I’d like to find calculations on the effect of hot/cold hydrogen on lift—might be a way to change the lift without releasing ballast or gas.
(Also, for this to work, 1kg of helium would occupy a balloon much larger than 10cm^3).
Bouyancy “is equivalent to the weight of the fluid that would otherwise occupy the submerged volume of the object, i.e. the displaced fluid.”
So compressing your lifting gas reduces the amount of air being displaced by its presence, reducing the buoyancy.
To take an actual real example:
Air has a weight of 1.222 kilograms/m^3 (dry, clean, standard pressure) Helium has a weight of about 0.167 kg/m^3.
That means that 1m^3 of helium lifts about 1kg. If you now compress that helium by a factor of 10, then 1m^3 of compressed helium would weigh 1.67kg - more than the sourrounding air and provide negative lift.
edit: get your unit and orders of magnitude right. :sigh:
I guess the aerogel would be dense enough compared to hydrogen that a conventional balloon would be better in all respects though.
Still curious as to whether it's been done already though.
- Helium and hydrogen “escape from everywhere”.
- Hydrogen burns
- Helium is obtained from natural gas and is rather expensive.
An aerogel made from biomaterials (think of a sort of bio-engineered coral) could possibly have enough strength and low density.
Edit: Wikipedia has a persuasive picture of some aerogel supporting a house brick!
Or lightweight solar panels, I suppose, feeding an air-source heat pump to boil the water. Normal Li-ion batteries sadly don't really have enough energy density (water takes 620 Wh/kg to boil; commercial energy-dense Li-ion is in the 200~300 Wh/kg range; at sea level and boiling point (=lowest possible temperature without it wanting to condense on every dust particle and other condensation nucleus) water vapor is half as dense as 20°C air so it could lift itself twice (and once it's used to lift itself already), and even a very good heat pump will have severe difficulty reaching a COP of 4.0). So I guess if you try really hard you could make a self-inflating steam balloon that takes off using Li-ion fed heat pumps and later stays airborne using solar cells to continue feeding the heat pumps...
Also - "water is lighter than air" is an odd claim.
But no, water is lighter than air. In it's gaseous form. Which requires substantial temperature or low pressure. The former you can make easily, the latter you encounter if you fly high enough.
The problem with hydrogen is that it was involved in a very public accident (the Hindenburg). However, only 1/3 of the people on it died and they did so on the ground because they couldn't escape from the fire [1]. The flammability of hydrogen can be mitigated just as the flammability of gasoline can be mitigated.
Also keep in mind the Ford Pinto is from 1970 while the Hindenburg was from 1930 and the Pinto's failure mode was _not unique_ to the Pinto; it was endemic of car designs of the time (doesn't mean the Pinto doing that design was smart, its just that cars were literally unsafe in any accident). Obviously in 50 years we've come a long way with gasoline safety; I expect we could make a significantly safer blimp nearly 100 years after the Hindenburg.