So I was watching “Heart of Gold”, an episode from Firefly. 

it struck me that mounting a chemical-based projectile weapon on a hovercraft may not be the best idea. 

The attraction of a hovercraft is that it has less friction involved. So a force pushing it forward would move easily, without having to work against the friction of the ground. In the same way, though, a force acting on it in any direction would send it careening off in the opposite direction. 

So standing on it and firing projectile weapons would probably be negligible, since the majority of return force of sending a bullet flying is acting against the human body. 

But when you mount a gun to the chassis, the majority of return force is acting on the structure itself. Every shot you fire acts against the vehicle’s forward momentum. 

It reminds me of that old Pauly Shore movie, “In the Army Now.” Or at least I think it was that. In any case, the army showcases a hovertank, and they mount a full cannon on it. It looks great, until the tank fires, sending it, indeed, careening away in the opposite direction from where it fired. And it’s a practical problem, really. 

So in order to account for the reverse force by a mounted weapon, your hovercraft would have to use more energy to keep itself stable. For something like a machine-gun, as in “Heart of Gold” it would ideally be synced to the firing of the gun, so that the appropriate forward movement from the engines would be active at the same time that the reverse force applied itself. 

A system like this was developed for World War 1 fighter planes. Interrupter gear allowed machine guns to fire through the propellers without shooting the fans themselves off. So maybe for a hovercraft you could just keep your forward motion regular, and slave the rate of machinegun fire to that rate. 

Hover weapons in science fiction manifest in different ways. But if you have an energy weapon, like a laser, that could work since it has no kinetic force involved. 

The old 90s strategy game Dark Reign had hovertanks with railguns mounted on them. This could also work. Railguns work by propelling a magnetic projectile through an electromagnetic coil. The principle is demonstrated in school science rooms, but the kinetic energy even a small projectile can build is highly destructive. 

This is different from the way guns work today. They involve using a chemical, earlier gunpowder but today nitroglycerin, to basically explode in the barrel and launch the bullet through the barrel towards a target. That explosion is what causes the significant recoil of the weapon. 

So a railgun would have no recoil. All the kinetic energy involved in firing the projectile is sent with it, instead of being recoiled against the weapon or the bearer of the weapon. 

… I think way too much about this. In this particular instance it didn’t really matter in the writing of Firefly, and it was a great episode as it was and didn’t need physics involved. In addition, it added to the style of the show to have men on horseback, and the leader in a hovercraft. 

But I like to think about these things.