AAA mortars!

The killer is the low muzzle velocity of mortars. Aitcraft tend to move rather fast and change course when in danger of being shot at.
 
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Japanese got 600m height out of an AA Mortar with a muzzle velocity of 80-150m/s, don't have a proper source just MV of the shell, just used MV of standard mortars used to launch it. US 81mm mortars could hit 210m/s, 107mm 250m/s, so I'd say 105mm and higher necessary for 1000m plus, could be wrong and could do it with less. But still expect 105mm plus

Issue with a 24" mortar is the size. There was a 24" mortar deployed in WWII on a self propelled chassis, that was 124 tons. Presumably a naval mount would weigh similar when all was said and done with navalising it and handling recoil. Issue is it weighs as much as a pair of twin 5" DP mounts, and I'd consider the longer range of the twin 5" much more useful. I'd say AA mortar mounts would have to be small enough not to displace a Bofors Twin or larger weapon, if they were to be practical

Mind you I am of the opinion this would probably not work
The 124 tons seems a bit excessive, so lets look at a few things.

When I was in the Berlin Brigade, our companies mortar platoon had, IIRC, two different guns, one was (I think) the 81mm, while the other was (I think) the 4.2" and that is/is not the 107mm? I know that both were man portable, so let us do a quick mental exercise here.

Take the notional 24" and 36" AAA Mortars I have been on about, and let us look at space/weight. Now, a single 24" mount is going to take up a certain amount of deck space, and if we were to posit a similar space being taken up by an array of roughly 4" mortars, we could probably get away with three concentric circles of such tubes, from deck space taken up perspective, and this notional array could have 6 tubes in the inner ring, 12 tubes in the center ring, and 18 tubes in the outer ring, for a total of 36 tubes/mount. Of course, this isn't going to actually work out this way, but just to give us an idea, let's go with this for now. Ok, so if each 107mm tube weighs 100 lbs, then the total weight of tubes alone is going to be 3600 lbs, and this then gives us 1.8 tons as the starting place for our mount. Let's say that we take the weight of the tube as being a total of 10% of the actual mounts total weight, and thus come up with 18 tons total weight for an alternative to the 24" mount.

For the notional 36" mount, let's say that gives us a mount with 4 concentric circles, with 6-12-18-24 tubes in each ring, respectively. This would then give us 60 tubes for the 36" alternative mount, and this would then give us 60 100 lbs tubes, for 6,000 lbs/3 tons, and then a total mount weight of some 30 tons or so.

So now, let's amend the OP, to posit either a system of massive mortars in the AAA air defense role, or, alternatively, a multi-tube array system of AAA mortars in the air defense role. the single big-barrel mortars would probably be far less of a hassle to load and operate, but the munitions might prove to be a problem, while the massed array of smaller mortars would be a nightmare to load and operate, could the simplicity of their design prove to be the better of the two systems?

I'm a total novice in the realm of alternative air defense for 1930-1945 warships, so I have to speculate without any factual basis to really model things on, but am attempting to do the best I can, in the hopes of getting enough of a discussion going on that folks more knowledgeable than I will join in and steer us in the right direction.

That being said, could we see an interesting, and perhaps viable additional AAA anti-aircraft defense capability coming into service in the early years of WWII, based upon massed mortar fire being used to fill an incoming aircrafts attack profile with huge numbers of explosive projectiles? A notional DD, with ten of the multi-tube arrays (36 barrel type), could put some 360 mortar rounds in the path of an incoming aircraft, per volley, while the same DD, with the (60 tube array), could put some 600 mortar rounds out, per volley. Rate of fire would be an issue, of course, but my main concern is training gunners not to shoot at an incoming aircraft at all, but rather the volume of space their attack profile should have them transiting through, and then filling said volume with massed fragmentation projectiles.
 
The Americans did experiment with the 12" mortars of the Manila Bay defenses. Those were field modifications of existing anti-ship shells. It didn't work out.
 
The 124 tons seems a bit excessive, so lets look at a few things.

When I was in the Berlin Brigade, our companies mortar platoon had, IIRC, two different guns, one was (I think) the 81mm, while the other was (I think) the 4.2" and that is/is not the 107mm? I know that both were man portable, so let us do a quick mental exercise here.

Take the notional 24" and 36" AAA Mortars I have been on about, and let us look at space/weight. Now, a single 24" mount is going to take up a certain amount of deck space, and if we were to posit a similar space being taken up by an array of roughly 4" mortars, we could probably get away with three concentric circles of such tubes, from deck space taken up perspective, and this notional array could have 6 tubes in the inner ring, 12 tubes in the center ring, and 18 tubes in the outer ring, for a total of 36 tubes/mount. Of course, this isn't going to actually work out this way, but just to give us an idea, let's go with this for now. Ok, so if each 107mm tube weighs 100 lbs, then the total weight of tubes alone is going to be 3600 lbs, and this then gives us 1.8 tons as the starting place for our mount. Let's say that we take the weight of the tube as being a total of 10% of the actual mounts total weight, and thus come up with 18 tons total weight for an alternative to the 24" mount.

