Bonzo Ammunition

“Thsese so called proximity fuses, made in the United States….proved potent against the small unmanned aircraft (V-1) with which we were assailed in 1944” – Winston Churchill

Bonzo referred to shells fused with the Proximity fuse T.98. Prior to the introduction of the Proximity fuse, the method of detonating an AA shell was either by a percussion fuse (i.e. by impact), timer fuse or altimeter. All required great accuracy in gun laying and control. The Proximity fuse used radio waves to sense the target, which then caused detonation. It still required accuracy in gun laying; but all the gunner had to do now was ensure the trajectory of the shell would at some moment pass close by the target.

Churchill, at the start of the War realized the danger of dive bombers to the Navy. He also realized the War would be one of technology – what he called the “Wizard War”. He recognized the value of a fuse that would detonate automatically when the projectile passed near to the target. As the nose of a shell was quite small, he favored developing such a fuse for the 3” rocket, which head was much roomier. Two fuses were developed. One used photo-electric (P.E) cells to produce an electrical impulse to detonate the rocket whenever there was a change in light, such as the shade of the enemy plane. The second was a Radio fuse which would be detonated by a radio device on the ground. By 1942, the British had a working P.E fuse and had decided to develop a true radio proximity fuse, i.e. using a tiny radar set in the fuse to explode the warhead when the projectile passed close to the aircraft.

The Americans, watching events unfold in France during June 1940 realized they were not ready to fight a technological war. Consequently the National Defense Research Committee (NDRC) was established, to provide funds and contract out technological developments and research. One of the first requests for NRDC funds was to develop a proximity fuse for the Navy. After Pearl Harbour there was a sense of urgency to counter the threat of enemy air power from preventing the American Navy from operating.

The Proximity fuse was made possible by the work of British firms, particularly Pye Radio and E.M.I which developed a valve small enough to go into a fuse and was robust enough to withstand the shock of discharge from the gun. However the Americans succeeded in perfecting the proximity fuse ahead of the British; although the British continued with their own research, they decided to rely on the American fuse.

The first proximity fuses, which were also referred to as Variable Time (VT) fuses, were ready for the American Navy by the end of 1942. This was the Mark 32 fuse; a corresponding fuse for the British Navy’s 4.5” guns, the Mark 33, was ready by mid 1943. It was also decided to develop a fuse for the American 90mm AA gun (and again a parallel effort to adapt it for the corresponding British 3.7” gun). This required miniaturization of the fuse and lead to the development of a new family of fuses – the Mark 45 type.

Above: Left - early Mark 45 type VT fuse. Right - the mock V1 used to establish the sensitivity of VT fuses

When the Americans were shown intelligence of the V1 in 1943, within days they had produced a mockup of a V1 at the New Mexico Experimental Range to establish the sensitivity of the VT fuses against the model and actually began to manufacture special VT fuses to meet the threat. If the Americans had not taken this initiative it is unlikely a VT fuse would have been ready to meet the V1 threat.

In early 1944, the Americans discussed supplying the new VT fuse to the British. The fuse was one of the Mark 45 types, referred to as the T98 fuse. The consensus was to supply the fuse when the V1 offensive began. General H.H Arnold USAAF was against supplying the fuse to the British in case it fell in to enemy hands, allowing the enemy to build their own version and use it against the tight bomber formations and knock the Eight Air Force out of the sky. It was finally agreed to issue the fuse to the British provided a “secret” classification was adopted and only if fired over water.

The first shells arrived in the UK at the start of the V1 offensive. Arrangements were made to construct a shelter for them in Savernake Forest in the Salisbury Plain. The Americans were puzzled by the British decision to employ the AA guns around the outskirts of London rather than along the coast. They were also surprised that no requests from AA Command had been made for the VT fuse. It transpired that General Pile had not been informed that Proximity fuses were being produced for him! Once he was aware of the fuse he quickly requested them. The British also came to the decision to move the guns to the coast; this would also allow for the firing of VT fuses which at that time were restricted to firing over water only. Guns began firing them from mid-July onwards, after the move from the Kentish Gun Belt to the Coastal Gun Belt.

General Pile noted the following about the fuse:

“…the problem of bursting the shells at the right height was effectively solved by the ever increasing use of the proximity fuse. This would explode at almost anything. Birds and even heavy clouds sometimes set it off prematurely, but it seldom failed to explode in close proximity to the target. Some gunners used to bewail the fact that, since they could not now get their shells to burst nearer than 20 yards from the target, it made their good shooting look indifferent. On the other hand, it made a lot of indifferent shooting look first class, so this argument cut both ways.”

Apparently the reference to clouds setting of the shell prematurely was misleading – American operational research posts stationed with the AA batteries reported as many premature bursts in unclouded skies as in clouded skies, persuading the gunners they could use the ammunition in bad weather.

A British report on “anti-Diver” firing with Bonzo calculated that the chances of destroying a “bird” engaged with BONZO was 80% and for Fuse 208 fuse around 25%, and that it would take 140 rounds of Bonzo per “bird” destroyed compared to 550 rounds with Fuse 208. This compared reasonably to figures supplied by 71 AA Brigade which claimed that the rounds per “bird” destroyed for Bonzo was 95 and for Fuse 208, 400 rounds. A rate as low as 70 Bonzo rounds per “bird” had been claimed.

When statistically analyzed, taking into account the actual number of rounds per “bird” (combined total of T.98 and Fuse 208), the number of targets presented (i.e. actually within range of the guns) and the fraction of T.98 fired to Fuse 208, this gave a probability of hitting a “bird” with a single Bonzo round of 1/80 and 1/480 with a single round fused with 208.

The report also referred to the problem of Proximity shells bursting “short”, particularly with early lots. One lot of T.98 fuses was fitted a plastic cap to try and reduce bursts short but this made no difference and the use of this lot was discontinued. It was also noted that 75% of Bonzo shells burst when entering a Z barrage burst. The practice of eight gun batteries using a mix of Bonzo and 208 fused shells also caused problems, the fragments of bursting shells fused with 208 causing the Bonzo rounds to burst. Severe restrictions had to be placed on the use of Bonzo because of the doubts about the SD (Self Destruct) mechanism and the evidence of the arming of the fuse earlier than expected– firing was generally not permitted over land.

Finally it was feared the Germans would realize some sort of new ammunition was in use as the VT shells produced a pattern of bursts different from timed-fuses, which exploded haphazardly over the sky. The VT shells formed a curtain of bursts over the water at the same height around the V1 as it approached the coast. This was visible from the British coast and was close enough for the Germans to see it from France. Later, intelligence showed that the Germans had noticed the curious pattern of bursts but had drawn no conclusions from it.

Reference:
The Deadly Fuze, R.B. Baldwin, Presidio Press, 1980

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