It was tasked with countering supersonic bomber aircraft launching stand-off missiles, and thus had to have very long range and high-speed capabilities.
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.
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.
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.
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.
Instead, Green Sparkler used command guidance for much of the mission, switching to an active radar seeker in the last 10 miles (16 km) of the approach.
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.
[3] Although Blue Envoy, and Green Sparkler, shared many broadly similar features with the Red Duster, it was an entirely different design in detail.
In order to deal with the skin friction heating of its Mach 3 performance, the entire missile was made of stainless steel rather than aluminium.
Instead, it was "lofted" on a near-vertical ascent into the high atmosphere, where it then tipped over to horizontal where it could coast in the thin air for long distances.
Roy Hawkins of the Royal Aircraft Establishment experimented with many different planforms before adding a further forward extension of the wing with an initial sweep at 82 degrees before meeting the original layout aft the engine inlets.
Blue Envoy was designed to be launched long before the target became visible to the missile's radar receiver and thus had to use command guidance for an extended period of flight.
Computers on the ground would send signals to the missile to fly it toward the approximate intercept location, and then as it approached, feed it information on where to look for the target.
[12] By 1957 the programme had defined the final shape of the missile and flown sub-scale models, had completed development and test-flown the 18" engines on the Bristol XTV.9, renamed BET.9 for Blue Envoy Test,[13] and the new radars were about to enter production.
Although there were no remaining issues to solve and production could begin, in April 1957 the project was cancelled as part of the suggestions of the 1957 Defence White Paper.
The war would be won or lost long before the Warsaw Pact forces reached the English Channel, so a conventional invasion was simply not a consideration.
Moreover, both the Navy and RAF were watching the shift from high-altitude bombers to lower-altitude strike aircraft, where the massive performance of the Blue Envoy would not be particularly useful as the radar horizon might be on the order of 10 miles (16 km).
Don Rowley, Director of the Guided Weapons Division, was quoted saying: When Blue Envoy was cancelled we were on our beam-ends: that was our most dangerous period.
Bristol and Ferranti engineers came up with the plan of using these parts of Blue Envoy on a new version of Red Duster - by this time known as the Bloodhound - which would offer a reasonable improvement in performance for very low development cost.
By September 1959 a small, ramjet-powered upper stage with a large solid-fuel booster had been produced, similar to the contemporary US design, RIM-50 Typhon.