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stupid question regarding projectiles
10

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

Ah .......

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

Note also that the cannon in tanks IS often smoothbore because they can shoot specialized sabot rounds (small diameter projectile/very large diameter powder chaber, ultra high speed, heavy-weight uranium and tungsten rounds!) with customized computerized optics and aiming computers and finned stabilizers.

The cannon in regular artillery IS rifled because those are NOT direct line-of-sight high-speed sabot rounds.

RE: stupid question regarding projectiles

Rifling also serves the purpose of providing a 'seal' between the projectile and the barrel thus assuring that virtually all of the cumbusting gases will be used to force the projectile down the barrel. This should result in higher muzzle velocities than if it were a smoothbore which, by definition, requires some amount of clearance so that the projectile does not become jammed in the barrel. This clearance would allow some of the gases to pass around the projectile thus reducing the efficiency of the 'gun'. Granted, you could use 'wadding' around the projectile but this would complicate the process and would be a variable which would affect the consistency of the projectile's velocity and thus the accuracy of the 'gun'.

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RE: stupid question regarding projectiles

John, aren't you a bit backwards in your logic?

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.

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RE: stupid question regarding projectiles

Quote (rconner)

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?

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.

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RE: stupid question regarding projectiles

FWIW, we have rebuilt rifling machines for Remington and can tell you typical rifling is between 10-18" per revolution. This small(?) amount of twist may help define how much energy is spent in rotating the projectile; seems like it would be minuscule compare to the high joules of the powder explosion but I have no real idea......

RE: stupid question regarding projectiles

Until the development of metal cartridges virtually ALL rifles were muzzle loading, even those that used Minié balls. But until the development of the Minié ball rifles used patched balls, but as I stated, while this did provide a 'seal' is was not as consistent or as effective and therefore reduced the efficiency and thus the velocity of the projectile as it left the barrel. The Minié ball wasn't only more effective because it was more accurate but also because it's muzzle velocity was higher due to a better seal between the projectile and the rifled barrel.

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

John, similar to the minie ball I believe most modern mortar rounds expand during firing to create the seal with the barrel. Actually I believe the term is an obturating ring/band not driving band which may imply rifling.

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RE: stupid question regarding projectiles

(OP)
Thanks to all who have responded to this question. So far I see one "no", though sort of qualified that the "seal" is better with riflings (while I had intended the seal to be the same with regard to effective clearance etc, whether or not that is possible, as Kenat has guessed from my clumsy wording, I went ahead and awarded a star). I also awarded another star to the "yes" answer, but since the score appears deadlocked must ask some more questions:

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

rconner, you are correct. I am often astounded at the answers here. People don't read the question.

RE: stupid question regarding projectiles

Air drag coef of a sphere is 0.5
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

3
The OP is simply asking if you could have the bullet come out of the rifle at a higher velocity if you didn't waste some of its energy in making it rotate inside the barrel.

RE: stupid question regarding projectiles

Quote (rconner)

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?)

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.

Quote (rconner)

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.

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.

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RE: stupid question regarding projectiles

The seal on modern projectiles is done with a copper obdurating ring: http://en.wikipedia.org/wiki/Obturating_ring

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.

TTFN
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RE: stupid question regarding projectiles

The new guided bullets system doesn't use rifling (counterproductive)

RE: stupid question regarding projectiles

@JohnR ... you had me at "cumbusting gases" (see 1st post, 18:25, 19th March)

Quando Omni Flunkus Moritati

RE: stupid question regarding projectiles

What about impact force.

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

i'd've thought that the bigger effect was reduction in drag ... a spinning bullet will have a lower Cd than an equivalent non-spinning one, and so arrive at the target with more velocity.

Quando Omni Flunkus Moritati

RE: stupid question regarding projectiles

Many of the answers thus far have been a circuitous walk in the park, but didn't really touch on the OP's question. :>) This was discussed in a post several years ago. My personal belief is that any friction, whether it's from rifling, wadding, a seal, or what-have-you, is going to reduce velocity, but only very slightly. The explosion taking place to drive the projectile will be "damped" by opposing force (friction), with projectile weight also entering into the equation, and thereby reducing potential velocity. It's simple physics, at this point. Bullet shape, drag coefficients, etc have nothing whatever to do with velocity potential until exiting the muzzle.

