The over-under engine arrangement featured an Armstrong Siddeley "Viper" turbojet engine fitted to the upper compartment and a de Havilland "Spectre" rocket booster in the lower compartment. The former was rated at 1,640lb thrust output with the latter adding 8,000lb of thrust to the mix. Because of the combination powerplant scheme in play, much of the fuselage and some of the wings were used to house the needed fuel reserves - a kerosene-based compound was used for the turbojet and a volatile "High Test Peroxide" (HTP) mix for the rocket motor. To aspirate the turbojet there were smallish intakes set to either shoulder of the pilot just outside of the cockpit aft framing. Despite the output power of the Viper, the aircraft would still require support from the Spectre rocket for take-off actions.
Dimensions of this impressive aircraft equaled an overall length of 45 feet, a wingspan of 25 feet and a height of 10.9 feet. Empty weight was 7,400lb against a Maximum Take-Off Weight (MTOW) of 18,400lb. Its primary limitation, aside from its dangerous fuel mix and high operating speeds, was its seven minute endurance window offered from the rocket booster - a failing of the wartime Me 163 as well.
The Program Falters yet Progresses
In a relatively short amount of time, the SR.53 program hit several snags: changing technologies and battlefield requirements worked against the slowly-gestating design which was also being delayed due to its inherent complexity. Such was the slow progress of the program that the third flyable prototype, XD153, was cancelled outright in January of 1954 (as mentioned earlier, the Avro Model 720 joined it in 1955). Taxiing trials with the XD145 prototypes did not begin until May of 1957 and a first-flight ultimately followed on May 16th of that year. XD151 was then showcased through several flybys over the heads of amazed onlookers at the Farnborough Air Show that September.
In May of 1958, XD145 was flown supersonically (Mach 1.14) for the first time. In June of 1958, an aborted take-off of XD151 cost both the aircraft and its pilot. Unable to stop, the aircraft ran over the end of the runway and hit a pole before being permanently stopped by a concrete fence. This force of impact then led the full fuel tanks to rupture and ignite. In all, this example recorded just 5.15 hours in the air covering eleven total flights.
This fatal disaster forced the grounding of prototype XD145 which was not allowed to go airborne again until February of 1959 and, even then, there were restrictions placed upon it to not have a repeat of XD151's encounter. In September of 1959, Mach 1.48 was achieved.
All told, the SR-53 held exceptional performance traits for its time: Its maximum listed speed reached Mach 2.1 - 2.2 and the service ceiling was 67,000 feet with a rate-of-climb of 52,800 feet-per-second.
The End of the Road
From the outset, the SR.53 was intended as the production-quality interceptor the Royal Air Force (RAF) sought but the design was eventually downgraded to an experimental platform for data-collecting. The P.177, which was to have some commonality with the SR.53, was now moved into the prime position to become an operational form one day. It was to be a dimensionally larger and more powerful, radar-equipped interceptor should it have come to fruition. However, waning interest on the part of authorities meant that the P.177 initiative was ended in 1957 following a British defense review stating that unmanned aircraft should now be the primary consideration for its fighting services.
With the death knell struck against the P.177, there stood little value in retaining the now-wholly-experimental SR.53. The aircraft was spared from the scrap heap for a little longer and used in continued flight testing due to its excellent performance capabilities (particularly in reaching up to high altitudes). Forty-six total flights were had by XD145. Despite its dangerous qualities, pilots remembered their time in the XD145 with fondness, stating its good control feel and stability. XD145 was eventually saved from scrapping and preserved as a museum showpiece (Royal Air Force Museum Cosford).
At the end of the day, there proved little need for a mixed powerplant interceptor in a world where missile technology had advanced to the point that it would drive strategic bombers to low-altitude flight paths in time. Furthermore, jet technology advanced along its own lines well enough which further negated rocket-powered interceptor forms. As such, the SR.53 was something of a limited technological accomplishment for its time, becoming nothing more than a footnote in the deep history of British aviation as a new age of warfare dawned.
Other Western powers, including the United States and France, attempted mixed-powerplant interceptors but no one form ever made it to operational status. The aforementioned Avro Model 720 was completed only in a wind tunnel model, showcasing a planform not unlike the much larger passenger-minded Concorde airliner still to come. The Republic XF-91 "Thunderceptor" (detailed elsewhere on this site), with its strange-looking inverse tapered wing mainplanes , was one American contribution to the field and two prototypes were built for the program. The SNCASO "Trident" and SNCASE "Durandal" were notable French contributions - both are detailed elsewhere on this site.
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