Okay, so I’m contemplating firing a gun in a vacuum. So of course I searched the archive to be sure that a gun would even fire there (it would, I gather). I’ve pretty much figured that I’ll have to have my back firmly against something pretty big or I’ll go flying off when I pull the trigger. So far so good. But while perusing this old thread:
This was the first I’d run across that idea, so I searched some more to see if there was any other discussion, since there are a few “firing guns in outer space” type threads. But I didn’t find anything.
So what’s the story? Will everyone in the fight only get to fire once or twice before their guns melt? This doesn’t make intuitive sense to me. It seems to me that if this were true I could step outside my space station in jeans and t-shirt and not get cold, since I woudn’t be losing heat. In fact, I don’t see why anything at all would ever cool down in space, except maybe very, very slowly, given that space isn’t exactly a perfect vacuum. What’s the deal here?
Just so it’s clear, this has nothing to do with being in a vacuum, and everything to do with zero-gravity. If you’re talking about a vacuum on Earth, you would handle recoil just the same way as in atmosphere on Earth.
As for losing heat in space, according to my calculations, humans would radiate heat at about 600 Calories per hour. So, you can cool down eventually via radiation.
Things cool down in space due to radiation. Every body above absolute zero radiates in the EM spectrum. Humans mainly radiate in the infrared. So, yes, you’d cool down in space, if the sun were not shining on you.
Right–sorry, I wasn’t clear there. The vacuum I’m working with is outside a space station, so it’s also zero-gravity.
Okay, and QED said more or less the same thing. That sounds like I would expect. Except I’m not entirely sure how to translate “600 Calories per hour” into something I understand very well–sorry, that’s what a degree in music gets you. If I’m in my t-shirt outside the station, at normal body temp, how cold am I a minute later? Or, more to the point (since I guess I’m not just hot, my body is generating heat, which complicates things) if I take my cup of tea outside and it’s–I don’t know, how hot is a cup of just-made tea? How long would it be before it froze?
And actually, that question is still kind of off what I’m looking for-- is it the case that a gun cooling down after firing would take longer than, say, inside my space station (where there’s air), and would this mean my guns would melt, or burn the people firing them in a gunfight?
How much of that is taken in by the gun, and how much is expelled?
How many calories can the average gun metal take before it warps?
Also, as a personal sidenote, how would vacuum affect the chambering of a semi-automatic pistol?
My instincts tell me that the “severe warping” you mention really shouldn’t happen after so few shots. If that were so, fully-automatic weapons would have to wick off heat into the air at a fantastic rate, before the heat in the air around the barrel had time to dissipate. Besides, radiation isn’t the sole means of shedding heat in a vacuum; when you fire the gun, it’s going to vent a lot of particles and gasses, all of which will be carrying heat out with themselves. If those gasses linger around the gun for a short time (during which they’ll also be losing heat by radiation), you’ve got a medium for heat loss by conduction/convection.
The only thing I could come up with on Google was this article comparing heat buildup/dissipation between a carbon-fiber barrel and a steel barrel. As you’ll note from the charts, immediately after firing fifteen .22-250 rounds through the traditional steel barrel, the temperature rose 48 degrees, from 54 to 102 degrees Fahrenheit. Now, it’s possible that there was incredible heat during firing that dissipated very quickly, but based on the slowless of residual heat dissipation, I’d have to think not. After 10 minutes of resting in Earth’s lovely air-filled atmosphere, the temperature had dropped only 17 degrees. The author says, “I know from experience that it could take as much as 90 minutes for a barrel to return to ambient temperature.” I suppose that it may take longer for a gun barrel to cool in space, but unless you’re shooting hundreds of rounds, you shouldn’t reach a steel-melting temperature to begin with.
And all of the above notwithstanding, let us not forget the words of the Master, who noted that, “space will make objects that are floating around in it cold–in some cases, very cold. Space is what’s known as a “temperature sink,” meaning it sucks heat out of things.”
I don’t know if you’re pondering this for a science-fiction story you’re writing, but if you are, why not just put heat sinks on the gun, or a compressor-based liquid-cooling system?
I was asking so I could figure out if I needed to do just that. I know very little about guns to begin with, but I’ve got two kinds involved in the outside gunfight–a number of machine-gun type weapons, that fire a long burst of shots, and a pistol, one shot at a time thing. I’m not planning on going into great detail describing any of them (except the pistol, which has to meet certain requirments earlier in the story), and I can always order my equipment from Clarkeian Technologies, Inc. of course, but I’d like to get the basic physics of the situation down so I know what I’m dealing with. I was kind of dismayed by the idea of the “big radiation fins” mentioned in the post I quoted–I want guns a single person can carry, and I want them to fire more than once or twice. And for various reasons, the pistol can’t have a big heat sink on it.
Not that most people will care much about it, and it’s not as though I’m not playing fast and loose with things like faster than light travel and a galactic empire with an economy that has no visible means of support and such, but I’ve found that when I read a story that gets the little details right I can accept most of the rest of it–it’s the little things that really throw me out of a story. I’ll buy a werewolf health inspector walking into a restaurant on a writer’s say-so, but tell me he signed off on a refrigeration unit in the kitchen that was running over 40 degrees F and I’ll put the book down. If that makes any sense.
Which is why I’m trying to understand what the details are, so I can figure out what I can ignore and what I should address, if only in my notes.
FWIW-
The cooling device need not make the gun a squad-operated weapon.
