Designs in Detail The F138/SR53
As mentioned, the original Operational Requirement in July 1951 did not specify a jet engine; the aircraft was to be purely rocket propelled. A meeting of the OR Committee at this time said that ‘it would be a very local defence weapon’ and so ‘The possibility of fitting a small turbine to assist the landing was then discussed but ruled out on the score of weight’. The armament was to be a battery of 2-inch air-to-air rockets. As an indication of the number of aircraft firms that there were at that time, tenders were invited from Bristol, de Havilland, Fairey, AVRoe, and Short Brothers. Copies for information were sent to Armstrong Whitworth, Blackburn, Boulton Paul, English Electric, Gloster, Percival, Saunders Roe, Supermarine, Westland, Folland, Handley Page and Scottish Aviation!
In the minutes of the F.124 tender Design Conference in July 1952, it was announced that ‘General agreement was reached that only the Saunders Roe, AVRoe and Bristol A design remained in the competition..and that ‘Saunders Roe had submitted very good designs on two previous occasions and he felt that their design team were so good that it would be a mistake for it to be disbanded as would be the case unless the firm received a contract soon.’
There are two curious points about this: the fact that it had taken a year to evaluate the designs, and the comment on the Saunders Roe design team: contracts were often awarded for seemingly obscure reasons. The recommendation was made that three prototypes be ordered each from Avro and from Saunders Roe.
However, the huge increase in the defence budget in 1950 could not be sustained, and economy again became the watchword. This meant that three prototypes could not be afforded; despite RAF preference for the AVRoe design, the Ministry of Supply decided to press on with the Saunders Roe design, but with only two prototypes.
But there is an interesting comment from the OR committee sometime later, in June 1953:
… the changes in requirement that have been brought in from time to time have moved the design some way from the basic conception of a simple rocket aircraft – and there is some danger, in my opinion, that the final weapon will be less effective than it might be.
Indeed, in a sense this remark could be said to be the essence of the whole story.
However, work progressed rapidly with the SR53: in October 1952 there was a structures meeting between Saunders Roe and the RAE, and a preliminary mock up meeting in Cowes in September 1953.
The final delay in the completion of the first aircraft, XD 145, was delivery of the Spectre I motor: this could not be delivered to Cowes before mid-December 1955. The motor had earlier been installed in Canberra for flight trials. By mid- June 1956 the aircraft was completed, then dissembled for transport to the Aeroplane and Armament Establishment at Boscombe Down. Here it was put together again, and the first rocket engine firing was on 16 January 1957. The first flight took place on 16 May. The second aircraft, XD 151, was first flown on 18 December 1957.
The SR53 was intended to be a lead in for the P177, and the cancellation of the P177 meant that it had lost its purpose. However, it was felt that in many ways that the SR53 was a unique aircraft that could be used for aerodynamic research, rather as the X planes in America. Accordingly, proposals were put forward for enhancing the performance2. It was estimated that the Spectre 1 rocket motor in its then state of 7,000 lb thrust, and S. I. of 190s gave a maximum velocity of Mach 1.8 at 60,000 ft and a maximum height of 76,000 ft. Thus a meeting in May gave some options for further development:
(i) Spectre 5 engine 95,000 ft or M1.8;
(ii) Twin Spectre 125,000 ft or M2.3;
(iii) Spectre 5 and airlaunch at 40,000 ft and M0.8 gives 115,000 ft or M2.9.
The Spectre 5 was an improved version of the rocket motor; the Twin Spectre, as its name suggests, stacked two such motors together.
Figure 31 shows various scenarios for improving the performance of the SR53, and the results that might be obtained.3 It must be said, however, that some parts of this scenario do look a little optimistic. A further problem was that the SR53 as designed was not that well suited for the task, and most of this kind of research had already been done by 1958. As it stood, there were constraints on the airframe, being all aluminium. Kinetic heating meant that certain key parts of the airframe would have to be replaced by stainless steel, and work was done on this by the design team at Osborne House on the Isle of Wight.
