I wondered if it was possible to have a balloon inflated with nitrogen (inexpensive, common, and non-flammable), knowing that the weak aerostatic thrust would perhaps be sufficient to compensate for the mass of the envelope, and also ensure stability.
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Could we build airships again using nitrogen? It’s lighter than air, or is the buoyancy not enough because it’s 78% air?
Two representative answers:
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Science teacher (2001–present)5y
The density of nitrogen at 20ºC and 1 atmosphere of pressure is 1.165 grams per liter.
The density of air at 20ºC and 1 atmosphere of pressure is 1.205 grams per liter.
While pure nitrogen is less dense than air, it’s not enough of a difference to allow a nitrogen-filled airship to lift any significant weight.
The Hindenburg held a whopping 200,000 cubic meters (200 million liters) of hydrogen gas. If you had a blimp of the same size, but filled with nitrogen instead of hydrogen, its internal mass would be 233 million grams (233 metric tons). Assuming the skin of the blimp is negligibly thin, it will displace 200 million liters of air, with a mass of 241 million grams (241 metric tons). So the buoyant force acting on this gas-filled bladder would be equivalent to the weight of 8 metric tons.
The Hindenburg’s average gross weight — that is, the weight of the ship without hydrogen — was 215 tons. Ergo, a Hindenburg-sized blimp, filled with nitrogen instead of hydrogen or helium, would be woefully inadequate to lift its own weight, let alone any passengers or cargo.
Yes, nitrogen is slightly lighter than air, all other things being equal. But for an airship, you need a gas that is much lighter than air. Nitrogen just won’t cut it.
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Has PhD in fluid dynamics from Caltech
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Could we build airships again using nitrogen? It’s lighter than air, or is the buoyancy not enough because it’s 78% air?
No. The 78% you are quoting is NOT the ratio of nitrogen density to air density. It is the fraction of air that is nitrogen. The other 22% is mostly oxygen and a few other traces gases. The density ratio - the thing that matters for buoyancy - is more like 97%. Good luck taking advantage of that 3% difference. In principle, this could be made to float. But it’s a rather poor starting point.
For all balloons supposed to work in pumping mode (Wind Fisher, Omnidea, this one and others), the aerostatic thrust should be approximately neutral to facilitate the recovery reel-in phases, and also ensure stability.
There are no passengers, and possibly no aloft motors if the @WindFisher system was applied, just the envelope and the tethers.
Now let us try with several sizes, knowing the square law for the area, and the cube law for the volume leading to the aerostatic thrust, which is noted in kg to simplify, knowing that for 1 m³, it is about 0.035 kg. Below are some rough calculations for cylindrical balloons made of a film of 0.2 kg/m².
50 m span and 10 m in diameter.
Mass: about 350-400 kg.
Volume: 3925 m³.
Aerostatic thrust: 137 kg. It is not enough!
100 m span and 20 m diameter balloon.
Mass: 1400-1500 kg.
Volume: 31400 m³.
Aerostatic thrust: 1100 kg. Just a little more effort!
200 m span and 40 m diameter balloon.
Mass: 6000-6500 kg.
Volume: 251200 m³.
Aerostatic thrust: 8800 kg. We are a little above, but not if we count the tethers, and the discs around the balloon.
And for such dimensions it will not be possible to pressurize the gas contained in order to have a sufficiently rigid shape to avoid overconsumption for the rotation motors, unless the thickness and mass of the envelope film are increased, or/and some light rigid rods should be integrated.
So using nitrogen is therefore a possibility but not a probability. For giant balloons like this, it would perhaps be a better (or a worse) alternative to the heating wire which requires an electric cable from the ground station.