3 ms·

Any idea why they can't use hydrogen and oxygen directly? Or are they just using a smaller quantity of helium as a 'working refrigerant'?

• pja 12y ago
  Liquid Oxygen is (surprise!) a powerful oxidisation agent whilst Hydrogen will cause early failure of the metal parts via hydrogen embrittlement. Liquid helium is inert and much less likely to cause problems I'd guess.
  • digi_owl 12y ago
    This seeems to use gaseous helium but other than that it seems to be close to the reason. The last thing you want is for any of those heat exchangers to break, as you are effectively mixing the components of rocket fuel. The main reason though seems to be the temperatures involved. They are exposing hot air to near liquid helium (thanks to being cooled the the liquid hydrogen), thus cooling the air before it reaches the compressor etc that will make it suitable for the rocket engine proper. At the same time they seem to be heating the hydrogen up. Easier to ignite a gas, or something close to it, than liquid. Just look at how your typical car engine works (never mind a fuel air explosive).
• baq 12y ago
  the hypothetical aurora spy plane also hypothetically uses exactly this solution.