For the notional 36" mount, let's say that gives us a mount with 4 concentric circles, with 6-12-18-24 tubes in each ring, respectively. This would then give us 60 tubes for the 36" alternative mount, and this would then give us 60 100 lbs tubes, for 6,000 lbs/3 tons, and then a total mount weight of some 30 tons or so.

So now, let's amend the OP, to posit either a system of massive mortars in the AAA air defense role, or, alternatively, a multi-tube array system of AAA mortars in the air defense role. the single big-barrel mortars would probably be far less of a hassle to load and operate, but the munitions might prove to be a problem, while the massed array of smaller mortars would be a nightmare to load and operate, could the simplicity of their design prove to be the better of the two systems?

I'm a total novice in the realm of alternative air defense for 1930-1945 warships, so I have to speculate without any factual basis to really model things on, but am attempting to do the best I can, in the hopes of getting enough of a discussion going on that folks more knowledgeable than I will join in and steer us in the right direction.

That being said, could we see an interesting, and perhaps viable additional AAA anti-aircraft defense capability coming into service in the early years of WWII, based upon massed mortar fire being used to fill an incoming aircrafts attack profile with huge numbers of explosive projectiles? A notional DD, with ten of the multi-tube arrays (36 barrel type), could put some 360 mortar rounds in the path of an incoming aircraft, per volley, while the same DD, with the (60 tube array), could put some 600 mortar rounds out, per volley. Rate of fire would be an issue, of course, but my main concern is training gunners not to shoot at an incoming aircraft at all, but rather the volume of space their attack profile should have them transiting through, and then filling said volume with massed fragmentation projectiles.
Deckspace for 10 4" 36 Barrel type would mean serious sacrifices. Deckspace was valuable, take a Gearing class DD, 390.5 feet long, 2600 tons standard. As designed armed with 3x2 5"/38 DP, 2x2, 2x4 40mm Bofors, 11 20mm Oerlikons, 2 Depth Charge Racks, 6 K gun depth charge throwers, 2 5 tube 21" torpedo tubes. It has no spare deckspace, when an additional quad 40mm Bofors was added they had to get rid of the rear set of torpedo tubes

To be aimable your 36 tube array has to be powered. Just going to assume it takes up about as much space as a Hedgehog, closest thing in OTL mounted shipboard, which when powered displaced a Twin Bofors mount. Ergo fitting a Gearing size DD would require removing all the Bofors and Torpedos most likely, may need to drop some depth charges or main gun mount too to fit 10
 
The Americans did experiment with the 12" mortars of the Manila Bay defenses. Those were field modifications of existing anti-ship shells. It didn't work out.

Crews put in timed fuses in an attempt to get the HE rounds to explode in the air rather than after they buried themselves in the ground. Didn't work well.

AFAIK there was no attempt to use them as AA rounds.
 
Well, the first problem you run into with this idea, @Shadow Master , is that with mortars you're stuck with an AA barrage and barrages are pretty crap as an AA defense.

The second, as has been already pointed out, is that the low muzzle velocity of these things makes them rather useless against dive bombers and torpedo bombers. Against torpedo bombers, the low velocity of these weapons means you're using arcing fire, which is really bad against aircraft. You want as close to direct fire as possible. Against dive bombers, people have already pointed out the ceiling, so I'll just point out that the 20mm Oerlikon, the standard light, last-ditch autocannon of the Allied powers, has a ceiling of 10,000 feet. The shortest-range weapon in the late-war arsenal still has more than enough AA ceiling to menace dive bombers in their dives.

On top of that, no matter what the mortar isn't going to be able to reach up and touch dive bombers before they reach their dives, which is the ideal time to shoot at them to begin with. So what this means is that your notional AA mortars can't stop the standard anti-ship airplanes from releasing their weapons, which means you're competing with light autocannon in the 20mm range here. 40mm autocannon, on the other hand, can reach up and touch dive bombers before they hit their dives; the Bofors, for instance, has almost as much ceiling as contemporary dive bombers do.

Which leads us to the third problem, which is that in every respect that impacts ship footprint these mortars are complete pants compared to a decent 20mm autocannon. Whether it's the 24" mortar or the massive clusters, the deck space requirements are enormous, the topweight requirements are enormous, the ammunition is bulky, you will need remote power control for this thing, which is more weight and space requirements, and reloading is going to be a nightmare that simply amplifies all the other problems. If you're doing muzzle-loading - and I really don't see how you can avoid that - you either have to space out the mortars so the crew can walk around them, which dramatically ups the deck space footprint of each tube, or you need to lug around a fucking crane, as the land-based 24" mortar had to do.

And the nail in the coffin? Fire control. If this thing is going to be of any use it's going to have to be tied to the central AA fire control system. And that's still not much use, because due to the low muzzle velocity of these mortars you're trying to predict things a half minute in advance and that just doesn't work for targets as fast as aircraft. It works for battleships, but battleships are slower, less nimble, and move on only two dimensions instead of three.
 
I recall reading about British ammunition for mortars means for use against helicopters. It was an article from either the 80's or 90's.
Can't find the article though. So AAA usage from mortars may not be such a crazy idea after all.
 
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