The Avro 684 was a proposed British high altitude heavy bomber of the Second World War, based on Avro's successful Lancaster.[1]
Due to the increasing vulnerability of contemporary heavy bombers, the need to fly at ever higher altitudes to avoid anti-aircraft fire and fighter interception became paramount. Avro developed the Lancaster to fulfil the requirement for such a high altitude bomber completing a brochure in August 1941, for the Avro 684 Stratosphere Bomber. Roy Chadwick decided to design a bomber that would dispense with defensive armament and rely on altitude and speed to evade enemy defences and was essentially a Lancaster with the nose section containing a pressure cabin (similar to the cabin from the Vickers Wellington B Mark X) and a large 'chin' mounted air intake and heat exchanger assembly. Pressure in the cabin was to be maintained by a Rotol blower at the 10000feet equivalent while at 40000feet.[1]
To achieve this performance, Chadwick used an engine arrangement called the 'Master-Slave' layout. The four wing-mounted Rolls-Royce Merlin XX engines driving four-bladed 13abbr=onNaNabbr=on diameter Rotol propellers, were to be supercharged by a fuselage-mounted slave Rolls-Royce Merlin 45 engine driving a large supercharger supplying pressurised air via an intercooler. The output from the slave engine was to be controlled to supply air at 20000abbr=onNaNabbr=on equivalent pressure at altitudes between 20000abbr=onNaNabbr=on and 40000feet.[1]
At an all-up weight of 60,000 lb the Avro 684 would be capable of reaching a service ceiling of 42,000 ft,on the mission return reaching 49,600 ft.
Due to the pressure of constant developments to the Lancaster and design work on the York transport derivative of the Lancaster, design of the 684 was suspended.