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]

Originally Posted by 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

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]

Originally Posted by Jack Moroney

...it wasn' a friggin catapult.

Trebuchet?

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]

Originally Posted by Surgicalcric

Trebuchet?

Crip

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

[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]

Originally Posted by The Reaper

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.

Originally Posted by Pete

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.

Originally Posted by Team Sergeant

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.

Originally Posted by The Reaper

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

Originally Posted by one-zero

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?

[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 , 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?

[JLF]

Originally Posted by 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 , 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?

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