3 ms·

Forgive my ignorance, but how does cooling super hot air super fast make a jet engine go faster?

deleted 9y ago
[deleted]
Turing_Machine 9y ago
If I'm not mistaken, it doesn't directly make it go faster. What it does do is let you use less-exotic (i.e., lighter weight) materials for the front end of the engine without having them melt. Less weight -> more speed.
matheweis 9y ago
I think cooling it allows you to increase the density to the point that the air can be used as oxidizer for the rocket engines. But I am still confused though - part of the article talks about how the breakthrough is that it doesn't need to carry oxidizer because of the precompression / cooling system, but then it talks about how it could operate in orbit for 14 days, where no about of compression will get you enough oxygen...
- wcoenen 9y ago
  They mean it needs to carry less oxidizer. The engines switch from air-breathing to on-board liquid oxygen at some point.
  - valuearb 9y ago
    They have to switch to onboard LOX when intake air gets too hot to cool sufficiently. That occurs at Mach 5.5. You need to achieve Mach 30 to reach orbit, so all this fancy engineering can only going to be used for a small fraction of the journey to space.
    - vilhelm_s 9y ago
      It's a small fraction of the velocity, but a significant fraction of the mass. E.g. I think for the reusable missions, the Falcon 9 first stage cuts out around Mach 6, but the mass of the first stage (which is mostly liquid oxygen) is about 80% of the mass of the rocket.
zeusk 9y ago
because that cooling happens before combustion making the intake air denser and the difference in kinetic energy of intake vs exhaust larger. It is a fine line since you might end up spending more energy (in terms of drag mostly) than you can recoup by making the combustion more powerful/efficient. Also, cold air does not have 'more' oxygen than hot air - its just more dense and hence has more oxygen per unit volume.
semi-extrinsic 9y ago
Well, you probably know colder air has higher density (or conversely, that hot air expands). Now a jet turbine compressor stage, just like a turbo on your car, moves a fixed volume of air per second into the combustion chamber. But the actual combustion cares about the mass of air (well, oxygen). So a higher density/lower temperature gives you more power with the same turbo. This is why cars have intercoolers.

deleted 9y ago

[deleted]

If I'm not mistaken, it doesn't directly make it go faster. What it does do is let you use less-exotic (i.e., lighter weight) materials for the front end of the engine without having them melt. Less weight -> more speed.

matheweis 9y ago

I think cooling it allows you to increase the density to the point that the air can be used as oxidizer for the rocket engines. But I am still confused though - part of the article talks about how the breakthrough is that it doesn't need to carry oxidizer because of the precompression / cooling system, but then it talks about how it could operate in orbit for 14 days, where no about of compression will get you enough oxygen...

wcoenen 9y ago
They mean it needs to carry less oxidizer. The engines switch from air-breathing to on-board liquid oxygen at some point.
- valuearb 9y ago
  They have to switch to onboard LOX when intake air gets too hot to cool sufficiently. That occurs at Mach 5.5. You need to achieve Mach 30 to reach orbit, so all this fancy engineering can only going to be used for a small fraction of the journey to space.
  - vilhelm_s 9y ago
    It's a small fraction of the velocity, but a significant fraction of the mass. E.g. I think for the reusable missions, the Falcon 9 first stage cuts out around Mach 6, but the mass of the first stage (which is mostly liquid oxygen) is about 80% of the mass of the rocket.

zeusk 9y ago

because that cooling happens before combustion making the intake air denser and the difference in kinetic energy of intake vs exhaust larger. It is a fine line since you might end up spending more energy (in terms of drag mostly) than you can recoup by making the combustion more powerful/efficient. Also, cold air does not have 'more' oxygen than hot air - its just more dense and hence has more oxygen per unit volume.

semi-extrinsic 9y ago

Well, you probably know colder air has higher density (or conversely, that hot air expands). Now a jet turbine compressor stage, just like a turbo on your car, moves a fixed volume of air per second into the combustion chamber. But the actual combustion cares about the mass of air (well, oxygen). So a higher density/lower temperature gives you more power with the same turbo. This is why cars have intercoolers.