Hacker News Reader

Comment by dandare | Hacker News Reader

Can someone please explain how the gunner/computer established the range and speed of enemy aircraft? The article says "a gunner would focus a series of dots" but unless at least two gunners are focusing the same plane I have no idea how they calculated range and speed. Thanks.

View on HN

I used to be a gunner on an M1A1, and although we had laser range finders, we also practiced with the old style optical sights that were installed alongside the main gun. How range finding works with optical sites is you usually have a series of marks in the viewfinder that you use to physically measure what you're aiming at, so if the tank/plane/whatever is between particular two pips (or whatever; edit iirc the M1 had two curved lines you would placer over the target) you know that it's about x meters away. The pips are calculated knowing the average length of the object, so this is, in practice, close enough.

The procedure is; measure the target, then use cross marks on the cross hairs to raise the gun tube to the appropriate level, manually guess lead and defilade, fire, on the way. It obviously takes practice to get good at. I found it curious that the US Army continued to teach/train/use this method, as well as land navigation with compass/mapp instead of gps. The idea is to not rely on technology entirely.

Edit: turns out I am describing stadiametric rangefinding:

https://en.wikipedia.org/wiki/Stadiametric_rangefinding

What military doesn't teach navigation with a compass? I would also imagine that most, if not all, teach rangefinding to those who might need it; snipers/sharpshooters, gunners, etc.

My only military experience is from my compulsory service in the Finnish Defence Forces, where everyone does have to learn how to read maps and use a compass (we never used gps/other equivalent). I was in signals myself, so I didn't have any training for rangefinding.

I seem to recall that the gunner on one of the more successful German tank crews during WW2 basically had the sights set for a specific distance and then eyeballed it from there.

Regarding teaching land navigation by compass:

I teach light infantry tactics to new lieutenants for the US Army. There's a couple reasons to use old-school methods over relying on your GPS:

* Using a compass and a map gives you a much better feel for terrain. One of the common problems with lieutenants is a disregard for advantages and disadvantages of specific terrain. I feel that this is by far the most important point. Understanding and visualizing terrain from a map is a hugely valuable skill that impacts most points of an operation, specifically route planning, where to place your machine guns, and where to attack the objective from.

* GPS can be jammed. This point was driven home hard by the novel Ghost Fleet. Ghost Fleet is a white paper turned into a novel (because no one would read a whitepaper) on what a future war with China would look like. I'm pretty sure Ghost Fleet is also the impetus for the Naval Academy re-adding celestial navigation to their classwork.

* GPS doesn't always work, or takes a while to get enough satellites to get a fix. You know what really sucks: taking fire and not knowing where you are.

* Batteries are heavy. Yes, tech is improving and you typically only need AA batteries. But soldier weight for light infantry is and continues to be a challenge.

In the early aughts I wrote a simulation for training infantry to call in artillery strikes. It relied on compasses and maps for exactly these reasons. Another point is that understanding what the automatic tools are doing for you makes you better at understanding the automatic tools (and why they're failing when they fail).

In WWII we saw the introduction of practical optical coincidence rangefinding, starting with naval guns and quickly moving to direct-fire artillery, tank guns, and AA applications. One gunner, but binocular vision with the aiming mechanism using angle of incidence from each eye input to calculate current range.

https://commons.wikimedia.org/wiki/File:Polish_destroyer's_r...

https://en.wikipedia.org/wiki/Coincidence_rangefinder

I don't know about the B-29 system in particular, but gyroscopic gunsights used on most WW2 fighters were fairly straightforward. First, you set a selector on the sight to the wingspan of your target (usually there would be presets for common enemy aircraft). Then you twist a knob (typically on the throttle) that adjusts a ring in the sight until it matches the size of the target - this establishes the range. Finally a gyroscope provides input on how your aircraft is moving, and since you're typically maneuvering to follow your target this is an approximation of the target's movement.

There's a lot of approximation involved, but generally it's good enough to hit an airplane sized target, especially on US fighters where you typically have 4-8 machine guns spitting out 50+ bullets per second in a shotgun like pattern.

Some relevant links:

http://i651.photobucket.com/albums/uu236/ken1926/P-51DK_03.j...

http://www.lonesentry.com/blog/wp-content/uploads/2010/09/ta...

https://upload.wikimedia.org/wikipedia/commons/d/d7/P-47_gun...

Reply on news.ycombinator.com