Heat provides lift—and must be maintained
A thermal airship works by making the air inside its envelope—the large fabric chamber—hotter than the surrounding air. Heating makes that air expand; because the envelope holds it at roughly the same pressure as the atmosphere, the expanded air is less dense. The airship then displaces heavier outside air, and the difference supplies buoyant lift.
The lift is modest compared with what the same volume of helium can provide. That makes the craft’s weight and the mass of its envelope, burners, fuel, and equipment especially important: all must be supported by the relatively small density difference. The burner is not a one-time starter. As the air cools and contracts, buoyancy falls, so heat has to be added during flight.
A different kind of control problem
Changing the air’s temperature gives the pilot a way to adjust altitude. More heat increases buoyancy; letting the air cool reduces it. But the response is not instantaneous: the large volume of air takes time to warm or cool, and the craft’s motion and the weather also affect where it goes. Propellers can provide forward motion and help steer, while the heated envelope handles vertical lift.
This trades the need to carry a supply of lifting gas for an ongoing demand for fuel and heat. The result is a lighter-than-air craft whose ability to stay aloft depends on managing temperature throughout the flight—not simply filling a bag once and sealing it.
