this post was submitted on 15 Apr 2025
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You're looking for the SABRE hybrid engine! It's super cool: https://en.wikipedia.org/wiki/SABRE_%28rocket_engine%29
It flash freezes air to liquid using a mad heat exchanger, until the atmosphere is thin enough to warrant switching to liquid oxygen. It's better than what you describe, as it saves a tone of weight over separate jets and rockets! It was tested, and seems to work!
...But out timeline sucks, hence it was canceled in 2024 :(
https://www.flightglobal.com/aerospace/reaction-engines-to-close-as-cutting-edge-sabre-fails-to-advance/160565.article
Aerospike engines are awesome, but I'm skeptical of the X-33 TBH. It would've been cool if it had worked out.
Where you should be looking now is the "gun launch" startups. Once that's figured out, it's so much cheaper to launch "sturdy" payloads that way. Nuclear upper stages are a good option, too (with fission fragment drives being my personal favorite: https://www.projectrho.com/public_html/rocket/enginelist2.php#fissionfragment)