Avro 730

The Avro 730 was a planned Mach 3 reconnaissance aircraft and bomber for the Royal Air Force. If it had proceeded into service, the aircraft would have replaced the V bombers as the primary delivery system for Britain's nuclear deterrent. It was cancelled in 1957 along with other development on manned aircraft as part of the 1957 Defence White Paper.

Development
During the early Cold War, the RAF bomber fleet of V bombers was given the nuclear deterrent role for Britain. The need for a very-long range supersonic strategic reconnaissance aircraft to support the V bombers both pre- and post-bombing was identified; an Operational Requirement, OR.330, for such an aircraft was identified and a specification was drawn up in 1954 for an aircraft that would be capable of entering the Soviet Union and avoiding air defenses. The aircraft envisaged would have to be capable of maintaining Mach 2.5 at 60,000 ft (18,300 m), with the ability to reach Mach 3, and operate at a maximum range of 5,754 mi (9,260 km). At that point in time, it was the most ambitious high-performance aircraft internationally to date.

There were three major submissions from the aircraft manufacturers: the Handley Page HP.100, Vickers SP4, and the Avro Type 730. All were futuristic delta or needle shapes in appearance employing multiple engines, 12 on the HP.100, 16 mounted horizontally at the rear of the Vickers. Avro were given a contract in mid-1955 to develop their submission aircraft. As an aid to development, the Bristol Type 188 aircraft was built to test the wing shape and later the effects of prolonged supersonic flight on metal. Up to 10 prototypes of the aircraft were proposed, necessitated in part due to the change to incorporate a bombing capability into the aircraft.

The first prototype was to have been designated Avro 731, a three-eighths scale test aircraft, it had been scheduled to fly in 1959. The prototype was under construction when the minister, Duncan Sandys, announced the decision to cancel its development. It had been suspected that by the time the aircraft came into service a decade later, it would have been already vulnerable to Soviet advances in anti aircraft missile technology. The Bristol 188 project continued despite the cancellation of the 730. Aspects and influences of the Avro 730 had encouraged studies at the Royal Aircraft Establishment, Farnborough, into supersonic transport aircraft, which in turn would eventually contribute to the development effort behind Concorde.

Design
The Avro 730 was a tail-first aircraft, an approach which greatly reduced trim-drag while increasing lift at slower speeds. The aircraft had a long, thin fuselage with a high fineness ratio; a small tapered almost-rectangular wing was mounted centrally on the fuselage. Four Armstrong-Siddeley P.156 engines were carried, two each mounted over-under in pods at the extreme tips of the wings. No conventional canopy was fitted in order to maintain the fineness ratio, the cockpit featured only two small windows facing to the side, and used a retractable periscope for viewing during take-off and landing. A crew of three would be carried: pilot, navigator and radar operator.

This initial version was intended strictly for the reconnaissance role, using its "Red Drover" sideways-looking radar to find targets for attack by the V bomber force that would follow. As development progressed it became clear that the radar would not need as big an antenna as initially believed, freeing up considerable internal room. In response, the RAF started concentrating on the secondary bombing role carrying both the radar and also including a long bomb bay for either a weapon or additional fuel. A high-speed bomber requirement was also being studied at the time, OR.336, so the two projects were combined into the new RB.156 requirement. This led to a fairly major redesign.

Although the new version looked much like the original, it was larger overall and featured a new wing planform. In order to increase wing area extra "winglettes" were added outside of the engine pods and the entire planform was re-shaped to be more of a classic delta wing. The wing inside the engine pods, about ⅔ of the overall span, was swept at about 45°, the smaller area outside was more highly swept at about 60°. The forward sweep on the trailing edge was removed. The engine pods were now specified to carry four Armstrong-Siddeley P.176 engines each, for a total of eight. The pods were circular at the front and mounted a single large shock cone, and grew progressively more "square" to the rear, where they ended flush with the rear of the wing. The rest of the layout was generally the same as the earlier version, with the rectangular canards, "hidden" cockpit and large cropped-delta vertical fin at the extreme rear.

The new version also had reduction in crew to two members. The bomb bay was narrow but very long at 50 ft (15 m), and was intended to be armed with a nuclear-tipped stand-off missile. A suitable warhead started development as Blue Rosette.