rotation of the earth

[highspeedmdd]

I was talking, and of course drinking, with a Canadian CSM a couple of weeks back and started talking about shooting.

He told me something that sounded interesting and had not heard of before. But since I was not an 18B, nor a sniper, maybe this is old news to some.

He told me that their guys shooting .50 cal started computating the rotation of the earth into their formulas and were increasing their probabilities by as much as 12% at ranges over 1000 meters.

Just wanted to see what people think about that.

[The Reaper]

I have heard rumors of that as well, and I know that the Germans did it with their railroad guns during WW II, but that was at 60 or 80 KM.

If you don't do it for mortars with similar ranges (and much greater hang time), why would you do it for a bullet with a flight time of less than 3 seconds?

Good question though, let's see what the experts say.

TR

[Pete]

Call BS on that one.

The earth only rotates in one direction. Faster at the middle and very slow at the top and bottom (Thats relative speed). You would have to compute where you were and what direction you were shooting.

3 seconds shooting in a direction of 85 degrees at 45 deg North Latitude? Just shoot. Wind has a bigger impact.

Pete

[Go For Broke]

I have not heard of it for the mortars, but as an old 13 series, that was one of the variables we had to account for during manual cannon gunnery (which failed many a young 2LT)

From the Now FM 6-40 (Formerly TC 6-40)
(1) Range effects Some of the deviation from standard conditions affecting range are: Muzzle velocity. Projectile Weight. Range wind. Air temperature. Air density. Rotation of the earth (Para 3-3 Exterior Ballistics, 3, (1))

This Rotation of the Earth was used during computation of Safety and during METerological Data Corrections for a firing solution. Correction factors applied to Range (in meters) was Table H in the Tabular Firing Tables and Correction factors applied for Azimuth (in mils) was Table I for the TFTs. This information was based off of 0 degrees latitude, with additional modifiers based of distance from 0 degrees latitude. FYI - These TFTs are rather thick, with different information for various weights of projectiles, types of projectiles, and type of projectile launcher (105mm or 155mm (current))

This is in addition to all the other factors that were taken in during the computation of safety / firing solutions which included such things as ambient temperature, tube temperature, powder temperature, winds, tube wear, projectile weight, high angle or low angle fire, elevation, displacement from base piece, time of flight, muzzle velocity variations, etc...etc...to include how much copper was deposited on the lands and grooves of the tube

Pete hit it on the head, when he said that You would have to compute where you were {are} and what direction you were shooting {target location} for it to be really effective.

Just my humble $0.02

V/R,

[Team Sergeant]

Call BS on that one.

The earth only rotates in one direction. Faster at the middle and very slow at the top and bottom (Thats relative speed). You would have to compute where you were and what direction you were shooting.

3 seconds shooting in a direction of 85 degrees at 45 deg North Latitude? Just shoot. Wind has a bigger impact.

Pete


Exactly.

Also the basis that the space launches are cheaper the closer to the equator. And, the reason the shuttle does that "roll" early in the launch and angles out to space instead of "straight" up.

Not only would you have to know where you are in relation to the equator but the angle to target line in relation to the earth’s rotation, elevation of tgt and shooter etc.

If this were true we would/should have been calculating the gravitational effects of the moon long ago.......

I’m with Pete on this one.

Team Sergeant

[Jack Moroney (RIP)]

When I went through a sub-course on Field Artillery a long, long time ago they were preaching that there was an effect on long range artillery , and no you smart assess that might be checking out what a long, long time ago means, it wasn' a friggin catapult.

[Surgicalcric]

...it wasn' a friggin catapult.

Trebuchet? :D

Crip

[one-zero]

I'll call BS as well, especially regarding sniper application - BUT, I see great potential to screw with some folks at the range;)
Will add a bogus data page to the SOTIC logbook so when someone asks about it I can give an incredulous look and say "gotta get into the 'advanced course' where they teach you the SMU stuff..."

[Sdiver]

Trebuchet?

Crip

I was going to say a Smooth Bore Cannon, myself. :munchin

[The Reaper]

The longest range tube artillery actually used in combat that I am aware of was the Paris Gun, AKA the "Kaiser Wilhelm Geschütz".

It was capable of hurling a 94 kilogram (210 lb) shell to a range of 130 kilometres (81 miles) and a maximum altitude of 40 kilometres (25 miles).

At the start of its 170-second trajectory, each shell from the Paris Gun reached a speed of 1,600 metres per second (5,200 ft/s) (almost five times the speed of sound). This is firing a 210 lb. projectile at the speed of a 120mm sabot round.

