[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.