How well would a firearm like the M4 perform in outer space?

[Purplehood]

As I understand it, heat-dissipation in space is a big issue and not nearly as easy as we imagine. Large surfaces are required to speed-up the process. I imagine that a Machine-gun barrel in space would have a difficult time "cooling off".

[The Annoyed Man]

As I understand it, heat-dissipation in space is a big issue and not nearly as easy as we imagine. Large surfaces are required to speed-up the process. I imagine that a Machine-gun barrel in space would have a difficult time "cooling off".

Well yes, it would; but my hypothesis has to do with heat dissipation through radiant light. Light does not require a medium to transfer. Heat is energy, and the barrel heat is merely the product of the kinetic energy transfered into the barrel from both the friction of the bullet's passage, and the transfer of heat into the barrel from the combustion gases as they travel down the barrel behind the bullet. (There would also be some energy transfer due to the elastic expansion and return of the chamber under each detonation, but that would be a relatively small percentage of the total.) IF enough energy from friction and hot gas gets transfered into the barrel at a faster rate than it can dissipate that energy, it will glow red (then orange, then yellow, then white, if it gets hot enough). We know this to be true from simple observation. That glowing means that the pent up energy which has been transferred into the gun barrel has risen into the visible spectrum because it has been acquired faster than it can be dissipated.

My hypothesis is that, A) since we know from observing the stars that visible radiant energy does not require a substrate (atmosphere, water, etc.) to dissipate; and B) we know that a white hot barrel is hotter than a red hot barrel and a red hot barrel is hotter than one which no longer radiates light energy; and C) since we know that stars change color as they cool; then we can extrapolate that, once firing ceases, a red hot gun barrel will in fact cool in space at a relatively rapid rate until it has dissipated enough heat energy that it can no longer emit that energy as light. Up to that point, it will cool at a much more rapid rate than a star does simply because of the mass and energy differential between a star and a gun barrel. Beyond that point, the rate of heat loss will slow down dramatically because A) heat energy in the non-visible spectrum requires a substrate to transfer itself out of the object containing it; and B) the only matter in contact with the gun is that part of the astronaut's (insulated) space suit which is holding the gun. The same principle would apply if it is hard-mounted to the exterior of a spacecraft, in which case the heat energy is managed by the spacecraft's cooling system. If the hard-mounted gun is insulated from the spacecraft's cooling system, then it will cool at a very reduced rate once the heat's energy has decreased to below the visible spectrum.

Are there any flaws to my reasoning?

Now I've got to get back to work.

[Purplehood]

As I understand it, heat-dissipation in space is a big issue and not nearly as easy as we imagine. Large surfaces are required to speed-up the process. I imagine that a Machine-gun barrel in space would have a difficult time "cooling off".

Well yes, it would; but my hypothesis has to do with heat dissipation through radiant light. Light does not require a medium to transfer. Heat is energy, and the barrel heat is merely the product of the kinetic energy transfered into the barrel from both the friction of the bullet's passage, and the transfer of heat into the barrel from the combustion gases as they travel down the barrel behind the bullet. (There would also be some energy transfer due to the elastic expansion and return of the chamber under each detonation, but that would be a relatively small percentage of the total.) IF enough energy from friction and hot gas gets transfered into the barrel at a faster rate than it can dissipate that energy, it will glow red (then orange, then yellow, then white, if it gets hot enough). We know this to be true from simple observation. That glowing means that the pent up energy which has been transferred into the gun barrel has risen into the visible spectrum because it has been acquired faster than it can be dissipated.

My hypothesis is that, A) since we know from observing the stars that visible radiant energy does not require a substrate (atmosphere, water, etc.) to dissipate; and B) we know that a white hot barrel is hotter than a red hot barrel and a red hot barrel is hotter than one which no longer radiates light energy; and C) since we know that stars change color as they cool; then we can extrapolate that, once firing ceases, a red hot gun barrel will in fact cool in space at a relatively rapid rate until it has dissipated enough heat energy that it can no longer emit that energy as light. Up to that point, it will cool at a much more rapid rate than a star does simply because of the mass and energy differential between a star and a gun barrel. Beyond that point, the rate of heat loss will slow down dramatically because A) heat energy in the non-visible spectrum requires a substrate to transfer itself out of the object containing it; and B) the only matter in contact with the gun is that part of the astronaut's (insulated) space suit which is holding the gun. The same principle would apply if it is hard-mounted to the exterior of a spacecraft, in which case the heat energy is managed by the spacecraft's cooling system. If the hard-mounted gun is insulated from the spacecraft's cooling system, then it will cool at a very reduced rate once the heat's energy has decreased to below the visible spectrum.