The gun itself, which weighed 256 tons and was mounted on rails, had a 28 metre (92 ft) long, 210 millimetre (8.3 in) caliber rifled barrel with a 6 metre (20 ft) long smoothbore extension.

The projectile reached so high it was the first man-made object to reach the altitude of the stratosphere, thus virtually eliminating drag from air resistance, allowing the shell to achieve a range of over 130 kilometres (80 miles). The shells were propelled at such high velocity that each successive shot wore away a considerable amount of steel from the rifle bore. Each shell was sequentially numbered according to its increasing diameter, and had to be fired in numeric order lest the projectile lodge in the bore and the gun explode. After 65 shells had been fired, each of progressively larger caliber to allow for wear, the barrel was rebored to a caliber of 240 millimeters (9.4 in).

The Paris gun was used to shell Paris at a range of 75 miles (120 km). The distance was so far that the Coriolis effect — the rotation of the earth — was substantial enough to affect trajectory calculations. The gun was fired at an azimuth of 232 degrees (west-southwest) from Crépy-en Laon, which was at a latitude of 49.5 degrees North. The gunners had to account for the fact that the projectiles landed 393 metres (1,290 ft) short and 1,343 metres (4,406 ft) to the right of where they would have hit if there were no Coriolis effect.

A total of 320–367 shells were fired, killing 250 people and wounding 620, as well as causing considerable damage to property. 20 shells were fired an hour on a good day.

I do not remember that much of my geometry and trig classes, but a deviation of 1343 m. deflection and 393 m. range at 120 km, would indicate a deflection difference of roughly 11.2 m. per km, and a range deviation of 3.3 m. per km. This is all based primarily on time of flight of the round, and the time to travel the extreme range allows for the deviation, not the distance. If you were firing a mortar with an average of two and a half minutes of flight time, the results might be similar. Given that the average precision round takes just over one and a half seconds to reach 1000 m., I am going to go out on a limb here and say that the Coriolis effects on the rifle round over 1/100th of the flight time would be measurable, but of negligible significance considering the other more serious factors like range estimation and varying crosswinds. At the same time, as noted, the direction of the shot relative to the rotation and and the position on the planet would be significant in the Coriolis effect. I do believe that some ballistic computers allow for it though, and with some of the kills coming at 2,000 m. or more, perhaps it is significant at those very extended ranges.

Dr. Bull was working on an improved version of this type gun for the Iraqis to put projos into space when he was suddenly killed with five rounds to the back of the head.

TR

[frostfire]

Good question though, let's see what the experts say.

I'm waiting for longrange1947 and Gene Econ responses on this.

Disclaimer: Team Sergeant, I got chewed over this before and I did learn my lesson to be wary of manufacturer claims and the profit motive. Also in that thread , TR and longrange1947 IIRC saw the demo with the shooter DM and were not impressed. However, now I just want to point out that since there is a weapon system (PDA program) that takes into account earth rotation, it's plausible that the Canadian CSM was truthful.


The earth only rotates in one direction. Faster at the middle and very slow at the top and bottom (Thats relative speed). You would have to compute where you were and what direction you were shooting.
Not only would you have to know where you are in relation to the equator but the angle to target line in relation to the earth’s rotation, elevation of tgt and shooter etc.
At the same time, as noted, the direction of the shot relative to the rotation and and the position on the planet would be significant in the Coriolis effect. I do believe that some ballistic computers allow for it though, and with some of the kills coming at 2,000 m. or more, perhaps it is significant at those very extended ranges.

The following are from:
(copy and paste)cheytac.com/White%20Paper2007.pdf
(copy and paste)cheytac.com/CABC%20Version%201.0.pdf

Coriolis Correction: The earth’s rotation has an effect on both elevation [when shooting east and west or angles including an east/west component] and on windage [when shooting north and south or angles having a north/south component].

User will enter in the PDA Advanced Ballistic Calculator: Target Height [MILS], Target Size [feet], Slant Angle, Air Temp, Ammo Temp, Air Pressure, Latitude (This in addition to the Gun to Target angle and the range are required to calculate the Coriolis Effect. While the effect of the Coriolis is small, it is a factor at extreme ranges, at the equator or at extreme northern and souther latitudes. The longitude is not needed) Data were collected using Kestrel 4000, Leica Vector IV, and Suunto X6 wrist computer

I'll call BS as well, especially regarding sniper application -
I sure know zilch of actual operations, but I suppose entering all those data under high stress in denied territory is impractical.

