OR.202 was issued in July 1950. The requirement was for a "pilotless interceptor with a range of 150 miles to defend the UK against mass attacks". The Ministry of Supply colour code "Green Water" was applied to this project / weapons requirement.

Green Water was to be powered by rockets or ramjets and have the performance to provide defence against aircraft flying at up to 60000ft. The requirement was quite clear that although the missile should possess high performance, defence against V.2-type weapons was not considered necessary.

Green Water should be capable of "annihilating" a mass bomber attack or at least "inflicting sufficient casualties to deter the remainder or reduce the weight of the attack to tolerable proportions."

This requirement was drawn up less than a year after the Soviet Union exploded its first atomic bomb, so the prospect of a mass raid was foremost in the Air Staff's mind. The briefing document states 500 bombers at a rate of 60 per minute flying at a speed of 450kts. This scale of attack seems to originate in the extrapolation of WW2 Bomber Command tactics to the Soviet air force.

During discussions on the capability of Green Water, it became clear that by optimising the missile for long range / high altitude interception, the short range / low altitude envelope would present problems.

One such problem was guidance. Semi-active homing was considered, but there were no radars that could provide target illumination at such ranges. Two double guidance systems were proposed. The first involved an initial beam-riding phase, followed by an active terminal homing phase. Again radar was the limiting factor with pulsed radar giving a range of 68 miles while continuous wave guidance produced a range of 170 miles. The second system involved inertial guidance in the first phase and active terminal guidance in the second.

The main problem was to be low altitude defence. A 17-mile range SAGW has a minimum interception altitude of 2000ft, while a long range 170-mile SAGW's minimum altitude is 40000ft. Although CW guidance might help with this, it also depends on radar power and horizon. This meant that a second low altitude defence system would be required. Ramjet power was the preferred option for the long range SAGW.

1) 150 mile range missile unlikely to be used.

1) Best to use a single, low to high altitude system. The working party supported the current SAGW programmes, (Sea Slug, Red Shoes and Red Duster) which, despite being uneconomic, did strive to provide answers to the technical problems that faced SAGW development and might lead to a functional system.

3) More work required on "putting-on" of homing heads during the mid course switch to terminal homing. Discrimination of multiple targets data handling in heavy raids.

4) Low altitude interceptions

5) Radio Counter Measures

The outcome was that OR.202 is not economic, a missile range <56 miles was most economic. OR.202 provides no solution to low altitude defence, so most practical solution is to provide a low altitude weapon and extend it up and out. The effects of RCM must be taken into account, including the erosion of detection and tracking capability. SAGW R+D must proceed at with all haste, with particular emphasis on increasing radar power, low altitude detection, target discrimination and putting-on homing heads.