Vaccume in the atmosphere and mortars

[PLM]

This is on another forum, I used to have to deal with stuff like vaccumes in Physics class but I been outa school for a year now, man. I'm far too lazy but he thinks mortars are best used at the mile range, so much that he says they should be kept off the map of any map smaller than a mile long, which nobody would map in an FPS game.

Also, for mortars, the US M2 60mm mortar has a muzzle velocity of 434 feet per second, and the US M1 81mm mortar has a muzzle velocity of 560 feet per second. The M1's minimum range is 1700 feet, the M2's minimum range is 1000 feet. The on-map range would be 60-85 degrees for the M2 and and 75-85 degrees with the M1, but that's with 1 mile maps if the mortars are positioned on the map's edge. It'd be best to leave the mortars for an off-map thing.

For the M1, it takes 25-35 seconds for a mortar to hit the target. For the M2, it takes 19-27 seconds. I haven't found a good source of information to calculate for heavy artillery and howitzers. I'll have the German data in a second.

I disputed the minimum range.

If you're going to get less range than you're firing above 80 degrees, and you'll have to fire either from a hill or prop the baseplate of the mortar up on something. 100 meters is 328 feet, the minimum range is 85.5 feet, but that's at just over 89 degrees, which the M1 is incapable of acheiving on a flat surface(irl it's very dangerous to do that, as there's the risk of the mortar falling right back down on your position, or behind your position)

Also, remember that this is a vacuum, in atmospheric conditions the range is drastically reduced. By about 20%-30%. You could simulate this without coding in atmosphere, but this won't affect the ballistics on weaponry worth a damn and wouldn't be very physically accurate.

I have no clue what hes talking about a vaccum

I pointed out that mortars have been used in the 100-200-300 meter range and there's no reason they can't be.

This is on a flat surface in a vacuum. On a surface angled downwards(so the time will be decreased instead of increased when shortening range) and in an atmosphere(ie, on Earth), you could easily get those ranges. The shortest possible range would be 85 degrees, but this means that instead of going out, it's going up, and it'll take quite a while to come back down.

1700 feet is ~500 meters. 30% of 500 is 150.

[Michael Emrys]

Apparently he never heard of staged charges. Sounds like he just learned how to use a slide rule and is deliriously happy to play with it. To each his own...

Michael

[John D Salt]

Originally posted by Michael Emrys:

Apparently he never heard of staged charges. Sounds like he just learned how to use a slide rule and is deliriously happy to play with it.

Michael, old feller, you and me must be the only greybeards who still remember slide-rules.

All the best,

John.

[Dave H]

Originally posted by John D Salt:

Michael, old feller, you and me must be the only greybeards who still remember slide-rules.

All the best,

John.

Fortunately for me, that statement is not entirely accurate.

[Kingfish]

Slide rules? Isn't that part of the overall playground etiquette?

[Wartgamer]

Some data from a US Manual:

81mm M43 mortar round (light) 45 deg

zone 0 (cartridge only) 235 fps 541 yds

zone 1 (cart plus 1 charge) 332 fps 1020 yds

zone 2 (cart plus 2 charge) 410 fps 1500 yds

zone 3 (..........3 charge) 499 fps 2042 yds

zone 4 (..........4 charge) 572 fps 2517 yds

zone 5 (..........5 charge) 638 fps 2963 yds

zone 6 (..........6 charge) 700 fps 3288 yds

The reference to a vacuum is just stating that atmospheric conditions must be taken into account.

[ April 29, 2005, 09:35 AM: Message edited by: Wartgamer ]

[Michael Emrys]

Originally posted by Kingfish:

Slide rules? Isn't that part of the overall playground etiquette?

YOU are going to be in big trouble if you ever run out of batteries.

Michael

[Michael Emrys]

Originally posted by John D Salt:

</font><blockquote>quote:</font><hr />Originally posted by Michael Emrys:

Apparently he never heard of staged charges. Sounds like he just learned how to use a slide rule and is deliriously happy to play with it.

Michael, old feller, you and me must be the only greybeards who still remember slide-rules.</font>

[MikeyD]

I had at first thought that he had mis-stated 'vacuum' for 'drag', but it appears he really means the ABSENCE of drag in a raw calculation. X would have range Y without drag, but Y=20% with drag, or something like that.

I don't know if his math is any good, but that appears to be where he was going. I've learned from these forums that there can be an uncomfortable gap between theory and practice in published ballistics calculations on historical weapons.

[Pzman]

Originally posted by John D Salt:

</font><blockquote>quote:</font><hr />Originally posted by Michael Emrys:

Apparently he never heard of staged charges. Sounds like he just learned how to use a slide rule and is deliriously happy to play with it.

Michael, old feller, you and me must be the only greybeards who still remember slide-rules.

All the best,

John. </font>

[junk2drive]

First I learned an abacus, then a slide rule...

Geometry was my fav.

[flamingknives]

Assuming a vacuum makes ballistic calcs easier, as aerodynamic drag is a cast-iron SoB to calculate for the whole trajectory without a computer. In a vacuum, you can completely ignore it.

