Vehicle envelope with lightweight ultrafilm for minimal leakage (VELUM)

Indoor inspection and mapping missions in tunnels, industrial facilities, and subterranean environments require aerial platforms capable of long-duration operation in cluttered, humid, and navigation-denied conditions. While multirotor drones provide high maneuverability, their endurance and payload capacity are fundamentally limited by battery-powered lift. Small indoor lighter-than-air vehicles alleviate this constraint through buoyancy; however, at meter-scale volumes, envelope materials become a critical limitation, as they largely determine system mass, gas retention, durability, and resistance to handling and collisions. Commonly used films present persistent trade-offs: metallized polyester offers low gas permeability but limited mechanical robustness, whereas polyurethane is more durable but heavier and more permeable. This work introduces and experimentally validates a lightweight composite envelope material based on low-density polyethylene combined with a fluoro-siloxane barrier coating. The proposed treatment significantly improves helium retention while preserving flexibility and resistance to handling. Mechanical and functional testing show that the coated material achieves substantially higher tear resistance than metallized polyester and improved durability compared to polyurethane, while remaining considerably lighter. A cylindrical airship fabricated from this membrane was deployed in a semi-autonomous underground mapping mission, demonstrating reduced helium leakage, stable performance in humid conditions, and multi-day operation. These results show that fluoro-siloxane-coated polyethylene enables lightweight, durable, and gas-efficient envelopes, supporting persistent indoor operation of small lighter-than-air aerial platforms.