Take a very large hoberman sphere (see link) , and some very tough thin plastic covering. Cover the hoberman sphere with the plastic sheet when at it's largest extent.
Contract the hoberman sphere to it's smallest size and suck all the air out. At this point we have a hoberman sphere which, with it's plastic covering, is like a balloon but has a vacuum (or near vaccum) instead of air. Possibly add a little hydrogen/helium to reduce air pressure on exterior.
When the hoberman sphere is extended to it's maximum size again, it's volume signifantly increases, meaning it's density is reduced. Get the proportion of the sphere correct, build the hoberman sphere with ultra light high strength composites - as long as the size and density are correct you get an object than has lift (due to reduced density) on demand.
Expand it and it rises. Contract it and it sinks.
Possible uses?
1. High altitude, low orbit satellite?
2. Easy, cheap way to get to 20,000 ft for fun.
3. Make the plastic transparent, have a seat in an airtight bubble in the middle and find some way to propel it and you have a flying, quiet transport device.-- noblea, Dec 11 2004 Link to Hoberman sphere http://hoberman.com...here/minisphere.htmHoberman sphere details [noblea, Dec 11 2004] Vacuum blimp Inflated_20Shell_20for_20Vacuum_20Balloon [david_scothern, Dec 13 2004] Vacuum Balloon Inflated_20Shell_20for_20Vacuum_20Balloon [bungston, Dec 13 2004] Lighter than Air Solid Develop_20a_20lighter-than-air_20solid [bungston, Dec 13 2004] Just like magic.-- swamilad, Dec 12 2004 Well only in the same way air bubbles magically defy gravity and float upwards in a glass of lemonade.-- noblea, Dec 13 2004 You're right, it's not magical at all. What is, is the material that it is to be made from. I suspect you'll find a lot of discussion about vacuum spheres on this and other websites. Nobody has yet provided a realistic useable material.
The Hoberman bit is new, as far as I know.-- swamilad, Dec 13 2004 A vacuum blimp would have significantly less lift than a helium balloon of similar size. In our atmosphere, the difference in lift between helium and vacuum is just 14%, and then you need a totally rigid shell for the vacuum blimp, which isn't required for a helium balloon.-- david_scothern, Dec 13 2004 Sounds like a dogged effort, though.-- normzone, Dec 13 2004 With some modifications, i.e. pistons to drive the retractable shell, I think this device would be useful for submersible. Buoyancy of the vessel can be manipulated by changing the size of the bubble.-- neoearth, Dec 13 2004 Unbuildable but halfbaked, therefore bun. I liked the Hoberman sphere bit.-- wagster, Dec 13 2004 There are two excellent HB ideas which have fine discussion covering this concept. [mfd]-- bungston, Dec 13 2004 I love the submersible idea, and given that the idea came to me from bubbles in lemonade, it should have been my first idea. Guess I just wanna fly...-- noblea, Dec 14 2004 A well folded skin, something with a massive surface area under no tension, might be capable of huge expansion, on the scale required here.
I like it [+]-- zen_tom, Aug 31 2006 random, halfbakery