I'm looking for opinions on what you guys think were the sweet spot, or optimal, sizes for AA guns in the WWII period. I found a slightly related thread from 20(!!!) years ago Best AA weapon , so maybe the time is ripe to revisit the topic.
To start the topic with my own rough thinking, you have roughly two size classes, light/medium (grouping them together here and calling it light to avoid having to type "light/medium" all the time), and heavy. The light guns are contact fuzed, and thus rely on a direct hit on the target, whereas heavy AA use time or VT fuzes to explode a big shell hopefully close enough to the target that the blast and/ or shrapnel would bring it down. Furthermore, you have land-based vs. naval AA, with slightly different requirements particularly for heavy AA.
First, land-based light AA. Increasing speeds of aircraft add two major impacts driving the optimal size upwards. First, the higher speed of aircraft meant that the AA gun had less time to pump shells into the air, and thus in order to give the AA gun more time to shoot at the target and thus increased probability of hitting, it needed increased range. And while light AA was a point defense weapon, when protecting a big target like an airfield, longer range also gives better coverage with fewer guns. Secondly, the shorter engagement time meant that multiple hits were increasingly improbable, thus the desire for a shell powerful enough to shoot down an attacking aircraft with a single hit. It seems a single 30mm shell was usually enough to shoot down a single-engined aircraft of the period. Taking into account also twin-engined aircraft were used for such close range attacks, maybe something somewhat bigger. So with this reasoning one ends up at something like the 40mm Bofors, widely regarded as the best medium AA of the war. The German 37mm wasn't that far off in size, although it had a fairly low rate of fire. Fixed in the 37mm Flak 43, but relatively few of these were deployed before the war ended. 20mm was widely used, such as the German Flakvierling, but it seems it was somewhat on the small side. Germany also developed the self-propelled Kugelblitz, mounting two 30mm Mk 103 aircraft guns in a turret on a PzIV chassis. Seems it could have been a formidable weapon, but fortunately only a few prototypes were made before the war ended. Soon after WWII Oerlikon conducted a study on the optimal light AA calibre, resulting in the ubiquitous Oerlikon 35mm AA gun.
For land-based heavy AA, it seems the mainstay of most major combatants were in the 90mm range, such as the German 88mm, the British QF 3.7" (94mm), and the US 90mm. Germany also had 10.5 cm and 12.8 cm flak cannons, but apparently they were so heavy and unwieldy that they were used mostly in static mounts. The Germans were apparently satisfied with the 12.8cm gun per se, noting that it was more cost effective than the smaller systems. Apparently due to the bigger lethal radius of the bigger shell, and the longer range of the gun meaning that fewer batteries (with assorted fire control equipment and staff etc.) were needed to cover an area? Later on the Allied systems got VT fuzes and gun-laying radar (SCR-584) which dramatically increased their lethality.
For naval light AA, much of the same considerations apply as for land-based AA. The 20mm Oerlikon was widely used as it was much smaller and lighter than the 40mm, although particularly towards the end of the war the Kamikaze threat showed the limitations of the 20mm. The short range and limited hitting power of the 20mm meant the difference it made was mainly whether the aircraft was already on fire when it hit the ship.
For naval heavy AA, the mobility issues of the heavy land-based AA guns weren't that much of an issue, and thus naval AA guns tended to be bigger than their land-based equivalents. The gold standard being the US 5"/38, although the British 4.5" was evidently also well regarded. One thing pushing the size of naval heavy AA upwards was of course also that they were dual purpose (well, at least UK and US ones), also filling the purpose of anti-surface fire. However there were quite big differences between naval and land-based heavy AA. Level bombing having shown to be almost completely worthless in hitting moving ships, attacking aircraft pressed home attacks much closer than the big four-engined fleets devastating Europe and Japan. Also when approaching a target it seems dive bombers came in at a much lower altitude than the four-engined heavy bombers, and the aircraft were smaller and more able to take evasive action to avoid heavy AA. One person on another forum seems to have a hobby horse that the heavy DP AA armaments of the US and UK were to some extent wasteful. I'm not really sure what to make of it, but the argument is not entirely without merit. Basically it goes roughly as follows:
(And yes, after WWII the USN introduced a rapid-fire 3" with VT fuzes as a replacement for the Bofors.)
