The use of a fuel as a coolant is pretty common in modern high performance aircraft.

And motorcycle engines. Evaporating fuel happens to be a great coolant.

Cool. Any examples or details?

Most “v” block engines even if they are liquid-cooled direct more fuel to the rear cylinder(s) to cool the head, either with larger carburetor jets in older engines or with software in newer ones. If you think about it all ICE are cooled by fuel; if you fed them with boiled fuel they would overheat.

"if you fed them with boiled fuel they would overheat"

It isn't clear to me that they would.

I suppose if your cooling system was highly optimised the extra energy could tip the balance, or is that what you're saying? But then all ic engines would be cooled by their fuel, its kind of inherent to the process.

The SR-71 used it to cool its skin and keep it around 300°C during Mach-3 flight. It also used it to cool its engines.

Isn't it also the SR-71 that has to expand to become fuel tight, and so leaks fuel all over the runway at takeoff.

I wonder how do this kind of crazy engineering ideas happen. What is gained by not making the thing fuel tight unconditionally? Leaking fuel at takeoff sounds extremely dangerous!

> Leaking fuel at takeoff sounds extremely dangerous!

The fuel that was used had to withstand fairly extreme conditions before reaching the engine to be burned, which also made it pretty hard to ignite under ordinary conditions. https://en.wikipedia.org/wiki/JP-7

Expansion due to heat, the GP mentions a temperature of 300c.

Edit to add: Different materials expand by different amounts due to heat, and different parts of the aircraft are reaching different temperatures.

If you know all the bits are going to expand x amount in normal flight, then you work back to what size they are when cold, if the shrinkage/expansion is too great you possibly could go with a different material (with various tradeoffs) or you just accept the shrinkage as they did here.

It also helps that the SR-71 fuel was almost impossible to ignite. It couldn’t be spark ignited, instead they used triethylborane, an pyrophoric liquid that burns in contact with air. It’s also used to ignite rocket engines as its reactive enough to burn on contact with liquid oxygen.

"almost impossible to ignite" .... "pyrophoric liquid that burns in contact with air"

Those seem mutually exclusive?

Do you mean that triethylborane ignites the fuel?

I don't know why I'm getting down votes for this, its a reasonable question.

To answer my own question triethylborane is indeed used to ignite the main fuel.

https://www.quora.com/Why-was-Triethylborane-used-as-a-start...

You’re getting downvoted because you’re calling out an inconsistency that doesn’t exist. The fuel that doesn’t burn is separate from the pyrophoroc stuff used to start it.

If you know everything is going to expand significantly due to heating, I’m sure making it fuel tight at room temperature would involve significant engineering trade offs.

It's interesting that at the dawn of rocket engines the regenerative cooling was thought to be a bad idea. The reasoning was the hot side will expand too much and disconnect from cool side because of stresses. It's good von Braun actually used one...

It's lighter and you're inherently accommodating for thermal expansion that would happen anyway and possibly cause issues.

> I wonder how do this kind of crazy engineering ideas happen

Three words: the Cold War. See also: Project Sunshine, the Oak Ridge Experiments, SDI, etc. Once you start writing blank checks to the military-industrial complex the crazy ideas start pouring forth.

It is!

I suppose on a supersonic aircraft, you can't reject heat into the surrounding air, because that's already been heated up too far by your passage through it. Is this technique still in use in supersonic aircraft?