THE AC DELAY
The main disadvantage of the Time Pencil was that it could not be used underwater. A time fuse was therefore needed for use with limpets and other mines. This was met by the AC (acetone cellulose) Delay, which, in its Mark II form, was widely used. The AC Delay Mk II depended on the softening by a mixture of acetone and amyl acetate, followed by the breaking, of a cellulose acetate tension bar holding back a spring-loaded striker. (In the Mk IA, the striker was held back by a plastic washer.) The solvent was contained in a glass ampoule, which could be crushed by a thumbscrew operated through a watertight gland. The delay time depended on the composition of the solvent, and a coloured dye indicated how long the operative had got. The six options were: red – 4½ hrs; orange – 7½ hrs; yellow – 15 hrs; green – 26 hrs; blue – 42 hrs; violet – 5½ days. Again, these were temperature dependent and the above figures referred to 15°C. When assembled, the switch was screwed into the body of the limpet.
Somewhat similar problems to those experienced with the Time Pencils arose with the AC Mk II delay. In those days the precision moulding of plastics was not well controlled. The tension bars were supplied by Halex (of toothbrush fame) and in one early batch of pre-production bars examined at Station IX, out of 100 bars some 30 were misshapen or badly scored and only eight were free from internal strain. Station XII (Maj Bedford and Capt Ault) tightened up the specification and imposed stricter quality control to eliminate these discrepancies. But another problem arose. The delay times were controlled by changing the proportion of amyl acetate in the solution, and these times seemed to depend on the source of the amyl acetate. Savory and Moore, who supplied the ampoules, assured Station IX that their amyl acetate conformed to the appropriate British Standard based on the refractive index, boiling point and density. What was not appreciated until the BSS was examined carefully was that it allowed the amyl acetate to contain up to 20 per cent of butyl acetate! Various batches from different suppliers would contain varying amounts of butyl acetate and hence give varying delay times. Again, the vital importance of a precise specification and adequate quality control was brought home. There were also other important design problems affecting the ease of manufacture. The waterproofing depended on the use of a rubber sealing washer resting on a seating ring. The exact design of the thread and seating proved to be vital and although Station XII were reluctant to change the design, it proved to be necessary. As with the Time Pencil this problem involved close collaboration between Stations IX and XII in tightening the specification and ensuring adequate quality control.
Everett was also concerned with the development of a plastic version of the Time Pencil delayed action fuse. His notebooks include several references to the PTP (presumably the Plastic Time Pencil) and to the despatch to Station XII of working drawings of the switch. They have not been located. There is no indication of the way in which it was intended to work. One speculation is that the wire retaining the spring was looped over a copper pin and that corrosion took place at the contact. Among the timing trials there is an obscure reference to the effect of the ‘length of the copper pin’.
For ad hoc operations the section produced a soluble delay, a push/pull combination switch, an air-armed non-removal switch and a time-delay, referred to as the ‘Eureka’ clock.
It is the variety of devices worked upon but not put into production which gives a clue to the way Newitt’s ‘Baker Street Boys’, as the scientists were sometimes referred to, were thinking about the next series of weapons in the fight against the enemy. These included the pocket hand grenade, the hydrostatic switch, the parachute canopy delay opening switch, the Allways switch, the parachute grapnel for landing, a thermal switch, the magnesium/lead alloy delay, an anti-tank mine activated by vibration, a device for climbing wire rope, an anti-disturbance switch, the sodium alginate relay, an air speed switch, a centrifugal switch, the photo-electric switch, the hydrostatic ‘fog signal’ switch, a speed switch for limpets, a mechanical altimeter switch for ascent and another for descent, an AC pencil time fuse and the air leak delay (disc time delay).
From time to time there are references to an ‘air leak’ or ‘disc time’ delay based on the rate of diffusion of air through a porous disc which was under development at Station IX towards the end of the war. Newitt thought it might be the ultimate answer to the need for a temperature-independent time-delay. No further information has been located in the files. Although Everett was involved, he could not remember any details until a clue was found in a rough free-hand sketch in his notebook covering the period 2 March to 3 April 1945 (see title page). It would appear from this that the main body of the switch was to be divided into two compartments separated by a thin, curved, metal diaphragm (probably of phosphor-bronze) which would ‘click’ over into one of two positions depending on the pressure difference across the compartments. The diaphragm would carry a firing pin at its centre. One compartment was at atmospheric pressure and the other evacuated and sealed. When the seal was broken air could diffuse through a porous plug (or disc). When the pressure difference approached zero a compressed spring would force the diaphragm to click over and cause the firing pin to strike the percussion cap. Conversion of this basic idea into a practical device would have involved some very precise engineering. It is recalled that among the main problems were those of making porous ceramic plugs with well-defined and reproducible characteristics, and fabricating the metal diaphragm. Everett has a vague recollection that advice on the latter was sought from Negretti and Zambra – a company better known for its barographs. Other difficulties would have included the problem of how to ensure that the device had not leaked in storage: how to devise a safety mechanism. One can speculate that, had the war been prolonged, this basic idea would have been developed into a reliable, largely temperature-independent time delay. Newitt considered it probable that all normal requirements could be met by a single switch of this kind and preliminary laboratory tests suggested that an overall error of less than 12 per cent was possible: but the war ended before the switch could be developed.
