Epicyclic Motion and Bullet Stability

[HappyHellfire]

Hi, I'm Dr. John Stutz and I teach ballistics to engineers and scientist. Because of the warm welcome, I will continue my series on cool things in external ballistics that you might not know. If you are interested, I have a new bullet design that I am selling called Aerospike Bullets.

So, many of y'all know that spin stabilized bullets "wobble" as they fly. This is called epicyclic motion. An example of the motion of the nose of the bullet is shown below. A bullet is called stable if any disturbance in this motion (like wind or leaving the barrel) decays away and is unstable if it increases. We use what is called the Gyroscopic Stability (Sg) criteria to measure stability. If Sg>1 the bullet is stable and below it is not. There are devices called yaw inducing devices that we use to intentionally cause a large disturbance as the bullet leaves the barrel that exaggerate this motion.

Did you know that there is a second stability criteria? Anyone know what it is?

 

[BKA]

Hi, I'm Dr. John Stutz and I teach ballistics to engineers and scientist. Because of the warm welcome, I will continue my series on cool things in external ballistics that you might not know. If you are interested, I have a new bullet design that I am selling called Aerospike Bullets.

So, many of y'all know that spin stabilized bullets "wobble" as they fly. This is called epicyclic motion. An example of the motion of the nose of the bullet is shown below. A bullet is called stable if any disturbance in this motion (like wind or leaving the barrel) decays away and is unstable if it increases. We use what is called the Gyroscopic Stability (Sg) criteria to measure stability. If Sg>1 the bullet is stable and below it is not. There are devices called yaw inducing devices that we use to intentionally cause a large disturbance as the bullet leaves the barrel that exaggerate this motion.

Did you know that there is a second stability criteria? Anyone know what it is?
View attachment 1565450

Stability estimation – the theory
Traditionally, bullet stability is estimated by using quite simple equations like the Miller and Greenhill equations. Lapua Ballistics estimates the bullet stability by using both dynamic stability ’Sd’ and gyroscopic stability ’Sg’ during the full bullet trajectory. The value of the stability calculator is more in comparing different combinations with each other (twist rate, bullet to be used, muzzle velocity, environmental conditions, etc.) than trying to say simply if a bullet is stable or unstable.

Gyroscopic stability Sg means the ability of a rotating object to resist the change of its rotating axis. Sg is quite easy to calculate and its value at the muzzle has to be absolutely over 1.0 for a bullet to be stable. In practice some safety margin is needed there so the recommended minimum value is 1.4. Values clearly over 2.0 are unnecessary high because an overstabilized bullet has higher air drag due to the ”nose up” flight position and is more critical to small imperfections in the bullet shape and symmetry. Gyroscopic stability increases with flight time/distance because the rotational speed of the bullet does not decrease as fast as the forward velocity.

Dynamic stability Sd means the ability of the bullet to decrease a wobbling motion caused by an internal or external effect. In a dynamically stable bullet, the wobbling decreases with the time/distance and in a dynamically unstable bullet the wobbling increases with the time. Sd is much more difficult to evaluate than Sg and it needs 6 DOF calculations. Lapua Ballistics is one of the very few ballistics software solutions that is capable of utilizing this and possibly the only one completely free of charge. Most software are capable of estimating stability only by Sg which is usually reasonable good enough for short distances but not to long distances and to the transonic region – where Sd is the main factor for stability. The vertical axis of the stability curve expresses Sg and the horizontal axis Sd. The favorable stability region is expressed as light blue but high quality bullets often retain the stability also in the dark blue region.

Dynamic stability is affected by e.g. the bullet length and shape and the positions and distance between the center of mass (cm) and the center of pressure (cp). The distance between cm and cp is very important because it defines the aerodynamic moment vector that tries to turn the bullet crosswise. Air density is also important, the stability is better in a higher altitude and warm weather. The grooves (made by the lands in the barrel) on the bullet surface slightly affect the dynamic stability and decelerate the rotating speed faster compared to a smooth surface. Dynamic stability is usually the problem only at long distances and in the transonic &region.

Hi, I'm Dr. John Stutz and I teach ballistics to engineers and scientist. Because of the warm welcome, I will continue my series on cool things in external ballistics that you might not know. If you are interested, I have a new bullet design that I am selling called Aerospike Bullets.

