Why do mortar shells have fins, but not artillery shells?
In: Engineering
Some artillery shells do!
That said, most modern artillery is rifled, which improves the stability and accuracy of the shell.
Most mortars are smoothbore and so need the fins to achieve stability and accuracy instead.
An artillery fires big bullets, so they have a smooth barrel and the bullet fills up the tube.
Mortar shells aren’t shaped like a cylinder like a bullet is. It’s not designed to ride the sides of the barrel in tight fit. Which means you can put find on it.
In summation, artillery : bullet :: mortar : Rocket.
Artillery shells are fired from rifled barrels which make the bullets spin. This spin stabilises the bullet, making it fly front first instead of tumbling or wobbling.
Mortars are smooth-bore, so the projectile does not have spin. To prevent it from tumbling it has fins which shift the centre of aerodynamic pressure to the rear, far behind the centre of gravity. This makes the mortar projectile aerodynamically stable.
It is very important to prevent tumbling and wobbling, as it greatly reduces accuracy. It can even become so bad that the projectile does not hit the target head first, possibly causing the detonator to fail.
That said, some artillery shells DO have fins, for example APFSDS shells – but that is because they are fired from smooth bore barrels.
Shells are spin stabilized. They bite I to spiral grooves in the barrel and are spun up when fired. Mortars, since they have to drop down the barrel first to fire, can’t have rifles barrels so use fins. In addition spin stabilisation doesn’t work with large changes in direction, the spin wants to keep it pointed am the same way, a mortar just needs to face the oncoming air stream.
Mortar shells are generally smaller and fired at a higher angle, so the fins help stabilize their flight path. Artillery shells are more aerodynamic and are fired at a flatter trajectory, relying more on spin for stability. Different design for different purposes.
Artillery shells a stabilised by rifling. Grooves in the barrel that spin the round up as it exits by digging into the outside of the round.
This is great, but it has two problems.
The first is that it stabilises the shell to the orientation of the barrel, not the direction of flight. Fine if you are engaging a target roughly in line with where you are aiming, but if you are shooting almost directly up, the shell will come down *backwards*. Fins stabilise to the direction of travel.
The second problem is that it takes a lot of force to engage the rifling. Fine for a breach loaded shell that only engages the rifling while being forced out by the propellant charge, but mortar shells are fired by dropping them into the barrel and letting them slide down. Muzzle loading a rifled barrel requires a ram rod, which if you used on a mortar would mean that the first thing the mortar hit would be the person firing it.
Mortar shells need fins because they are fired at a steep angle and need to stay stable during their high arc trajectory. Artillery shells, on the other hand, are fired at a much flatter trajectory and use rotational stability from rifling in the barrel. Different tools for different jobs!
Mortar shells typically need fins because mortars are short-range and fired at high angles. The fins help stabilize them in flight. Artillery shells are generally fired at much higher speeds and longer distances, so they’re spin-stabilized by the rifling in the barrel instead. Different stabilizing methods for different types of firing situations.
Don’t mortar shells include explosives and artillery just kinetic weapons?
Mortar also is designed to be launched at extreme angles and almost float straight down. You want the head of the device to land first. The fins ensure that happens.
There has been a lot of talk about rifling stabilization, but dancing around rhe real reason. The speed of the shell dictates if fins or spin are best for stability.
Artillery, such as howitzer, are long range weapons that shoot at a high arc. The shell is a high speed projectile so rifling can engage and the spin will stabilize the round, letting it fly straight and true.
A mortar is a medium range weapon that shoots at a high arc. The shell flies slower than a howitzer so spin stabilization would do less. Fins are a much more economical solution.
The modern mortar is a result of economics more than anything else. Mortars of old looked somewhat like cooking pots attached to a board. These short cannons were meant to lob explosives over fortifications. A light charge for a relatively short range. The modern ones have the ammunition completely self contained. The propellant and the payload are all on the shell. The “mortar tube” is effectively just a pipe with a nail at the bottom to strike the propellant primer. No longer does your mortar team need powder and fuses in addition to ammo, you just need to give them crates of ammo.
But because mortars have always been slow projectiles, spin stability has always been beyond it. Fins, while providing more drag, impart more stability without reducing effectiveness.
Latest Answers