That’s where the vacuum comes in. Extract half of that hot air, seal the container, and you have 70% of the helium lifting force. (Math may be wrong there but y

That’s where the vacuum comes in. Extract half of that hot air, seal the container, and you have 70% of the helium lifting force. (Math may be wrong there but you get the idea) Really you don’t need hot air at all. If you have a massive vacuum contained by a rigid shell of some super light super strong material you’d be set. Density of air at sea level is around 1 kg/m^3, so 100 m^2 of material lifts 3 tons. Assuming one ton for material, one for propulsion, one for cargo, you end up needing a shell material that can weigh up to 1 kg/m^2 and can hold a near vacuum. Like I said earlier, “just” and engineering problem. The good news is economies of scale are on your side: 10000 m^2 of shell can lift 3 thousand tons. At that scale your shell can be up to 300 kg/m^2. A bit more reasonable.