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(Thinking of SABRE and it's heat exchanger).

As I understand it, Skylon would take off with full oxygen tanks.

Putting aside engineering difficulties, is there a thermodynamic reason why a hybrid rocket couldn't lift off with empty oxygen tanks and fill them as it ascends - in time to use the harvested oxygen when it switches to pure rocket mode?

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    $\begingroup$ The nitrogen in the air is an issue. It uses up tank space to no advantage. $\endgroup$ – Steve Linton Jun 30 '19 at 9:45
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    $\begingroup$ Using an air separation plant within the rocket to split air into liquid oxygen and nitrogen would be far too heavy and expensive and would need a lot of energy for liquifaction and separation. $\endgroup$ – Uwe Jun 30 '19 at 10:00
  • $\begingroup$ @Steve, & Uwe good points re Nitrogen. I admit I'm hand waving away the engineering issues involved in syphoning off pure oxygen from superheated air :-) $\endgroup$ – InkWiring Jun 30 '19 at 10:17
  • $\begingroup$ Storing superheated pressurized gaseous air in tanks would require very heavy tanks. Much heavier than the stored air itself. Cryogenic oxygen tanks filled before launch would be much lighter. $\endgroup$ – Uwe Jun 30 '19 at 14:43
  • $\begingroup$ In principle, yes. In practice, why? There does not seem to be any advantage to doing so. $\endgroup$ – Organic Marble Jul 3 '19 at 17:59
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In a way, that is sort of already what makes the saber engine so clever, it manages to get cryogenic oxidiser out of the atmosphere at less mass cost than it would take to lug it up there. At the moment, it only uses this to directly pass through to the engines (not via storage).

In theory if this could be made more efficient then it could create a surplus that could be stored. However, I think you are misguided in thinking this could be practically used as a substitute to filling up the LOX tanks on-the ground.

Yes, purely theoretically (engineering not an issue): what you state would be possible.

If weight/volume wasn't an issue for the machinery needed to compress, cool, and distill the super heated and super sonic atmospheric air you bump into. Nor was weight/volume for the enormous power-plant you'd need to run all of that. And you didn't need to use up much LH to do so: yes, you could use that system to fill the [partially-]empty LOx tanks on your way up.

However:

  • A) You'd be loosing an awful lot of momentum to all that air you intake. The fastest you can spit it out the back again by reacting it with LH is about 4.4kms (which is pretty good, but if you didn't manage to gain any use of the nitrogen, the average of what you intake falls to about Mach 2.5, which means this is harming your overall dV past this speed).

  • B) If you had near massless power generation and complex, high power, high force processing equipment: you could optimise space travel a lot more than what you suggest.

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  • $\begingroup$ Hi @drjpizzle, when the question popped into my head, I was mostly just intrigued by the conflicting thermodynamic and mass implications of taking off without the weight of the extra oxygen versus the energy cost of bringing atmospheric oxygen to a dead stop at cryogenic temperatures (as opposed to thinking it would be in any way practical) :-) I'll mark as answered and thanks very much for the input! $\endgroup$ – InkWiring Jul 4 '19 at 19:18

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