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design the aircraft with a degree of protection.
So the Yamatos don't get build, but the aircraft carriers instead.
As an island nation, remember how much the German subs troubled the UK in the Great war, and act accordingly.
compare the capabilities of own AAA outfit vs. the capabiliies of own dive bombers, the result will be that something better than 25mm is needed, and in increased numbers.
A more capable IJN also needs more fuel, that is a major road block.
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Considering Japan's ability in designing torpedoes, perhaps they should have an ASW homing torpedo like the US Fido earlier. And some more advance depth charge type throwers would have been helpful, like the Hedgehog. And a convoysystem should have been used from day 1 of the war.
Here is where I think we have too much foresight. According to post war reports the Japanese still felt at the end of the war that their 25mm was a capable AA weapon. And I wonder how much has to do with the weapon itself - fire control may have been part of the issue, as well as the fact that Japanese AA would get lesser results as US aircraft was more durable than Japanese aircraft. And with the 25mm, it had a good muzzle velocity and cyclical rate of fire. Maybe a hopper type loading system the the 40mm Bofors could work, or even making them belt fed. But the mounts on the 25mm seemed to be a big problem, too slow to traverse and too much vibration on the triple mounts. How much of this is 20/20 hindsight I don't know.
But the 5"/50 used by destroyers was horrible for AA, and you would think that would be easy to figure out. Too slow traverse/elevation, lack of maximum elevation, very difficult to load at high angles of elevation. The fact that these were competent anti-surface weapons but struggled as AA would have been rather obvious I would think. THe problems again though seem to be as much of a mount issue as a gun issue, so fix the mounts if possible or go with the 3.9"/60 gun.
For the Japanese Fido to work, much more is needed than a workable 'dumb' torpedo - namely the electronics that works for the task. The eagerness to have the 'Fido-like' torp was another thing - sub hunting was not something IJN was eager to do until too late.
The IJN was expecting from their destroyers to fight much more the surface targets/threats, rather than enemy A/C? They were not alone in this, of course.
that they don't have an engine powerful enough to provide for those requirements.
That requires having a strong armament, LMGs will struggle against the bombers. Also, fighters are of no use if they can't catch enemy bomber. The bomber will fight back with it's own guns - a fighter need to carry a degree of protection.
And this should be without the advantages of hindsight, but perhaps with the knowledge of some of the more enlightened leadership. As an example, Yammamoto was a supporter of air power and would probably like to have seen more emphasis placed on carriers than battleships.
Any idea why that was? Technological issues? I've wondered that myself and never really have found an answer. Lack of motivation for building such engine perhaps?
Looks like the Zero has 2 of these requirements down. We must remember the Zero was an early war plane that had to fight on through the war with little significant improvements.
The 2x 20mm + 2 x7.7 LMG are actually a pretty heavy armament for early war. Equivalent to maybe 6-7 x .50 cals if you believe the 3:1 ratio of 50's to make one 20mm.
The Ammo supply was a bit low for certain, though this was at least increased from 60 to 100 per gun pretty early on. And for it's time, it was indeed a fast plane. Biggest issue is lack of self sealing tanks or armor, and this indeed was it's achilles heel. And I think it was more of an issue against bombers than fighters - you can out maneuver an opponents fighter or be tough to get a good lock on. But against a bomber you are facing and aircraft who cannot train as much firepower on you as you can on it generally, but you will usually be subject to such lighter firepower even when you have out-maneuvered the bomber. Here armor (I would think most importantly in this situation the windscreen and fuel tanks) makes a marked difference.
Any idea why that was? Technological issues? I've wondered that myself and never really have found an answer. Lack of motivation for building such engine perhaps?
Things got better when the 99-2 was introduced, with better MV and ammo supply, but also a lower RoF - now 480-490 rpm vs. Hispano II at 600.
It's maybe 2:1, ratio of the IJA cannon vs. .50 BMG 'kill capacity'? The Japanese batteries were of far lower weight than USA ones, of course.
Strictly on power, not muzzle velocity and range, I'd give even the 99-1 a 2.5:1 vs a .50 Caliber. And a 7.7mm should probably be 1/3 of a .50
overall, probably equal to the 6 x.50 arrangement on many US fighters, other than the H)-1 was not the most accurate at range.