OR.337 was drawn up to provide the RAF with a fast response interceptor to combat the Soviet high altitude supersonic bomber threat. OR.337 had been an update of the original Air Staff requirement, OR.301, with the inclusion of a Royal Naval aspect. To help with development of such an aircraft Saunders Roe built the SR.53 demonstrator. This was a fairly successful prototype, or in modern parlance, a technology demonstrator but would be useless as a service aircraft as it lacked endurance. The service aircraft was to be selected from two competing designs: the SR.177 and the Avro 720. The main armament of these types would have been theFirestreak Infra Red AAM. These would have been mounted on the wingtips as seen in the photo above.
The mixed powerplant fighter came along at the wrong time, as during the mid 1950s, SAMs like Red Duster were starting to show promise. While the SR.53 would have been able to attack bombers at longer range, a 200mile range SAM called Blue Envoy was in development. Ultimately neither came to fruition as the threat changed from high altitude attacks, to low altitude, a regime where SAMs like Thunderbird and Bloodhound operated well.
The SR.53 was powered by a Bristol Siddeley Viper turbojet and a De HavillandSpectre rocket motor. (The image shows the top chamber of a Double Spectre)
In the event all work on mixed powerplant fighters ceased with the 1957 Defence White Paper and the SAMs took over.