September 30, 2018

Secondary Armament - Dual-Purpose Guns

The battleship had emerged from the First World War with two separate batteries of medium-caliber gun, one for shooting at surface targets, and one for air targets. The obvious inefficiency of having to carry two separate sets of guns prompted a search for a gun, and a mounting, which could fulfill both roles. Both destroyers and aircraft were getting larger, tougher and faster, and countering them required a gun with reasonably high muzzle velocity, a large shell, and a high rate of fire. The US and Britain both installed these weapons on their treaty battleships, and refitted their older battleships with similar batteries where possible.

Iowa's starboard DP mounts1

Another major driver of DP guns was improved AA fire control. The first generation of AA guns had relied on barrage fire, attempting to saturate a patch of sky the target airplane would fly through. During the interwar period, however, systems like the British High-Angle Control System and the American Mk 37 director2 were developed which would theoretically allow accurate engagement of aerial targets. However, this method required high muzzle velocity, as the target could maneuver in the dead time between aiming and the shell reaching the target. This fit well with the need for higher muzzle velocity to reduce the dead space for anti-destroyer fire.

4.5" secondary battery on HMS Valiant

The British, after a complicated process that saw them investigating 4", 4.7", 5.1" and 5.5" DP guns,3 selected a 4.5"/45 gun for their reconstructed capital ships. This weapon used a so-called "Between-decks" (BD) mounting, which penetrated below the deck the gun was on, instead of simply sitting on the deck like previous mounts had done. This gun was a reasonably powerful and effective weapon, but fired a fixed round that weighed 87 lbs, of which 55 lbs was the shell. As the 74 lb round of the 4.7" gun on the Nelsons had been found to tire the crew easily, this decision is hard to explain, and it limited the rate of fire to no more than 12 rpm.

Secondary 5.25" turret on HMS King George V

However, the power of the 4.5" gun against surface targets was considered suspect, and another weapon was sought for a new class of cruisers. The resulting 5.25"/50 was loaded separately, and the 80-lb shell was considered the largest that could be manually loaded. Unfortunately, the British had made the same mistake as on the 4.5", and in service the 5.25" was found to be cramped, which limited the rate of fire to no more than 7-8 rpm. The weight of the mounts limited the King George Vs to only eight twins, and the mounts themselves were unreliable. Worse, the power training and elevation gear was not fast enough to keep up with high-speed targets. Power operation was critical to these mounts, as they were much too heavy to handle manually in action.4 This meant that emergency power for the secondary battery was now vital, and deficiencies in this area were revealed during the last battle of Prince of Wales.

General arrangements of the twin 5"/38

In contrast to the struggles of the British, the Americans managed to solve the DP gun problem brilliantly, with the famous 5"/38, one of the greatest naval guns ever. In various single and twin mounts, it was fitted to everything from battleships to destroyers to merchant ships to aircraft carriers. First introduced into service in 1934 aboard the Farragut class destroyers, the 5"/38 was everything a weapon should be. Reliable, accurate, and powerful, it gave good service against surface and air targets, shooting down 30% of Japanese aircraft killed by AA fire during the war.5 The last guns were not retired from sea service with the Mexican Navy until 2014.

Missouri fires her 5" battery on her shakedown cruise

The Americans had done a good job evaluating the limits of loading, and the gun was separately loaded with a 55 lb shell and a cartridge which weighed about 30 lb when full of powder. Power ramming and well-placed trunions meant that the standard rate of fire of 15 rpm per gun could be maintained even at high angles, and a good crew could reach 22 rpm for a short time.6 Another key factor in reaching this rate of fire was the mount design. Traditionally, AA guns had been supplied with ammunition from a fixed position on the deck, which then went into the fuse-setter near the gun, and into the breech. This path could become tangled at high elevation or at extreme train angles, and on almost all 5"/38s, ammunition was instead supplied through a set of hoists at the center of the mount. These hoists meant that the ammunition was always supplied to the crew in a good position, and the integrated fuse-setters removed a step that had previously slowed the rate of fire significantly.7

Inside one of the 5" gunhouses on Iowa. Note hoists on the left, labeled 4 and 5.8

With a muzzle velocity of 2,600 ft/sec, the 5"/38 could hurl a projectile to a range of about 18,000 yds, and as high as 37,000 ft. In theory this makes Iowa a threat to airliners at cruising altitude, although in practice, range at that altitude was essentially zero. Several varieties of shell were available. The most widely used was AA Common, a high explosive fragment shell initially fitted with time fuses and later with proximity fuses which were essentially a tiny radar that would set the shell off when it got close enough.9 These shells could also be fitted with fuses for use against surface targets. Regular common shells also were used, with less explosive and more armor penetration. Other varieties of shell included white phosphorus for smoke generation and setting things on fire,10 star shells for night combat, and "window" or chaff for creating false radar targets. Iowa was completed with ten twin 5" mounts, five on each side of the superstructure. In the 80s, four of these mounts were removed to provide space and weight for the new missiles.

5" magazine on Massachusetts

The shells and charges are stored in magazines deep in the ship. When it's time to use them, they are passed up through chain hoists into the handling room. This room, directly under the mount, serves to pass the shells from the magazines into the hoists to the mount, and the powder into the scuttles leading up through the floor. This allowed the mounting designers to avoid the problems of a mechanical coupling between the rotating mount and the fixed paths up from the magazine. 5 men manned the upper handling room, while 8 occupied the magazines.