Wireless detonation also came under examination at Station IX. In February 1943 a Pole by the name of Sienkiewicz claimed to have invented a device for detonating explosives electrically at long range by means of wireless techniques. Letters passed between ‘C’, the head of SIS, and ‘CD’, the head of SOE, in which it was pointed out that the device was similar to, but not as good as, an invention already developed at Station IX. SOE had, in fact, rejected the Pole’s device a year previously.5
As a result of experience in the field it was recognised that a delay device need not in general be highly accurate – except in the case of short timings, when the accuracy could mean a matter of life or death to the agent – provided that they were 100 per cent reliable under the conditions encountered. The reproducibility of better than ± 20 per cent was regarded as satisfactory provided that the quality control excluded any faulty items. A further precaution was to use Time Pencils in pairs. Early in the war it was thought a saboteur would need time delays of from one day to in excess of six days and Time Pencils had been produced in a range from ten minutes to six days. But in the 1942–44 period the demand had been for shorter timings, very few being over six hours.
As a result of suggestions fed back from the field for a means of firing a number of charges simultaneously without the tiresome and hazardous business of directly linking them with instantaneous fuse, the Station had developed the sympathetic fuse, which responded to the pressure surge from a nearby explosion. Forms for use both in air and underwater were developed.
As an illustration of the diversity of problems and the advantages of a flexible organisation, it is recalled that the amateur clockmaker, the retired Lt Col H.H. King, became concerned with a completely different topic, that of the problems of dropping dogs by parachute. Details of this work have not been found and one can only surmise that his skills were called upon to devise a mechanism to release the dogs from their harnesses
once they were safely on the ground.
INCENDIARIES
From the outset, fire-raising was seen as a most effective means of sabotage. Given the right conditions for combustion, which were often simple and not difficult to arrange, the ignition from a single source might wreak enormous destruction. It is not surprising therefore that by the end of 1940 the anticipated requirements for small incendiary bombs and incendiary arrows were one million of each. Among the earlier incendiary devices inherited by SOE were a large incendiary bomb (2 lb, 0.9 kg) and a smaller (2 oz, 56 gm) device, the latter especially suited for use among loose inflammable material. Several of the incendiaries developed by Station IX were based on a mixture of aluminium and iron oxide (Thermite). When ignited with a chlorate initiator this can attain a temperature of 2,400°C. The molten iron so produced had been applied in peacetime in certain welding techniques. In any case it was a very efficient incendiary. Two such devices were available: the 1¾ lb (0.8 kg) Mk II Firepot and the 2½ lb (1.1 kg) Thermite Incendiary.
A number of smaller devices about which little is known and which probably had their origins at Station XV included: Incendiary Cigarettes (popular with the Italians!); Incendiary Ground Nuts, and Cough Mixture Incendiary Material.
Another device about which there is limited information was the Incendiary Arrow. It is said to have resembled a large safety match about 18 in long with a percussion fuse in the head. It could be fired from a bow or catapult with a range of about 50 yards. Although it was ordered in large quantities in late 1940, there is no reliable information on its use in the field.
Among the incendiary devices which appear in SOE archives but which may have been inherited in mid-1941 from Naval scientists is the Deckerette. This was described as a sandwich of raw rubber sheet impregnated with kerosene between two 2-in square celluloid leaves. Each celluloid sheet had a small disc of phosphorus wrapped in cotton attached to its outer side. Phosphorus is, of course, spontaneously flammable in air so the cotton wrapping would have provided only a degree of safety in its handling. This device was looked upon as being suitable for starting conflagrations in crops, forests, timber yards and warehouses.6
The project that engaged much of the attention of Walker, Bamford and Wilde was the design of the Pocket Incendiary. It was a small incendiary consisting of a plastic case filled with petroleum gel, and ignited with a 30-min. Time Pencil. It was a conveniently sized device which burned for about four minutes. When proper preparations had been made with respect to secondary fuel, draughts, convection, conduction and radiation this was a very effective device, so much so that larger incendiaries might not be necessary. It was a versatile piece of equipment which, when mounted on an instruction card and dropped in large numbers from the air, provided the basis for Operation Moon (later called ‘Braddock’), the full story of which is recounted in Chapter 15. Several million were produced but only about a quarter of a million were actually delivered to the field.
A very simple incendiary device was that known as the Tyesule. It was a gelatine capsule containing 2 oz (57 gm) of a petrol/ paraffin mixture. At one end the capsule was coated with a match composition which, when rubbed on a striker board, caused the device to ignite. With this, one had to choose targets carefully, for it did not provide any delay in which to make good an escape.
In confined spaces, where the supply of air was limited, use could be made of the Incendiary Block. About the size of a brick, this consisted of a wax block strapped to a cellulose acetate box containing sodium nitrate which decomposed to provide oxygen to fuel the flames.
