Meteors and rail guns. Physicists please.

OK, I am in some kind of comments war in Youtube where people have all kinds of crazy hypotheses about why the Navy railgun projectile tests have so much flaming hot air around the projectile when it is first launched.

I have been asserting that these flames are a plasma formed by compression heating of the air in front of the projectile. Dump a lot fof kenetic energy into the air, molecules are compressed because they don’t have time to be pushed aside, compression generates heat, and the railgun is so badass that it generates so much heat that the air is ripped apart into a plasma. I used the example of how a bicycle pump becomes warm to give an everyday example of compression heating. I pointed out that compression heating is why returning spacecraft get hot and also why meteors glow.

But AARGH! Some people say it’s friction. And I can see why. I always heard this friction with the air nonsense, and kinda believed it until I learned the truth. The SR-71 gets hot from COMPRESSION, not friction. The Shuttles got hot from compression, and so do meteors. And I think the glowing fire behind the railgun projectile is compression too. As I did research I learned that they actually do demonstrations of heating by friction in elementary school and tell kids that this is also why meteors get hot. Why do they do this when we know it’s compression?

And some folks actually think that meteors and the railgun projectile are actually ON FIRE! Yep, burning by combining with oxygen and that no flame-like phenomena could appear without combustion.

Please, SDMB, confirm to me I am not full of shit by asserting the Navy railgun is awesome enough to rip the air into plasma by compression alone.

I don’t know the answer but I’m interested to hear it… and I guess this is the clip in question:

In the case of powder-burning bullets and supersonic aircraft, the air is compressed, yes, but it’s also free to explode outwards away from the compression.

I watched the video. The flames seem to occur while the projectile is accelerated inside the gun’s barrel. There’s no flame or glowing of any kind after it has left the barrel.

It would be useful to know the projectile speed.

I think it may involve compression, but I think the sheer amount of heat involved is the main factor. Railguns work by magnetic pressure, there is a LOT of electricity involved in that, and as anybody who’s ever touched… well, pretty much any electronic device while it’s running can tell you, electricity gives off a lot of heat. I think the sheer amount of heat generated by the rails goes a long way towards heating the air to a plasma-like state.

It may not be the sole factor, but I think it’s probably the major factor.

Most of those flames in the video are post-barrel, but pre-exit from building.

From Popular Science:

“One big question this video begs is, what causes the giant fireball? Rail guns are supposed to be powered solely by electricity, and don’t use explosives of any kind for propellant. Babb told PopSci the answer: The flames are from pieces of the projectile disintegrating; the 7-pound slug is jammed so firmly between the rails that when it’s fired, pieces shear off and ignite in the air. There’s been some speculation online that the flames come from some sort of gas that’s been used to increase conductivity. Wrong: The EMRG uses no secondary propellant — just electricity. As a result, the breech can remain open during firing and the gun produces no blowback whatsoever. In fact, the researchers sometimes place cameras and mirrors inside the breech during tests to get a better sense of what’s going on.”

My guess is that some of this burning may not be as easily visible in the daylight as compared to the darker building.

I agree that in this case it is due to compression of air in the barrel of the gun. If it was due to friction, there would be a glowing trail behind the projectile after it left the barrel, and there is nothing of the kind. The pump analogy is actually quite accurate in this instance.

I would disagree about the plasma. The air is simply hot enough to glow. Based on the color I would say about 1000C.

Aerodynamic heating actually takes places because air is slowed down in the presence of a moving object (from the object’s point of reference). It is first slowed down by a compression wave to sonic velocity and then by laminar/turbulent flow in close proximity of the object. How much can be attributed to compression/friction would depend on the specific object and its speed.

I think that, in this context, “friction” is simply a general term meaning “dissipative forces which oppose relative motion”, and that the drag from compression of the air in front of the projectile is therefore a form of friction.

Very good point. Waste heat from the operation of the railgun itself. But that’s certainly not combustion of the alleged “friction”. I still think there has to be a lot of compression heating as well. The SR-71 Heats up mightily from compression, and the projectile is going much faster.

I really doubt the railgun projectile is burning. if it burned it would burn unevenly and there would be no hope of aiming it. The projectile dose have a sabot. Perhaps parts of the sabot might be burning?

I still think the flames are mostly caused by compression heating, and as I said above, waste heat from the railgun itself.

Thank you for explaining some of the details better than I did. But it really is more like an air pump than rubbing two sticks together, right? Seems like what’s going on with the railgun is more like the Gas Law equations than friction between two solid objects.

Really? Don’t the equations of friction involve “normal force” vectors and coefficients based on properties of solid materials? I hardly see how those apply to meteors, spacecraft, and rail guns. The gas laws and arcane equations of aerodynamics known only to a few.

if you call every force which prevents something from happening, you might as well say that friction keeps me from jumping to the moon. I kinda think it’s mostly gravity.

According to another site that links to the same video, “The flames that appear in the video of the test firing come from a combination of electricity arcing across the launcher, shavings of aluminum reacting with the air, and the bullet’s hypersonic flight”.

I agree with this. Friction is not a substance or a fundamental force of nature, and neither is compression. The projectile is rapidly displacing a lot of air molecules, and at the level of air molecules and their mutual spacing, this is a king hell violent game of billiards. That it leaves the molecules with a great deal of widely dispersed velocities should be no surprise. Forcing this into neat exclusive categories such as friction and compression is overextending the large scale equilibrium situations of continuum shear flow and continuum pressure increase.

The equations of dry solid friction involve that. Not all different kinds of friction behave in the same way.