Rocket Interceptors and the SR53.
The idea of the rocket interceptor came from the Air Ministry at the start of the 50s. It was almost certainly triggered by the explosion of the first Russian atomic bomb. The Air Ministry then realised the threat from the high flying jet bomber carrying nuclear weapons. Defence of cities such as London was well nigh impossible, but defence of UK airfields, and V bomber bases, might have been more practical.
In 1945 the R.A.F. and U.S.A.F. had indisputably the world's most powerful strategic bomber fleets. Yet they were on the point of becoming obsolete, and the factor that was driving them obsolete was the jet fighter. The increase in performance that the jet engine gave rendered the likes of the Lancaster and its derivatives hopelessly vulnerable. If airborne radar and guided weapons are added to that balance, it tilts even further away from the bomber.
One answer, of course, was to build jet powered bombers. The Air Ministry had been aware of this for some time, and before the war had ended, had issued the Operational Requirements that would lead to the V bombers. Similarly, the Americans, while having pushed their propellor driven designs as far as feasible, were also busy designing jet bombers, culminating in the B52, still in service.
In post war Europe, the strategic focus for the Western Allies switched very rapidly from Germany to Soviet Russia. The Soviet Air Force was also a formidable fighting machine, although it had evolved along lines more tactical than strategic. It had, though, on the drawing board, some impressive interceptor aircraft such as the Mig 15.
And the whole strategic equation was rewritten in 1945 with the advent of the atomic bomb. Now it became infintely more urgent to intercept a bomber before it reached its target. And the further problem was that any jet atomic bomber would be flying very high, very fast. The problem was to get an interceptor to that height quickly enough, and to give it sufficient speed differential to be able to manoeuvre into a position in order to be able to attack. It was further realised that such attack would probably be by guided weapons of some form - either infrared, heat seeking, or radar controlled.
Up until the 60s, the bomber's best defence had always been height. The higher the aircraft, the more difficult it is to spot and the more difficult it is to hit with conventional antiaircraft shells. Fir interceptor fighters, the choice was either to loiter at high altitudes, which, given their limited endurance, was not usually a feasible option, or to reach these heights as quickly as possible. In 1950, the performance of the jet engine was not sufficient to do this. The Germans had had some considerable success with their rocket propelled interceptor, the Me163, even though it came into service very late in the war, and was, to put it politely, of idiosyncratic design.
Hence the Air Ministry issued Operational Requirement 301 (see appendix A), and invited several firms - Blackburn, Westland, Fairey Aviation, Saunders Roe and Bristol to submit designs. The main points of the designs was that they should be relatively simple, use rockets for the main propulsion, but a turbojet to get home and land. Not all the designs featured the Spectre: the Avro design, which the R.A.F. preferred, used an engine called Screamer, using liquid oxygen and kerosine as fuels. This lost it some points on the technical side: handling liquid oxygen would not be as convenient as HTP, although LOX could be prepared from portable compressors if need be. However, if I were a pilot, I would not be that keen on flying with several hundred pounds of liquid oxygen behind me.
Most of the designs were reasonably conventional in appearance, although there were one or two oddities. The designs were submitted to the R.A.E., who then 'scored' them on various criteria. In the end, at the Tender Design Conference in July 1952, they chose the Saunders Roe design as being the most suitable. The Avro was the runner up, and both firms were told to go ahead with the idea of providing at least three prototypes. The philosophy at this time was to have prototypes from two competing designs , so that if one proved to be a turkey, then there was a back up. Accordingly, specification F137D was issued to AVRoe and F138D to Saunders Roe.
But the whole project fell foul of defence cuts in 1952, with the whole project under the axe at one stage. However, the SR design was revived, but with the number of prototypes cut from three to two. A preliminary mock up was produced at Cowes, with a conference in September 1953 to evaluate the design. The first prototype SR53, serial number XD145, was first flown on 16 May 1957, and went supersonic on 15 May 1958.
At the same time, work was proceeding on another interceptor fighter, although one of more conventional design. This was the English Electric F23/49, later to be named the Lightning, and in November 1954 there arose a project to fit it with 2 rocket motors each of 2250 pound thrust, using HTP and kerosene, with a burn time of 2.5 minutes. Napier were to provide the motors. Development continued for some years, with test flights of the rocket motors in Canberras, but this rocket augmentation was cancelled in 1958.
But whilst development had gone on with the SR53, the R.A.F. realised its limitations. The main of these was endurance and turbojet performance - it would be better to have something that would do more than just "get you home". Hence a improved design, the F177, was proposed, which replaced the Viper jet of the SR53 with the Gyron Junior. But other events were catching up with the SR53.
When the Air Ministry were considering the project initially, it was only seven years on from the first British jet flight by Whittle, and the jet's performance was not up to requirements. The Ministry were aiming for a development period of around another seven years, and in that time the performance of the jet engine would increase considerably. Indeed, in February 1955, a paper was prepared comparing various configurations of the SR53, the F177 and the F23/49. The latter was a conventional jet aircraft, and although its climb rate was not up to that of the rocket powered aircraft, it was not that far behind.
The second event was the development of guided weapons. In 1950, there was no other way of intercepting high flying bombers. But by the mid 50s, guided weapons such as Bloodhound, radar guided and with a range of fifty miles, were appearing on the scene. Their role was exactly that of the SR53, at a good deal lost cost. Indeed, the threat of guided missiles removed the bomber's previous safety of height, as the downing of Gary Power's U2, which wrecked the summit between Krushchev, Eisenhower and Macmillan. Doing the same with a manned interceptor was still much more difficult, as the Russians were to experience in their interception of Korean Airlines Flight 007 in 1987 - and that target was a Boeing 747 taking no evasive action whatsoever.
The third event was the advent of the ballistic missile. In 1950, the only feasible method of delivering nuclear warheads was by bomber. By 1957, Sputnik had been launched, and ballistic missiles were entering service in the USA and USSR. There was no defence against these weapons.
In other words, the SR53 was obsolete. First the F177 was cancelled, then the SR53, as a result of the 1957 and 1958 Defence White Papers. The Lightning was the only interceptor left to the R.A.F.. This policy was much vilified at the time, but in retrospect, was almost certainly correct.
The two prototypes, now flying, were then kept on for research and development. XD 151, the second prototype, crashed on take off on 6th June, 1958, the pilot being killed. Despite investigation by the Aircraft Investigation Board, the cause was never discovered. The take off was aborted just as the aircraft was leaving the ground, and the rocket and jet engines were cut. The investigation was unable to determine whether this was done by the pilot, or whether they had cut out through accident or failure. But the aircraft was going too fast to stop in the space left, and it hit obstructions at high speed and burst into flame.
After the accident, test flying was continued with XD 145. Sanders Roe put forward quite an ambitious programme for the craft, talking about airlaunching it, and using it with uprated motors, almost as an equivalent to the X-15. But R.A.E. and the Americans, who were providing some of the funds, were less impressed, feeling that the aircraft, although interesting and unique, was not suitable for such a programme. Indeed, Saunders Roe at one stage were talking of it in terms of doing research for re entry studies for a manned satellite, and for this, the SR53 with its aluminium airframe, would definitely have been unsuitable.Eventually in June 1960, the further flying programme was cancelled, and the machine handed over to the R.A.E. Fortunately it has been perserved, and can be seen in the Aviation Museum at R.A.F. Cosford.
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