stupid question regarding projectiles 10

[rconner]

I am aware that specifically designed "riflings" of a barrel or tube improve accuracy of at least relatively short projectiles (or vice versa on the projectile, causing it to rotate), I guess by a sort of gyroscopic-type effect that keeps the projectile from tumbling. This is probably a stupid question, but it would seem that energy required in contact with riflings to rotate a projectile could reduce e.g. muzzle velocity at least very slightly (by taking away just a little of the propellant energy?) Restated, if I had same energy source/charge, same projectile weight and an optimally designed rifled tube vs an optimally designed smoothbore, would in fact a projectile EXIT a smoothbore tube end at even just a little higher velocity?

 

[rconner]

Thanks again to everyone who responded. Before I asked these questions in the OP, I did do at least a brief web search. I found conflicting information and opinions, just like here. I even noticed one web site where a poster said categorically that a rifled barrel would result in higher muzzle velocity than a smoothbore, and claimed to have testing with fancy instruments to prove. I was just trying to smoke out/understand whatever principles would or could substantiate this concept (as that poster did not explain why). In spite of everyone's efforts, I'm not sure I clearly understand yet!

 

[IRstuff]

re: muzzle jump; I vaguely recall that thread. Assuming a spherical pressure wavefront leaving the barrel and using the last 100mm of a 7.62-mm weapon's barrel as a benchmark with a 24 m/s velocity increase in that distance, everything is basically done by 5-mm past the barrel, which means that the muzzle velocity might increase by no more than 1.5 m/s for a nominal muzzle velocity of 874 m/s.

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[ornerynorsk]

'Tis such a pleasure to banter with polite and knowledgeable engineers. . . . . It seems that I have fundamentally misunderstood the purpose of a forum. I had no idea that it was reserved for such a learned and noble class. It never said a word in the registration form about exclusion of experience and new thought.

I will show my uncredentialed, unruly self to the door of this exclusive club and humbly excuse myself. Apologies all around for having inconvenienced your minds in this crude and brutish manner.

Seriously now folks, your snobbery and disdain for those of us rich in experience but lacking in text book knowledge does not enhance your image as engineers. I have been in arms and ordnance for a pretty good share of 31 years, medical now, quite a switch, huh? It was my feeling that I had some worthwhile contribution to this thread, regardless if it was scientific or not, simply based on the fact that I have quite probably designed, built, shot, and handled more arms and ordnance than the combined lot of the participants of this forum, ranging from small caliber covert stuff up to 5 inch shipboard gun systems. You all have a wealth of knowledge that this "freshman" can't hold a candle to, but how many of you can build a submachine gun entirely from scratch without the benefit of blueprints or a CAD system, or put together experimental submunitions and projectiles on the test bench and then go out and send them downrange for observation of performance and effect. I've cut more chips on more materials on more projects than I care to remember, and there is nothing I like better than to talk shop and share my experiences with people who have similar interests. I've worked with some phenomenal engineers and designers in my day. It's very true that you all are very scientific and knowledgeable, but if you adopt the attitude that you have nothing to gain from someone else's experience, you might as well punch the clock and go home and take your place in front of the telly, because you're basically done contributing in any meaningful manner to the advancement of knowledge and technical discovery.

So with that, I'm done with this rant, time to get to work.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[btrueblood]

Agree, ornery. And I buy your theory, but I think we would need many nights over many beers to work out a test method to prove it. I can cite references for what I stated above, but it's not worth my time anymore, I didn't realize I was amongst snobs. And thanks too, Kenat, I'll be your second at the duel if you need me.

 

[Compositepro]

Ornerynorsk, my comment was not meant to be insulting. I don't know your background and assumed you are an engineer. When you make a statement that, in a technical sense, is nonsense on an engineering forum, you can expect to be challenged on it. It's not personal.

 

[IRstuff]

"However, I have a personal theory that the sudden breakaway of the projectile upon exiting the muzzle, thereby effectively eliminating nearly all friction, has a catapult effect, coupled with the gases created from the propellant actually exceeding the projectile velocity and overtaking the projectile for a brief moment. The physics and math required to prove or disprove my theory are beyond my current understanding, but it could be tested real-world with the right sensors and cameras."

