It seems to me that heated helium with syphoned pressure regulated to 1000 mbar or less would get you halfway to the Karman line prior to expenditure of precious fuel. Has anyone thought of this?
JP Aerospace are thinking about it. It would be assisted by a balloon station at 140,000 feet, from where it will start to slowly accelerate to orbital velocity using an ion engine. They are running airships commercially and have the altitude record. But I think that the Airship to Orbit is part of their under-financed not-for-profit activities.