Short Sperrin

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.

Design and development
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 140000 lb. 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 kn 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&mdash;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 1500 nmi distant at 500 kn 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 10000 lb 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 kn and a lower height over the target of 35000 ft to 45000 ft.

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.

Testing
The first prototype (serial VX158), powered by four Rolls-Royce Avon RA.2 engines of 6000 lbf of thrust and piloted by Tom Brooke-Smith, had its maiden flight on 10 August 1951. By this time, in the light of the latest knowledge, and the fact that the Valiant project was now proceeding well and only six months behind the Sperrin the judgement of the Air Ministry was that an insurance project was now no longer needed, and a decision was taken to order the Vickers Valiant instead of the Sperrin and the Sperrin project was cancelled, although the Ministry of Supply determined that the Sperrin would serve as a research aircraft. Work on the two prototypes was continued, with the second prototype (VX161) flying on 12 August 1952 with Sqn Ldr "Wally" Runciman at the controls, accompanied by Flight Test Development Engineer Malcolm Wild. It was fitted with more powerful Avon RA.3s of 6,500 lbf (28.1 kN) thrust.

The two Sperrins were used in a variety of research trials through the 1950s, including engine tests using VX158 as a testbed for the de Havilland Gyron turbojet - a large engine delivering 15,000 lbf (66.7 kN) thrust. The Gyron Gy1 replaced the lower Avon in the port nacelle (see image). For the first flight with this engine configuration on 7 July 1955. VX158 was piloted by Jock Eassie and Chris Beaumont. Testing with this asymmetric engine configuration continued until March 1956, when the single Gyron Gy1 was removed and two Gyron Gy2 engines, each providing 20000 lbf thrust, were fitted, one in each engine nacelle below the original Avon RA.2s. The first flight of VX158 with the new engine configuration took place on 26 June 1956, again with "Jock" Eassie and Chris Beaumont at the controls. During this flight the port outer undercarriage cover fell off; VX161 was flown over from Farnborough and its corresponding cover was used to repair VX158. VX161 never flew again and was scrapped at Sydenham in 1957. VX158 was flown at the Farnborough Airshow in 1956 with two Avons and two Gyrons fitted but "six months later the Gyron programme was discontinued and VX158 was scrapped at Hatfield in 1958.

A photograph of VX158 with both Gyrons fitted can be seen in C.H. Barnes' and D.N. James' definitive work, "Shorts Aircraft since 1900".

Among other test work, VX161 (which had a fully operational weapons bay) was involved in trials relating to bomb shapes with mock-ups of the Blue Danube nuclear bomb and the Blue Boar television-guided glider bomb.

Specifications first prototype


Across the two aircraft, the Sperrin had four different engine configurations:
 * 1) Four Rolls-Royce Avon RA.2 turbojets of 26.6 kN (6,000 lbf) thrust each: VX158
 * 2) Four Rolls-Royce Avon RA.3 turbojets of 29.0 kN (6,500 lbf) thrust each: VX161
 * 3) Three Rolls-Royce Avon RA. turbojets of 26.7 kN (6,000 lbf) thrust each (two on the starboard wing, one in the upper part of the port engine nacelle) and one de Havilland Gyron Gy1 turbojet of 66.7 kN (15,000 lbf) thrust in the lower part of the port engine nacelle: VX158
 * 4) Two Rolls-Royce Avon RA.2 turbojets combined with two de Havilland Gyron Gy2 turbojets of 89 kN (20,000 lbf) thrust each: VX158