>>12242186Engines work by taking in cold gasses, using fuel to heat them up, and then letting the high pressure hot gasses expand back to low pressure through a nozzle. This makes thrust.
If you're moving very fast, the air ramming into your engine is getting heated up by compression. This is a problem because if your engine can only handle 2000 degrees and the intake air is already at 1800 degrees, the combustion chamber of your engine will only be 200 degrees hotter than your intake, which means you get basically no thrust.
What the precooler does is take the incoming compressed hot air and removes that heat, so that the air can go into the combustion chamber at 100 degrees instead of 1800, getting you a temperature increase of 1900 degrees between compressor and combustion chamber, allowing you to produce significant thrust. The precooler needs a large supply of very cold fluid to work, which means you need to use your cryogenic fuel as your heat sink just like in a normal rocket engine. At very high speeds, the amount of coolant fuel required to keep the engine working is actually much more than the amount of fuel the engine needs to burn to operate, which results in a decrease in efficiency, but the fact that the engine is still air-breathing in that mode leaves it more efficient than a closed cycle rocket engine even when the hydrogen to air mixture is off stoichiometric by a factor of ten at least.
The two major issues with this technology are that the heat exchanger needs to somehow prevent ice formation (apparently solved, that was their breakthrough a while ago), and more importantly the fact that these things can still only get us to mach 5 or 6 before needing to switch to rocket mode anyway, because beyond those speeds cooling the air enough to allow the engine to run becomes less efficient than just using a rocket. Orbital velocity is ~mach 25, so this super complex engine cycle only gets you 1/5th the way to LEO.
Better to just use reusable TSTO.
