Operational Requirement F.155

Operational Requirement F.155 was a specification issued by the British Ministry of Supply for an interceptor aircraft to defend the United Kingdom from high-flying supersonic bombers.

Although a nuclear threat from Soviet nuclear-armed bombers was identified as early as in 1955, F.155 calling for supersonic interceptors (in service by 1962) was superseded by the 1957 Defence White Paper. The paper was a major review of military spending and one of its elements was the cancellation of nearly all manned fighter projects as a radical change had occurred in strategic threats with the expectation that intercontinental ballistic missiles and low-level strike would replace high flying bombers.

During the early 1950s, the RAF's front line defence was converting from Gloster Meteor (a design that first flew during the Second World War) to Gloster Javelin all-weather fighters with the anticipation of new interceptors coming into service in the near-future. These included the English Electric Lightning (point defence interceptor) and Saunders-Roe SR.177 interceptors in development. While the Lightning embodied largely conventional jet engine technology, the SR.177 was a "mixed" rocket/jet design that used rocket power for delivering high speed and maximum altitude performance. Due to the length of design and development, for the interim the RAF also considered purchase of the Canadian Avro CF-105 Arrow projected to be operational by 1961.

Operation Requirement F.155 specified exacting demands:

The Ministry of Supply made clear in the requirement that the plane and missiles should be treated as a "weapon system" i.e., a cohesive whole. The armament specifications were covered by a separate Operational Requirement: OR.1131. Two missiles were identified in OR.1131:

A smaller version of Red Hebe would eventually also come into the equation. Blue Vesta was a 150 kg (330 lb) infra-red guided weapon for attack from a pursuit angle. The Blue Vesta was largely displaced by the development of Blue Jay Mark 4 (which would enter service as Red Top). Red Hebe was nearly 600 kg (1,300 lb) but being radar guided, it would be able to hit its target from any firing angle including a collision course intercept.

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