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Hydrogen reduces the lifting force of a pure vacuum by less than 10% and solves the problem of needing a stronger-than-diamond super lightweight shell. So, unti
by dr_dshiv 3y ago
Hydrogen reduces the lifting force of a pure vacuum by less than 10% and solves the problem of needing a stronger-than-diamond super lightweight shell. So, until you can make a shell with a mass less than the total amount of hydrogen, it doesn’t make much sense.
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.
- Xylakant 3y agoThere is already a way to change the lift without releasing gas: pump the gas into a compressed container - it reduces the volume and thus the specific lift.
- WirelessGigabit 3y agoSo 1 kg of Helium in a balloon with a volume of 10cm^3 has more lifting power than that 1 kg of Helium in a balloon of 5cm^3?
- the_third_wave 3y agoLifting power is defined by the mass of displaced medium minus the mass of the displacing medium. Change your units from cubic centimetres to cubic metres and the answer is "yes".
- snewman 3y agoYes, although a better way of saying it is that it is not the helium which has lifting power, it is the balloon. A balloon has lifting power equal to the weight of the air displaced by the volume of the balloon. From this, you must subtract the weight of the balloon and its contents. Helium is one of the few materials that allows you to wind up with a positive net lift. (Also, for this to work, 1kg of helium would occupy a balloon much larger than 10cm^3).
- stetrain 3y agoYes, like how a 1kg 10cm^3 block of wood will have more buoyancy in water than a 1kg 5cm^3 block. 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.
- Xylakant 3y agoYes - the latter is more dense by a factor of 8 (2^3) and thus has less lifting power. 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:
- scotty79 3y agoThe helium is only to keep the baloon of given volume from collapsing.
- dr_dshiv 3y agoSuch a good way to think of it!
- harimau777 3y agoIt looks like this sort of thing is already used by submarines to control their depth. https://en.wikipedia.org/wiki/Variable_buoyancy_pressure_vessel https://en.wikipedia.org/wiki/Variable_buoyancy_pressure_ves...
- TylerE 3y agoThis assumes a flexible envelope. Most airships are rigid. Volume doesn't change. Indeed, being rigid is what makes them airships and not blimps.
- Xylakant 3y agoRigid airships still have flexible gas bags (cells) inside. They just have a rigid hull around them.
- namibj 3y agoGoogle Loon or whatever they called the stratospheric balloons carrying solar powered cell/wifi base stations that made their way around the earth with the Jetstream or w/e used a bladder in the balloon that was inflated/deflated with ambient air. Similar to a ship taking/releasing ballast water in ballast tanks.
- dcminter 3y agoI'd been wondering if an aerogel sphere with a mylar coating (constructed under vacuum) would be sufficiently low density to float - the aerogel would let it resist the pressure and the mylar would seal it. 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.
- Scalene2 3y agoMaking an aerogel sphere in a vacuum would be quite the challenge.
- dcminter 3y agoI mean you need a vacuum to create an aerogel in the first place!
- dsign 3y agoI have done a bit of research on that. I don’t think that current aerogels are engineered to withstand full atmospheric pressure. With that said, the approach would make sense for a number of reasons: - 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.
- dcminter 3y agoI thought the silica based ones were pretty strong under compression? Edit: Wikipedia has a persuasive picture of some aerogel supporting a house brick!
- namibj 3y agoWater is lighter than air and only needs a modest temperature to have it's vapor pressure exceed ambient pressure at high altitude. Bonus for self-inflating when you reach altitude/letting you accumulate thermal energy in some form and take off by dumping the heat into your liquid water bladder to inflate your water vapor balloon. Wonder how well/cheaply (in weight for $affordable budget) you could insulate the balloon to curb your thermal power demand. Maybe enough that you could use your liquid water bladder with a little insulation as a thermal mass to run an absorption heat pump (Type2: warm power, cold sink, hot target output) off of ambient temperature during the day, cooling the thermal mass back down during the night. 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...
- aaron695 3y ago[dead]
- harimau777 3y agoIsn't the main problem with hydrogen its flammability? Or is that something that is exaggerated?
- rob74 3y agoAccording to the Wikipedia article, even using helium only reduces the lift by 14%...
- lsaferite 3y agoIsn't the objective to have livable space though? If the space is filled with helium or hydrogen then you cannot live in the space.
- WithinReason 3y agoFilling it with a vacuum however...
- lsaferite 3y agoHaha. Sorry, I was reading some of the links posted here where they talked about using slightly warmer air and must have gotten lost.
- lesuorac 3y agoIt's exaggerated. 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. [1]: https://en.wikipedia.org/wiki/Hindenburg_disaster https://en.wikipedia.org/wiki/Hindenburg_disaster