Saturday, 30 June 2012

Appendix XVI: Navigating World War 2 bombers

Alan Dower Blumlein (1903 - 1942) was a British electronics engineer credited with inventing the recording and playback of stereo sound. He was honoured in April 2015 for his pioneering work at a special ceremony held at E.M.I.'s Abbey Road studio in London. What is probably more significant and less well-known is the role he played in the development of H2S airborne radar, a system widely credited as a factor in shortening the Second World War.

Alan died aged 38 on the 7th June 1942 during a secret trial of H2S when all on board the Halifax bomber in which he was flying were killed when it crashed at Welsh Bicknor in Herefordshire. Blumlein's role in the project was a closely guarded secret at the time and consequently only a brief announcement of his death was made some years later.

 After the Battle of Britain. RAF Bomber Command had begun night attacks against German cities. Navigation in the dark was very difficult, particularly if there was cloud cover over thee ground, and at first crews had to rely on 'dead reckoning' - a method of estimating position by speed, flying time and compass settings. Unpredictable winds could upset the finest calculations.

Although they had reported good results from the raids, an independent analysis in 1941 based on daylight air reconnaissance showed that approximately half of all bombs fell on open country and only one bomb in ten hit the target - and 'hitting the target' meant it fell within five miles! In the crucial Ruhr valley, the industrial heartland of Nazi Germany, only one in fifteen was on target and at a cost of over 700 British bombers.

Raids on Germany were more or less suspended until 1942 when the 'Area bombing' strategy was introduced and radio electronics promised some improvement in targeting. A radio navigation system called 'Gee' was developed and introduced from about March 1942 as well as another medium-range navigation system called 'Oboe' which was mostly used on De Haviland Mosquitoes and was able to guide single aircraft more accurately to the target.

Towards the end of 1943 'Gee' was refined as 'Gee-H' using an aircraft-mounted transmitter in conjunction with those on the ground back in Britain. All of these systems were dependent on radio transmitters back home and were limited in range, not least because of the curvature of the Earth. Berlin, for example, was out of range.

H2S was the first airborne ground-scanning radar system and it performed its first experimental flight on the 23rd April 1942, with the radar mounted in a Halifax bomber. It was designed to identify targets on the ground for night and all-weather bombing and its first operational use was on the 30th January 1943 when 13 'Pathfinder' bombers set out to mark targets in Hamburg. It was initially fitted to Stirling and Halifax bombers, mounted in a 'radome' (radar dome) beneath the aircraft. In the summer of 1943 the RAF began to equip Lancasters with the new navigation aid.

The Mallon crew's Lancaster AA-W with H2S radome fitted beneath, clear perspex aft.

H2S was vital in the air battle for Berlin, a series of large raids on the German capital and other cities from November 1943 until March 1944, as Berlin was out of range of Gee and Oboe and often obscured by cloud in the winter. The H2S sets available at the start of the battle were not particularly effective and it was not until the 2nd December 1944 when the H2S Mark III, which could identify open and built-up spaces, was successfully used for the first time on operations that it became possible to bomb Berlin accurately.

A modification of H2S called 'Fishpond' used the radar scanner in the aircraft to identify enemy aircraft to a range of up to 30 miles.

H2S had a surprisingly long life span, equipping almost all Bomber Command aircraft during the lae war period and beyond. An advanced version, the Mark IX, was developed for the post-war V bomber fleet. It was last used in anger during the Falklands War in 1982 in the Avro Vulcan, and some units remained in service on the Handley Page Victor until 1993.

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