In cases where the fuel for the conflagration was enclosed, such as in a fuel tank, a combined explosive– incendiary device was tested. In the case of a diesel oil tank the explosive ruptured the tank after a preset delay and dispersed the oil in a fine mist which was ignited by the simultaneously fired incendiary device.
Meanwhile, the search for alternative incendiary materials was being carried out by Walker, Bamford and their group on an oil/nitrate incendiary and on phosphorus/iodine gel. Other incendiary devices included a Directional Incendiary, a Magnesium Flash Incendiary, a Flash Grenade and a Jet Thermite Bomb as well as a Pocket Smoke Generator. Unfortunately, details of these devices are unavailable; although considerable progress was made, none was put into production. Bamford also conducted research aimed at setting fire to Chinese junks being used by the Japanese. This involved causing a dust explosion of magnesium powder which would be spread onto all the surrounding wooden surfaces and make the fire almost impossible to extinguish. In trials, mock-ups could be burnt to the waterline very rapidly. Interest was shown in this principle being adapted for an air-delivered weapon but the Japanese surrender occurred before work on it was completed.
DESTRUCTION OF DOCUMENTS
The need for a means of destroying documents quickly and completely had long been recognised. This arose when, for example, an embassy had to be vacated at short notice in the face of an invading enemy. There must have been many examples as the Germans advanced across Europe. An equally important and frequent need arose when an agent or a courier was captured in possession of a briefcase containing secret documents, such as coded messages. A first requirement was that the document case could be destroyed rapidly by the agent or be booby-trapped so that if opened subsequently then an incendiary device was activated and destroyed the contents. In this connection it is important to realise that burning of tightly packed sheets, as in a book (please do not try it!), is extremely difficult – very often the edges of each page are singed, leaving the bulk unaffected. This means that sensitive material should not be packed tightly and that adequate incendiary material must be incorporated.
Station XV responded to this need by devising a number of incendiary briefcases and document boxes containing a variety of incendiary materials.7 In the early days such devices were not always subjected to user trials, and in any case many of them were ‘one-off’ devices produced to the personal needs of the agent. When later extensive tests were made, a number of shortcomings were found in some of the devices, which proved dangerous even when tested in a user trial. There is one well-documented example of a serious failure in the field recorded by Sweet-Escott. ‘Poor David (Smiley) took with him an agent’s briefcase, one of the proudest inventions of our devices department in London. It was fitted with a switch which made it explode if the holder were attacked, thereby destroying the secret documents inside. Something went wrong with it when he was in Siam and it blew up, badly damaging his forearm. No doctor could be found there and he had to be flown out.’ When he arrived back in India he was still picking maggots out of his arm.8
Among the proposed improvements in the efficiency of destroying papers was that of typing on to lightly nitrated paper, or using this inter-leaved with the documents. Tests of this paper showed indeed that when a single sheet was ignited it burned fiercely for a few seconds and left practically no ash. But if more than a few sheets were used, especially if they were packed closely, they tended to detonate. This was demonstrated in a user trial by rolling a sheet into a loose ball, igniting it and then trying to stamp it out. The result was a detonation and a sore foot. It was soon realised, in addition, that this nitrated paper had a short shelf-life – despite requests from time to time it was never produced in quantity although it was listed in the SOE catalogue.
Station IX took up the challenge to make a document case which it was virtually impossible to open without it igniting. It was realised, for example, that, in the simpler versions, examination by X-rays could reveal the nature of the booby trap. If the device was contained in a metal case, then a hacksaw or diamond wheel could be used to open it. Taking all these points into consideration a group at Station IX under Maj Walker and the physicists Wilson and Chibnall set about creating the ‘ultimate’ document case. This turned out to be a cylindrical metal container some 4 in in diameter and 10 in long, the hinging and opening components being protected by one or more keys or combination loc
ks opened either mechanically or electrically. The documents were to be rolled round a Thermite incendiary charge. To protect against X-ray examination the case was double-walled, the cavity being filled with a paste containing a lead compound (probably lead oxide – although this is speculation); to protect against sectioning, the metal walls were incorporated in an electrical circuit so that a hacksaw blade would short-circuit the two walls and set off a detonator. It is not now known what other precautions were taken, but every attempt was made to ensure that the contents could not be extracted unharmed. The result was a somewhat heavy device which would certainly not be cheap to produce. Nevertheless it was considered that there might be circumstances when transporting extra top-secret material in which the expense was justified.
A good deal of interest was shown by several Government organisations, in particular MI5, where Lord Rothschild was an acknowledged expert on the defusing of sabotage devices and booby traps, having been awarded the George Medal for his work.9 He visited Station IX several times during the development of the incendiary document case and expressed a strong desire to test his skills on this new challenge.
Everett was deputed to take one of the prototypes to show Rothschild in his office in St James’. This was not entirely simple since, on the appointed day, no transport was available from The Frythe to London. The only way was to flount the law and carry an explosive device by public transport. Everett got a lift to Welwyn Garden City carrying the document case in a brown paper bag. He then took a train to Liverpool Street with the parcel bouncing ominously on the string baggage rack above his head, crossed London by Underground and presented himself to Rothschild. After explaining in detail all the components of the case, Everett cautioned Rothschild strongly against any attempt to open it.
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