Light speed projectiles

darkmaster · Post by **darkmaster** » Wed Nov 14, 2018 5:00 am

So, this belated NaNo, I kind of stumbled into a really strong submission. By happenstance I started watching this live reading of an r/HFY submission. At the beginning of November the reader had been in the middle of a break to work on their thesis and so I went to Reddit to finish the story and happened upon an open invitation to also write stories in the universe. He's a real nice guy, retired commissioned officer, and he's been providing me with details about the world.

One thing he mentioned in a document he sent me, was that standard military issue is a coil gun that fires projectiles at relativistic speeds. I think he's under the impression that because the projectile is propelled via electromagnetic impulse there wouldn't be any recoil. Of course that's not how physics works.

I have a pretty good idea what would happen to anyone actually firing such a weapon, but I was wondering if any of the big brains here at the den could give me the numbers on that. I'm really interested to know how hard that gun is pushing back as the projectile leaves the barrel.

TiaC · Post by **TiaC** » Wed Nov 14, 2018 5:21 am

Well, this is conservation of momentum. Now, relativistic speeds mean that the mass of the projectile will increase as it accelerates in some nasty bit of calculus. Without that, the gun would have the same momentum in recoil as the projectile, and would travel at (speed of the projectile) * (mass of projectile/mass of gun). Energy can then be derived from that (E=.5mv^2). With a gunpowder weapon, the acceleration would be highest at first and trail off, but I'm not sure if the same would be true of a coil gun.

Hicks · Post by **Hicks** » Wed Nov 14, 2018 6:25 am

https://what-if.xkcd.com/1/

darkmaster · Post by **darkmaster** » Wed Nov 14, 2018 6:41 am

A coil gun is a projectile weapon system that accelerates a projectile via magnetism inside the barrel rather than a chemical reaction. So similar to the function of a firearm.

Alright correct me if I'm mathing wrong. But.

299,792,458*(4.02/4127.69)=291951.877921

E=.5(4127.69)(85235899021.6)

E=175,913,680,000,000 joule which makes 129,747,271,908,073 foot pounds of energy imparted upon the weapons user.

I just got visions of marine battalions exploding upon firing their weapons.

darkmaster · Post by **darkmaster** » Wed Nov 14, 2018 6:46 am

Hicks wrote:https://what-if.xkcd.com/1/

Neat, thanks XKCD.

TiaC · Post by **TiaC** » Wed Nov 14, 2018 7:06 am

darkmaster wrote:A coil gun is a projectile weapon system that accelerates a projectile via magnetism inside the barrel rather than a chemical reaction. So similar to the function of a firearm.

Alright correct me if I'm mathing wrong. But.

299,792,458*(4.02/4127.69)=291951.877921

E=.5(4127.69)(85235899021.6)

E=175,913,680,000,000 joule which makes 129,747,271,908,073 foot pounds of energy imparted upon the weapons user.

I just got visions of marine battalions exploding upon firing their weapons.

Your math is completely wrong, as you are using newtonian physics at relativistic speeds. If you were to accelerate a projectile to c, it would have infinite mass and therefore you would need infinite energy.

If we assume low relativistic speeds it seems that it starts to matter at around .15c (source).

darkmaster · Post by **darkmaster** » Wed Nov 14, 2018 7:22 am

Never mind, I forgot a decimal point.

TiaC · Post by **TiaC** » Wed Nov 14, 2018 7:30 am

Probably, but I'm not a physicist so I can't say for sure.

The concerns raised in the xkcd are still an issue, as you are so far past the speed of sound that unless you are in a hard vacuum, your projectile will be destroyed by the stress and its energy will start to diffuse.

Lasers will probably work better. If you are really doing hard sci-fi, then infantry wouldn't exist, humans are just too slow and fragile to keep up with drones.

darkmaster · Post by **darkmaster** » Wed Nov 14, 2018 7:36 am

I mean, how hard are we talking? But no, not hard sifi, FTL exists in form of gravitationaly induced space-time contraction. So, you know, magic might as well just exist.

Drones are an interesting idea though, because they would probably be cheaper than human lives too.

darkmaster · Post by **darkmaster** » Wed Nov 14, 2018 8:18 am

Okay, I redid my math at actual .01c, instead of 01c.

3,000,000m/s(4.02g/4127.69g)=2,554.17m/s

2,554.17m/s^2=6,523,784.39m/s

E=.5(4127.69g)(6,523,784.39m/s)
E=13,464,079,794.4J

Which comes to a mere 7,324,431,457.35 foot-pound

Thaluikhain · Post by **Thaluikhain** » Wed Nov 14, 2018 10:04 am

Hicks wrote:https://what-if.xkcd.com/1/

XKCD misses a bit here:

"Suppose you’re watching from a hilltop outside the city. The first thing you see is a blinding light, far outshining the sun. This gradually fades over the course of a few seconds, and a growing fireball rises into a mushroom cloud. Then, with a great roar, the blast wave arrives, tearing up trees and shredding houses."

Between the light and the blast, you also get the heat flash. And there can be quite a while between that and the blast, maybe long enough for people to think things are over. Duck and cover and stay covered.