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There is an air breathing rocket engine that's exactly what you want on a plane: https://en.m.wikipedia.org/wiki/SABRE_%28rocket_engine%29 https://en.m.wikiped

There is an air breathing rocket engine that's exactly what you want on a plane: https://en.m.wikipedia.org/wiki/SABRE_%28rocket_engine%29 https://en.m.wikipedia.org/wiki/SABRE_%28rocket_engine%29 https://www.gov.uk/government/publications/synergetic-air-breathing-rocket-engine-sabre-programme-evaluation-report-2022/synergetic-air-breathing-rocket-engine-sabre-programme-evaluation-report-2022 https://www.gov.uk/government/publications/synergetic-air-br... The answer is "its too expensive, unsafe and basically unnecessary." Even Concorde (with conventional jets) proved to be uneconomical when luxury commercial aviation was lucrative, and US rocket powered X-Planes like the X-15 never made it out of prototypes. Giant turbofans are good enough.

keepamovin 3y ago
I guess I was thinking, non-air breathing that carries its own oxidizer, so it’s a real rocket but on a jet
- brucethemoose2 3y ago
  Yeah, not going to work. Oxidizer is the vast majority of weight on rockets: https://www.marssociety.ca/2021/01/07/rocket-physics-the-rocket-equation/ https://www.marssociety.ca/2021/01/07/rocket-physics-the-roc... TL;DR Rockets on launchpads have about the same fuel weight/body weight ratio as a soda can, with the soda being the fuel. A rocket airplane would similarly need to be mostly fuel.
  - keepamovin 3y ago
    So...you couldn't just like launch it then fly it...because of the weight? You couldn't add anything else to it, so it wouldn't be useful? Couldn't you just like find a more efficient fuel/oxidizer mix? Or is this like fundamental chemical energy density? Does it make any difference if you just have to fly in atmosphere, and not escape Earth's gravity? Seems like it should be easier...so you could have more payload.
    - brucethemoose2 3y ago
      > So...you couldn't just like launch it then fly it...because of the weight? Y Precisely. Rockets use liquid oxygen and cryogenic, extremely weight efficient fuels (usually liquid hydrogen or liquid methane). They are pretty much at the edge of practical chemical energy density. > Does it make any difference if you just have to fly in atmosphere, and not escape Earth's gravity? But they have to fly for a much longer time, with atmospheric drag. If you look up examples like the Me 163 and the X15, you will see that their range is extremely short: https://en.wikipedia.org/wiki/Messerschmitt_Me_163_Komet https://en.wikipedia.org/wiki/Messerschmitt_Me_163_Komet https://en.wikipedia.org/wiki/North_American_X-15 https://en.wikipedia.org/wiki/North_American_X-15 The reasons rockets use rockets (and not jets) is because their flight time in the atmosphere is even shorter, and they need a ton of thrust in that short period, not because they are particularly efficient.
      - keepamovin 3y ago
        Excellent. Thank you very much! :)

keepamovin 3y ago

I guess I was thinking, non-air breathing that carries its own oxidizer, so it’s a real rocket but on a jet

brucethemoose2 3y ago
Yeah, not going to work. Oxidizer is the vast majority of weight on rockets: https://www.marssociety.ca/2021/01/07/rocket-physics-the-rocket-equation/ https://www.marssociety.ca/2021/01/07/rocket-physics-the-roc... TL;DR Rockets on launchpads have about the same fuel weight/body weight ratio as a soda can, with the soda being the fuel. A rocket airplane would similarly need to be mostly fuel.
- keepamovin 3y ago
  So...you couldn't just like launch it then fly it...because of the weight? You couldn't add anything else to it, so it wouldn't be useful? Couldn't you just like find a more efficient fuel/oxidizer mix? Or is this like fundamental chemical energy density? Does it make any difference if you just have to fly in atmosphere, and not escape Earth's gravity? Seems like it should be easier...so you could have more payload.
  - brucethemoose2 3y ago
    > So...you couldn't just like launch it then fly it...because of the weight? Y Precisely. Rockets use liquid oxygen and cryogenic, extremely weight efficient fuels (usually liquid hydrogen or liquid methane). They are pretty much at the edge of practical chemical energy density. > Does it make any difference if you just have to fly in atmosphere, and not escape Earth's gravity? But they have to fly for a much longer time, with atmospheric drag. If you look up examples like the Me 163 and the X15, you will see that their range is extremely short: https://en.wikipedia.org/wiki/Messerschmitt_Me_163_Komet https://en.wikipedia.org/wiki/Messerschmitt_Me_163_Komet https://en.wikipedia.org/wiki/North_American_X-15 https://en.wikipedia.org/wiki/North_American_X-15 The reasons rockets use rockets (and not jets) is because their flight time in the atmosphere is even shorter, and they need a ton of thrust in that short period, not because they are particularly efficient.
    - keepamovin 3y ago
      Excellent. Thank you very much! :)