[Larry]

During January 1945 the Fighter Interception Development Squadron (of the Night Fighter Development Wing) carried out operational trials at RAF Ford and later Manston under the code name of Operation Vapour to combat low-level Heinkel He111H-22 aircraft of III/KG 53 carrying air launched V1 flying bombs, which were proving difficult to intercept. The modus operandi typically involved the Heinkels leaving the bases in Holland at night and in bad weather, and flying out over the North Sea at a height of less than 300ft. Once the Heinkels neared the coast of East Anglia, they would increase speed, pop up to 1,500ft and release their V1 flying bombs towards London before turning for home at low level. In addition to the problems of their detection at low level, the Heinkels often cruised at 120-160 mph, which some RAF pilots complained was almost the same as the stalling speed of their Mosquito night fighters, which had been sent to intercept them!

So to assist in countering the Heinkels of III/KG53, the Fighter Interception Development Squadron borrowed a Coastal Command Wellington, equipped with a modified ASV Mk VI radar set and PPI to act as an ACI aircraft. After initial trials in daylight over the Channel, very low level patrols off the coast of northern Holland were carried out at night by the Wellington in formation with four or five Mosquito night fighters, which were relieved in turn by further night fighters after two hours on patrol. In order for the night fighters to locate and keep station with the Wellington, the aircraft was fitted with a special homing beacon, possibly ‘Rebecca / Eureka’.

The radar controller in the Wellington was a civilian scientist from TRE named E.J. Smith, from New Zealand. He had been seconded to the RAF as a Scientific Observer and had previously carried out early ground controlled interception experiments for TRE at the Worth Matravers Chain Home Low radar station, which led to the development programme for GCI. During Operation Vapour the RAF night fighter crews were very impressed with E.J. Smith’s ability to give accurate vectors from the blips on his PPI. Although the results of the trials were slightly disappointing, as radar returns from the sea extended to seven miles radius when flying as low as 500 ft, the maximum range obtained of fourteen miles was nearly twice that of the AI Mk X radar fitted to the Mosquito night fighters. Despite encouraging results from Operation Vapour, the Luftwaffe stopped air launching flying bombs in mid January 1945 so after a few weeks, the operational trials ended. Further ACI trials were recommended with ASV Mk X (AN/APS-5) in a Liberator aircraft, ASV Mk XVII in a Warwick and a modified H2S Mk IV fitted with a 200 Kwatt transmitter and a six foot aerial array but the war ended before these could be carried out.