The manufacturing principle of hot air balloon was simple. There were only obstacles they needed to overcome: the combustion equipment and airbag.
The first difficulty was the hot air balloon must depend on firewood or charcoal to heat the air without the pressure vessel providing the gas. Never mind the low efficiency, the balloon would not be able to fly long distances. That was also why people still choose to use hydrogen balloons in actual combat when they had already discovered the manufacturing principle of hot air balloon. Roland gave up on producing the combustion equipment and relied on Anna to do the heating work.
