[dmurray]

It's crazy to me that once they had this high-performance jet fuel, they also used it as a coolant and as hydraulic fluid.

I guess the main requirement for the latter two purposes is that it doesn't break down at really high temperatures. The jet fuel needs to be stable at high temperatures too (additionally, you need to be able to burn it to make the engine go) so once you put all this research into finding the right jet fuel you have a liquid that will do OK on the other jobs.

Edit: and also it was used for lubrication! Good fuels definitely do not necessarily make good lubricants...it must just be really hard to find anything with the right thermal stability.

[throwaway0a5e]

> they also used it as a coolant and as hydraulic fluid.

Potentially a much smaller headache than pumping one or two more fluids around inside a jet engine and keeping them from breaking down. The jet fuel only needs to not break down for a single pass through the engine. A closed lube or cooling system would need to not break down for multiple passes (i.e. much longer duration at high temperature).

>it must just be really hard to find anything with the right thermal stability.

A total loss lubrication system doesn't need to be actively cooled (assuming your lubricant input is of suitable temperature) which saves a ton of weight and complexity which is why rocket and jet engines often use the fuel as a lubricant.

[masklinn]

FWIW JP-7 was not the only hydraulic fluid, it was only used as hydraulic fluid in a few places where that was convenient, or possibly where they'd have needed a different hydraulic fluid due to the temperature.

[iggldiggl]

The F-1 rocket engine on the first stage of the Saturn V used a similar arrangement, with the fuel also doing double duty as hydraulic fluid, coolant and lubricant.

[2trill2spill]

That's pretty standard on most liquid propellant rocket engines.

[euler_angles]

Many tactical aircraft to this day use their fuel for the same purposes. I know the F-35 uses its fuel as a heat sink. The F-35B variant specifically uses fuel as the hydraulic working fluid (aka "fueldraulics")for much of the work of the 3 bearing swivel nozzle.

[kube-system]

Most two-stroke internal combustion engines are also lubricated by the fuel.

[op00to]

Well, sort of. Every 2 stroke engine I've seen needed oil in the fuel, so if you just ran straight petrol you'd have a problem.

[kube-system]

According to the article, the lubricants in JP-7 are also additives.

[raverbashing]

I wonder how do you actually make it catch fire when you need it to combust

[wil421]

Compress the fuel in a small area and then apply some high voltage. It’s probably not much different than a spark plug in a car.

[masklinn]

They actually injected triethylborane to act as a match and initiate combustion (TEB auto-ignites at freezing temperatures and burns ridiculously hot).

The plane had a limited number of TEB "shots" and those were needed after each afterburner use, or when restarting the engine in mid-air (as crew needed to do under some conditions), so they had to be carefully managed.