Several scenarios were sketched out: with no armament, more fuel could be carried. The jet engine could be removed, and with a more powerful rocket engine and still more fuel, the flight envelope could be extended further. Also suggested was the use of solid fuel Mayfly rockets to assist take-off, and, in the piece de resistance, air launching from a Valiant was proposed. All this would have made XD154 Britain’s answer to the X-15! The Valiant could take the aircraft up to 40,000 ft at a speed of Mach 0.8. Unlike the American X-15, which was underslung, the proposal was to mount the SR53 on top of the Valiant.
In the meantime, flight trials went ahead with XD145 flying supersonic for the first time at around 45,000 ft on its 31st flight, in May 1958. But disaster overtook the programme in June when XD151 crashed during an aborted take-off on its 12th flight. A long and thorough investigation followed. Here is an excerpt from the report4.
TRUE SPIKED – FT./SEC.
Figure 31. The SR53 as a research aircraft – although some of the ideas do seem on the optimistic side for such an aircraft.
The aircraft taxied out at 1200, and the Spectre was started at 1203. Approximately 5 seconds elapsed before the engine went ‘hot’ but this is understood to be normal. The aircraft lined up on the runway and after cockpit checks were completed, and 10° flap selected the aircraft commenced to take off. The aircraft accelerated normally and the nose wheel was raised. About 30 seconds after the pilot had reported ‘hot’, he was heard to call ‘Panic Stations’ and then a moment later, ‘Come and get me will you’. The anti-spin parachute was seen to stream, but the aircraft ran off the end of the runway. Upon impact with a runway marker light pole, a chain fence with concrete posts and finally a large marker light the aircraft broke up and caught fire. The pilot was killed.
An analysis of the film shows that the take-off was abandoned at a very critical stage when the aircraft was half way down the runway and on the point of becoming airborne. The rocket is shown to have ceased to run hot i. e. no flame, at this point. It could not be determined whether or not the rocket was deliberately throttled back by the pilot. There is no sign of the airbrakes having been opened and the aircraft left the runway at an estimated speed of about 145 knots.
The aircraft was on its take-off run, just becoming airborne, when the rocket motor abruptly cut. Whether the pilot took this action was never established, like so many other details of the accident. The aircraft braked hard, but overran the runway. It might still have survived but for a wing catching an obstruction, a runway lamp post. As the aircraft disintegrated, the rocket fuel ignited in a fierce conflagration. No cause for the accident was ever established, and there was no evidence of pilot error. Saunders Roe’s chief test pilot, John Booth, was killed in the accident.
With the investigation producing no clear result, the flight test programme continued, and XD145 made a total of 56 flights, or 22 hours flying time. Peter Lamb, Booth’s successor, described the SR53 as ‘an extremely docile and exceedingly pleasant aircraft to fly’, which, given the kick the rocket engine must have produced, says a lot about the aircraft. It reached a maximum speed of Mach 1.33, not an exceptional speed, altitudes of up to 55,000 ft, but certainly lived up to expectations with a climb rate of 29,000 ft per minute.
By the time the Ministry had decided to go ahead with the possible research project, Saunders Roe had been taken over by Westland. Westland’s policy was to drop fixed wing aircraft development to concentrate on helicopters (and Saunders Roe would later become, for a time, the British National Hovercraft Company). Saunders Roe’s Chief Designer, Maurice Brennan, responsible for all the fixed wing designs, had moved to Hawker Siddeley. The Ministry talked to Hawker Siddeley, but concluded that
… it was not clear for some time whether Westland would be willing to take on this work using the existing Saunders Roe team for the purpose, but they eventually decided to concentrate their activities on helicopter work and decline all fixed wing business. We had no alternative but reluctantly to accept their decision. However, Saunders Roe’s Chief Designer had by this time joined Hawker Siddeley and asked the latter to consider taking the job on. Having examined the matter with them, we reached the conclusion that we could not obtain by this means the programme of work that we wanted within the amount we had set aside for it.
The programme was finally closed in July 1960.
XD154 was set aside at RPE Westcott, and fortunately has been preserved. It is now in the Aerospace Museum at Cosford, with many other famous prototypes including the TSR 2 and Bristol 188.