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?

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    $\begingroup$ "Has anyone thought of this?" Yes, and basically the conclusion is 'not worth the hassle'. The real challenge to reaching orbit is speed, not altitude (the kinetic energy of an object in low orbit is approximately 35 times that of the gravitational potential energy). $\endgroup$ – Andrew Thompson Dec 21 '14 at 8:51
  • $\begingroup$ what about slow thrust being applied over a long duration, to allow cetrifugal force to push out against gravity up to the 18k orbit velocity? Seems to me it would be easier to do it in rarified atmosphere, then trying to thrust something from zero to orbit in a five minute window. If the goal is orbit, who cares if it takes a day to get there. With a completely re-usable vehicle, high traditional payload capacity, (dirigibles) with low maintenance and consumables, how can that not be worth the hassle? $\endgroup$ – TomSwift Dec 21 '14 at 9:03
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    $\begingroup$ "If the goal is orbit, who cares if it takes a day to get there." The people that make the fuselage that needs to handle such a whopping heat load, for one. Generally if the gas you're in can support either winged flight or dirigibles, it is thick enough to be a significant problem to a vehicle traveling at high speed. $\endgroup$ – Andrew Thompson Dec 21 '14 at 9:07
  • $\begingroup$ With a lifting body design, and composite airframe via 3D printed titanium or tungsten "lattice", filled with evacuated aerogel, and carbon-carbon sheathing, (all of which can be done with 3D printing these days, minus the evacuated tank the printer would sit in) I can't see that the airframe and skin would be much of a problem to manufacture. It would have the strength and possibly the heat resistant properties you are saying it needs, and weigh next to nothing. Mind you, I'm no expert in this field, just curious. cnet.com/news/breakthrough-material-is-barely-more-than-air $\endgroup$ – TomSwift Dec 21 '14 at 9:26
  • $\begingroup$ @AndrewThompson The expense of gaining altitude isn't just the difference in potential energy. There is also gravity loss suffered during the vertical ascent. However I agree that balloons aren't worth the hassle. $\endgroup$ – HopDavid Dec 21 '14 at 14:53

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.

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  • $\begingroup$ I've watched their progress, but it's very basic under a simplistic design. It seems to me they would get more bang-for-the-buck using Q thrusters, which was mentioned recently to be able to generate 20N per KW, (Harold "Sonny" White claimed, they have developed while working on warp drive testing at NASA) without a gas propellant. $\endgroup$ – TomSwift Dec 21 '14 at 9:36
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    $\begingroup$ @TomSwift Please note that this is a question-and-answer site. Comments should be used to request and give clarification, not to start a discussion. $\endgroup$ – David Richerby Dec 21 '14 at 11:15

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