Anti-handling Device - History

History

Technology to incorporate sophisticated anti-handling mechanisms in fuzes has existed since at least 1940 e.g. the Luftwaffe's ZUS-40 anti-removal fuze (of which 3 slightly different versions existed) which was used during the London Blitz and elsewhere. ZUS-40s were designed to fit underneath most Luftwaffe bomb fuzes. When a delayed-action bomb containing a ZUS-40 was dropped on a target, the massive jolt after it hit the ground freed a ball-bearing inside the ZUS-40, thereby arming a spring-loaded firing pin. However, so long as the main bomb fuze remained inside its fuze well, the cocked firing pin in the ZUS-40 was prevented from springing forward. ZUS-40s were often fitted underneath a type 17 clockwork long delay fuze, which gave up to 72 hours delayed detonation. Rendering safe a type 17 fuze was normally a simple and straightforward process i.e. unscrew the fuze locking ring, remove the fuze from its pocket in the side of the bomb and unscrew the gaine. However, fitting a ZUS-40 underneath a type 17 fuze made the render-safe process much more complicated and dangerous. Removing the main time-delay fuze more than 2 cms from its fuze pocket (without neutralising the anti-handling device underneath) automatically released the cocked firing pin inside the ZUS-40, which sprang forward to strike a large percussion cap, thereby causing detonation of the bomb and the death of anyone kneeling beside it. Because the ZUS-40 was designed to be concealed underneath a conventional bomb fuze, it was very difficult to know whether a particular bomb was fitted with an anti-handling device or not. In any case, many electrically fired German bomb fuzes (which could be fitted above a ZUS-40) already had a tiny pendulum-based "trembler" device inside them, which closed the circuit and triggered detonation if the bomb was subjected to rough handling. Some German anti-handling fuzes were even more sophisticated and therefore particularly dangerous to EOD personnel e.g. the type 50 and 50BY fuzes. These were normally fitted to 250/500 kg bombs and contained two mercury tilt switches which detected movement across vertical and horizontal axes. The fuzes fully armed themselves approximately 30 seconds after hitting the ground. Subsequently, if the bomb was moved in any way, the mercury switch completed an electrical circuit and triggered detonation. To complicate matters still further, German bombs could have two separate fuze pockets fitted, with different fuze types screwed into each one. As a result, one bomb could incorporate two separate anti-handling devices working independently of each other e.g. a type 17 clockwork fuze with a ZUS-40 hidden underneath it screwed into one fuze pocket, and a type 50BY in the other. Although the designs of these anti-handling fuzes varied, all were specifically designed to kill bomb disposal personnel who had the task of rendering them safe.

Allied forces developed their own designs of anti-handling devices during World War II. For example, the American M123A1, M124A1, M125 and M131 series of chemical long delay tail-fuzes (which remained in service until circa 1960s) were used in air-dropped bombs. Frequently fitted to M64 (500 lb), M65 (1000 lb) and M66 (2000 lb) general-purpose bombs, these fuzes were primarily designed to operate as chemical long-delay fuzes, with settings ranging between 15 minutes and 144 hours. The time delay mechanism was simple but effective: after being dropped from the aircraft a small propellor at the rear of the bomb revolved, gradually screwing a metal rod into the fuze, crushing an ampoule of acetone solvent contained within it. When this happened the fuze was fully armed and the timer countdown had started. The acetone soaked into an absorbent pad next to a celluloid disk which held back a spring-loaded firing pin from a percussion cap connected to an adjacent detonator. Acetone slowly dissolved the celluloid disk, gradually weakening it until the cocked firing pin was released and the bomb detonated. The time delay of the fuze varied according to the acetone concentration and the thickness of the celluloid disk. Removing a chemical long delay fuze from a bomb after it had been dropped would have been a straightforward process had it not been for the fact that there was an integral anti-withdrawal mechanism designed to kill anyone who tried to render the bomb safe: fuzes such as the M123 (and its derivatives) contained a tiny ball-bearing at the lower end which slid out of a recess when the fuze armed itself a few seconds after being released from an aircraft. The ball-bearing jammed into the screw-threads inside the fuze well, preventing the fuze from being removed. Because the lower end of the fuze was locked in place deep inside the bomb (where it was hard get at) this posed major problems for enemy EOD personnel: attempting to unscrew a fully armed chemical long-delay fuze caused it to separate into two separate fuze assemblies i.e. upper and lower. This action automatically triggered detonation by releasing the cocked firing pin in the lower fuze assembly, with lethal results for anyone nearby. Although many decades have passed, unexploded bombs dating from World War II with chemical long-delay fuzes fitted remain extremely hazardous to EOD personnel. This is because corrosion makes the fuze mechanism (which is still holding back the spring-loaded firing pin from a percussion cap) much more sensitive to disturbance. There is a high risk that even minor movement (e.g. gently rotating the bomb casing to gain better access to the rear end) will release the firing pin.

The British "Number 37 Long Delay Pistol" (used by RAF Bomber Command during World War II) was another chemical long delay fuze which used a very similar type of anti-removal mechanism. A later design of British nose fuze called the number 845 Mk 2 operated purely in anti-disturbance mode. It contained a mercury switch which triggered detonation if the bomb was moved after a 20 second arming delay, which started when the bomb hit the ground.

Since then, many nations have produced munitions with fuzes which have some form of anti-handling function. Alternatively, they have produced munitions with features which make it very easy to add an anti-tamper function e.g. extra (but empty) threaded fuze wells on anti-tank landmines, into which the detonators on booby-trap firing devices (plus booster attachments) can be screwed.