Need Help Settling a Debate About Guns in Space

[scorpiooooooh]

The debate is about the propagation of buckshot in the vacuum of space, which is similar to a question someone posted here in Astrophysics back in 2006. The discussion is taking place in a gamer forum where the participants are players of a game titled Halo: Reach. What started out as a simple random rant posted by someone in the forum has turned into a debate hotly contested by a bunch of folks whom clearly lack a comprehensive education in physics (including myself, guilty as charged).

The original question (rant) was this:

"why don't shotguns have infinite (correction: very long) range in space/anti-gravity rooms?

vacuum / zero g = no friction = nothing to slow down bullets = the velocity won't drop for very long periods of time"

On one side of the argument, some believe the buckshot should propagate in exactly the same manner as it would in Earth's atmosphere at sea level.

On the other side of the argument, some believe that the vacuum of space drastically changes the dynamics and that the buckshot will maintain a substantially tighter grouping over great distance.

For those who are not familiar with shotguns, a shotgun shell consists of (a) a plastic or paper shell casing, (b) the brass at the base of the shell, (c) a primer that is inserted in the center of the brass base, (d) the powder charge, (e) the wad which consists of three parts - the powder wad, the cushion, and the shot cup. (f) Lastly, in this case, 0000 (quad-ought) buckshot buffered with a granular filler of saw dust or plastic.

The shot cup has four slits along the length. The purpose of the slits is to cause the shot cup to expand (bloom) the moment it exits the muzzle, essentially slamming on the brakes, forcing it to peel away from the buckshot.

The folks at Bungie provide elaborate descriptions of the weapons used in the game.
The specs of the shotgun and ammo are as follow.

M45 TS (Tactical Shotgun) - 8 Gage 3" Magnum - 24" barrel - Mfg. Misriah Armory

Shotgun Shell - Hippo Mag (Magnum) - 8 Gage - Shot size, 0000 (quad-ought) - 15 balls of shot per shell - Shot size 9.5 mm (nominally) - Shot weight 2 oz. - Mfg. Soelkraft.

From what I have gathered reading archived posts here in physicsforum and other sites is this:

* Gunpowder will burn/explode in space (even under water as long as the powder remains dry) because an oxidizer is already present in it's chemical compound. The low pressure environment probably influences the dynamics of burn rate for the gun powder.

* The vacuum of space in near Earth, mid-Earth and far Earth orbital zones present negligible opposing forces (as compared to Earth's atmosphere at sea level) to everything that exits the muzzle of any firearm. Interplanetary and deep space present exponentially less matter per cm sq. Caveat - While gravity is always a factor, the scenario presented assumes what is commonly referred to a zero gravity.

* The propellant gasses should instantly dissipate omnidirectionally into the extreme low pressure environment of outer space the moment it exits the barrel. The difference in pressure between space and the propellant gasses is go great that the energy of the gasses becomes irrelevant outside of the barrel. (this is relevant as some folks postulate that the exiting gasses would influence the spread pattern from behind because the gasses continue to push the load after it has exited the muzzle).

* Without an atmosphere, there will not be sufficient drag to cause the shot cup to bloom. Therefore, the peeling away of shot cup may not occur at all or it will occur at a substantially greater distance from the muzzle. Conventional ballistics cease to be relevant outside Earth's atmosphere.

You have probably reckoned by now that I support the side of the debate that postulates the shot will remain in a tight cluster and loose little energy over greater distance. After reading the poster's question, my first thought was "what a stupid question". Having given it some thought, it turns out to be not as stupid or easy to figure out. Is there any real world application to finding definitive answers about shotguns in space? Maybe not in gunsmithing anytime soon, but the science may be applied to other real world situations. If nothing else, the debate is a good learning experience that promotes critical thinking and science. In my book, that alone is pretty worthwhile.


Your time and consideration in helping resolve the debate will be very much appreciated.

Scorpio

 

[tony873004]

It can't act exactly like it does on Earth's surface because of the atmosphere. Low Earth orbit has some atmosphere. That's why we woke this morning to news of NASA's weather satellite crashing to Earth.

To begin to answer your question, we have to define what me mean by "in space". In space could mean simply above atmosphere. Or it could mean "in orbit" in which case it would be more correct to say "in orbit".

Spaceship1, the winner of the X-prize went up about 70 miles, where the air is so thin it is negligible during short periods of time. But it never achieved orbit. After reaching max altitude, it fell back to the ground. If the pilot happened to open the cockpit and fire a shotgun, parallel to the ground, the buckshot would have rapidly distanced itself from the shooter in a very linear fashion. The pilot would not see the buckshot curve towards the ground, as he would if he fired the shotgun while standing on the ground. That's because they're both freefalling at the same rate (even if they're still on the way up).

