Start with a glass balloon filled with Hydrogen and then line the inside with a thin layer of aerogel for insulation, then put a thin dark coating on the inside of the aerogel. At one point add a one way valve to release over pressure.
Release it at sea level and as the sun heats the dark layer it will then heat the hydrogen. The over pressure gets released by the valve at the bottom. The Aerogel is there to retain convection heat over night, but as the H2 is pumped out of the balloon it should get more and more lift. The gas will cool at night, but hopefully not too much to collapse the balloon. The valve should let air in if the pressure difference exceeds a limit.-- MisterQED, Oct 23 2008 Aerogel porosity http://www.scienced...5e8d4d5ea59a275c55e [2 fries shy of a happy meal, Oct 26 2008] Why glass? For the thinness required to be bouyant, that glass is going to have to be exceedingly spherical and without structural flaws, to avoid shattering as night falls.-- BunsenHoneydew, Oct 24 2008 First glass in VERY strong stuff, brittle but strong. Second it is pretty good at stopping the passage of the H2. Third it is a material that we have a lot of experience with. Lastly the valve at the bottom will reverse if too much differential occurs from cooling. This will cause a loss of altitude, but if set correctly should keep the structure from collapsing.
If too much temperature/pressure differential occurs, then later models should have valves modified to allow some over pressure during the day to lessen nightly pressure losses.
Maybe the coating could be made photosensitive to absorb the most heat during the day, but then be a poor IR radiator at night?-- MisterQED, Oct 24 2008 Still, you're letting hydrogen out, and air in, with each cycle. That's not going to help your bouyancy-- BunsenHoneydew, Oct 24 2008 //Still, you're letting hydrogen out, and air in, with each cycle. That's not going to help your bouyancy// No the idea is to never let any H2 out unless you don't need it anymore due to the altitude. If the shell can withstand a -3psi pressure diff, but the thermal cycle causes a 5psi shift, then set the valve to only release gas after a 2psi over pressure or redesign for better insulation to lower the pressure swing.-- MisterQED, Oct 24 2008 So we have a complicated floating balloon. What is the benefit of this over a hydrogen balloon? It's heavier? More expensive? Not temperature compensated?-- swimr, Oct 24 2008 [QED], you don't want to let air in when there is hydrogen in there. The part I thought was brilliant was the physics of using solar heating to increase then release the pressure and thereby increase the bouyancy, but I realize this would be a losing proposition if the container falls then implodes due to cooling, which then explodes due to air mixing with hydrogen. Perhaps this device could be used as a weapon of random-walk sorts though.-- quantum_flux, Oct 24 2008 // a weapon of some sort //
NOW you're talking .....
Lithium deuteride ? Did someone mention Lithium Deuteride ?-- 8th of 7, Oct 24 2008 No, with a correctly set valve you should never let air in. The idea is as close as you can get to a vacuum balloon and solves the launching issue.
It is a lighter than Hydrogen balloon because the lift comes from solar heated partial pressure Hydrogen, the next best thing to pure vacuum. The glass envelope is insulated to even out the thermal cycles, but will end up warmer inside than the environment for the same reason that solar hot water heaters work.
But really all that is only for lower altitudes. If the glass envelope can withstand -1.5 psi then it can handle pure vacuum above 12km because the air pressure is only 1.5psi, if it can handle only -.5 then it has to reach 20km.
This idea was the solution for the vacuum balloon anno and it uses no unubtanium.-- MisterQED, Oct 24 2008 Name revised from "Hot Hydrogen Balloon" to better illustrate the idea.-- MisterQED, Oct 26 2008 Could you not just seal the aerogel to render it impermeable to Hydrogen and lose the glass? It doesn't appear to be very porous stuff to begin with.-- 2 fries shy of a happy meal, Oct 26 2008 It is porous enough the let air out, so H2 would go thru easily. It also isn't stiff enough, so the sphere would dent and then catastrophically collapse.-- MisterQED, Oct 27 2008 This is another vacuum balloon idea, with all the flaws discussed before. First, a vacuum has not much more lifting power than hydrogen. Air has a molecular weight of 29 and hydrogen 2, so hydrogen already has 93% of the lifting capacity of a vacuum. Second, glass is very heavy compared to air or to the films used in balloons, and any glass balloon thick enough to resist even a few psi would never float--for instance, have you ever seen a light bulb float off?
A glass sphere one foot in diameter with walls 1/16" thick weighs 2.6 pounds. With a vacuum inside, it weighs 2.56 pounds. Let it go, and it's not going anywhere but down.-- ldischler, Oct 27 2008 random, halfbakery