//free\\ | Google Gravity Balloon

Rather than a sphere, Loon used a lobed structure (like a pumpkin) with a tendon network. This shape allows pressure-induced stress to distribute along the seams, not the film. The film itself was a 0.076 mm thick co-extruded polyethylene with a specific UV-resistant additive. The seams were reinforced with load tapes.

Mathematically, the pressure differential (\Delta P) is limited by the meridional stress (\sigma) in the lobes: [ \Delta P = \frac{2 \sigma t}{R_{curv}} ] where (t) is film thickness and (R_{curv}) is lobe radius. By keeping (R_{curv}) small (many lobes), Loon could handle (\Delta P) up to 200 Pa without bursting. Unlike airships or drones, Loon had no propulsion. How do you steer a balloon? You change its altitude to catch different wind currents. The stratosphere has multiple layers of wind moving in different directions (e.g., west-to-east at 20 km, east-to-west at 25 km). 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: Rather than a sphere, Loon used a lobed