[G]

Dr. Bull was working on an improved version of this type gun for the Iraqis to put projos into space when he was suddenly killed with five rounds to the back of the head.

Suicide? :D

[longrange1947]

The rotation of the earth is a VERY minor problem with rounds fired at normal ranges by the .50. I call BS on the increase in hit probablity. At that range the factors effecting hit probablity are great and the rotation is minor and not worth the 12 per cent he is claiming. At 1000 meters the effect, uhhhh that 'C' word :D , is minimal and not worth the effort. At longer ranges maybe but as stated the unknown effect of the wind at the longer ranges is greater than rotation.

My bet is that the CSM is refering to the PDA programs that snipers have today that can take the effect into account. Turning on and off the effect will show the difference and it is not great. For those with great curiosity it is worth .25 moa at 1600 meters wth the .50 cal. Yes that is a quarter of a minute of angle. 1 mph of wind is worth more than that.

81mm mortars did not compute the effect but the 4.2 mortar did on the old slide rule, there is that old enough? :munchin

[JLF]

The rotation of the earth is a VERY minor problem with rounds fired at normal ranges by the .50. I call BS on the increase in hit probablity. At that range the factors effecting hit probablity are great and the rotation is minor and not worth the 12 per cent he is claiming. At 1000 meters the effect, uhhhh that 'C' word :D , is minimal and not worth the effort. At longer ranges maybe but as stated the unknown effect of the wind at the longer ranges is greater than rotation.

My bet is that the CSM is refering to the PDA programs that snipers have today that can take the effect into account. Turning on and off the effect will show the difference and it is not great. For those with great curiosity it is worth .25 moa at 1600 meters wth the .50 cal. Yes that is a quarter of a minute of angle. 1 mph of wind is worth more than that.

81mm mortars did not compute the effect but the 4.2 mortar did on the old slide rule, there is that old enough? :munchin

We use the Horus ballistic software and it does allow the option to correct for Coriolis effect which is very minor, but at that level of computing we just consider it "FM". I was recently at a private RND evaluation in Paulden, Arizona for consideration of a .338 system and talked to a MARSOC Instructor personality who stated that the Marine Corps are developing a ballistic system that incorporates the entire suite of sniper tools into a total package system. I believe they're calling it "One Shot." It ties the GPS, Kestrel, LRF, and Ballistic PDA together and he said it will consider Coriolis and Magnus effects. I'm amazed how far technology has come. But I still keep my trusty Mil-Dot Master and Slope Doper handy.

[The Reaper]

I would offer the opinion that if it takes no more effort to enter the shot data into the ballistic computer or PDA, fine, tell me what the total correction for windage and elevation is and let me get on with it.

If I were trying to crunch numbers manually, the target will have moved on by the time I got the correct answer. I can't hold .25 moa at 1600 meters from a field firing position anyway.

My novice shooting opinion is that at that range, most first shots are going to be misses for ranging, winds, slope, target movement, etc. anyway, better to watch point of impact closely and get a good corrected second round out before he hears the first, and don't worry about Coriolis effects.

Just my .02, now that we have heard from LR, I suspect that Gene will be along with some good points shortly.

TR

[kachingchingpow]

speedy...

Coriolis effect will have about as much impact on getting your ex's Durango out of my driveway as it would on field ballistic dope.

You got punked :D

[Bill Harsey]

what I've learned here:
when hunting and shooting on a north-south axis the earth will rotate a tree in front of the running elk just as the trigger is squeezed.

[HOLLiS]

what I've learned here:
when hunting and shooting on a north-south axis the earth will rotate a tree in front of the running elk just as the trigger is squeezed.


Bill, thank you for explaining all of this, I was wondering if my two remaining brain cells where still functioning. Over the years I have had shots like that and never knew why. I will tape a compass to my hunting rifle barrel with a note attached, "shooing N or S is a no no". I hope I got it right. :confused:


Hopefully, I fill my freezer with some tasty Roosevelt Elk meat this season.

Best,

H.

[Rumblyguts]

Bill, thank you for explaining all of this, I was wondering if my two remaining brain cells where still functioning. Over the years I have had shots like that and never knew why. I will tape a compass to my hunting rifle barrel with a note attached, "shooing N or S is a no no". I hope I got it right. :confused:


Hopefully, I fill my freezer with some tasty Roosevelt Elk meat this season.

