Operation Outward - Design

Design

The balloons had no guidance control and operated only on a timing fuse. Each balloon was about 2.4 metres in diameter and carried a simple timing and regulating mechanism. A double-walled can contained mineral oil in an inner chamber and a roll of hemp cord and piano wire in an outer chamber. At deployment, a slow-burning fuse was lit, calibrated to the estimated time to arrive over German-controlled territory. At launch the balloon rose and expanded in size until an internal cord tightened, releasing some gas and preventing further increase in altitude beyond 25,000 feet (7620 m); the balloon would begin a slow descent. After several hours, the fuse would burn through the cord holding the trailing wire. The payload consisted of about 200 metres of light hemp cord secured to the balloon at one end and tied to about 90 metres of steel wire at the other. This would unroll as the balloon sank to working altitude of about 300 metres. A stopper on the canister of mineral oil was also released, so that it would slowly drip out and lighten the load on the balloon, to assist in maintaining altitude.

The plan was that the wire tail would be dragged for many miles (kilometres) across the countryside, eventually encountering a high-voltage transmission line. A phase-to-phase short circuit would be initiated; during trials, arcs 4 metres long were initiated by the piano wire. The arc would burn for some time before the transmission line protection operated; there was a good chance the circuit breaker itself would be damaged, and the line conductors might burn through due to arcing, causing a line to collapse and require repairs. German efforts to protect transmission lines from attack were unsuccessful; neither a new type of line conductor clamp, nor different overcurrent protection settings had any useful effect.

Just less than half of the Operation Outward balloons carried wire; the rest carried an incendiary bomb intended to set fires in forests.