Blue Envoy

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 Mach 3 (3,675 km/h; 2,284 mph) with a maximum range of over 200 miles (320 km).

Blue Envoy
Place of originUnited Kingdom
Service history
In serviceNever entered service
Production history
ManufacturerBristol Aeroplane Co.
WarheadContinuous-rod warhead
Possible small nuclear
Proximity fuse

Engine2× Bristol BRJ.811 ramjets,
4× Borzoi solid fuel boosters
150 miles[1]
SpeedMach 3[2]
Semi-active radar homing
Fixed installation

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."[3]

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 to 75 miles (121 km) 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.


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.[4]

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).[5] 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 20 miles (32 km), 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 rocket-powered Red Shoes and Bristol Aerospace's ramjet-powered Red Duster. The Ministry of Supply decided to develop both systems, although at the time they were very similar designs and even sharing the same Marconi designed radar systems. Red Shoes emerged as the 30 miles (48 km) ranged Thunderbird, while Red Duster became the 40 miles (64 km) ranged Bloodhound.

Green Sparkler

Work on the Stage 2 missile did not begin until sometime later, initially under the name Green Sparker. 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 targets, the missile had to have long range.[3]

Green Sparker had a design range of over 200 nautical miles (370 km; 230 mi), 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 10 miles (16 km) 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.

Some consideration was given to the US's BOMARC for the Stage 2 role. This was ultimately rejected because while the BOMARC had an active radar seeker, it was (at that time) a simple pulse unit that would be very easy to jam. They also considered the 300 nautical miles (560 km; 350 mi) range excessive, given that the AMES Type 80 radars that would be providing initial aiming had a range of just over 200 nmi.

As the Soviets introduced new bomber designs, it appeared that there would be a period in the late 1950s where the Stage 1 missiles would not be adequate while the Stage 2 missile would still be under development. This led to the introduction of the "vulgar fractions"; Stage 1 12 and Stage 1 34. Stage 1 12 was an updated Thunderbird with new radars, while Stage 1 34 was a slightly modified version of Green Sparkler using semi-active guidance instead of an active seeker and thus offering a shorter maximum range on the order of 150 nautical miles (280 km; 170 mi).

By this time the Royal Navy was concerned about similar stand-off missile attacks against their ships. They developed a requirement for a similarly long-ranged missile that could launch from ships. Bristol's design was the only entry for this new design, known as Blue Envoy.[3]

Blue Envoy

Although Blue Envoy, and Green Sparker, 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. The speed was measured by a thermometer, adjusting the ramjet power to keep the skin temperature under 620 °F (327 °C).[6] To reach those speeds, a larger 18 inches (460 mm) diameter ramjet engine was required. Overall, the fuselage was not much larger than Red Duster, and did not carry appreciably more fuel.[7]

To reach the required range, increased from Red Duster's 40 miles (64 km) to Blue Envoy's 150 miles (240 km), the missile did not fly directly at its targets. Instead, it was "lofted" on a near-vertical ascent into the high atmosphere, where it could coast in the thin air for long distances. Control at these altitudes was difficult, and while Blue Envoy retained Red Duster's "twist-n-steer" guidance system, it had much larger tailless compound delta wings in place of the original smaller clipped delta wings and separate tail surfaces. Vertical stabilizers were mounted about 23 along the wing span, closer to the tips.[7]

The initial layout, developed by Dietrich Küchemann, had the main portion of the wing swept at 75 degrees, lowered to 42 degrees outboard of the vertical stabilizers. This proved to cause interference with the air intakes for the engines, so Roy Hawkins of the Royal Aircraft Establishment experimented with many different planforms before settling on a forward sweep at 82 degrees.[8]

Controlling the missile during its initial launch and climb was a difficult problem. Normally, missiles use some form of proportional navigation, an algorithm that determines a near-perfect interception vector based on nothing more than the angular velocity of the target relative to the missile. However, 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 the flight. The missile would be flown toward the approximate intercept location, and then as it approached, fed information on where to look for the target.[7]

Ferranti began development of a small digital computer to perform these intercept calculations. The computer would first be fed the target location from a new remote tactical control radar, already under development as Orange Yeoman. The computer would then calculate an approximate intercept point and feed that information to the missile's autopilot. The computer also sent the current angular location of the target relative to the missile, the "angle error", so the missile could keep the receiver aimed in the right direction, listening for the signal of the guidance radar. Some thought was also given to using the computer to directly control the missile's control surfaces.[9]

