Operational Requirement F.155

Operational Requirement F.155 was a specification issued by the British Ministry of Supply on 15 January 1955 for an interceptor aircraft to defend the United Kingdom from Soviet high-flying nuclear-armed supersonic bombers.

They generally featured two engines, mounted a powerful new aircraft interception (AI) radar, and would be armed with improved versions of the de Havilland Firestreak missile or an even larger radar-guided design known as "Red Hebe".

This paper considered the introduction of new Soviet medium range ballistic missiles (MRBMs) that were being deployed in East Germany; these were armed with chemical warheads, but it was clear that nuclear-armed versions would be available by the mid-1960s.

The paper led to the cancellation of nearly all crewed fighter projects as well as most other air defence programs as a radical change had occurred in strategic threats with the expectation that missiles and low-level strike would replace high-flying bombers.

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.

Their first proposal was a single-engined fighter development of the existing FD2, which was felt to have good export potential although it did not meet the RAF criteria laid down in O.R.

A fighter-bomber derivative of this aircraft was also envisaged, with Bristol Olympus 21R engines coupled with a large ventral drop tank giving a much-improved radius of action at low altitude.

Going on the basis of the smallest aircraft that could do the job, Hawker's design (the P.1103) used a single, albeit powerful, engine – a 25,000 lb (111.1 kN) development of the Gyron.

A razor-thin straight wing carried the engines in nacelles – two Gyron Juniors on each side – with a rocket booster under the long narrow fuselage.

[1] Vickers-Armstrong submitted the Type 559; an unorthodox canard design with a massive chin air intake, split vertically, for two reheated Gyron engines placed, as in the English Electric Lightning, one above the other.

Two Red Hebe or Blue Jay missiles were mounted alongside the upper part of the fuselage between the canard and the mainplane, which had end-plates incorporating twin rudders.

The P.8 was larger overall, carried a crew of two, moved the weapons to the wingtips and undercarriage to the fuselage but featured the same vertically-stacked engine and nose intake layout as the Lightning.