- use lighter than air balloon to life an aircraft to cruising altitude
- transition to powered flight while collapsing the balloon (by compressing the gas into liquid form?). Or maybe detaching from it?
- cruise
- at destination, glide towards touch down?
Interesting thought, but that would mean you've got aircraft ascending slowly right through the entire airspace with limited control. Difficult to imagine it would be workable at busy airports.
> Or maybe detaching from it?
Consuming a balloon's worth of helium every flight is a non-starter, and hydrogen is probably unacceptable to passengers. Landing a balloon is also not cheap or easy, even without passengers.
> at destination, glide towards touch down?
Almost all planes essentially do this already, they'll reduce thrust and start descending a long way out.
If you could first lift the rocket to a high altitude, you could get away with far less fuel. The smaller mass would mean faster acceleration (think a rocket fired from a jet fighter).
note: I'm not a rocket scientist or physicist :-)