To start the topic with my own rough thinking, you have roughly two size classes, light/medium (grouping them together here and calling it light to avoid having to type "light/medium" all the time), and heavy. The light guns are contact fuzed, and thus rely on a direct hit on the target, whereas heavy AA use time or VT fuzes to explode a big shell hopefully close enough to the target that the blast and/ or shrapnel would bring it down. Furthermore, you have land-based vs. naval AA, with slightly different requirements particularly for heavy AA.
First, land-based light AA. Increasing speeds of aircraft add two major impacts driving the optimal size upwards. First, the higher speed of aircraft meant that the AA gun had less time to pump shells into the air, and thus in order to give the AA gun more time to shoot at the target and thus increased probability of hitting, it needed increased range. And while light AA was a point defense weapon, when protecting a big target like an airfield, longer range also gives better coverage with fewer guns. Secondly, the shorter engagement time meant that multiple hits were increasingly improbable, thus the desire for a shell powerful enough to shoot down an attacking aircraft with a single hit. It seems a single 30mm shell was usually enough to shoot down a single-engined aircraft of the period. Taking into account also twin-engined aircraft were used for such close range attacks, maybe something somewhat bigger. So with this reasoning one ends up at something like the 40mm Bofors, widely regarded as the best medium AA of the war. The German 37mm wasn't that far off in size, although it had a fairly low rate of fire. Fixed in the 37mm Flak 43, but relatively few of these were deployed before the war ended. 20mm was widely used, such as the German Flakvierling, but it seems it was somewhat on the small side. Germany also developed the self-propelled Kugelblitz, mounting two 30mm Mk 103 aircraft guns in a turret on a PzIV chassis. Seems it could have been a formidable weapon, but fortunately only a few prototypes were made before the war ended. Soon after WWII Oerlikon conducted a study on the optimal light AA calibre, resulting in the ubiquitous Oerlikon 35mm AA gun.
For land-based heavy AA, it seems the mainstay of most major combatants were in the 90mm range, such as the German 88mm, the British QF 3.7" (94mm), and the US 90mm. Germany also had 10.5 cm and 12.8 cm flak cannons, but apparently they were so heavy and unwieldy that they were used mostly in static mounts. The Germans were apparently satisfied with the 12.8cm gun per se, noting that it was more cost effective than the smaller systems. Apparently due to the bigger lethal radius of the bigger shell, and the longer range of the gun meaning that fewer batteries (with assorted fire control equipment and staff etc.) were needed to cover an area? Later on the Allied systems got VT fuzes and gun-laying radar (SCR-584) which dramatically increased their lethality.
For naval light AA, much of the same considerations apply as for land-based AA. The 20mm Oerlikon was widely used as it was much smaller and lighter than the 40mm, although particularly towards the end of the war the Kamikaze threat showed the limitations of the 20mm. The short range and limited hitting power of the 20mm meant the difference it made was mainly whether the aircraft was already on fire when it hit the ship.
For naval heavy AA, the mobility issues of the heavy land-based AA guns weren't that much of an issue, and thus naval AA guns tended to be bigger than their land-based equivalents. The gold standard being the US 5"/38, although the British 4.5" was evidently also well regarded. One thing pushing the size of naval heavy AA upwards was of course also that they were dual purpose (well, at least UK and US ones), also filling the purpose of anti-surface fire. However there were quite big differences between naval and land-based heavy AA. Level bombing having shown to be almost completely worthless in hitting moving ships, attacking aircraft pressed home attacks much closer than the big four-engined fleets devastating Europe and Japan. Also when approaching a target it seems dive bombers came in at a much lower altitude than the four-engined heavy bombers, and the aircraft were smaller and more able to take evasive action to avoid heavy AA. One person on another forum seems to have a hobby horse that the heavy DP AA armaments of the US and UK were to some extent wasteful. I'm not really sure what to make of it, but the argument is not entirely without merit. Basically it goes roughly as follows:
- Anything beyond roughly 3" is a waste. The extra range bigger guns gives is not needed for naval combat. Planes didn't fly that high, and also the small and nimble aircraft could do evasive maneuvers so that at the time of flight required to reach longer distances meant that shooting at such ranges was mostly a waste of ammunition. While a 3" shell has a smaller lethal radius than a bigger one, the gun is also much smaller and lighter, allowing more of them to be mounted on the ship for the same weight.
- Adding high-angle capability to guns adds a lot of extra space and weight to the mounts. However fuze setters etc. are not that heavy, so one can relatively cheaply add AA capability to 4-6" guns if one wants it, as long as one doesn't insist on high-angle capability.
(And yes, after WWII the USN introduced a rapid-fire 3" with VT fuzes as a replacement for the Bofors.)
