Loon required —a fully sealed, rigid envelope that maintains internal pressure higher than the external atmosphere at all times. The challenge: as the sun heats the balloon, internal pressure rises, stressing the polyethylene film.
Project Loon was born from a counterintuitive question: What if the cell tower floated? google gravity balloon
Loon’s envelope used helium. To lift a 15 kg payload (electronics + batteries) plus a 15 kg envelope, the balloon required displacing ~30 kg of air. At 20 km altitude (pressure ≈ 50 hPa), the volume needed is: Loon required —a fully sealed, rigid envelope that
That’s a sphere ~8.7 meters in diameter—roughly a tennis court’s width. The final Loon balloons used a pumpkin-shaped lenticular envelope to reduce drag and manage stress. Traditional weather balloons are zero-pressure : they have an open duct at the bottom. As gas expands (daytime heating, rising altitude), excess vents out. At night, the balloon contracts and descends. This is fine for a 2-hour radiosonde flight but disastrous for a 100-day mission. Loon’s envelope used helium