compound balloon
A variation of the hot-air balloon that is lifted by two separate balloons of different types, usually suspended one over the other.
compound balloon: two balloons, two lift methods, one basket
A compound balloon combines a conventional hot-air balloon with a gas balloon, typically a helium-filled envelope, stacked vertically above it. The hot-air section provides controllable lift through burner heat, while the gas balloon offers steady supplementary lift. Both envelopes are tethered to a shared basket or frame, creating a hybrid system that exploits the advantages of each method: the gas balloon's passive buoyancy for sustained altitude, and the hot-air balloon's active thermal control for precise descent and landing.
In typical configurations, the helium balloon sits above the hot-air envelope. The gas balloon may be smaller than the hot-air balloon by a significant margin, often contributing 15 to 30 percent of total static lift, while the hot-air section provides the majority. Connection points between the two must distribute loads evenly; stress concentrations at attachment fittings are a primary failure mode. The pilot manages both systems independently: the burner for the hot-air balloon and ballast or a parachute vent for the helium envelope.
Operational advantages include extended flight duration, higher ceiling capability, and reduced fuel consumption compared to a pure hot-air balloon of equivalent payload. The gas balloon's constant lift reduces the burner duty cycle, which saves propane and extends flight time. Disadvantages include greater complexity in rigging, higher initial cost, regulatory scrutiny (gas balloons face stricter envelope certification), and increased weight aloft. Recovery and packing become more time-intensive after landing.
Maintenance priorities differ between the two envelopes. Hot-air fabric deteriorates from heat exposure and UV damage; inspection focuses on seams, corner attachments, and the crown. Gas balloon envelopes, typically polyurethane or polyester laminates, suffer from pinhole leaks, weld seam failures, and degradation at load tapes. Both require dedicated crew training: mismanagement of the gas balloon's pressure during descent can rupture the envelope or cause rapid deflation.
The term is rare in modern aviation and appears mainly in historical records and experimental craft documentation. Most contemporary long-duration ballooning relies on either pure hot-air systems with improved burner efficiency or pure gas balloons with envelope innovations. Compound balloons remain a niche configuration, built occasionally by enthusiasts or for specialized altitude or endurance records.