Blue Envoy (a Rainbow Code name) was a British project to develop a ramjet-powered surface-to-air missile. It was tasked with countering supersonic bomber aircraft launching stand-off missiles, and thus had to have very long range and high-speed capabilities. The final design was expected to fly at with a maximum range of over .
Development started as Green Sparkler sometime in the early 1950s. Green Sparkler featured active radar homing, but it was quickly decided this was beyond the state of the art. Replacing the active homing with semi-active radar homing produced Blue Envoy. The designs were otherwise similar, and similar to the US CIM-10 Bomarc as well.
Test launches of sub-scale models were carried out successfully, and development of the new ramjet engines and seeker electronics was well advanced when the project was cancelled in April 1957 as part of the 1957 Defence White Paper. Its cancellation made Blue Envoy "possibly the most enigmatic project in the field of 1950s United Kingdom weapons development."
An impromptu meeting between the contractors led to a proposal to use the guidance system and ramjets to upgrade the Bloodhound Mk. I missile design. This private proposal was accepted and became Bloodhound Mk. II, which increased range from to and offered much greater performance against low-level targets and radar jamming efforts. The Bloodhound Mk. II would ultimately serve as Britain's primary air defence missile into the 1990s.
History
ROTOR plan
During the late 1940s a series of events prompted the complete reformation of the British air defence system. This led to ROTOR, which was designed to provide widespread radar coverage of the entire British Isles and defend that airspace using a combination of interceptor aircraft and anti-aircraft artillery.
In 1953, as part of continual modifications to the ROTOR concept, the anti-aircraft artillery was to be replaced by surface-to-air missiles (SAMs), or as they are known in the UK, surface-to-air guided weapons (SAGW). As SAMs were new technology, it was planned these would be deployed in two stages, an interim Stage 1 design with range on the order of , and some time after that, a greatly improved Stage 2 missile with much longer range.
Two designs were entered for the Stage 1 missile contract, English Electric's Red Shoes and Bristol Aerospace's Red Duster. The Royal Aircraft Establishment (RAE), who was in overall control of missile development, was interested in seeing ramjet propulsion developed, and suggested Red Duster move to this form of power. Otherwise the two systems were very similar designs, even sharing the same Marconi designed radar systems. Red Shoes emerged as the ranged Thunderbird, while Red Duster became the ranged Bloodhound.
Green Sparkler
Work on the Stage 2 missile did not begin until sometime later, initially under the name Green Sparkler. Stage 2 was tasked with effectively countering bomber aircraft flying at supersonic speeds at very high altitudes that were potentially launching stand-off missiles from hundreds of miles range. In order to stop these attacks before they reached their launching points, the missile had to have long range. This, in turn, demanded high speed as there would only be a short time between detection on radar and the aircraft reaching their launching areas.
Green Sparkler had a design range of over . This was beyond what could be effectively guided using a semi-active radar homing like the ones used on Thunderbird and Bloodhound. Instead, Green Sparkler used command guidance for much of the mission, switching to an active radar seeker in the last of the approach. Two seekers were considered, one using a continuous wave radar with separate transmit and receive dishes in the nose, and another using a pulse doppler radar using a single dish. Both were to also offer home-on-jamming.
Some consideration was given to using the US's BOMARC for the Stage 2 role. This was ultimately rejected; while the BOMARC had the desired active radar seeker, it was (at that time) a simple non-doppler pulse unit that would be very easy to jam using the recently introduced carcinotron. They also considered the range excessive, given that the AMES Type 80 radars that would be providing initial aiming had a range of just over 200 nmi, meaning this very large missile's range performance would be somewhat wasted.