Quote of the Day
Old men want to date a nurse with a purse.
— Petra from Texas, a retired woman I met on my recent Alaskan cruise. She was discussing her opinion of dating older men. Basically, older men want a woman with money and will care for them.
I have watched a lot of old WW2 combat footage, and I have noticed that many of the machine guns and fighter planes had similar reticles. A reticle is a fine-grid of lines used in conjunction with an eyepiece to assist in taking measurements or with accurately pointing an instrument. Figure 1 shows a reticle similar to what I have seen in numerous combat scenes.
I became curious when I saw that there were numerous variations this basic design for these reticles. To get some insight into these reticles, I began reading some old WW2 manuals I found on the Internet. During my reading, I discovered that reticles are earlier versions of modern telescopic used by target shooters and hunters today (link). The ultimate realization of this type of reticle is made by Horus.
This post will summarize the results of my reading into a short explanation of how these reticles were used.
These reticles are designed to serve two purposes:
- measure the range to the target
The reticles of optical sights have long been used to estimate the range to a target using stadiametric methods, which I have covered in other posts
- determine the required lead for a target of a given speed.
If you assume the target and projectile move at constant speeds, you can use the reticle to estimate the amount of deflection (aka lead) required. This method of lead determination is the same as what I have discussed with WW2 straight-running torpedoes. The Wikipedia also has an excellent discussion of stadiametric ranging.
I found a close up of a common WW2 reticle called the Mk 10 in this manual. The sight is composed of the following parts:
- An outer ring of 4 inch diameter called the 200 knot ring.
- An inner ring of 2 inch diameter called the 100 knot ring.
- A tiny circular ring in the middle that is used to center the front sight.
You can also find reticles that have a third ring called "300 knot ring." Figure 3 shows a 20 mm cannon with a 3-ring reticle.
Manual Ranging Example
Figure 4 shows how the reticle was used to estimate the range to an aircraft, in this case an HE 177, which measured 50 feet between the wingtip and fuselage centerline. The gunner needed to know the type of airplane and its dimensions in order to estimate the range to the target.
Lead Determination Example
Figure 5 illustrates the reasoning behind calling the rings "100 knot" and "200 knot". If we assume that the target and projectile move at constant speed, the amount of deflection θ required to hit the target is constant. We can show that θ is constant by observing that for any time of projectile flight, the ranges covered by target and the projectile form similar triangles. This means that the the deflection angle θ will be constant.
This means we can construct reticle rings that will provide the gunner with the amount of deflection required to hit a moving target of a given speed.
The effective use of these rings requires a very well trained gunner. The gunner must be able to:
- estimate the target speed
- identify the dimensions of the specific target types
- compensate for the projectiles drop under gravity (something I have not covered in this post)
During WW2, the gunners spent much time practicing with the reticle so that using the reticle became a reflex action, i.e. done without conscious thought.