DaveC426913 said:
But it's also incredibly strong. Is it strong enough to form a lattice for a dirigible that's filled with vacuum?
Yes and no.
Having investigated the possibilities previously, the first "vacuum airship" will actually be a reduced-pressure, a spherical "depression airship", not a true vacuum airship. I will see how much I can remember.
As a first consideration, the internal pressure will only be reduced by less than one psi below the local atmospheric pressure. That limit will reduce the peak stress on the envelope and structure. It also dictates the pressure of the maximum ceiling altitude, at which it does become a true vacuum airship. The differential pressure will also dictate the size, as wind pressure from gusts could approach the differential envelope pressure. I would expect turbulence, or a storm gust to create an external ram air pressure of maybe half a psi, that should not cause a collapse.
An initial spherical prototype may have a hollow core. There is no need for structural compression at the centre of the sphere, if those forces can be diverted, to flow around a thick-walled, hollow internal structure.
There will need to be an impervious envelope that is not under as much hoop tension as a party balloon. That envelope should be a stretchy film, that hangs from the asperities, of the convex hull, of the foam structure. Each bubble in the foam surface, will cause a local spherical depression of the envelope film, (like a golf ball, but for a different reason).
The imperviousness of the envelope is relative, as the evacuation pump needs to handle the leakage gas flow across the membrane at the maximum differential pressure of one psi.
As the evacuation pump extracts air from inside the envelope, the airship will rise. As air is allowed to reenter, the airship will descend. If it falls, air will need to pass into the airship, through a safety valve, automatically maintaining the one psi maximum differential pressure. That is to prevent a structural implosion.
The internal air can probably not be dry, so the weight penalty of liquid water must be avoided. Condensation of liquid water will occur on all the internal foam surfaces, unless that is engineered, hydrophobic, and managed correctly.
Water vapour has a lower density than dry air, so the elimination of water from the internal gas-mix could be counterproductive. That raises a possible objection, that the airship contains a lighter than air lifting gas, namely water vapour, (cold, but not condensed, steam).