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

Spinning projectiles have a higher Cd than the equivalent non-spinning shape, due to higher relative velocities at the surface of the projectile. But a spinning bullet tends to stay stable in flight, relative to a non-spinning bullet, and thus has a lower Cd than the precessing, tumbling, unstable round.

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

Quote (btrueblood)


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.

Something I suspect that they had learned from their recent 'dust-up' with Colonial irregulars on the other side of the Atlantic winky smile

John R. Baker, P.E.
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RE: stupid question regarding projectiles

I am no weapons expert, by any means, but don't the rapid-fire fully-automatic machine pistols (aka UZI) have smoothbore barrels for exaxtly this reason?

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

There is no modern firearm (up to about 40 mm) that I'm aware of that has a smoothbore, save for shotguns. Every UZI, Sten, MAC, etc that I've handled has your conventionally rifled barrel.

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

(OP)
I guess it could be argued I do indeed spend some or inordinate time in my own little theoretical world, trying to at least think I understand at least a few first principles of why things do like they do (in keeping at least pretty much with my feeble understanding of natural “laws”, like conservation of energy etc.) I guess I should also go ahead and explain that I am not (at least at this time!) “designing projectiles”, as another has assumed. Please do not read any more into my ramblings other than what I write. As I now have two sort of categorically opposite principles that have recently been expressed re at least "Cd" (though maybe feeling just a little more love overall now re aye to the original post question), I guess must ask some further, though also likely stupid, questions prompted by those most recent responses:

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

The purpose of spinning the projectile is to stabilize it. Without the spin the projectile will tumble which is a much higher drag situation. Fins are an alternative method to give stability to objects that are no so easy to spin. There would be no point to having helical fins as the two methods of stabilization are redundant and would surely be higher drag.

----------------------------------------

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RE: stupid question regarding projectiles

rconner - don't read too much into the discipline tags both btrue and I have backgrounds in aero.

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.

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RE: stupid question regarding projectiles

Here is a table of small calibre rifle bullets, with different spin rates.

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

Cheers

H

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RE: stupid question regarding projectiles

Great link Pud, thanks!

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

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... The physics and math required to prove or disprove my theory are beyond my current understanding

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

Just to confuse the issue, the fletching (feathers) on arrows are often set with a twist to provide rotation to the arrow and stabilize its flight. I think many archers would disagree that "Fins are an alternative method to give stability to objects that are no so easy to spin. There would be no point to having helical fins as the two methods of stabilization are redundant and would surely be higher drag."

RE: stupid question regarding projectiles

or inversely to the coyote effect??? LOL. If you stop and think about it though, the projectile is accelerating the entire distance of the barrel. Upon exit from the barrel, the gases are still accelerating for a brief moment before dispersing. I believe this gives the projectile a little extra kick, especially providing the friction has just dropped to near zero, or is actually at zero, considering the expanding gases briefly envelope the projectile at the moment of exit.

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

Spinning a finned projectile is unhelpful enough that several APFSDS manufacturers incorporate a "slip band" into the design of rounds for discharge through a rifled barrel.

A.

RE: stupid question regarding projectiles

Slowly spinning a fin-stabilized projectile will generally make it more accurate. The aerodynamics of a projectile will never be perfectly symmetrical. The imperfection will cause a constant curvature of the flight path. Slow spin will change this to a helical path which will end-up much closer to target.

"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

Can't we just apply equations to this stuff? Knowing the data, just plug in the info and get the answers. Seems a simple case of

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

"This is clearly not a scientific statement." Nor was it intended to be.

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

Quote (ornerynorsk)


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.

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

Oh crap... I thought this was the Pub, so I was giving leeway. I just realized this is the Mechanical Engineering forum! If your statement was non-scientific, it has no place here.

-handleman, CSWP (The new, easy test)

RE: stupid question regarding projectiles

(OP)
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!

RE: stupid question regarding projectiles

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.

TTFN
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RE: stupid question regarding projectiles

2
'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.

RE: stupid question regarding projectiles

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.

RE: stupid question regarding projectiles

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.

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."

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.

TTFN
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RE: stupid question regarding projectiles

Quote (IRstuff)


...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 winky smile

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RE: stupid question regarding projectiles

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.

RE: stupid question regarding projectiles

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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RE: stupid question regarding projectiles

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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