If we consider reusable rockets, the F9 uses the thick atmosphere to slow down. What’s the fuel penalty for F9 to land at 19,000 feet?
And I always thought the point of Kilimanjaro was to rocket sled up the slope and launch already at some speed.
I'm interested if an upper-stage with perhaps 3 or 5 engines would gain any useful ISP improvement (on the way up) with a higher expansion-ratio* central landing engine, and/or any engine mass reduction** that might end up in an improvement in mass fraction.
*enabled by a high altitude landing pad
**enabled by better T/W after staging. i.e. all engines thrusting for longer after staging, instead of just the vacuum engines. Might this allow the engine to be slightly downsized?
And what constrains landing engine expansion ratio, landing air pressure or 'effective air pressure' during supersonic retro-propulsion?