H2S - the round clock-like dial was where the map appeared

The cause of the crash which killed the inventor of H2S, Alan Blumlein, was established as insufficiently tightened tappets in the Halifax engine. In view of Bennett’s comments on RAF Defford’s attitude to maintenance, see below, it seems possible that over-maintenance was responsible.

Bennett tells us a lot about the development of H2S in his autobiography, Pathfinder, mentioning the Blumlein crash and describing what came after it.

It had been hoped that [Gee] would give sufficient accuracy in navigation to make blind bombing of the Ruhr practical. Unfortunately this was not quite so, but its value as a navigational aid was tremendous. In particular its use in coming back directly to home aerodromes was a tremendous help, and saved many stupid flying accidents such as had occurred in earlier days.

Gee was the first radar device used on bombers, and was most valuable. The hope for the Pathfinders, however, lay in two other devices. The first of these was an airborne radar ground reflection system known as H2S. The second was subsequently given the code name Oboe.

On the former I went to work immediately, for it seemed to me to be such an excellent device with such tremendous potentialities that it should be top priority. A few days before my appointment to command Pathfinders, the first aircraft fitted with a test rig of this device had crashed and killed all on board. This was a sad loss, and a great setback.

I immediately went down to the Telecommunications Research Establishment (TRE) at Great Malvern  (in the college), and I worked there and at RAF Defford, its aerodrome, on and off for the next few weeks.

Defford had the vague idea that aeroplanes, even in wartime, had to be wrapped in cotton wool, seldom flown, but in the glorious name of ‘inspection’ repeatedly pulled to pieces and put together again. In short, they had the most old-fashioned pre-war RAF conception of maintenance, and no idea whatever of getting on with the job. The maintenance personnel very soon fell down on the job, so I signalled to Bomber Command to let me have some men from 4 Group who were accustomed to maintaining Halifaxes. A small detachment arrived, and provided the tremendous help that we needed.

The scientists (‘boffins’) rallied round magnificently. The H2S team was headed by Professor Dee, assisted by Dr (now Professor) Bernard Lovell, with J P W Houchin and B J O’Kane at Defford doing the practical side. These boffins used to fly with me, and certainly put in a wonderful effort in getting H2S into being so quickly. In general, such elaborate device as H2S, with its eight boxes each chock-a-block full of complicated ‘circuitry’, would have taken one to two years to develop, yet so far as the main principles were concerned the whole system was complete within one month, and we had done an enormous amount of flying with fairly good results.

From PATHFINDER, by D C T Bennett
Bennett considered the contribution of Professor Dee and Bernard Lovell, ‘who gave us the means of seeing through clouds’, to be so important that their photographs appear amongst the very few which are printed in the first edition of his book (1958, Frederick Muller, London).