> other non-combustive gases

What options exist other than helium?

There isn't really anything else -- if you look at a periodic table, you can see the next inert gas is Neon, but it is heavier than Nitrogen (which makes up most of the atmosphere).

For the sake of argument, would it be possible to use vacuum? Maybe some kind of spongy structure, with no single points of failure, but lighter than air by way of being mostly nothing?

I was wondering the same thing. It would have to be a sphere, a thick shell constructed of an incompressible material such that the inward pressure causes compression. Some kind of honeycomb structure?

The weight of the shell depends on the area (goes up as the square of the radius). The lift depends on the volume (goes up as the cube). Seems like for a large enough size you might be able to get positive lift.

The inward pressure per square inch of shell is constant (14.7 lbs. per sq. in. at sea level with a total vacuum inside), so as long as the inward pressure is distributed evenly it doesn't seem like it has to be stronger just because it is larger.

Inside would only have to be a partial vacuum. You can heat the gas inside to reduce the pressure differential, which might not be too costly if the shell is fairly thick and provides thermal insulation. Here again the volume vs. area works in your favor.

Conclusion: Probably Buckminster Fuller could come up with something.

No, because there is no known material (even exotic stuff like nanotubes) that can hold a vacuum AND be lighter than the contained volume equivilant of air.

Some analysis on this topic has been done: https://en.wikipedia.org/wiki/Vacuum_airship#Material_constr...

If your spongy structure is capable of withstanding a half decent vacuum it will likely be much heavier than the volume of that structure filled with air.

But it would still be only marginally lighter than hydrogen or helium, and require a much more sturdy support mechanism.

Neon is actually less dense than Nitrogen. According to Wikipedia:

  Nitrogen..Density at stp (0 °C and 101.325 kPa) : 1.251 g/L
  Neon......Density at stp (0 °C and 101.325 kPa) : 0.9002 g/L

Nitrogen has a lower atomic number than Neon, but on the other hand Nitrogen is diatomic, which I imagine accounts for the greater density.

Neon is insanely expensive however.

But just imagine if you ran some high-voltage current through it..

Yeah lightning strikes could look cool.

Nitrogen itself is not easy to set alight, though.

But it would take an enormous volume of nitrogen to achieve significant positive buoyancy when the nitrogen is enclosed by much of anything. Air's mass is 28.97g/mol, vs. 28g/mol for N2. So, based on my vague recollection of chemistry, that means you only get 1g of container/cargo/engine for every 22.4L of gas enclosed.

> that means you only get 1g of container/cargo/engine for every 22.4L of gas enclosed.

For reference, the Hindenburg carried about 200 million liters of gas. A nitrogen craft would need approximately 1.52 million liters to carry a single 150#/68kg human being, not counting the additional gas needed for the craft itself.

So if you want to build a Hindenburg-size craft to carry about half a dozen people it might actually be plausible!

None. I thought. Methane is also flammable and a vacuum is not a gas and not currently possible.

But wiki says Ammonia (Hard to combust)-

https://en.wikipedia.org/wiki/Lighter_than_air

Off planet you obviously have other options.

But a complex molecule might work I assume?