The Double Spectre rocket engine propelled Blue Steel test vehicles. The operational Blue Steel stand-off bomb carried Britain's nuclear deterrent between 1964 and 1975. Each bomb was carried beneath a Vulcan or Victor aircraft, air-launched at altitude and then propelled by a Stentor rocket engine to accelerate to a maximum velocity of Mach 2. The Double Spectre burned kerosene fuel in hydrogen peroxide. It consisted of two combustion chambers: the upper being capable of variable thrust; the lower of fixed. Single Spectre engines were developed in the 1950s for the experimental rocket-powered SR-53 interceptor aircraft.
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