How did the Upkeep bomb work?

Short answer

Upkeep weighed 9,250 lb (4,200 kg), of which 6,600 lb (3,000 kg) was Torpex. It was dropped at 60 ft (18 m) above the water with a backspin of 500 rpm: the mine skipped across the surface until it struck the dam wall as its forward speed ceased. No. 617 Squadron used it against the Ruhr dams in May 1943.

Published: November 5, 2026 · Updated: November 5, 2026 · Edited by Above Ten Thousand · Editorial policy

Lancaster B Mk I R5852 of No. 83 Squadron in flight over Lincolnshire, June 1942

A weapon designed to bounce

Upkeep was a mine with a total weight of 9,250 lb (4,200 kg). Of that figure, 6,600 lb (3,000 kg) was Torpex, the explosive charge. The weapon was held inside the aircraft by a large pair of calipers, triangulated supporting arms that opened from each end of the mine to release it. Before release it was given a backspin of 500 rpm.

The release profile

The drop was made precisely at 60 ft (18 m) above the water's surface. From that release point, the mine skipped across the surface of the water. It ended up striking the dam wall when its forward speed ceased.

The night of 16-17 May 1943

On the night of 16-17 May 1943, Operation Chastise attacked dams in the German Ruhr valley using Upkeep. The unit involved was No. 617 Squadron. Two of the dams were breached and caused widespread flooding, damage and loss of life. Of the 19 attacking aircraft, eight did not return, taking with them 53 of the 113 RAF crew members.