Red Top (missile)

Even before the original Firestreak entered service, improvements were being studied to increase its performance. Still known by its rainbow code "Blue Jay", Blue Jay Mk. II introduced an improved seeker and more powerful motor known as Magpie II. Mk. III derated the motor to limit acceleration when launched at high speed in order to avoid overheating due to aerodynamic friction. Neither project was proceeded with.[1]

Looking for a further improved weapon for the Operational Requirement F.155 interceptors, in 1955 the Air Ministry issued OR.1131 for an all-aspect design capability against enemy aircraft travelling at Mach 2. De Havilland responded with Blue Jay Mk. IV, which was later given its own rainbow code, "Blue Vesta".[2]

Blue Vesta adopted the PbTe seeker of Mk. II and further upgraded the motor to the new Magpie III. To handle the aerodynamic heating issues, the fins were made of steel rather than aluminium, and featured cut-away sections to keep the rear portions of the surfaces out of the Mach cones, a feature they referred to as "mach tips".[2][lower-alpha 1]

Work on Blue Vesta was curtailed after 1956 as the Royal Aircraft Establishment (RAE) concluded that the closing speeds of two Mach 2+ aircraft would be so rapid that the missile would have no chance to be launched while it was still within the range of its seeker. They suggested moving to the much larger radar-guided Red Hebe, although some low-level work on Blue Vesta's underlying technologies continued.[2]

Red Top mounted on the forward fuselage of a Lightning

F.155 was canceled in the aftermath of the release of the 1957 Defence White Paper. It was decided to proceed with the development of the English Electric Lightning largely because it was already close to being complete and there were near-term threats that could not be countered by existing aircraft.[lower-alpha 2] Blue Vesta was reactivated in July 1957 to arm this aircraft. For security reasons, yet another rainbow code was assigned in November 1957 and it became "Red Top".[3]

Firestreak's valve-powered electronics were replaced by transistorized versions, which were smaller and greatly improved reliability. More importantly, they no longer required active cooling, which in Firestreak had been accomplished with an aircraft-mounted system using ammonia, presenting a ground-handling safety issue. Removing the bottles and all of the plumbing for the cooling systems freed up room in the electronics section and the aircraft-mounted weapon pack.[4]

Elimination of the cooling for the electronics also had the side-effect of removing the cooling for the seeker head, which had been used to improve its performance. For Red Top, the all-aspect lead telluride (PbTe) seeker developed for Blue Vesta was replaced by a much less expensive indium antimonide (InSb) system known as "Violet Banner". This seeker lacked the sensitivity of the PbTe model and did not offer general all-aspect capability; while it worked against supersonic targets warmed by skin friction, it would not work against subsonic targets and required a tail-aspect in these cases.[5] This loss in performance was offset to some degree by increasing the field of view from 30 to 60 degrees,[4] allowing the fighter much greater tactical freedom.

In contrast to Blue Vesta, which retained most of the original Firestreak physical layout, Red Top significantly rationalised the design. The fuselage was straightened, removing the boat-tail layout of the Firestreak. Previously the Magpie motor was in a bottle in the centre of the missile, and its exhaust reached the rear through a long tailpipe. The warhead was wrapped around the pipe, but that left too little room for the control fin actuators, which were instead controlled from nose-mounted actuators using long pushrods.[3]

In Red Top, the smaller electronics package allowed the warhead to be moved forward, leaving room for the control actuators to be mounted directly on the fins and thus removing the need for the pushrods. Moving the actuators to the rear, along with the air bottle that powered them, still took up much less room than the warhead, allowing the rocket motor to be made larger by extending it rearward. This resulted in significantly higher performance than the original design.[3]

With the warhead moved forward and the guidance electronics now taking up less room, the warhead was enlarged to 31 kg (68.3 lb) from Firestreak's already prodigious 22.7 kg (50 lb). It also used an expanding-rod warhead in place of the earlier blast fragmentation type. This move also left room at the rear of the fuselage for one of the two rows of the IR proximity fuse, improving its view of targets. The new "Green Garland" fuse required smaller rectangular "windows", compared to the Firestreak's triangular windows, further simplifying the layout.[3]

The Red Top was much faster[6] and had greater range and maneuvrability than the Firestreak, and its infrared seeker enabled a wider range of engagement angles. "Unlike modern [1990s] missiles, Red Top and Firestreak could only be fired outside cloud, and in winter, skies were rarely clear over the UK."[7]