Given an excel spreadsheet, the aerodanamic drag equations, the constant acceleration equations the mass of the bomb, the drag coefficient of the bomb and it's muzzle velocity, it ought to be reasonably simple to calculate a range for any given angle.

[MikeyD]

Funny thing is, drag is CAUSED by a vacuum! ...more or less.

The shell (or car or bicycle) speeding along pushes the air molecules aside. It takes some time for the air to collapse back in behind the shell and this creates a microvacuum at the shells base. That's why Bull's base-bleed artillery shells have the increased range. The microvacuum behind the shell in filled by - something - exiting a hole in the shell's rear, eliminating the drag.

Watch a few NOVA programs on PBS and you become a danger to others

[flamingknives]

Indeed. Although the contribution to drag by the nose pressure - the bow wave, if you like - is significantly greater than base drag.

Base drag can also be attenuated by a boat-tail. Due to it's shape, a mortar bomb will have little base drag but significant nose drag.

[Edit: base drag is due to vacuum, nose drag is down to overpressure]

[PLM]

I'm very confused. Poor me

[flamingknives]

By what, exactly? (Don't say 'everything', as that's not very useful) Let us know what you're not getting, and your level of understanding on the subject, and it makes it much easier to explain.

[YankeeDog]

PLM:

The short and simple version is that whoever posted those comments has his head up his @ss.

Air resistance would reduce range some, but the big mistake he makes is assuming that mortars can fire at only one muzzle velocity. For example for the 81mm M2, he quotes a muzzle velocity of 560fps.

As Wartgamer's table shows, the 81mm M2 can be fired with 7 different "charge" settings. To fire a mortar with more than base charge, additional propellent is attached to the tail of the bomb before it's dropped down the tube. So actually, the muzzle velocity of an M2 can be anywhere between 235fps & 700fps.

So, assuming Wartgamer's table is correct (and it looks about right to me), on charge 0, the 81mm M2 have an Muzzle Velocity of only 235fps, about half what your misguided poster used in his calculations.

Obviously, the slower the shell goes, the less time it will spend in the air, and the closer you can drop the shell to your firing position.

I'm to lazy to do it myself, but I suspect if he re-ran his calculations with 235fps muzzle velocity & 80 degree muzzle angle, and then subtracted 20% or so as a rough estimate of air resistance, he'd end up with a minimum range very close to the 100m or so usually quoted for such weapons.

[Wartgamer]

All of that data was using a set 45 degree angle of fire. I would assume this gives the greatest range and using a fire chart, you would back off angle to decrease range.

Using non-air resistance newtonian physics, you should get:

235 fps yields approx 576 yards range 10.6 sec(~6% range loss)

700 fps yields approx 5104 yards range 31 secs (~35% range loss)

This is just envelope-back calcs but I can crunch it better. Clearly 3 times the velocity (or 3 times the flight time also) has resulted in approx 6 times the range loss.

[flamingknives]

Originally posted by Wartgamer:

Using non-air resistance newtonian physics, you should get:

That's not a term I'm familiar with. What does it mean?

I'd hazard that you're assuming a constant resistance for the bomb.

[Wartgamer]

neglecting air resistance. moon artillery with earths gravity.

[MikeyD]

Something that will have to be taken into consideration when the CMx2 title "Space Lobsters of Doom" comes out.

[flamingknives]

Right. Having said how easy it is, I decided to have a go.

As a result, we see this graph of the trajectory of an M821 or M889 81mm mortar bomb.

Muzzle velocity: 66m/s

Bomb mass: 4.14kg

Launch angle: 82.5 degrees

Spreadsheet

I've accounted for air drag and gravity in this calc in increments of 0.1 seconds. While I've kept the drag coefficient constant at subsonic velocities, it doesn't change much. That said, the value of Cd is the crudest fudge known to man, and any results must be taken with this in mind.

Impact is at 120m/s and 86 degrees down angle.

[Wartgamer]

I'd hazard that you're assuming a constant resistance for the bomb.

No, thats incorrect.

I am using a comparison of no-resistance from air (pure newtonian equations) and known data (US 81mm mortar rounds.

Given these two, one can calculate some effects from air resistance.

[Wartgamer]

Originally posted by flamingknives:

Right. Having said how easy it is, I decided to have a go.

As a result, we see this graph of the trajectory of an M821 or M889 81mm mortar bomb.

Muzzle velocity: 66m/s

Bomb mass: 4.14kg

Launch angle: 82.5 degrees

Spreadsheet

I've accounted for air drag and gravity in this calc in increments of 0.1 seconds. While I've kept the drag coefficient constant at subsonic velocities, it doesn't change much. That said, the value of Cd is the crudest fudge known to man, and any results must be taken with this in mind.

Impact is at 120m/s and 86 degrees down angle.

The impact velocity is greater than the launch velocity? Me thinks not.

[flamingknives]

You could have said "Assuming a vacuum", and it would have been clear, rather than make up something about 'non-air'

"neglecting air resistance" would also be good.

*stops before going off on usual tirade about unclear terminology*

*Breathes*

Everything's clear now. Thanks for clarifying.