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?

 

[racookpe1978]

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.

 

[racookpe1978]

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.

 

[JohnRBaker]

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'.

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
Siemens PLM:
UG/NX Museum:

To an Engineer, the glass is twice as big as it needs to be.

 

[KENAT]

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.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[KENAT]

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.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[mikekilroy]

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......

 

[JohnRBaker]

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.

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
Siemens PLM:
UG/NX Museum:

To an Engineer, the glass is twice as big as it needs to be.

 

[KENAT]

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.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[rconner]

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.

 

[renderu]

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

 

[racookpe1978]

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.

 

[renderu]

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.

 

[KENAT]

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.

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.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[IRstuff]

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
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[cloa]

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

 

[rb1957]

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

Quando Omni Flunkus Moritati

 

[Walterke]

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?

NX 7.5.5.4 with Teamcenter 8 on win7 64
Intel Xeon @3.2GHz
8GB RAM
Nvidia Quadro 2000

 

[rb1957]

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

 

[ornerynorsk]

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.