The Short SA.4 Sperrin (named after the Sperrin Mountains, a range of hills in Northern Ireland) was a British jet bomber design of the early 1950s built by Short Brothers and Harland of Belfast, popularly abbreviated "Shorts". It first flew in 1951. The design had always been a fall-back option in case the more advanced jet designs of the V bombers were delayed, and it was not put into production because these swept-wing designs (such as the Vickers Valiant) were by then available. The Sperrin prototypes were however valuable for research data on large jet aircraft.
The Air Ministry issued a specification on 11 August 1947 B.14/46 for a "medium-range bomber landplane" that could carry a "10,000 pound [4,500 kilogram] bomb to a target 1,500 nautical miles [2,780 kilometers] from a base which may be anywhere in the world", with the stipulation it should be simple enough to maintain at overseas bases. The exact requirements also included a weight of 140,000 lb (64,000 kg). The B.35/46 specification required that the fully laden weight would be under 100,000 lb (45 tonnes), the bomber have a cruising speed of 500 knots (930 km/h) and that the service ceiling would be 50,000 ft (15,200 m). This request would be the foundation of the V bombers.
At the same time, the British authorities felt there was a need for an independent strategic bombing capability—in other words that they should not be reliant upon the American Strategic Air Command. In late 1948, the Air Ministry issued their specification B.35/46 for an advanced jet bomber that should be the equal of anything that either the Soviet Union or the Americans would have. The exact requirements included that the fully laden weight would be under 100,000 lb (45 tonnes), the ability to fly to a target 1,500 nautical miles (2,800 km) distant at 500 knots (930 km/h) with a service ceiling of 50,000 feet (15,200 m) and again that it should be simple enough to maintain at overseas bases. A further stipulation that a nuclear bomb (a "special" in RAF jargon), weighing 10,000 lb (4,500 kg) and measuring 30 ft (length) and 10 ft (diameter), could be accommodated. This request would be the foundation of the V bombers.
However, the Air Ministry accepted that the requirement might prove to be difficult to achieve in the time-scale required and prepared for a fall-back position by re-drafting B.14/46 as an "insurance" specification against failure to speedily develop the more advanced types that evolved into the Vickers Valiant, Avro Vulcan and Handley Page Victor., as this was to be a less ambitious conventional type of aircraft, with unswept wings and some sacrifice in performance. The only significant performance differences between B.14/46 and the more advanced B.35/46 were a lower speed of 435 knots (806 km/h) and a lower height over the target of 35,000 ft (11,000 m) to 45,000 ft (14,000 m).
Under this requirement, the Air Ministry placed a contract for two flying prototypes and a static airframe with Shorts. The design, known initially as SA.4 and later, as the "Sperrin", had more in common with the Second World War designs than the new jet age. It was straight winged, although the leading edge was slightly swept. The engines were mounted in nacelles mid-wing, two engines per wing, with one engine stacked above the other. The airframe was built largely of aluminium alloys with a tricycle undercarriage (nosewheel and two, four-wheel bogies), the nose gear retracting backward and the main gear in the wings towards the fuselage.
The SA.4 was designed for a crew of five: pilot, copilot, bombardier ("air bomber"), navigator and radio operator. The prone bombardier's position was a tube extending forward of the cockpit above the radome; the crew compartment being pressurized. These positions were fitted with opaque nosecones, as the Sperrins were never used for live bombing. An ejection seat and accompanying hatch was fitted for the pilot alone. The three crew positions behind the pilots faced backward with the crew entrance below.
As a possible production aircraft, the Sperrins were built on production jigs, which slowed their construction.
