lil-miss-choc:

beveryveryquiet:

fuckyeahfirefly:

uncdan:

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. 

I don’t usually post these types of things here at FYF, but this one was so well thought-out and had historical research behind it and I loved it too much not to put here. Apologies from Laurel.

The assertion that railguns don’t have recoil is unfortunately not true. It’s a closed system; you’ve got an force causing a projectile to go forwards rapidly, and a consequent opposing force in the gun. Even if it’s a railgun, bolt that thing down, or both the rail and the projectile will shoot off in opposite directions as we conserve momentum.

The reason why a hand-held weapon is useful is that the impulse is small. A 147grain 9mm round will have a momentum of perhaps 5kgm/s at most. A 200kg hovercraft (2 small men + kit - I’m being generous with how light this kit is) at 15m/s (33mph) will have a momentum of 3000kgm/s. If we assume momentum is conserved, the change is around 0.0016%.

In comparison a .50 BMG at 50grams, 860m/s muzzle velocity has a momentum of 43kgm/s, or around 1.4% of the hovercraft’s, so still probably not enough to worry about. That impulse could begin to need steering correction if it was shot perpendicular to the direction of the hovercraft, though - the uncorrected result would be a side drift of 20cm per second, which could start to stack up if you were firing repeatedly. I do like the idea of an engine-based corrective system, which could look at the angle and add a bit of steering to compensate.

But again, these calculations hold whether you’re using a railgun or a chemical propellant.

A human can be used as a crude squishy meatbag damping system, like a fancy recoil damping system attached to the chassis. This reduces the force by increasing the duration of the impulse, and is probably preferable to the propulsion system to a more sudden change. But just having a human is in the way doesn’t stop the momentum transfer to the hovercraft, because Newton said so. Whether you choose a big, old-fashioned explody gun or a fancy electro-kapow device, it doesn’t make a difference. Whilst we’re on it, lasers also have recoil, but you’re firing tiny, tiny photons so the momentum is minuscule - a laser gun to slice and burn a hole through flesh these might be a better alternative instead of trying to punch a metal lump through a human at high speed.

Now I feel bad, because this was a fairly major post in response to something in a TV show. Hmm. Firefly is awesome.

Don’t feel bad - the point I was going to make was the planes that fired through the propellors were actually used in the Second World War. I know this because my granddad was one of the technicians who fixed them. They were still unreliable, though. When pilots got in from a fight, the techs asked them if they’d fired the gun successfully (ie without shooting off their own propellor). If they had, then no-one was allowed to touch the mechanism until the plane went out again.

Another random interesting fact: the tips of a propellor on a WWII aircraft actually exceeded the speed of sound, though the planes themselves did not.

Others responded regarding recoil and railguns, so I don’t need to add more. As always, thank you for the corrections! I sadly stopped doing Physics in IB, so it’s been a while. 

I’m pretty sure that interrupter gear was developed in the First World War, though. A lot of WW2 planes had their machineguns mounted in the wings. Not all, but many, thus avoiding the problem of shooting through the props, since they could connect the cockpit trigger electrically, whereas WW1 pilots still had to manually use one hand to pilot and another hand to shoot the machine gun. Kind of an invention of necessity. 

I’m not doubting that they were further developed in WW2, but I’m pretty sure the first instances of it, however unreliable, were from WW1. 

And I just checked Wiki to make sure, and yes. The British were mounting interrupter gear or synchonisers onto their Sopwith Camels in 1916. The French and Germans had been working on it before that, too.