The fact that most streak camera images show blow-by around the projectile at the muzzle essentially disproves the "catapult" effect, since the gas that escapes around the projectile contribution nothing to its acceleration. Moreover, gases tend to take the path of least resistance, and the projectile provide an inertial obstacle, which causes the gases to go around the projectile anyway, not counting the fact that the gases want to expand in a spherical wavefront. The only reason gases provide acceleration to the projectile in the barrel is because it's sealed and the gases have nowhere to go. As for testing, the "right camera" would need to run a minimum of about 160,000 fps, given that the round is already moving 5.5 mm/frame; and any sort of measurement would need to run more like one million fps. That would get about .86 mm/frame of relative motion. Kurzzeit.com did it once in 2008

http://www.kurzzeit.com/eng/startseite.htm

If someone downloads the video, it would be possible to do a frame by frame relative motion measurement.

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[JohnRBaker]

...it would be possible to do a frame by frame relative motion measurement.

I've actually done that, NOT for anything nearly as fast as a bullet, but rather a test sample being stretched to the point of failure in a Tinius Olsen testing machine, back when I was helping a doctoral candidate with his thesis (I worked for the ME-EM department as a photographer my last two years in engineering school). Back then (this was in 1970/71) all we had was a rather nice 16mm camera which could be run up to about 180 frames per second. I developed untold feet of fine-grained black & white film which was then blown-up, frame-by-frame, to create actual photos from which you could not only see but measure the deformation of the test samples which were etched with a grid so that you could 'see' the progression of the elongating effect on the sample, right up to the point of failure. Hey, it put food on the family table ;-)

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[ornerynorsk]

No personal offense was taken (likewise, I hope, he says after taking a good solid whack at the wasp's nest!), and I do concede that I was somewhat lacking in my description of why I believe my theory to be valid.

Here is why I believe the projectile continues to accelerate for distance "x" as it leaves the muzzle:
1. Projectile continues to accelerate up to the point of muzzle exit. This is indisputable unless there is grossly excessive barrel length involved.
2. As the projectile begins to leave the muzzle, friction begins to drop off. This is not an instantaneous event.
3. Pistol bullets tend to have a contact length of ~1 caliber, more or less depending on actual bullet design and weight. Rifle bullets are ~1.75, more or less.
4. Because the exit of the contact portion of the projectile is not an instantaneous event, occuring over a distance, it continues to accelerate further as it is leaving the muzzle.
5. Upon complete exit, the bullet now has zero friction relative to the barrel, and the gas now has freedom to disperse, but this also is not an instantaneous event. It will take a number of microseconds to disperse.
6. Until the gas disperses to the point where it is no longer providing propulsion to the projectile, it is still providing propulsion to the projectile. (huh?!?!) It is evident that some propulsion has being given as the shock wave and gases from the burning propellant quickly overtake the projectile. Just like holding your hand over a garden hose, the water is dispersing, but you can feel the force of it. IRStuff, this basically is what I am contending gives the "catapult" effect, for lack of a better term on my part. I've watched countless high speed frame grabs, and this is very typical, not just a random Youtube fluke.
7. The question is, with a given set of parameters, at what distance from the muzzle does it stop accelerating? Dunno, sounds like more than freshman physics, but maybe not. I'm comfortable being proven wrong, I just don't have the mathematical ability at the level required to either prove or disprove it.

Looking forward to further thoughts and comments on this.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[IRstuff]

Again, the gas going past the projectile is lost acceleration, so not a whole acceleration to be gained. Since this is never going to end, I've done the frame by frame. As near as I can tell, there is almost no change in velocity
First frame:

Last frame:

Graph:

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[GregLocock]

Incidentally I see values for Cd being quoted above. Given that Cd varies substantially (say 40%) with mach number, especially in the transonic region, and (almost) any longbore without a silencer is operating above M1, the Cd of any projectile, of whatever shape, and whatever axis of rotation, will vary during its flight.



Cheers

Greg Locock


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