Many rifles today have “fluted” barrels. The fluting is grooves cut lengthwise on the outside of the barrel. This is claimed to aid in cooling because it increases the surface area exposed to air. Bead blasting is also said to help, for the same reason. There is some debate on how effective this is, but for a sci-fi novel, I think the theory is sound enough. It sure sells a lot of custom barrels. I have even seen some barrels with “radial fluting”; grooves cut around the circumference (think o-ring grooves).
For the machinegun, consider the Fabrique Nationale MAG58, known as the M240 in the US. It’s in 7.62 NATO and one of the finest light MGs ever made. I’m a gunner in an M1 Abrams tank. At my first gunnery, I fired a continuous burst that I figured was about 40-50 rounds without a jam. Turns out it was 196 rounds :eek:
Put a muzzle brake on both guns to help tame recoil. Gases exiting the barrel cause part of the recoil. A brake will vent the gases to the side or even slightly to the rear, rather than straight forward. Put more vents going up to counter the gun’s tendency to rise in a full-auto burst.
I wouldn’t lend any credence to the barrel-warping scenario. As has been mentioned, convection acts rather too slowly to be saving all the earth-bound gun barrels.
I’d say, if the gun doesn’t depend on special cooling on the ground, it’s find in space. (If rather un-suited as to re-coil, etc). Note that several guns do overheat on the ground, so it’s not like that’s anything special.
Also, several varieties of earth-bound machine guns are air/water cooled, or require barrel changes when they overheat. These would remain concerns in space.
Wow!! How big is it? Is there a picture of it, or something similar? And, since my original question was about heat–I imagine it gets pretty hot if you fire it awhile! Does it ever get too hot to work with, or can you just fire away as long as you have ammo?
Now see, I had no idea this sort of thing existed. That sounds like a great idea–what does it look like? Just vents at the end of the barrel, or what? My main character doesn’t know anything about guns, either (imagine that!) but she finds herself with one, and that one I have time and opportunity to describe.
when comparing heat loss of humans to metal/inert objects, keep in mind that most of the heat loss from humans is caused by evaporation of liquids. In a vacuum, exposed bodily fluids (sweat, moisture in the lungs and nasal passages etc.) will boil away at body temp. due to the lowered atmospheric pressure. This boiling will shed heat at a very quick rate.
Heat sinks such as metal blocks with many grooves or fins won’t work nearly as effectively in space as in atmospher. Although the added mass will be able to absorb a little more heat, the main function of heat sinks is the dissipation of heat by creating more surface area for contact with more air. No air will mean no heat dissipation through convection. You’ll still be left with radiation as your only method of heat loss.
My WAG would be that your best bet would be some kind of liquid cooling system - could even be an after market addon to earth based weapons brought to orbit. Or if you’re looking for something a little more exotic - some kinda gun that fires tiny self contained rockets, which would do most of their acceleration (and heat build up) after they’ve left the gun barrel.
One more thing to consider is the recoil overall. Chances are, a single shot from a gun would not push you that far back, even in zero gravity. Sure, you might move a little, but remember that the gun is going to exert the same force on you as it does on the bullet…and the bullet’s only about 4 grams…your body mass should be quite difficult to move. Going through some quick calculations…assuming you fire a 5.56 mm NATO round (4g), at a pretty high velocity of 900 m/sec (which is about right), assuming it reaches final velocity in .003 seconds (I have no idea how fast a bullet really accelerates), and assuming you weigh 180 lbs, the recoil should theoretically accelerate your body at a whopping rate of: ** 0.015 m/s/s**
So, you’d accelerate at about 1/650 the rate of gravity. I don’t think I’d worry too much about the “bracing yourself for the recoil.” This is assuming I didn’t do a massive screwup in my calculations, which I don’t think I did.
I’ll see if I can get close-up pics of ours. Don’t know if that will be possible though.
About heating the barrel: I don’t know how long a burst can be fired before the gun is damaged (I don’t plan on finding out either. I really don’t want to pay for a broken MG that I can’t have). One thing that can happen is a “runaway gun” (will not stop shooting). The chamber can get so hot that heat soaks through the cartridge and ignites the powder.
Well, the answer here is slightly complicated. The amount of heat that a human loses by radiation is indeed much less than what he or she produces by metabolism. This should be obvious when you think that here on Earth we lose heat by convection as well as by radiation, but we don’t cool down. If that were all there was to it, a person who stepped out of a space station would indeed heat up, not cool down (at least until he or she died of asphyxiation, when the production of metabolic heat would cease and cooling would commence). That’s why an astronaut in a space suit needs quite a lot of cooling.
But radiation is not the whole story. Space is not only empty of air, it is also empty of water vapour. And the saturated vapour pressure of water at human body temperature is considerably above zero. So it you went out without a spacesuit your body moisture would evaporate away to space (some of it would even boil). And in evaporating it would absorb latent heat of evaporation from the rest of your body. You would freeze-dry (after you asphyxiated, that is).
I recall once reading the account of an astronaut (Mercury program, IIRC) who ws accidentally exposed to a vaccuum. He passed out from anoxia after about 15 seconds. And he reported (after resuscitation, obviously) that the last thing he remembered before passing out was the feeling of the saliva boiling in his mouth.
Would greater area cooling fins radiate more as well? I’m not saying it’s significant, but if so they would do something. They’d have to point out, not just be a honeycomb of course.