). Nasty stuff, but room temperature.

https://en.m.wikipedia.org/wiki/Proton_(rocket_family)

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SpaceX is unique in cooling down a room temperature fuel. Other users of RP-1 just use it at room temperature.

In addition to Proton, several Chinese and Indian launchers use storable propellants.

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  • $\begingroup$ Still needs LOX... $\endgroup$
    – Antzi
    Commented Feb 13, 2018 at 7:50
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    $\begingroup$ "Are there any currently operating launch systems that do not make an attempt to chill the fuel component" - the question ignores LOX, so I'll do that too. $\endgroup$
    – Hobbes
    Commented Feb 13, 2018 at 7:54
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H2O2 and RP1 was used in the British Black Arrow orbital launcher.

Beal Aerospace was working on Kerosine/Peroxide (KerOxide) engines before they went under.

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Which fuel?

  • RP1 (refined petroleum) runs at ambient temps, but is cooled often by he cryogenic oxygen carried aboard. By chilling it, it's density improves 9%.

  • Liquid hydrogen must be stored at 20 degree's Kelvin (-423 degrees Fahrenheit)

  • With a boiling point of 360 degrees Kelvin, Hydrazine based fuels are usually kept at "room temperature" for decent storage.

  • Solid rocket fuels have no real temp. The Shuttles Solid fueled boosters used Aluminum slurry/perchlorate which was kept at basically room temp.

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  • $\begingroup$ The question was about propellants, both fuel and oxidizer. Writing about RP-1 only is easy. $\endgroup$
    – Uwe
    Commented Oct 26, 2021 at 9:12
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    $\begingroup$ "Solid rocket fuels have no real temp" What? $\endgroup$ Commented Oct 26, 2021 at 9:57

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