Plan view of a 5" twin mount

A total of 14 men occupied the gunhouse proper, which a lot of visitors find hard to believe when they're looking inside. The mount captain controlled the entire mount, while each gun had its own captain responsible for maintaining it and fixing it if anything went wrong. The pointer and trainer were to take over if the remote control failed, either matching the pointers or keeping their sights on the target. The sights were set by the sight setter, who sits between the guns. Next to him was the fuse setter, responsible for setting the time fuses if the automatic control was offline for some reason. A checker made sure that the sights are on target. The primary responsibility for loading each gun fell to three men: the projectile-man, powder-man, and hot case man. The first two were responsible for placing the shell and charge onto the loading tray and ramming them, while the last-named made sure that the hot casings ejected were not left to rattle around inside the gunhouse, burning the crew.

Sister Bean with two single 5" mounts on Cassin Young

The 5"/38 was probably the finest gun of its type ever built, and gave the American battleship unmatched capability against air, surface and land targets. But it itself was not enough when faced with the air threats of the day. Lighter, faster-tracking weapons were required, and we'll take a look at them next time.

1 Note the Eagle, Globe, and Anchor on Mount 56. One of the secondary mounts was traditionally manned by the Marine detachment, and was marked as such. Photo courtesy of The Fatherly One.

2 I intend to examine both of these systems in some detail. They worked on rather different principles, and their relative merits can be evaluated by the British adoption of the Mk 37 aboard Vanguard.

3 A twin 4" was widely used, but not on their battleships.

4 On my first day as a tour guide, I got to elevate the guns of Mount 55 by hand, but it was for their use in a salute to the San Pedro Boat Parade that evening, and they didn't elevate very fast.

5 This was split evenly between guns firing time-fused and proximity-fused ammunition, although only half as many rounds of the later were fired.

6 This was not commonly done, as the fuses were set based on an assumed dead time between leaving the hoist and being fired. Shooting early or late could throw them off, reducing the chance of a kill significantly. Proximity fuses solved this problem when they were introduced in 1943.

7 The British only fitted hoist fuse-setters to a few of their guns, which usually fired about 25% faster than the same guns with manual fuse-setting gear.

8 This picture, and all but the diagram in the rest of the post, are mine.

9 The proximity fuse, also known as the VT fuse (for variable-time) was one of the great unsung innovations of the war, easily more important than things like the V-2. I'll go into much greater detail at some point.

10 The US military has only ever used WP for marking targets. That's their story, and they're sticking to it.


  1. September 30, 2018David W said...

    Now I see the inspiration for the anti-fighter mounts on the Death Star in Star Wars! I'm surprised they were that blatant about making the futuristic lasers look and work just like the real explosive weapons.

    On a more relevant subject: note 6 suggests that the fuses were on a strict timer, that started when the shell left the hoist. I had previously thought that fuses were timed from the instant of firing, either by the temperature or acceleration of firing itself. That seems both more accurate and safer than a timer with an assumed dead time. More accurate since you're only paying attention to time since leaving the gun, and safer because, well, the obvious. Would hate to have an issue with your gun that led to your own rounds blowing up inside the gunhouse because you couldn't fire them in time! Why was this done this way?

  2. September 30, 2018bean said...

    The timer was indeed started when the gun fired, and set based on the time of flight. But ToF is constantly changing, and the computer had to take the time from when the shell left the hoist into account when setting it.

  3. September 30, 2018David W said...

    Oh, I see - the assumption was in where the target plane would be by the time the shell was fired, not the whole chain of everything. Much more rational than my initial interpretation. I'm surprised that made enough of a difference in accuracy to outweigh 50% extra shots!

  4. September 30, 2018bean said...

    It usually wasn't a cost of 50%. 22 rpm is a good crew firing a short burst, and couldn't have been sustained. I suspect that the dead time was configurable, and it could have been set for 22 rpm if that was the maximum sustainable rate of fire. Actually, the biggest problem would be variable RoF. If they're consistently fast, the spotter in the director would just dial in a correction, because the shells would be bursting short. If they don't have a consistent rhythm, spotting isn't going to help much and you'll end up chasing targets all over the sky.

  5. October 04, 2019Jade Nekotenshi said...

    It's interesting to note that everything old is new again, with electronic time fuzes having made a reappearance. Of course, the clever new hack is to measure the muzzle velocity of each round and set the fuze electronically after the shell has been fired.

  6. October 04, 2019bean said...

    After the shell has been fired? Haven't heard of that. If you can do that (sources, please, mostly because I want the juicy details) then it makes sense because it's less vulnerable to countermeasures.

  7. October 04, 2019John Schilling said...

    Oerlikon 35mm GDM-008 "Millenium Gun", see e.g.

    They've (re)invented a true shrapnel round for antiair/antimissile use, and as Jade notes they inductively set the fuse just after the shell leaves the barrel, using the measured rather than predicted muzzle velocity.

  8. October 04, 2019bean said...

    Very interesting. It doesn't seem to be wildly popular, which probably means there's some hidden drawback (besides expensive ammo, that is) which is why it hasn't seen more adoption. Friedman doesn't have any more details, except that the shrapnel is actually 157 spin-stabilized penetrators, not just a regular burst.

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