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While I get that one is a vertically launched rocket and the other is a space plane, is it even wildly possible that there could be a vertical intake version of the SABRE engine? Maybe it can be used as a strap-on booster, which is attached to a normal rocket and uses Reaction Engine's pre-cooler tech to reduce fuel requirements and therefore increase the payload. It would stay in jet/air breathing mode as usual before mach 2 or whatever speed it needs to switch to rocket mode.

Or is it infeasible to use such jet+rocket engines in a vertical rocket launch configuration?

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    $\begingroup$ Rockets grab altitude as quickly as possible. The air breathing ability is good for only a couple of minutes and adds a bunch of mass. Almost certainly a bad tradeoff. $\endgroup$ – zeta-band Mar 27 '19 at 19:21
  • $\begingroup$ Somewhat indirectly, that’s what Virgin Galactic does with a carrier aircraft using regular aircraft engines going up to the altitude where the actual “spacecraft” is then launched, using a different technology to get to sub-orbital flight. But we’re quite far from a regular rocket actually launching into orbit (or further). $\endgroup$ – jcaron Jan 1 at 11:11
  • $\begingroup$ Incidentally, I am pretty sure that SABRE isn't really a jet engine in air-breathing mode. $\endgroup$ – ikrase Jan 1 at 11:13
  • $\begingroup$ Something I thought was interesting is when Bezos founded Blue Origin he and his friend started by considering every way they could think of to launch something into space, thinking there must be a better way than an old-fashioned rocket. Then they started designing rockets. $\endgroup$ – Greg Jan 1 at 13:37
  • $\begingroup$ @ikrase it isn't, really...the obvious difference is that it pipes compressed air through a bunch of plumbing instead of having the combustion chamber immediately behind a compression fan, but the bigger functional difference is that the compressor isn't powered by combustion in the engine, but by helium being heated in the compressor and cooled by boiling off liquid hydrogen. $\endgroup$ – Christopher James Huff Jan 2 at 5:07
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Air-breathing engines are usually combined with wings or lifting bodies because they inherently have lower thrust to weight ratios...there's just no way to beat the power and thrust density of something combining liquid propellants and blasting the combustion products out a nozzle with something that has to use ambient air streaming past the vehicle at high speed as an oxidizer. Rockets are much better than jet engines at thrust and acceleration, you strap rockets to aircraft to get the aircraft off the ground faster. Strapping jet engines to a rocket isn't going to work very well.

Making it worse, rockets launching vertically leave the significant atmosphere in a couple minutes, and reach airspeeds where airbreathing is impractical even sooner. To contribute substantially, air breathing engines have to be used with a low trajectory that keeps them in the atmosphere longer, and they have to be the primary source of thrust in that period or you're just flying a rocket very inefficiently while carrying useless jet engines. Again, that means wings or lifting bodies.

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