Best,

H.
Wait, WAIT!...

Now you've intorduced the variable of magnetism with relation to the distance of the compass from the barrel! :confused:

And just when this was making sense... :;)

[The Reaper]

Here is an excellent scientific explanation of how bullets fly, for anyone who is interested in the technical explanation of the forces acting upon a projectile in flight:

http://www.nennstiel-ruprecht.de/bullfly/fomo.htm#header_forcesmoments

The actual formula for computing the Coriolis effect on a projo is here:

http://www.nennstiel-ruprecht.de/bullfly/coriolf.htm#header

Uggh, too hard, physics of ballistics make Zog's head hurt. Need Reader's Digest version.

Their summation of the effect is "The magnitude of the fictitious Coriolis force is so small that it is usually completely neglected and - as a rule of thumb - only has to be considered in ballistics for ranges of 20 km or more (artillery shells)."

TR

[kgoerz]

A little off topic. Someone on this board showed me a simple way to explain why we read winds 2/3 the distance to the target, LongRange. He was explaining to the Students why the wind effects the Bullet at this point more then any other .
He said "with the wind blowing, throw a ball straight up in the air, notice the wind will move the ball in that split second it becomes weightless" Thats why we read winds where the Bullet reaches it's max trajectory. Bullet becomes weightless like the ball. At this point the effects of wind overtake the effects of gravity.
This is also why the wind speed at your shooting position is of little importance. Thought this was a good analogy and always used it successfully. Of course I never give him credit for this analogy;)

If you don't have an answer for a Student for why his groups are all over the place. Need a little time to figure it out. A good one to use is "It's possible your Barrel Harmonics are off" Big word for something might be touching your free floated Barrel. Just like (PPIM) Pre Primer Ignition Movement. Jerking the Trigger.

[Peregrino]

This is fun! And nobody's mentioned "spin drift" yet. :munchin I do remember that from the 4.2 mortar class in the heavy portion of the Weapons "Q" course. Peregrino

[The Reaper]

This is fun! And nobody's mentioned "spin drift" yet. :munchin I do remember that from the 4.2 mortar class in the heavy portion of the Weapons "Q" course. Peregrino

Actually, they have.

It ties the GPS, Kestrel, LRF, and Ballistic PDA together and he said it will consider Coriolis and Magnus effects.

The Magnus effect is the technical term for spin drift.

TR

[frostfire]

Reaper Sir, thanks for the links. Those are what I've been looking for for years. My google-fu needs tons of tune-ups.


...we read winds 2/3 the distance to the target.....
...wind effects the Bullet at this point more then any other...
...wind will move the ball in that split second it becomes weightless...
...we read winds where the Bullet reaches it's max trajectory...
...Bullet becomes weightless like the ball...
...At this point the effects of wind overtake the effects of gravity.
...wind speed at your shooting position is of little importance...

ugh...nerd time! I'll be here all night. So I better share the headache. I may have to PM longrange on this when it gets too long and convoluted

1. The tradeoff puzzle. The wind affects the bullet more the further it is from the muzzle as it is losing its kinetic energy/forward momentum. On the other hand, while the effect of wind is small at muzzle, the smallest deviation caused by the wind will propagate over distance.
So the correct answer is at 2/3 the distance ??? Why?

2. Since trajectory is the path a moving object follows through space, what is max trajectory? The end of the path? The point where bullet is weightless? The ball analogy works where force of gravity = upward force (that split second 0 gravity state at max height), but then the bullet will also have to be at the max height and that is 1/2 of the distance, not 2/3 ???

I've almost given up 100% testing scientific reasonings at the range and accepted what the old salts said that lrs is more of an art than science. Indeed, paying attention to basics, being in the "zone" and hard holds give more results that xyz physics, but still...."curiosity kills the cat, but satisfaction brings it back"

[The Reaper]

2. Since trajectory is the path a moving object follows through space, what is max trajectory?

Max ordinate, the apogee of the bullet's path.

TR

[frostfire]

ok, after reading several ballistics for dummies, I learnt that maximum ordinate is beyond the midrange. Hence, 2/3 the distance makes perfect sense.
Should have thought of this earlier, as the bullet slows down, gravity and air friction have more effect.

I'll just have to take it that wind at 2/3 distance dominates over any propagated deviation from wind at muzzle. Now back to reading flags and setting the scope to see mirage at 2/3 range. So much for toss-grass-in-the-air method.....living and (always) learning.

[The Reaper]

Once the bullet goes subsonic, it really gets squirrelly and prone to deflection.

