Why are cannons never seen pointing skyward to fire to a steep ballistic arc?

by circle_square_leaf

It seems that cannons would be able to greatly increase their range by firing a steep parabolic arc, like a mortar.

I've seen primitive mortars used as seige weapons, squat things with loose projectiles, sometimes explosive projectiles. I wonder why the same concept couldn't be used for a proper cannon; longer barrel, precisely measured powder, tight projectile, etc.

I can understand why a flat firing angle would be preferable from a ship, but from a fortress, or perhaps protecting coastal settlement from attacking ships, why wouldn't they ever fire along a high ballistic arc?

hborrgg

Just to clarify something quickly, cannons generally were used alongside actual mortar peices which were frequently shot in an arc at very high angles of 45 degrees or more. This also seems to have been how most of the earliest gunpowder siege artillery was used. Bert Hall in Weapons and Warfare in Renaissance Europe notes that descriptions of sieges during the mid Hundred Years War typically mention the stone balls crashing through the roofs of buildings rather than being used to break down walls.

As for why the longer barreled cannons were typically only fired at low angles, it has to do with accuracy and aerodynamics.

Precisely estimating the range of a distant target on uneven ground was a very difficult thing to do. And the farther away the target was, the smaller the margin of error had to be. Typically this would have to be done by simply watching where the previous shot fell and making adjustments to the gun's angle after each one, but this took time and was made difficult if the shots flew inconstantly due to imperfections of the gun, recoil, inconsistent powder, or the magnus effect, or if it became difficult to see where exactly each shot landed due to smoke or terrain. Other methods existed, one mentioned in 16th century manuals involved pointing two guns directly at the target and then carefully measuring the angle and distance between each one to solve for an imaginary triangle, but again this wasn't always precise and could take time without access to calculators. As a result the most accurate way to shoot was to just get close enough that a gunner could point his sights directly at the target without having to worry about the precise distance, i.e. "point blank range." The point blank range could be greatly extended by making much higher velocity cannons with much longer barrels and more powerful gunpowder.

A round projectile wasn't very aerodynamic however, and at higher velocities, especially when shot faster than the speed of sound, would lose speed very quickly due to air resistance. Even before they had the correct math to describe it, early gunners noted that higher velocity or "violent" shots seemed to travel with an almost flat trajectory for a certain distance before it suddenly started to drop off very quickly. When Benjamin Robins first discovered a way to measure the instantaneous velocity of a cannonball in flight during the 1740s, his experiments showed that a British 24-pounder should have been able to shoot more than 15 miles based on it's muzzle velocity, but the actual maximum range was less than 3 miles due to air resistance.

Basically early gunmakers could increase their weapons' point-blank range quite a bit by increasing velocity, but this was very inefficient when it came to long-range performance. Additionally, one of the simplest ways to improve muzzle velocity was to design artillery that fired smaller or lighter cannonballs, despite these having even worse aerodynamics overall. Shooting a lighter ball or with a higher velocity can also increase the impact that forces like the magnus effect can have on the projectile's trajectory, further decreasing precision at it's maximum range. It also guarantees that at longer ranges the projectile is likely to strike the ground at a high angle, which when shooting at something like a wall, a ship, or a line of soldiers results in a much smaller "killing zone" which is the margin of error by which the gun's angle may be off but still hit the target. Conversely, shooting a high-velocity round ball at a low angle to flat ground or water could sometimes make the "killing zone" even larger if it continues to bounce or roll for a while after first striking the ground. Here's an illustration of this concept from P. B. Hughe's Firepower, note that in the first example a shot from the cannon is likely to hit a man standing anywhere within a 900 yard area without having to make any adjustments to the angle of the cannon.

The other main advantage of a higher velocity cannon was the damage it could do. K.E.= (1/2)mv^2, meaning that a relatively small, iron cannonball traveling at a very high velocity could potentially do as much damage as a much larger cannonball with more mass but a lower velocity, and could do so for much cheaper and a higher rate of fire. But again this would be limited to very short ranges due to how quickly the lighter projectile would lose speed due to air resistance. In the 16th century, though their guns could shoot much further, to effectively batter down the walls of a town or fortress it was still considered best to bring the artillery within just 150-200 yards using trenches. In naval combat it meant that a captain had to be very cautious about when he fired his cannons and how many since their effect was noted to be far more devastating when fired at extremely close range right before boarding. If he fired off too many of his guns from too far away, then he might not have time to reload before the opposing ship closed the distance and retaliated with a far more devastating broadside.

So to sum up, firing at steep angles just wasn't something most cannons were optimized to do. For shooting at a high angle over the walls of a town you instead want a mortar which can shoot a very large, heavy projectile at a very low velocity so that it loses far less of its energy due to air resistance and lands with much greater precision at the end of its arc. Guns which shot at a much higher velocity could be much more accurate and much more powerful overall, but only at shorter ranges when shooting straight ahead.