stupid question regarding projectiles
stupid question regarding projectiles
(OP)
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?





RE: stupid question regarding projectiles
But how could you "aim" such a "ideal" projectile? (A thrown bit of metal at high speed that misses its target is more then useless - it means you have marched hundreds of miles carrying excessive weight for nothing, and have spent time, money and gunpowder shooting at something that you miss, rather than something you want to hit. Even if you are huntig, not at war, now you have alerted the target food (a deer or squirrel or elephant) or enemy (lion or tiger or bear), who now can either run away, go hide, or is running at you ready to kill while you are reloading a second (equally useless) projectile!
However, you are also forgetting the economy of shape: a conical elongated bullet that engages in the rifled slots is slower, but more efficient w/r to air drag. A spherical "roll-down-he-barrel" bullet gets gasses go by that are inefficient, and while create more air drag as it flies towards the target less accurately randomly spinning.
Net: A "boat-shaped" aerodynamic spinning bullet will arrive at its target faster and more often than a round bullet. Ask the hundreds of thousands of civil war soldiers hit by Minie bullets, the millions killed and wounded by aerodynamic bullets in WW1 and WW2 compared to the inefficient and far slower round balls.
RE: stupid question regarding projectiles
The cannon in regular artillery IS rifled because those are NOT direct line-of-sight high-speed sabot rounds.
RE: stupid question regarding projectiles
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RE: stupid question regarding projectiles
It is possible to create a seal in a smooth bore - modern mortars (most muzzle loading) and some modern tank guns do it routinely - just not with a simple 'round ball'.
Rifling doesn't cause the seal, but because it requires the bullet (or a driving band etc.) to deform into the rifling for it to spin then a fairly good seal gets formed.
I think maybe you're confusing issues related to muzzle loading to those related to rifling, while there is some relation they are separate issues.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: stupid question regarding projectiles
Sorry, as to the OP, question all other things being equal and 'optimally designed' covering the seal of the projectile to barrel etc. then yes at the point it exits the barrel I'd expect the smoothbore to be going a little quicker as you haven't expended any energy spinning the projectile.
This isn't really the reason many modern tank guns are smooth bore though - it's more to do with barrel wear issues if I recall correctly.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: stupid question regarding projectiles
RE: stupid question regarding projectiles
As for modern smoothbore weapons, such as mortars and tanks, granted they are very effective but I think you will find that there is much less of a 'seal' than you may think. For example, mortar rounds are designed to be 'dropped' into the end of a mortar tube and the system depends on the free-falling round striking the bottom of the tube with enough velocity and force that the fixed firing pin will initiate the primer that ignites the mortar round's small propellant charge. There is no need for rifling since mortar rounds are fin stabilized and therefore does not depend on spinning to maintain a proper trajectory. That being said, there must be sufficient clearance between the body of the mortar round and the inside of the tube so that the round does indeed fall freely. And given the conditions under which these weapons are being used these clearances must provide sufficient tolerance so that dirt or slight imperfections, due to handling and abuse, in either the outside of the round or the inside of the mortar tube does not interfere with this gravity dependent loading and firing scheme.
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RE: stupid question regarding projectiles
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: stupid question regarding projectiles
JohnR, if go ahead and say the "seal" is somehow equal (varying clearances as nec'y to do this, is your answer still the same (would it also not e.g be possible to put a soft disk on the back of a projectile to allow initial clearance but effectively expand say to get same initial clearance and seal in a smooth bore?)
racook, if I go ahead and further qualify that the "shape" of the projectiles are identical and I am not really "aiming" at anything, when you say "but more efficient w/r to air drag" do you mean that the projectile from a rifled bore would arrive at a really big target at the same time, just a little slower, or faster (let's further say the projectiles from both barrels somehow don't wobble or tumble, but one does purposefully rotate and one does not). One more question, what exactly would cause the rotating bullet to have less drag, if one were to arbitrarily assume both projectiles are somehow perfectly shaped and don't wobble? A star awaits.
Any others, it appears I need some tiebreakers or supporting links with regard to effects just on muzzle velocity.
RE: stupid question regarding projectiles
RE: stupid question regarding projectiles
Of a "round nosed bullet, with square end on the cylinder = 0.295
Air drag coef of a smooth "boat-tailed" bullet is slightly over 0.045
the sphere is going to be tumbled and irregularly 'rolling" and twisting through the air (like a slowing spinning curve ball) due to the casting and forming irregularities on the sphere.
Thus - and I'm not going to let you get away with ignoring "accuracy" in the design of ammo! -
the better aerodynamics of a spinning bullet being kept from tumbling by its length and the rifling will arrive not only at its target faster than a sphere, but more often than its competitor. 8<) A non-rifled cylinder will not be spin-stabilized and so will will tumble even worse than a sphere and be even shorter ranged than a sphere.