So, many of y'all know that spin stabilized bullets "wobble" as they fly. This is called epicyclic motion. An example of the motion of the nose of the bullet is shown below. A bullet is called stable if any disturbance in this motion (like wind or leaving the barrel) decays away and is unstable if it increases. We use what is called the Gyroscopic Stability (Sg) criteria to measure stability. If Sg>1 the bullet is stable and below it is not. There are devices called yaw inducing devices that we use to intentionally cause a large disturbance as the bullet leaves the barrel that exaggerate this motion.

Did you know that there is a second stability criteria? Anyone know what it is?
View attachment 1565450

 

[BKA]

Im just a hack, the above is direct from the Lapua website

 

[HappyHellfire]

The equations that you mentioned "Miller and Greenhill equations" are simplified forms of the full solution. That is called Linear Projectile Theory and was developed over 40 years before and after WW2.

When you plot the gyroscopic and dynamic stability criteria together you get what we call the "Stability Cup". If you plot your gun and bullet muzzle conditions you can accurately predict stability if the solution falls within the cup.

 

[Rat Rifle]

How much Jump do You recommend with Your Mono Bullets ?

 

[HappyHellfire]

How much Jump do You recommend with Your Mono Bullets ?

There are multiple types of jump. Sometimes I don't know the common use of the terms. Can you be a bit more clear?

 

[Rat Rifle]

There are multiple types of jump. Sometimes I don't know the common use of the terms. Can you be a bit more clear?

Yes. Barnes has been offering solid copper bullets for years. They recommend a minimum of .050” jump to the lands to avoid a huge pressure spike that denser than lead core bullets cause. Jump is the term used for how much running start the bullet gets before reaching the rifle bore lands. A good thing to be aware of when producing bullets.

 

[Dave M.]

Hi, I'm Dr. John Stutz and I teach ballistics to engineers and scientist. Because of the warm welcome, I will continue my series on cool things in external ballistics that you might not know. If you are interested, I have a new bullet design that I am selling called Aerospike Bullets.

So, many of y'all know that spin stabilized bullets "wobble" as they fly. This is called epicyclic motion. An example of the motion of the nose of the bullet is shown below. A bullet is called stable if any disturbance in this motion (like wind or leaving the barrel) decays away and is unstable if it increases. We use what is called the Gyroscopic Stability (Sg) criteria to measure stability. If Sg>1 the bullet is stable and below it is not. There are devices called yaw inducing devices that we use to intentionally cause a large disturbance as the bullet leaves the barrel that exaggerate this motion.

Did you know that there is a second stability criteria? Anyone know what it is?
View attachment 1565450

Did you cut and paste that out of one of Bryan Litz’ books??
Who do you continue to start new threads without providing a shred of evidence on you bullets??? This truly may be the best “punk” ever, or this is the single worst example of how a new company should break into any market.

 

[HappyHellfire]

Yes. Barnes has been offering solid copper bullets for years. They recommend a minimum of .050” jump to the lands to avoid a huge pressure spike that denser than lead core bullets cause. Jump is the term used for how much running start the bullet gets before reaching the rifle bore lands. A good thing to be aware of when producing bullets.

Ok, got it. I have been loading with the ogive (nose) touching the lands. I have not measured or noticed any issues with barrel pressure spikes.

In external ballistics we refer to jump as either gun jump due to things like barrel whip or aerodynamic jump from things like cross wind across the barrel.

 

[Rat Rifle]

Ok, got it. I have been loading with the ogive (nose) touching the lands. I have not measured or noticed any issues with barrel pressure spikes.

In external ballistics we refer to jump as either gun jump due to things like barrel whip or aerodynamic jump from things like cross wind across the barrel.

Sir, Respectfully, Your off to a rough start with this crowd

 

[HappyHellfire]

Sir, Respectfully, Your off to a rough start with this crowd

Don't I know it. I fell in love with external ballistics twenty years ago and have spent most of that time learning everything I can.

Von Braun was a damn Nazi but as a rocket scientist was welcomed with open arms in this country. We have buildings named after him in Huntsville, AL. Unfortunately, most ballisticians are not really held in high regard.