The main warhead developed for Blue Envoy was a continuous rod warhead, although some consideration was given to a small nuclear warhead under the code name "Blue Fox",[7] which weighted about 450 pounds (200 kg) and had a yield around 5 to 10 kiloton. Another weapon being developed for the missile role was "Pixie", even smaller at around 250 pounds (110 kg) and 1 kiloton.[10]


Sub-scale models of the Blue Envoy vehicle were being tested when the project was cancelled in April 1957 as part of the suggestions of the 1957 Defence White Paper.[7]

Up to this time, UK war plans were based on the concept of the "three-day war", in which a conventional war in Europe quickly escalated to an all-out nuclear war among all the nuclear-armed powers. The war would be over long before the Warsaw Pact forces reached the channel and a conventional invasion was simply not a consideration. The late stages of the war would be fought between Soviet bombers and RAF interceptors, and the interceptors would either destroy the bombers hundreds of miles from shore, or the UK would be destroyed.[11]

The White Paper considered the effects of the introduction of nuclear-armed ballistic missiles to warfighting scenarios. The UK was within the range of medium range ballistic missiles (MRBMs) stationed in East Germany, which had a flight time on the order of 15 minutes or less. Unlike the ICBMs being developed by the US and USSR, medium-range missiles were simpler and cheaper. It was expected that by the early-to-mid 1960s, the main attack on the UK would be carried out by these missiles. There was no credible scenario where they would use only bombers; if an attack by bombers was detected, this would only signal that missiles were soon to follow.

As there was no defence against ballistic missiles, the only possible counter was deterrence. The UK's V bomber deterrent was highly vulnerable while on the ground, so any signal of an attack required their immediate launch. In such an environment, defence systems like Blue Envoy did not make much sense; in any scenario where the Blue Envoy might be used against bombers, the V bombers would have to be launched anyway because missiles were sure to follow. In that case you simply launch on warning, in which case Blue Envoy would be defending empty airfields. The logic was considered so convincing that any attempt to defend the deterrent force was eventually abandoned.[6]

There were also problems with the design itself. Experiments with stainless steel construction on the Bristol 188 had demonstrated this material was much more difficult to work with than expected. Further, the Navy was planning a new series of smaller ships, and Blue Envoy would be far too large to be carried by them. The RAF was also 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).[6]

Bloodhound Mark II

The cancellation of Blue Envoy caught Bristol by surprise, and they had no other ongoing projects to keep the missile division running. Don Rowley was quoted saying:

At the time of its cancellation, development of its radar systems and ramjet engines was largely complete. 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.[7]

The proposal proved interesting enough that it was ordered into production in spite of the very low priority for air defenses after 1957. The resulting Bloodhound Mark II entered service in 1965. Many changes were made as part of this process. The new 18-inch engines were added to the design, providing more thrust and allowing higher weights. This capacity was used to increase the fuel storage by extending the missile's fuselage until it was even longer than Blue Envoy. This almost doubled the range from the Mark I's roughly 40 miles (64 km) to about 75 miles (121 km). Another major change was that the seeker now used the new AMES Type 86 and AMES Type 87 radars, which were continuous-wave radars that could track targets very close to the ground and were much more resistant to jamming.[6]

These changes made Bloodhound a much more formidable weapon, and in this form, it served into the 1990s.[6]

See also

Aircraft of comparable role, configuration and era



  1. "Bristol Blue Envoy". Skomer. Archived from the original on 2012-04-12. Retrieved 2012-09-28.
  2. "Blue Envoy". This is Rocket Science. Archived from the original on 2010-01-07. Retrieved 2012-09-28.
  3. Aylen 2012, p. 6.
  4. Gough 1993, pp. 50-53,64.
  5. Gough 1993, p. 64.
  6. Gibson & Buttler 2007, p. 59.
  7. Aylen 2012, p. 7.
  8. Gibson & Buttler 2007, p. 57.
  9. Aylen 2012, pp. 6-7.
  10. Moore, Richard (2001). The Royal Navy and Nuclear Weapons. Psychology Press. p. 111.
  11. McCamley, Nick (2013). Cold War Secret Nuclear Bunkers. Pen and Sword. p. 90.


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