Now, on tank guns, if you fire a fin-stabilized sabot round from a smooth bore, the large casing of the sabot allows you to use a much, much larger amount of powder to throw a very small diameter heavy projectile. The heavier but smaller nosed projectile will get through armor plate BECAUSE of its high speed and small area that it is hitting. More psi on the armor when hitting the target = more penetration (deeper penetration) of armor plate.
People's skin, not being armor plated, doesn't need the high speed rounds that an anti-tank weapon needs. Higher speed on an anti-tank round simplifies aiming errors too: less deflection by the wiond, by gravity over a shorter time of flight.
RE: stupid question regarding projectiles
RE: stupid question regarding projectiles
This is what the obturating ring in a mortar does rconner. Plus for experimental purposes where durability, rate of fire, cost per shot... aren't significant concerns then especially if you go to breach loading you could have a projectile that fit in a smoothbore like a gas syringe.
racookpe & John (and arguably me by responding to them) are thinking beyond your simple question to practical application making various assumptions along the way.
If you're only concerned about muzzle velocity then you don't need to worry about aerodynamic affects after it exits the barrel. However, for practical applications this is obviously a concern.
Trouble is, I don't think there's any way you can reasonably make that assumption. It's not just the form of the projectile that's the issue, forcing perturbations from the atmosphere, firing or possibly just the effect of gravity would tend to make the projectile wobble unless I'm missing something. Making the projectile significantly 'nose heavy' (center of mass forward of center of pressure) might do the trick - and is effectively what the tank rounds do but in a more efficient manner. However, it has other trade offs for most real world applications.
If somehow you can make that assumption then the non spinning round might be considered to have better performance as it would have a tiny bit less surface friction, I think without putting too much effort into it.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: stupid question regarding projectiles
A previous Prodas simulation of a 105-mm M735 APFSDS round shows that the resistance is greatest only in the first millisecond, as the round starts down the barrel and where less than 5% of the propellant has been consumed. Therefore, at best, you'd only get a 5% improvement in muzzle velocity from that, and about 3% from resistance in the remaining 5.6 milliseconds in the barrel.
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RE: stupid question regarding projectiles
RE: stupid question regarding projectiles
Quando Omni Flunkus Moritati
RE: stupid question regarding projectiles
Even though the rifled bullet might fly slower, will the impact force be the same/more because the energy from the rotation will also be used on the impact?
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RE: stupid question regarding projectiles
Quando Omni Flunkus Moritati
RE: stupid question regarding projectiles
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.
I've spent more than a few hours over the years messing around with ballistics, experimental projectiles and cartridges, and building firearms completely from scratch, and I'll stick to my theory until someone proves it wrong.
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.
RE: stupid question regarding projectiles
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: stupid question regarding projectiles
To the OP question, yes, you will get slightly better muzzle velocity for an unrifled round, or to go to your original question, better propellant efficiency, all else being equal (some propellant energy must be expended to create the rotation of the bullet). But, in terms of mass, a typical high velocity rifle round has about 10x more bullet mass than powder mass - so saving a few % of a few % of total system weight doesn't make it worth worrying about.
The British proved (in defeating Napoleon) that the better accuracy of rifles vs. smoothbores is worth the effort. Early rifles used greased leather patches that caused a much tighter fit of bullet to bore (to ensure the rifling had a consistent effect), and this tight fit meant they were much more difficult to load than a musket. Napoleon felt the rifle's rate of fire was too slow (1-2 rounds per minute vs. up to 5 or 6 rpm for muskets) for the battle tactics of the time. The Brits realized that squads of skirmishers and snipers with rifles could take out officers and noncoms during the battle with accurate ranged fire, and so disrupt the enemy's ranks. They did this repeatedly throughout the Peninsular wars and up through the battle of Waterloo, often winning battles against numerically superior forces. Bottom line, is since the 1800s, the slower rifles were king.
RE: stupid question regarding projectiles
Something I suspect that they had learned from their recent 'dust-up' with Colonial irregulars on the other side of the Atlantic
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RE: stupid question regarding projectiles
When you are spitting out 10 rounds per second, you can make up for accuracy with volume. My understanding is that these weapons aren't much use over around 25 yards, anyway.
rp
RE: stupid question regarding projectiles
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.
RE: stupid question regarding projectiles
rb1957 (and I have noticed your selected field/moniker), if it is indeed true that the, "spinning bullet will have a lower Cd than an equivalent non-spinning one" why is this so and in terms of first principles? Developing this concept further or re-stating, and extending the thinking to say some sort of at least primitive rocketry understanding (though this not my current intent, either), if one were not worried about dizziness or any conceivable effect of added “g’s” on any occupant(s) why wouldn’t “ya’ll” (I shouldn’t lump all of you together, but betraying my southern conditioning) pre-spin rockets somehow on the launching pad (one would think this could somehow be done, and with mostly external energy not consuming the weight of the propellant the rocket has to carry) before blast-off, or provide the rocket with helical fins to make it spin, if the rotation truly makes it easier to pass through the air? One more, if just the rotation itself decreases the drag or friction between the projectile material and air, if you could somehow pre-spin a projectile in say a smooth-bore exclusive of the charge energy, would that projectile then exit the muzzle quicker than the same projectile and barrel (and materials etc.) that is not purposefully “spun”? [I had these questions formulated before the latest response of Kenat, so I will go ahead and ask them.]