Fortunately, I'm a pretty stubborn person. Thanks for the thought.

 

[Thauglor]

Sir, Respectfully, Your off to a rough start with this crowd

Yes cuz he makes crazy claims like 40% improved range for the same terminal energy without a shred of evidence. And to think it was almost all to a longer boat tail that none of the other companies ever looked at. And then he keeps posting only details on drag coefficient without ever saying anything about BC, on top of which his data doesn't even match his curves so he throws away data to make his bullets look better

He is also broadcasting his company without being a paid sponsor, so saying a big capital FU to all the other companies thay pay to advertise

 

[HappyHellfire]

Yes cuz he makes crazy claims like 40% improved range for the same terminal energy without a shred of evidence. And to think it was almost all to a longer boat tail that not of the other companies ever looked at. And then he keeps posting only details on drag coefficient without ever saying anything about BC, on top of which his data doesn't even match his curves so he throws away data to make his bullets look better

He is also broadcasting his company without being a paid sponsor, so saying a big capital FU to all the other companies thay pay to advertise

I'm sorry if my bullets offend you. I'm sorry that I don't know your forum culture. You seem very sensitive about this an I will try to be more careful with your feelings.

 

[Coyotefurharvester]

Most machined monolithic bullets have a minimum velocity for expansion due to contact. Typically tested with ballistic gel, what is the minimum velocity for effective expansion with your hunting bullets?

 

[tac284]

Dr. Guffy is that you?

 

[HappyHellfire]

Most machined monolithic bullets have a minimum velocity for expansion due to contact. Typically tested with ballistic gel, what is the minimum velocity for effective expansion with your hunting bullets?

I haven't done any terminal ballistic testing. My field is external ballistics but I'm more than willing to learn how to conduct the test if it is something needed.

That reminds me of a joke. External ballistics is done by aerospace engineers. Internal ballistics is done by chemical engineers. Terminal ballistics is done by psychotic engineers.

 

[jackieschmidt]

I’m going to try and remember all of this at the Gulf Coast Region VFS Championship this coming Sunday.

Seriously, in the world of extreme accuracy, nothing beats success in The Competitive Arena. Get your bullets in the hands of some competent shooters, and win some matches.

 

[HappyHellfire]

I’m going to try and remember all of this at the Gulf Coast Region VFS Championship this coming Sunday.

Seriously, in the world of extreme accuracy, nothing beats success in The Competitive Arena. Get your bullets in the hands of some competent shooters, and win some matches.

It may take a while but that's the plan. Thanks.

 

[jelenko]

It may take a while but that's the plan. Thanks.

You're not doing yourself any favors posting without getting some backing.
Honestly, I'm gobsmacked that you keep creating posts that lead off on some almost arcane theoretical topic.

I get you're stubborn, but not coming across as learning from feedback.

 

[Dave M.]

Hi, I'm Dr. John Stutz and I teach ballistics to engineers and scientist. Because of the warm welcome, I will continue my series on cool things in external ballistics that you might not know. If you are interested, I have a new bullet design that I am selling called Aerospike Bullets.

So, many of y'all know that spin stabilized bullets "wobble" as they fly. This is called epicyclic motion. An example of the motion of the nose of the bullet is shown below. A bullet is called stable if any disturbance in this motion (like wind or leaving the barrel) decays away and is unstable if it increases. We use what is called the Gyroscopic Stability (Sg) criteria to measure stability. If Sg>1 the bullet is stable and below it is not. There are devices called yaw inducing devices that we use to intentionally cause a large disturbance as the bullet leaves the barrel that exaggerate this motion.

Did you know that there is a second stability criteria? Anyone know what it is?
View attachment 1565450

Did you cut and paste that out of one of Bryan Litz’ books??
Who do you continue to start new threads without providing a shred of evidence on you bullets??? This truly may be the best “punk” ever, or this is the single worst example of how a new company should break into any market.

Ok, got it. I have been loading with the ogive (nose) touching the lands. I have not measured or noticed any issues with barrel pressure spikes.

In external ballistics we refer to jump as either gun jump due to things like barrel whip or aerodynamic jump from things like cross wind across the barrel.

please spend more time with competition shooters and reloaders and you will learn the lingo.