Orbit is different. To be in orbit means you have enough velocity parallel to the ground that the Earth curves away from you as you drop, and you fall around the Earth. If you fired a shotgun in orbit, one of 3 things would happen:

1). The buckshot would hit Earth. This depends largely on the direction you fired it. If you were in Low Earth orbit, and you fired in a retrograde orbit (figuratively speaking, out the back window), the buckshot would hit Earth within 45 minutes, probably sooner.

2). The buckshot would orbit the Earth in an orbit that was different from your orbit, but intersected your orbit at one point. If you fired from Low-Earth orbit, it would slowly sprial down to Earth over a period of years, just like the NASA weather satellite.

3). The buckshot would escape Earth and directly orbit the Sun. In Low-Earth orbit, this is unrealistic. It needs about 3km/s of velocity to escape Earth. I don't know how fast buckshot travels, but I imagine it's a lot slower than this. In higher orbits, escape velocity drops. So if you went out far enough, your buckshot could leave Earth altogether, again dependent of the direction you fired the gun.

I'm just guessing, but I imagine that the lack of air to expand the shot cup would keep the balls in a tighter configuration, at least initially. Unlike firing from the ground, where the balls are destined to hit the ground in just a few seconds or less, in space they have minutes, hours, years, or eternity to spread out (depending on the direction of the shot).

 

[DrStupid]

vacuum / zero g = no friction = nothing to slow down bullets

That's correct. It's Newtons first law [see http://en.wikipedia.org/wiki/Newton's_laws_of_motion]

Without an atmosphere, there will not be sufficient drag to cause the shot cup to bloom.

I think that doesn't matter. The shot cup will be destroyed by the explosion of the powder and its remains will no longer influence the pellets.

I support the side of the debate that postulates the shot will remain in a tight cluster and loose little energy over greater distance

You are right in regard to the energy but the cluster will expand similar to a shot on Earth and as there is nothing that keeps the pellets together it will not stop expanding. This limits the effective range. If the target is to far away it would only be hit by a single pellet (at the most).

 

[256bits]

For those who are not familiar with shotguns, a shotgun shell consists of (a) a plastic or paper shell casing, (b) the brass at the base of the shell, (c) a primer that is inserted in the center of the brass base, (d) the powder charge, (e) the wad which consists of three parts - the powder wad, the cushion, and the shot cup. (f) Lastly, in this case, 0000 (quad-ought) buckshot buffered with a granular filler of saw dust or plastic.

The shot cup has four slits along the length. The purpose of the slits is to cause the shot cup to expand (bloom) the moment it exits the muzzle, essentially slamming on the brakes, forcing it to peel away from the buckshot

Where did you get the idea that this is the type of shell configuration?
Not all shells have a "wad-cushion-cup" or mono-wad all in one configuration, and the wad may be separate from the cushion and no cup at all. The shell casing be it either plastic or paper is crimped at the end to hold the shot in.

The spread pattern is dictated by the choke of the barrel, from no choke having the greatest spread. If the barrel is rifled ( ie the groves within the barrel like a rifle ), the spread pattern with be like a donut, but most shotguns have do not use this. The atmosphere is not the cause of the spread pattern.

As the shot leaves the barrel, it spreads out due to slight tranverse or sideways velocity of each individual shot. the spread may be something like 10 inches at 20 yards depending on the choke.
As the target becomes more distant the number of pellets that would make contact and the destruction falls drastically. About 40 yards is effective with no choke.

With the shot used in your game situation ie 15 shot per cartrgide, after a certain distance the spread patern will be so wide that each individual pellet could justl pass by the target and you will be lucky to have a hit with just one pellet.

 

[Chronos]

The pellets are moving apart as they leaved the barrel. No getting around that. Not as quickly without an atmosphere, but, they will disperse.

 

[Drakkith]

I think the key difference is that without friction from a significant amount of air, the pellets will continue unimpeded for very long distances, but continue to spread out as they go. So the "range per pellet" is enormous, yet after a certain distance the spread is so great that your chances of hitting a man sized object is slim.

 

[scorpiooooooh]

A visual aid.

 

[gregtomko]

I think the question was about the spread of the shot as it travels outward. The forward velocity would decrease less while the lateral velocity, because of its relatively smaller magnitude, would remain more similar in lower atmospheric pressure. The pattern would be a tighter distribution per distance traveled the farther you get from sea level.

 

[gregtomko]

if you need help with your debate, without equations, you can think of it in a simple thought experiment. Most people can understand when you double your speed, your wind resistance is squared. If there is no air, there is no wind resistance, so there is no penalty for increased velocity. The forward velocity and the lateral velocity will remain constant, and the pattern will be tighter. The more air, the more the greater forward velocity is opposed relative to the lesser lateral velocity. If the wind resistance causes the shell to expand as it exits the muzzle, this tightening will only be more prevalent.