Are there any flaws to my reasoning?

Now I've got to get back to work.

Not really since I am not sure but I think you agree with me in general.

[Steve133]

As far as the effectiveness of lubricants in space, I have an anecdotal but I'm-pretty-sure-it's-true story to offer. As some of you know, my family was part of the Caltech/JPL community. My parents were professors there, and my father in law was a JPL engineer who designed and built guidance packages for a number of spacecraft. One of my good friends who was one of my racetrack pit-partners was also a technician at the Carnegie Institute facility on the Caltech campus, and they were building a gyroscope to be used in one of these guidance packages that was to be sent into space—I think it might have been the Hubble Telescope. The problem was how to lubricate the bearing points of the spinning gyroscope's axis. This is less of a critical issue for a gyroscope that is going to return to earth because of the sheer length of time the lubricant will have to hold up for one that is not going to return. They had a meeting about how to procure a lubricant with the properties of tackiness so that it would adhere well to the parts to be lubricated without migrating to places that did not need it, low volatility so that it would not evaporate away, the ability to absorb tremendous shearing forces, and yet the ability to maintain a thin film at the points of contact between moving parts. Brian got up, left the meeting, went out to the parking lot, and retrieved the can of Bel-Ray motorcycle chain lube from under the seat of his motorcycle. He brought it into the meeting, explained its properties, demonstrated it, and—according to what he told me—the decision was reached to use tiny amounts of motorcycle chain lube at the points of contact. Instead of spending thousands of dollars on research and sophisticated materials, a $3.95 can of chain lube saved the day.

I love those kinds of stories, and I have another one involving my father in law, who saved the government from spending a million dollars on a spacecraft part with a $1.98 piece of wood.

Yeah, last time I was hands-on with any space hardware (which was about 5 years ago), we just used Braycote for everything. Not quite as mundane as motorcycle chain oil, but not exactly gee-whiz exotic super-science stuff either. Cost and availability seem almost completely independent of how well something will work in space - some really expensive stuff just plain won't work, but sometimes the stuff you buy off the shelf at the auto parts store is just fine.

Regarding the idea of heat-transfer, if you'll let me return to my physics-major roots for a bit:

There are 3 mechanisms of heat transfer: conduction (which we're all familiar with - works by direct contact), convection (which we're probably still all familiar with - depends upon circulation of a working fluid), and radiation (which, as TAM pointed out, doesn't require any sort of transmission medium). In a vacuum, radiation is the only one that works. Unfortunately, it's also the least efficient. So TAM, your basic premise is correct. One subtlety is that you don't need to be visibly glowing to radiate energy - low-energy wavelengths like infrared work okay, too. It's been a while since I've had to think about this, but pretty sure that the Stefan-Boltzmann Law says that you radiate more energy at higher temperatures (which makes sense), so you'd reject more heat once the thing got hot enough to start glowing, but you'd be doing so before then as well. It would still probably take a pretty long while to cool down once you reached that point.

For reference, we use giant radiator panels to keep the International Space Station cool (I actually work with those) - radiator systems are a pretty basic part of spacecraft design. If you purpose-built a firearm to use in space, you'd probably have to design a cooling system for it.

[C-dub]

Revolver. Problem solved.

[Scott in Houston]

I do think picking up the brass afterwards would be a big problem, though.

That could get me to shoot steel and buy a nice magnet. :)

[fizteach]

As far as recoil goes, conservation of momentum (which comes from F=ma) holds true in space just as it does on the earth.

I agree with TAM and Steve, as radiation would be the mechanism of heat transfer. Space actually contains dust and gas that can also allow some conduction to occur.

"The Stefan–Boltzmann law, also known as Stefan's law, is a relation which described the power radiated from a black body in terms of its temperature." from Wikipedia

I don't think the M4 would be classified as a black body.

Great discussion. Let's talk about some more physics

[The Annoyed Man]

One subtlety is that you don't need to be visibly glowing to radiate energy - low-energy wavelengths like infrared work okay, too. It's been a while since I've had to think about this, but pretty sure that the Stefan-Boltzmann Law says that you radiate more energy at higher temperatures (which makes sense), so you'd reject more heat once the thing got hot enough to start glowing, but you'd be doing so before then as well. It would still probably take a pretty long while to cool down once you reached that point.

