The Bristol Hercules was a 14-cylinder sleeve valve radial engine designed by Sir Roy Fedden and produced from 1939 by BAC. It powered Bristol's own Beaufighter but was more commonly used on bombers. From the 1375 hp Hercules I to the 1735 hp Hercules XVII produced late in the war, the variants powered the Avro Lancaster B.II, the Handley Page Halifax, the Short Stirling, the Vickers Wellesley and the Vickers Wellington, among others. The sleeve valve engine was an efficient configuration that allowed the use of lower octane fuels for the same compression ratio. It was clever, and it worked. Gordon Wilson provides a biography of this ingenious workhorse, designed and modified under the pressure of wartime, He has had exclusive access to the restoration of a Handley Page Halifax, which has provided picture details unavailable elsewhere.