September 01, 2023

Open Thread 138

It's time once again for our regular open thread. Talk about whatever you want, so long as it isn't culture war.

Overhauls are Underwater Protection Part 2, A Brief Overview of the United States Fleet and for 2022, The American Secrecy System and Submarine Espionage.


  1. September 14, 2023muddywaters said...

    Something that feels like an unlikely coincidence, but I don't think actually is: fire control has several errors that are similar enough in size that they all matter, but that have such different physical origins that they seemingly have no reason to be. - The intrinsic spread of the gun itself, i.e. the spread it would have if used as land artillery (stationary gun and target). - Pointing error, i.e. how accurately you can point at a target from a rolling ship. Before 1917 this is basically pure human skill, though with a ~4-fold improvement around 1905 from better techniques (including practice aids) and better sights. - Rangefinder error. - The enemy dodging. This is a hard limit on unguided weapons: while minimizing the observation-to-firing time does help, at long range they can literally dodge while the shell is in the air. I don't think this is actually a big coincidence because I've also seen its equivalent in other technologies; the actual explanation is plausibly some combination of the biggest error getting the most work on reducing it until several errors become nearly equal, and us choosing to talk about it rather than some technology that only has one main error because that would be boring.

  2. September 15, 2023muddywaters said...

    It also feels like a coincidence that we just barely got to practically perfect fire control (i.e. the main limit is that the enemy might dodge while the shell is in the air) then quickly moved on to guided weapons (which don't have that limit). However, this possibly wasn't actually as close as it seems: ship-to-ship combat was at least close to that limit by ~1920 (except possibly for the "can't dodge and accurately shoot back at the same time" problem, which I don't understand enough to have an informed opinion on), while anti-air systems (smaller, tighter-turning targets = this limit requires faster-reacting systems to reach) continued to improve after 1945. It also has a plausible explanation, in that radar and better servos were useful for both better fire control and guided weapons.

  3. September 15, 2023bean said...

    Huh. I hadn't thought about either of those, but in fairness I haven't been in the fire control world much in the last few years. I suspect that it's a combination of attacking the biggest error and happenstance that the errors came out to about the same size.

    As for timing on the arrival of guided weapons, I think the common factor there is electronics. Yes, in theory you could shoot that well on the old electromechanical system, but I wouldn't be surprised if some of the same technology that went into the stable vertical also helped with guidance, and radar was obviously big in both.

    Also worth noting that very few AA systems actually tried to shoot at turning targets. Generally, if you're under air attack and the target is turning, you've done your job of disrupting the attack already.

  4. September 15, 2023Anonymous said...


    Generally, if you're under air attack and the target is turning, you've done your job of disrupting the attack already.

    True, but OTOH making the enemy replace a plane and pilot while keeping your stuff is better than merely keeping your stuff.

  5. September 17, 2023muddywaters said...

    in theory you could shoot that well on the old electromechanical system

    It seems plausible that in surface combat in good conditions (skilled operators and good visibility), a 1945 fire control system was only moderately better than a 1915 system, but that the difference could be much larger in bad conditions.

    In particular, c.1903-1917 systems required very skilled layers/pointers for good accuracy, either firing the guns precisely when the ship's roll took the sight across the target (rough estimates from the practice scores suggest that a c.1910 battleship gunlayer could do this to ~0.07sec accuracy, giving ~10% hits at 10km in typical sea conditions), or continuously moving the guns to keep the sight on the target. They hence put significant effort into giving their gunlayers lots of, and the right kinds of, practice and allowing their best layers to control multiple guns. Around 1917, this was replaced by gyro firing / selected level, i.e. automatically firing the guns at the instant in the ship's roll where they are correctly aimed. This seems to have been only a moderate gain in accuracy over the old system with an experienced layer, but could plausibly reach this accuracy even when operated by a recent recruit. This might be important if wartime fleet growth meant there weren't enough experienced crews for all the ships.

    It also didn't require the target to be visible at the instant of firing, which was useful in bad visibility. (Bad visibility is more common than you might think, as a light mist that you might not even notice in everyday non-seagoing life is bad visibility compared to dreadnought battle ranges. In the North Sea this level of visibility is the norm, not the exception.) This only worked for a short time after the target went out of sight, before the rangekeeper's predicted target position accumulated too much error, or the target turned. However, a few more shots is better than nothing, and it could be more than a few if the target repeatedly went in and out of view in smoke/splashes/patchy mist/rain.

    Later on, RPC gun mounts removed another step that had previously required skill, and radar added the ability to engage targets that had never been in sight.

    very few AA systems actually tried to shoot at turning targets

    They might not try to shoot while the target is turning, but faster-reacting systems are better on targets that have recently turned. The Mk37/Mk1A has a "the target just turned" button (p.26-28), that makes it converge faster at a cost in accuracy/stability.

    However, faster-reacting fire control systems are also useful in other situations. A target may be detected when it is already within range (to quote the above document, "most air targets that are visible at the time of initiating action against them are undesirably close"). If the target you were firing at is no longer a threat, either because you destroyed it or because it has already dropped its only bomb/torpedo, you may want to quickly switch targets.

    This need for fast reactions may be why tachymetric systems seem to have been primarily developed for anti-air use. (Tachymetric systems only require the user to hold the sights and rangefinder/radar range on the target, and automatically determine target motion from the rate of change of this input. Older systems required manually adjusting the estimated target motion to match the observed rates, which was simpler to build, but slower to use.) The tachymetric mode of an original Mk37/Mk1 was only recommended for air targets (p.42). However, the Mk37/Mk1A was tachymetric in both anti-air and anti-surface modes, and HMS Vanguard's AFCT Mk10, possibly the only pure anti-surface system developed late enough for the option to be available, was tachymetric.

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