Although Red Top was a relatively straightforward upgrade to Firestreak, Duncan Sandys almost cancelled it as well. He felt that Firestreak would be acceptable during the short period before the Bloodhound Mk. II SAM entered service in the early 1960s and eliminated the need for interceptor aircraft altogether. The Air Staff argued this point, ultimately convincing him that Red Top's head-on attack profile against new Soviet supersonic bombers known to be in development was an urgent requirement.[4]

At a February 1958 meeting of the Controller of Guided Weapons and Electronics, the group earmarked Red Top for both Lightning and the Fleet Air Arm's de Havilland Sea Vixen. It was expected that Red Top would offer a dramatic leap in performance for Sea Vixen, as its enlarged motor significantly improved its performance when launched subsonically, the relative improvement being less when launched from a supersonic Lightning. There was also some discussion of mounting four Red Top on Blackburn Buccaneer, likely the B.112 version that had been proposed to replace Sea Vixen for long-duration combat air patrol.[4]

Independently, in 1959 Jon Fozard of Hawker Aircraft also considered using Red Top on the Hawker Siddeley P.1127. The concept fit the aircraft with a more powerful engine, added the AIRPASS radar from the Lightning, and carried a pair of Red Top missiles.[8]

Red Top on display at the RAF Museum Cosford.

Hawker Siddeley Red Top missile mounted on an English Electric Lightning at the RAF Museum at Hendon, London.

Red Top testing began using the new fuselage layout with the original Firestreak faceted nosecone and leftover Magpie III motors from the Blue Vesta program. Ten such lash-ups had been fired by June 1959. Guidance tests with the new motor and seeker were carried out from an English Electric Canberra beginning in early 1960 and firing from the Lightning in September 1961.[8]

The Red Top entered service on both the Lightning and Sea Vixen in 1964. It remained in service until the final retirement of the Lightning in 1988.[9] Unusually, the missile that the Red Top was intended to replace – Firestreak – also remained in service on the Lightning until 1988. This was because Red Top's larger wing area required the Lightning to have a larger fin to maintain stability at high speeds, leaving Firestreak on the older models that were already in service.[8]

While the development of Red Top was being carried out, another adaptation of the original Firestreak was being considered to produce a semi-active radar homing version that would allow a single airframe to be converted from IR to radar by changing the nose section. Initially known as Blue Jay Mk. V, this became "Blue Dolphin", but this was cancelled in 1958.[4]

A longer-ranged Red Top Mk. 2 was also proposed, replacing the Linnet rocket with a liquid fuel rocket running on MADI/RFNA, likely the de Havilland Spartan.[8]

When plans began to introduce the McDonnell Douglas Phantom in UK service, the issue was raised about the inclusion of Red Top on that platform. It was agreed from the start that the primary weapon for this platform would be the AIM-7 Sparrow, but the choice of a secondary weapon was more contentious, with arguments being made for both Red Top or the AIM-9 Sidewinder. Those favouring the Red Top, mostly within the Ministry of Defence, pointed to its much higher performance against supersonic targets, especially in frontal engagements where it could be fired at ranges over 20,000 yards (18 km) under favourable conditions.[10]

In contrast, the Director of Surface and Amphibious Warfare (DSAW) noted several problems. The main issue was that the weapon would not be connected to the Phantom's radar for cueing, which would require expensive modifications to the aircraft. This greatly lowered its performance at many shot angles, to the point where it was no more effective than Sidewinder. If cueing was added, then it would also suffer from the same problems that Sparrow would in terms of electronic countermeasures or a radar failure on the aircraft, which was the entire argument for including a secondary IR-based weapon. That being the case, the cost of Red Top, at ￡18,000 had little to offset the Sidewinder at ￡3,500 to ￡3,900. Additionally, Red Top was heavier, especially if one considered the mounting system, which would reduce the loiter time of the aircraft.[11] These arguments won the day and the Phantom carried Sidewinder in service.

• Gibson, Chris; Buttler, Tony (2007). British Secret Projects: Hypersonics, Ramjets and Missiles. Midland Publishing. pp. 47–53. ISBN 978-1-85780-258-0.CS1 maint: ref=harv (link)