RE: stupid question regarding projectiles
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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.
RE: stupid question regarding projectiles
Some rockets - as in the military kind not putting man on the moon - are indeed spin stabilized. This has been done in various ways including if my memory serves twisted launch rails, canted fins and twisted nozzles/vanes in the exhaust. However, this was done for accuracy not some attempt to reduce drag as far as I know or can understand.
As btrue says, spinning the projectile increases relative surface velocity which will increase drag. However this isn't really applicable in a tight fitting bore as the aero drag isn't really a factor. I've started to lose track though rconner if you really only care about muzzle velocity per your OP or free flight performance. If you only care about muzzle velocity forget all the aero talk.
You have got me thinking about the difference in energy lost as friction between a spinning or non spinning projectile in a 'tight' barrel. I'm thinking that again any point on the surface of the spinning projectile actually travels further so if work = force*distance then the spinning projectile should lose more energy. However, I'm assuming that the coefficient of friction isn't varying with velocity.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: stupid question regarding projectiles
Taking the .223 Remington (aka 5.56 NATO), with a muzzle energy of 1209 ft-lbf, spinning at 198000 rpm, the energy contained due to spin is a mere 2.08 ft-lbs.
Table here of other small calibre stuff with more info:
http://www.varmintal.com/17hmr.htm#Energy
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H
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RE: stupid question regarding projectiles
Heavier bullet weights also need tighter rifling twist (more spin) to optimally stabilize them. Mil issue M16's, M4's and other 5.56 firearms in which a 55 grain projectile will be standard issue usually have a 1:9 twist (1 rotation in 9 inches), whereas match barrels for accuracy and competition (and ground squirrels!) using 62+ grain weight projectiles will generally need a 1:7 twist. The same holds generally true for any caliber. Each caliber/weight group has a definite sweet spot as far as rifling twist goes. There are also barrel makers offering progressive or variable rate rifling, but I've never messed around with those.
Never minding the circuitous walk in the park, here is an interesting link to one of many German innovations in arms, an idea to attain higher velocity: http://en.wikipedia.org/wiki/2.8_cm_sPzB_41
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.
RE: stupid question regarding projectiles
It ought to work identically to the "Coyote Effect" wherein failure to realize that you've fallen off a cliff postpones your acceleration.
RE: stupid question regarding projectiles
RE: stupid question regarding projectiles
This isn't the best video available, but it's one I could find quickly (while still at work) http://www.youtube.com/watch?v=J6lA3ZsmJ4c
It also demonstrates the slightly imperfect gas seal between the bullet and bore.
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.
RE: stupid question regarding projectiles
A.
RE: stupid question regarding projectiles
However for an arrow or some types of unguided rockets it's the chosen technique.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: stupid question regarding projectiles
"the sudden breakaway of the projectile upon exiting the muzzle, thereby effectively eliminating nearly all friction, has a catapult effect..." This is clearly not a scientific statement. Muzzle blast providing thrust to a bullet after is leaves the barrel was discussed in another thread.
RE: stupid question regarding projectiles
E(avail)=E(accel)-E(twist)......
E(twist) is simply related to T=jw/t where:
- T is amount of Torque required to accelerate the
- j inertial mass of the bullet from
- 0 to w rad/sec rotational speed on exit of barrel,
- in a time of t sec while twisting/accelerating.
Seems if one REALLY wants to calculate percent of energy LOST due the twist they could easily research the data to fill in the blanks above and get their answer.
RE: stupid question regarding projectiles
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.
RE: stupid question regarding projectiles
The physics and math are not beyond freshman/sophomore level...
You have a lot of experience working with things you don't understand...
This is not the mind of an engineer.
-handleman, CSWP (The new, easy test)
RE: stupid question regarding projectiles
-handleman, CSWP (The new, easy test)
RE: stupid question regarding projectiles
RE: stupid question regarding projectiles
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RE: stupid question regarding projectiles
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.
RE: stupid question regarding projectiles
RE: stupid question regarding projectiles
RE: stupid question regarding projectiles
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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RE: stupid question regarding projectiles
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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RE: stupid question regarding projectiles
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.
RE: stupid question regarding projectiles
First frame:
Last frame:
Graph:
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RE: stupid question regarding projectiles
Cheers
Greg Locock
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