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?

 

[KENAT]

"spinning bullet will have a lower Cd than an equivalent non-spinning one" rb1957 - is this real world where the spinning bullet should be more stable, or in rconner's theoretical world?

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[btrueblood]

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.

 

[JohnRBaker]

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 ;-)

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[redpicker]

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

 

[ornerynorsk]

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.

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[rconner]

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.]

 

[dgallup]

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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[KENAT]

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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[Pud]

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

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[ornerynorsk]

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.

 

[ivymike]

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.

 

[77JQX]

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."

 

[ornerynorsk]

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.

 

[zeusfaber]

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.

 

[KENAT]

Zeus, yes for that application spinning a finned projectile is unhelpful.

However for an arrow or some types of unguided rockets it's the chosen technique.

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[Compositepro]

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.

 

[mikekilroy]

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.

 

[ornerynorsk]

"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.

 

[handleman]

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)

 

[handleman]

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)