TR

[longrange1947]

This is fun! And nobody's mentioned "spin drift" yet. :munchin I do remember that from the 4.2 mortar class in the heavy portion of the Weapons "Q" course. Peregrino

Reference Reaper below your post. :D

You guys are starting to make my head hurt! Reaper! Dam man, that formula will kill you and the effect is just too slight to be worth computing unless you are going ballistic in sniping. :D

Interestingly, a subsonic bullet actually has a better BC then it did just prior to transition. Go figure! :munchin

It is squirrely as heck though.

[The Reaper]

I am still trying to wrap my head around a variable BC as the bullet flies.:D

TR

[longrange1947]

I am still trying to wrap my head around a variable BC as the bullet flies.

That is why alot of the ballistic "predictor" programs fail as they do not take into account that the BC changes with velocity. Sierra, atleast, shows three to four BCs with speed envelopes to get a basic prediciton. Presicion Workbench uses a modeled curve based on actual performance of your bullet/weapon combination at specific ranges.

As the velocity approaches trans sonic it falls off rapidly and then during subsonic shift it jumps back up. It flies really well but is very suseptable to any changes in air resistance as from wind shift to thermals. The round is spinning at a higher ratio to forward velocity so the round is now nose high and overstablized (gyroscopic) making it even more suseptable to violent tilts in axis, just as a gyroscope tilts once upset.

Predicitng bullet flight at those speeds is right next to impossible as so many things come into play.

[The Reaper]

I saw a great article about ballistics in Very High Power magazine.

They went to Yuma and got the YPG instrumentation to help them track .50 bullets in flight.

One of the things they looked at was the attitude of the bullet at impact. Turns out that as you state, the bullets fly nose high from max ordinate on (or just never turn nose down from their initial launch attitude).

Very weird.

TR

[HOLLiS]

That is why alot of the ballistic "predictor" programs fail as they do not take into account that the BC changes with velocity. Sierra, atleast, shows three to four BCs with speed envelopes to get a basic prediciton. Presicion Workbench uses a modeled curve based on actual performance of your bullet/weapon combination at specific ranges.

As the velocity approaches trans sonic it falls off rapidly and then during subsonic shift it jumps back up. It flies really well but is very suseptable to any changes in air resistance as from wind shift to thermals. The round is spinning at a higher ratio to forward velocity so the round is now nose high and overstablized (gyroscopic) making it even more suseptable to violent tilts in axis, just as a gyroscope tilts once upset.

Predicitng bullet flight at those speeds is right next to impossible as so many things come into play.


Thank you, that is what I thought/leanred. I met some Greenill formula fanatics that told me I was FOS.

Thanks for the excellent read. My biggest mistake was sleeping too much in calculus class.

[kgoerz]

That is why alot of the ballistic "predictor" programs fail as they do not take into account that the BC changes with velocity. Sierra, atleast, shows three to four BCs with speed envelopes to get a basic prediciton. Presicion Workbench uses a modeled curve based on actual performance of your bullet/weapon combination at specific ranges.

As the velocity approaches trans sonic it falls off rapidly and then during subsonic shift it jumps back up. It flies really well but is very suseptable to any changes in air resistance as from wind shift to thermals. The round is spinning at a higher ratio to forward velocity so the round is now nose high and overstablized (gyroscopic) making it even more suseptable to violent tilts in axis, just as a gyroscope tilts once upset.

Predicitng bullet flight at those speeds is right next to impossible as so many things come into play.

I'm stealing this one also....Is Ed still out there or are you the only civilian working there? I heard he retired, or was sick. Ill be back late June. Might just work locally after this. I never thought I would say I'm getting tired of Bogotá. Let me know if you need anything. I'm getting some gear made here. Just for personal use or a gifts, not selling it. Me and Doc have something for ya.
It's still cheaper to have stuff made here but not as cheap as it use to be. We don't use Commando DE Selva as much, couple of other Gear makers have sprung up.

[Team Sergeant]

One of the things they looked at was the attitude of the bullet at impact. Turns out that as you state, the bullets fly nose high from max ordinate on (or just never turn nose down from their initial launch attitude).

Very weird.

TR

My guess would be that as the bullet slows gravity starts to take affect and changes the angle of the pressure wave moving in front of the bullet.

This pressure wave might keep the nose up at slower velocities where at the higher velocities the pressure wave would be equally distributed.

Just a guess. ;)

TS

[longrange1947]

KG - Ed retired last year. He is shooting for the Sierra Rifle Team and is on the Palma team so training for his matches. I have no idea how many bullets he sends down range but he fires about 100 rds a day just in .22 to stay up on the basics.

Give me a buzz when you get back in the area and I'll buy you a beer. :)

Not too sure I am ready for whatever you and NDD have dreamed up. :D