As I was watching Tom Mueller describe the Merlin engine, he said that the RP-1 propellant is pumped through ribs in the nozzle to regeneratively cool the engine. Also, a video about Copenhagen suborbital's TM65 rocket engine shows how the alcohol fuel is used to cool the engine. Since both of these engines use liquid oxygen as their oxidizer, why would they use the fuel to cool the engine. Why not use the very cold liquified oxygen instead of the room temperature hydrocarbon fuels?

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    $\begingroup$ In short "If stuff goes wrong, the fuel would burn, where the oxidiser explodes" $\endgroup$ – Criggie May 4 '17 at 6:43
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    $\begingroup$ Does raising the temperature of the fuel increase the efficiency in the combustion chamber? If so, this could be a case of killing two birds with one stone. $\endgroup$ – Michael Richardson May 4 '17 at 17:17
  • $\begingroup$ In normal internal combustion engines, you use extra fuel to cool the engine. Allowing it to run lean (more oxygen, less fuel) will cause it to run hotter. $\endgroup$ – rpmerf May 4 '17 at 20:54
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    $\begingroup$ Fuel doesn't burn without an oxidiser. The rocket isn't made of oxidisers, so it's perfectly safe to use fuel as coolant. Oxidisers burn fuels. The thing is, at high temperatures and with strong oxidisers like liquid oxygen, almost everything is a fuel. Remember rust? That's oxygen oxidising iron/steel (and it does the same thing with pretty much any metal). Now imagine a much more powerful oxidiser than atmospheric oxygen - that's how you get things like thermite. Fuels are dangerous in air, since there's plenty of oxidiser around to let them burn or explode; oxidisers are always dangerous. $\endgroup$ – Luaan May 5 '17 at 8:22
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    $\begingroup$ Saying that LOX is colder is a fallacy, since at the temperature of the combustion cooling zone, the temperature will be hundreds of degrees, at which point oxygen is a gas under any pressure. You are not using the intrinsic temperature of the propellants as a cooling factor, but the heat transport capability, which is higher in the liquid. $\endgroup$ – MikeW May 9 '17 at 11:25

Whatever you use as a coolant will become hot. Hot oxygen will (a) vaporize, making the plumbing somewhat more difficult, and (b) react with and erode (or maybe even ignite) the cooling channels, unless they're made of special materials.

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    $\begingroup$ Not sure about the vaporize part - the h2 coolant in the SSME certainly did - but the reactive oxygen part is spot on. $\endgroup$ – Organic Marble May 4 '17 at 4:02
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    $\begingroup$ To rephrase b): "Because oxidizer tends to oxidize stuff". $\endgroup$ – Jörg W Mittag May 4 '17 at 7:52
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    $\begingroup$ Most engines operate at pressures well over the critical pressure therefore neither fuel nor oxidizer will vaporate. E.g. Merlin operates at 6,14 MPa or 9,72 MPa chamber pressure while oxygens critical point is 5,04 MPa. An example for fuel would be hexane with a critical pressure of 2,99 MPa. $\endgroup$ – Christoph May 4 '17 at 8:45
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    $\begingroup$ Put another way, without careful metallurgy and design, your propellant mixture may end up swinging toward engine-rich combustion... $\endgroup$ – Tristan May 4 '17 at 14:04
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    $\begingroup$ Don't worry, @Tristan, that will correct itself once you run out of engine. $\endgroup$ – a CVn May 5 '17 at 16:54

The specific heat capacity of the fuel is higher than that of the liquid oxygen at the operating temperature/pressure, hence for the same rate of flow, using the fuel cools better ... and apart from any of the above-mentioned liquid/vapour/pressure issues.

The operating temperature range when using kerosene / RP-1 is also much greater than if using LOX.

As per my comment above, any "coolant" is not used because it's intrinsically cold, but because it can transport heat effectively in the required region at the required temperature range.

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    $\begingroup$ I wondered about that, but that's actually not true at least for RP-1/LOx, as far as I can tell - LOx's specific heat capacity at its boiling point is something like 20% higher than RP-1 at 273 K. Those were at 1 atm though, I know the pressure in the cooling channels is much higher - were you able to find data suggesting they switch around at ~10 MPa? $\endgroup$ – pericynthion May 4 '17 at 17:55
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    $\begingroup$ @pericynthion: However, Cp for liquid O2 is the incorrect figure, since at the temperature encountered, the oxygen would be a supercritical fluid. $\endgroup$ – MikeW May 10 '17 at 9:56

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