The Proximity (Variable-Time) Fuze

   Of all the developments in anti-aircraft weaponry during the war years, the proximity, or "VT" fuze, as it was known, deserves the most special mention. It cured one of the most pressing problems in anti-aircraft gunnery by ships --  accuracy. Before it, the most effective AA proceedure to be adhered to by surface ships was barrage fire: as soon as it could be reasonably expected that fire might actually hit the enemy planes, a cordon of steel, time-fuzed shells detonating at different altitudes regardless of presence of enemy planes, or their non-existence there, would be send up by heavy AA. This could hardly be considered very effective, especially since without radar, neither height nor distance could be measured or estimated with great accuracy.

    The VT fuze designed to ignore these problems by carrying along its own radar  -- effectively, it was a tiny radar coupled to a shell, detonating it when the radar, which was side-looking, detected a target.  
The main problem in the design was making the radar (employing tiny glass vacuum tubes) rugged enough to survive the shock of firing -- 20,000g (g: normal gravitation of Earth, 9.81m/sec2) in a 127mm L/38 gun. It could not demand special treatment such as careful handling, which could not be expected under combat conditions of stressed gunmen, and which would not be desirable from a rate-of-fire aspect.  

    Development of the VT fuze, an idea which originated in the mid-1920s, began in August of 1940, with the Naval Research & Development Center  in charge of the design. They completed a first design in May, 1941, with a first working shell fired in June. Two months later, a proposal was put forward to immediately begin production of the new fuze although it was by no means perfect, on the grounds that each month's delay could cost the Navy a cruiser, each third month a battleship.

    In January of 1942, the first batch of production rounds exceeding 50% reliability was manufactured, and through the summer, test firings continued, with production of the Mk 32 VT fuze beginning in November, 1942. From early January, 1943, the fuze was used in combat, its first victim likely having been a Rabaul-based IJN dive-bomber shot down by Helena on 4 January off Guadalcanal.

    Several upgrades were effected on the Mk 32 during the war causing many re-designations, which will not be covered here. Let it be said that these upgrades affected mainly the reliability of the shell and its adaption to lighter calibers. Only the 127mm L/38, 152mm L47 (in the AA role it played in the Worcester class postwar cruisers) and 76mm L/50, of the USN's guns, were suitable to this shell. The British used it in their 4.5" and 5.25" guns, and several U.S. and British Army guns employed the fuze.

    As to its effectiveness: VT-fuzed shells made up about two fifths to half of the 127mm ammunition expended during 1943 - 1945. For each plane it shot down, it required about 500 rounds, fewer for kamikaze planes, more for conventional attacks, the reason being the simpler trajectory of a kamikaze. By contrast, the non-VT fuzed 127mm results showed an expenditure of 2,000 rounds per plane, slightly less for regular attacks (probably related to the swift rate of change of the kamikaze's altitude, which the VT fuze needn't care about). Both the 40mm and 20mm guns killed more planes, but their range was much shorter, especially when protecting other ships. This reason partially explains why the post-war Navy eventually (by mid-1950 at latest) discarded the smaller weapons and concentrated exclusively on 76mm and 127mm guns.