You've just help make the point that there are a number on this forum who are a sight smarter than I am. The funny thing is, I actually did consider the infrared spectrum and then discarded it because I guesstimated that it involved a fairly small part of the total radiation package, particularly when compared to the visible spectrum, and concluded therefore that it played a smaller part in dissipating heat than the visible spectrum did. So Steve133, if I had thought this all the way through, would it have been more correct to say that heat energy radiates light at all temperatures (perhaps all the way down to 1º Kelvin, assuming a net positive temperature differential between the gun barrel and its environment), but that this light is not in the visible spectrum at lower temperatures?

My second question is: was I wrong in believing that, given temperatures high enough to radiate visible light, the steel of the barrel would dissipate that heat more rapidly than at lower temperatures where no visible light radiates? In other words, is the rate of heat dissipation constant, regardless of the temperature, or does it dissipate on a curve as it decreases?

My third question is: is the light (at whatever wavelength) merely a "symptom" of the energy state, or does it actually cause cooling? I base this question on the notion that light radiation = energy loss, and energy loss = lower temperature. Is that a correct notion?

I confess that my physics education ended after a semester of "cowboy physics" at A&M, which was a required course for my major in Biomedical Science at the time. Anything else I know was learned either from hanging around a lot of really smart people when I was growing up, or simply from observations made during practical application of solutions on a racetrack. You know that a motorcycle engine is really hot when an open class racer pulls into the hot lane for a tire swap during a 24 hour endurance race, and the entire cylinder block is glowing a dull red, and the header pipes are bright orange just downstream from the exhaust ports.

[7075-T7]

And my second question is: was I wrong in believing that, given temperatures high enough to radiate visible light, the steel of the barrel would dissipate that heat more rapidly than at lower temperatures where no visible light radiates? In other words, is the rate of heat dissipation constant, regardless of the temperature, or does it dissipate on a curve as it decreases?

Yes, in fact it's dependent on Temperature to the fourth power. More specifically it which depends on the emissivity of the surface, a perfect blackbody radiator has an emissivity 1.0, while highly polished gold has an emissivity of 0.02. The level of total energy transmission is directly proportional to the emissivity of the surface.

List of emissivities: http://www.infrared-thermography.com/material.htm" onclick="window.open(this.href);return false;

The emissivity of a material not only dictates how much energy it radiates when heated, but also how much energy is absorbed with something hot is around it. Gold foil was used on the lunar landers as an insulation material since it would not absorb much heat from the much higher intensity solar radiation, but would also not emit heat as fast when the surface was facing the "coldness" of space.

This secondary part of the radiation equation is why a lot of infrared thermometers and most FLIR cameras aren't really accurate at telling you what the real temperature is, unles you can adjust the emissivity value to closely match the surface that you're measuring. (Yes, I have been known to look up the value for anodized aluminum and adjust the emissivity value in my infrared thermometer to get the most accurate temperature). Humans are around e=.98, which I always found interesting.

[Steve133]

This discussion has officially passed the threshold of conversations that I can meaningfully participate in on my phone during quick breaks at work, so I'll limit my response to the following for right now:

I don't think the M4 would be classified as a black body.

Great discussion. Let's talk about some more physics

Dude, I was a physics major. EVERYTHING (stars, trains, horses, M4s, etc.) is a spherical blackbody in a frictionless vacuum. And all constants are some power of ten multiplied by "about 3".

The "spherical cow" approach is close enough for government work, and certainly close enough for Internet spitballing about guns in space.

[Purplehood]

Dude!

[Dadtodabone]

This discussion has officially passed the threshold of conversations that I can meaningfully participate in on my phone during quick breaks at work, so I'll limit my response to the following for right now:

I don't think the M4 would be classified as a black body.

Great discussion. Let's talk about some more physics

Dude, I was a physics major. EVERYTHING (stars, trains, horses, M4s, etc.) is a spherical blackbody in a frictionless vacuum. And all constants are some power of ten multiplied by "about 3".

The "spherical cow" approach is close enough for government work, and certainly close enough for Internet spitballing about guns in space.

On the assumption of a spherical cow:
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[fizteach]

This discussion has officially passed the threshold of conversations that I can meaningfully participate in on my phone during quick breaks at work, so I'll limit my response to the following for right now:

I don't think the M4 would be classified as a black body.

Great discussion. Let's talk about some more physics

Dude, I was a physics major. EVERYTHING (stars, trains, horses, M4s, etc.) is a spherical blackbody in a frictionless vacuum. And all constants are some power of ten multiplied by "about 3".

The "spherical cow" approach is close enough for government work, and certainly close enough for Internet spitballing about guns in space.

It's a good thing I didn't state unequivocably that the object would not be a black body. A simple as I recall statement would have sufficed rather than condescension.

But that's just the way I approach things.