United states navy anti-aircraft developments in World War 2

Perhaps no naval force in the world had endured a more intensive aerial campaign than the US Navy of WW2. During the course of the war, a naval report concluded that between 7500 and 8000 attacks were made on ships penetrating the combat air patrol. No less than 1300, aircraft were witnessed destroyed. Given the ratio of aircraft falling in sight of the ships to those unable to return to base or suffering total loss after landing rendered a figure of about 2800 being the total.^[1]

Early Developments

WW1 had been a war in which air warfare blossomed but had not matured to the point of being a real threat to naval forces. Assumptions that a few small relatively small caliber naval guns could manage to keep enemy aircraft beyond a range where harm might be expected. In 1939 radio controlled drones became available to the US Navy in quantity allowing a more realistic testing of existing anti-aircraft suites against actual flying and maneuvering targets.^[2] The results were sobering to an unexpected degree. The United States was still emerging from the effects of the Great Depression and funds for the military had been sparse. To the degree that powder fused shells were still 50% of the shells used.^[3] The US Navy found that a significant portion of its shells were duds or low order detonations (incomplete detonation of the explosive contained by the shell). Virtually every major country involved in combat in World War 2 invested in aircraft development. The cost of aircraft research and development was small and the results could be large.^[4] So rapid was the performance leaps of evolving that the British HAC's fire control system was obsolete and designing a successor very difficult for the British establishment.^[5] Electronics would prove to be an enabler for effective anti-aircraft systems and both the US and Great Britain had a growing electronics industry.^[6] In 1939 radio controlled drones became available to actually test existing systems in British and American service. The results were disappointing by any measure. High-level maneuvering drones were virtually immune to shipboard AA systems. The US drones could simulate dive bombing which showed the dire need for autocannons. Japan introduced powered gliders in 1940 as drones but apparently was unable to dive bomb.^[7] There is no evidence of other powers using drones in this application at all. It may have caused a major underestimation of the threat and an inflated view of their AA systems.^[8]

The problem

The fire control problem in AA is similar to the fire control problem with two additional problems. The 1st is that the target is moving in three dimensions, not two. The second is that speeds of the target in this age were an order of magnitude faster. The problem requires that range, altitude, and speed, must be found very quickly. The results have to be fed into a predictor (mechanical computer in this case) and the resulting elevation, bearing, and fuse setting have to be sent to the gun mount. The entire engagement might only last 2 to 3 minutes at most. For the autocannon, both the ranges and time factor were considerably compressed. Were this not enough, the rate of change in all 3 dimensions also has to be measured and predicted. In the example, should the attacking aircraft be turning and diving then the altitude, range, speed, and bearing will all be changing. Further complicating the problem is that the AA platform itself is pitching, yawing, and moving.

System of systems

Effective naval anti-aircraft systems are very complex. Each element of an AA system at sea is often a system of its own. First, an aircraft has to be detected. Then an accurate range, bearing, and altitude must be ascertained. Next, a rate of change both in range, bearing, and altitude, must be determined. Appropriate lead has to be computed so as to place the shell where the plane will be. This computation has to be transmitted effectively to the gun mount. The computation has to be continuously updated through the time of the engagement. The gun itself has to accurately deliver shells to the predicted area where the plane will be. The shell itself has to explode at the right time and deliver enough fragments to damage the plane in the case of larger shells, or actually hit the aircraft in the case of smaller shells. The gun mount itself has to be powered and smoothly track to the inputs of the fire control system. The fire control system in more advanced systems must have a stabilized plane of reference (hereafter called a stable element). This was no minor task in a warship.

Guns

When the US Navy began to rearm in 1939 in many ships the primary short ranged gun was the M2 .50 caliber machine gun. While effective in fighters at 300 to 400 yards this is point blank range in naval anti-aircraft ranges. Production of the Swiss Oerlikon 20mm had already started to provide production for the British and this was adopted in exchange for the M2 machine guns.^[9] In the December 1941 to January 1942 time frame production had risen to not only cover all British requirements but also allowed 812 units to be actually delivered to the US Navy.^[10] By the end of 1942 the 20mm had accounted for 42% of all aircraft destroyed by the US Navy's shipboard AA. However, the King Board had noted that the balance was shifting towards the larger guns used by the fleet. The US Navy had intended to use the British Pom-Pom, however, the weapon required the use of cordite which BuOrd had found objectionable for US service.^[11] Further investigation revealed that US powders would not work in the Pom-Pom.^[12] Bureau of Ordnance was well aware of the Bofors 40mm gun. The firm York Safe and Lock was negotiating with Bofors to attain the rights to the air-cooled version of the weapon. At the same time Henry Howard, an engineer, and businessman became aware of it and contacted RAMD W. R. Furlong Chief of the Bureau of Ordnance. He ordered the Bofors weapon system to be investigated. York Safe and Lock would be used as the contracting agent. The system had to be redesigned for both the English measurement system and mass production. As the original documents recommended hand filing and drilling to shape.^[13] As early as 1928 the US Navy saw the need to replace the .50 caliber machine gun with something heavier. The 1.1"/75 (28 mm) Mark 1 was designed. Placed in quadruple mounts with a 500 rpm rate of fire it would have fit the requirements. However, the gun was suffering teething issues being prone to jamming. While this could have been solved the weight of the system was equal to that of the quad mount Bofors 40mm while lacking the range and power that the Bofors provided. The gun was relegated to smaller less vital ships by the end of the war.^[14] The 5"/38 naval gun rounded out the US Navy's AA suite. A dual propose mount it was used in both the surface and AA roles with great success. Mated with the Mark 37 director and the proximity fuse it could routinely knock drones out of the sky at ranges as far as 13,000 yards.^[15] A 3"/50 MK 22 semiautomatic dual gun was produced but not employed before the end of the war and therefore beyond the scope of this article. However early marks of the 3"/50 were employed in destroyer escorts and on merchant ships. 3″/50 caliber guns (Marks 10, 17, 18, and 20) first entered service in 1915 as a refit to {{USS|Texas|BB-35}}, and were subsequently mounted on many types of ships as the need for anti-aircraft protection was recognized. During World War II, they were the primary gun armament on destroyer escorts, patrol frigates, submarine chasers, minesweepers, some fleet submarines, and other auxiliary vessels, and were used as a secondary dual-purpose battery on some other types of ships, including some older battleships. They also replaced the original low-angle 4"/50 caliber guns (Mark 9) on "flush-deck" {{sclass-|Wickes|destroyer|5}} and {{sclass-|Clemson|destroyer|1}}s to provide better anti-aircraft protection. The gun was also used on specialist destroyer conversions; the "AVD" seaplane tender conversions received two guns; the "APD" high-speed transports, "DM" minelayers, and "DMS" minesweeper conversions received three guns, and those retaining destroyer classification received six.^[16]

Rangefinding

The first devices deployed for general uses was the altiscope, a verticle coincidence rangefinder. While easy to use it lacked a great deal in a number of respects they were not good in all weathers and they could not be used until the aircraft was well above the horizon. They were not of great use at a far distance.^[17] The officer in charge of the station had spotting scope provided for the purpose of observing the airburst as to the regard of fuse setting. While not intended for the purpose it turned out to be a fair stereoscopic rangefinder. In practice, the spotting/stroescopic spotting scope was of more use than the Altiscope. The Bureau of Ordnance took notice and designed a purpose-built stereoscopic rangefinder and incorporated them into the Mk 28 and 33 directors.^[18]

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