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Dadtodabone wrote:I'll go with Penny, I love Fig Newtons.

I'll go with Penny for almost any reason whatsoever.

Steve133 wrote: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:

fizteach wrote: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.

Good Lord! Sheldon Cooper has gone and gotten himself a CHL!

Q: How many theoretical physicists specializing in general relativity does it take to change a light bulb?
A: Two. One to hold the bulb and one to rotate the universe.

Q: What is the simplest way to observe the optical Doppler effect?
A: Go out at and look at cars. The lights of the ones approaching you are white, while the lights of the ones moving away from you are red.

The Heineken Uncertainty Principle says "You can never be sure how many beers you had last night."

According to Einstein's Theory of Relatives, the probability of in-laws visiting you is directly proportional to how much you feel like being left alone.

Einstein's favorite limerick was:
There was an old lady called Wright
who could travel much faster than light.
She departed one day
in a relative way
and returned on the previous night.

A student riding in a train looks up and sees Einstein sitting next to him. Excited he asks, "Excuse me, professor. Does Boston stop at this train?"

There is a sign in Munich that says, "Heisenberg might have slept here."

I'll be here all week. Don't forget to tip your waiter.

Steve133 wrote: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.

Purplehood wrote: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.

Scott in Houston wrote:

C-dub wrote: Cooling, isn't it pretty cold in space? Why would heat build up be an issue?

This is one thing people don't realize about space. Yes, it's cold, but there's very little heat transference. You need contact for that. Heat would actually leave an object much quicker in ice water than in space due to contact and heat transference. In space, an object actually retains its heat fairly well relatively speaking.

I'm no engineer/physicist, but I think it would be a matter of how hot the barrel was, wouldn't it? Machine gun barrels can glow red under extreme use, and they give off part of that heat energy as light. Stars give off part of their heat energy as light, and that doesn't need contact with a substrate to transfer, does it? The light from stars transfers without there being any atmosphere between the star and earth. In fact, we experience some of that transfer as heat. So here is my hypothesis:

• A red-hot gun barrel would cool in space at a relatively rapid rate until it had given off enough energy to stop emitting light, at which point the rate of cooling would be greatly reduced as the heat transfers through the bearer's hands and is dealt with by the spacesuit's cooling apparatus. The problem of course is that the insulating layers of spacesuits would impede that heat transference.

Like I said, I'm no rocket scientist, so I'd be interested to know from someone better informed if my guess is correct or not.

Steve133 wrote:Of course, just making sure the cartridge propellant burns is only part of it - most lubricants aren't vacuum-rated, so there's a chance that whatever was used on that gun would boil off or flash-freeze, so the action could lock up..

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.

C-dub wrote:I also wondered if it would fire at all since oxygen would be needed, but IDK since they will fire underwater. Is there enough oxygen underwater for that or is it the oxygen trapped inside the sealed case of the round? Hmm. Oh, I just went back and noticed the author mentioning an oxidizer.

The recoil thing is also interesting.

Cooling, isn't it pretty cold in space? Why would heat build up be an issue?

The propellants contain oxidizers.

How well would a firearm like the M4 perform in outer space?
http://www.quora.com/Physics/How-well-w ... ans2627496

How well would a firearm like the M4 perform in outer space?
I wonder what would happen to a firearm when fired in outer space, the most problematic thing I can imagine is the heat buildup

Frank Heile, PhD in Physics then career in SW Eng
106 votes by Don van der Drift, In PhD Physics program for 2.5 years, former Physics researcher at LBNL, Sietse de Boer, Ryan Carlyle, (more)
The vacuum of outer space will not be a problem for firing a bullet. Guns do not need oxygen to work. The "gunpowder" or whatever explosive is in the cartridge that holds the bullet is completely self contained and does not depend on the atmosphere. It has an oxidizer mixed with the fuel and is perfectly capable of firing in a vacuum. Even the primer which is struck by the pin of the firearm is completely self contained and will work in a vacuum.

The gun will work (very slightly) better in space. The bullet will not have to push and compress the air in the barrel as it exits the gun. Air will not slow down the bullet as it travels, so the range of the gun would essentially be infinite.

There's more. I just thought this was an amusing treatise on the subject. The scary part is that the Rooskies pulled a Crazy Ivan and actually TRIED shooting a gun in space, having test-fired a gun from their Salyut space station.

.......wonder what they were shootin' at.....