Mark 13 torpedo

The Mark 13 torpedo was the U.S. Navy's most common aerial torpedo of World War II. It was the first American torpedo to be originally designed for launching from aircraft only. They were also used on PT boats.

Originating in a 1925 design study, the Mark 13 was subject to changing USN requirements through its early years with resulting on-and-off development. Early models — even when dropped low to the water at slow speeds — were prone to running on the surface, or not running at all. By late 1944, the design had been modified to allow reliable drops from as high as 2,400 ft (730 m), at speeds up to 410 knots (760 km/h). The final Mark 13 weighed 2,216 lb (1,005 kg); 600 lb (270 kg) of the high explosive Torpex.

The Mark 13 was designed with unusually squat dimensions for its type: diameter was 22.5 inches (570 mm) and length 13 feet 5 inches (4.09 m). In the water, the Mark 13 could reach a speed of 33.5 knots (62.0 km/h; 38.6 mph) for up to 6,300 yards (5,800 m). The Mark 13 ran 12.8 knots (23.7 km/h; 14.7 mph) slower than the Mark 14 torpedo, a characteristic which, along with a lesser mass, lesser negative buoyancy and the lack of a magnetic influence feature in its Mark IV exploder, meant that it did not suffer from some of the same problems as its larger siblings. 17,000 were produced during the war.

By 1942, poor combat performance had made it apparent that there were problems with the Mark 13:

Despite the complications that were attending the other phases of torpedo development, the Bureau of Ordnance considered the aircraft torpedo problem so important that it was assigned the highest priority at the Newport Station. The improvements and modifications of 1942 and 1943 still left the weapon unpopular, however, and production problems were as great as those stemming from incomplete development. In mid-1943 an analysis of 105 torpedoes dropped at speeds in excess of 150 knots (280 km/h) showed clearly why aviators distrusted the Mark 13: 36 percent ran cold, 20 percent sank, 20 percent had poor deflection performance, 18 percent gave unsatisfactory depth performance, 2 percent ran on the surface, and only 31 percent gave a satisfactory run. The total in excess of 100 percent proved that many torpedoes were subject to more than one of the defects, just as the bulk of the problems were still due to the effects of poor air stabilization on water behavior. Better performance at reduced aircraft speeds was small comfort since aviators could not be held down by paper restrictions that imposed serious and dangerous handicaps in combat. And even when they accepted the limitations, the water entry behavior of the torpedo produced frequent hooking and broaching. Time promised to complicate the problem still further. Unsatisfactory for existing planes, the torpedo would certainly fail to utilize the potentialities of aircraft then under development. Confronted with such a problem, the Bureau felt that it had two alternatives: it could accept the Mark 13 as an interim weapon with recognized tactical limitations and initiate the design of a new torpedo, or it could concentrate on eliminating the known defects in the existing weapon. To attempt both might spread effort too thin to assure success in either venture. The first alternative involved predictable delay, since the Bureau estimated that 2 years would be required to move a weapon from conception to production. On the other hand, 12 known defects seemed to preclude immediate success in converting the Mark 13 into an effective aircraft torpedo. Neither alternative was considered alone, so the Bureau decided to increase its resources and follow both at once. The National Defense Research Committee was appealed to for aid, and in late 1942 it accepted a double-barreled order from the Bureau. The Committee was given a blank check to produce a new aircraft torpedo, the Mark 25, for tactical use at 350 knots (650 km/h) launching speed, and it agreed to aid the Bureau in making immediate improvements to the Mark 13.

The Committee assigned the California Institute of Technology to undertake the first systematic study of the dynamics of aerial launched torpedoes. Tank tests using scale models revealed that the "low and slow" approach that had been presumed necessary for a successful drop was actually counterproductive: striking the water at a flat angle frequently caused the after body of the torpedo to "slap", damaging the mechanism. Full scale testing simulated aerial torpedo drops under controlled conditions by pneumatically launching full size torpedoes down a 300 feet (91 m) slide on California's Morris Dam into a mountain lake known for its clarity, allowing all aspects of the water entry to be examined utilizing high-speed photography. Fragile or vulnerable components were improved, tested, refined, and tested again. Improved components were shipped to Newport Rhode Island for air drop testing – 4,300 drops in all. The Caltech study led to the development of "drag rings" that slowed and stabilized the torpedo in flight and cushioned its impact with the water and "shroud rings" (also known as the "ring tail") that reinforced the vulnerable tail fins. They also tested and developed a box-shaped wooden tail that stabilized the torpedo in flight and absorbed energy as it was stripped off as the torpedo entered the water, based on the Kyoban series of similar aerodynamic tails, first developed in 1936 by the Japanese for their Type 91 torpedo used at the attack on Pearl Harbor, but first observed at the Battle of the Coral Sea on 8 May 1942.

Experiment soon revealed that optimum water entry angles were approximately 22-32 degrees relative to the plane of the surface: the torpedo might plunge as deep as 50 feet (15 m) but it would return to its set depth and bearing if the mechanism was undamaged. This enabled the US Navy to develop a series of attack profiles that varied the combination of speed and altitude to produce the ideal 22-32 degree water entry angle. For the Grumman TBF Avenger torpedo bomber this meant drop altitudes as high as 800 feet (240 m) and drop speeds as high as 260 knots (480 km/h) which the Avenger could achieve by diving to the release point. Multiple attack profile options also allowed strike planners to de-conflict attack routes by assigning each torpedo squadron a different attack profile, greatly reducing the risk of mid-air collision over the target. Finally, there was the added benefit of increased range, as the torpedo traveled a significant distance in the air before entering the water (up to 1,000 yards (910 m) when released at 800 feet (240 m) and 300 mph (480 km/h)). Combined with radar that delivered the exact range to the target, the results proved to be remarkable:

New planes outdated [the] Torpedo Mark 13, but drag rings and stabilizers renewed its usefulness. Throughout 1943 torpedo performance remained poor, but the following year witnessed a revolution in the behavior of the Mark 13. Minor changes to the propeller blades and a reduction in gyro damage helped, but the greatest improvement resulted from the stabilizing effects of two appendages--the drag ring and the shroud ring.