Why do mortar shells have fins, but not artillery shells?

Question

Why do mortar shells have fins, but not artillery shells?

In: Engineering

Answer 1

You got the answers about the presence of rifling or not and spin stabilization.

Spin stabilization is standard for 155 mm artillery like NATO countries uses. That being said, there are some fin stabilized artillery shells.

For some example of artillery finners:

There’s the PGK fuze: https://en.wikipedia.org/wiki/M1156_Precision_Guidance_Kit which basically adds some guidance to shells using fins on the fuze. The projectile is still spinning, and the fins are kind of stubby/not where you’d expect them, but they are there.

Then, there is the M982 Excalibur which has proper fins and trajectory control: https://en.wikipedia.org/wiki/M982_Excalibur

The M107 and M759 which are ye ol’ typical artillery projectiles you were likely thinking of do not have fins.

Answer 2

There are some good answers already in here. Yes, mortars are smoothbore and rifled projectiles don’t need fins, yes, drop firing a projectile means that you need a looser fit between barrel and bullet, which means fins will engage better.

But, there is another important bit to consider. When you’re loading a round, especially if you’re loading it manually, it’s far easier if you aren’t loading the thickest part first. You can do it much faster and are less likely to miss the opening. You want to put a narrower bit into the barrel first, then have it widen out to the full diameter. Most standard rifled bullets are somewhat pointy at the front and flat at the back. When you load from the rear of the barrel, the point goes into the hole first, and acts as a guide to feed the rest of the bullet into position as it gets bigger around. The flat back then acts to catch the gases and make sure they expand the bullet into the rifling grooves instead of trying to go around the bullet.

For muzzle loaded mortars, you want a somewhat pointy bit on the back, so they can load the round smoother and easier. And if we’re going to have a narrow bit at the back end anyway, that’s going to direct gases to the edges to try to get past the bullet, which will reduce the efficacy of rifling as well, so why not just go with fins instead?