Discussion in 'Gunners' started by cozza, Jan 22, 2010.
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blimey you are into a whole world of aerodynamic hurt if you want to sort this out properly.
The basics are moderately simple if you know the cross-sectional area of the projectile, its co-efficient of friction then you should be able to calculate the additional force on the projecticle from the wind. Add this into the basic trajectory equations and you have a method of working out the effect of the wind, hence compensating for it.
That is the basic method, there is a whole load of heavy maths involved with effects of spin, or fin, stabilisation plus the fact that the wind is unlikely to be steady in either direction or speed during your flight path.
The method currently used is that some spotty bloke (I've met him) works out all the possible variations and sticks it into a nice computer package...this is then given to the guys at the gun end and they input the variable and it tells them the QE and charge.
The mortar guys have a handy table which they look it up on, I think rather than a computer...but it is the same set of maths just on a lookup table (circular thing last i saw one).
If you are inasne about working out the maths then there a number of books that give you the direction to go in.
When I was in, it was all Firing Tables, met, MV's for each charge, Equilibrium Yaw etc. OK, we had FACE and later, AFDC, but still needed to imput the initial data and met was hourly.
Haven't got a clue what they use now, but I would assume the basic data is roughly the same.
Last I played with Light Gun (about 2 years ago now) it was terminal with a GPS unit on the top of the barrel (with gyros I assume) and met input as you say. Firing tables still exist and were supplied as a backup, personally I used the firing tables more but that was mainly as the terminals never worked on trials and I was more interested in the ammunition than the target.
Basic data is the same.....the computer systems used to be a copy of the firing tables in softcopy but I think it is the otherway round now with the terminal containing a ballistic model and the tables printed from the model.
We already have one. Their called 'Mortar Fire Controllers', normally in their late twenties or thirties. They're brilliant...
What happens is - mortar fires, bomb heads up, comes down and lands. If the wind changes where we think its gonna land, the MFC calls and adjustment...
If it ain't broke - no need to fix it, fella!
Thanks S_R, very interesting info. How do the Gyro's cope with recoil presssures? I would have thought they would be too delicate, or are they locked in the Gymbals once the initial data from GPS has been imputted?
Not sure to be honest, I dealt with ammunition not the gun, think it fitted on the top of the compensator and so wouldn't be directly affected by the barrel recoil (the compensator housing being "stationary" in relation to the gun structure)
We only even got it working once to be honest, never got any of the working kit for trials and most of the trials were from a range stand anyway (only fired 2 or 3 from a real gun).
Don't forget that the wind will be doing different things throughout a projectile's trajectory, which is likely to be significant in terms of both range and altitude; and that all of this changes over the duration of flight (which is also likely to be significant).
indeed in addition to the fact that the wind force acting on the projectile will change as the projectile is sped up in the direction of the force (as the relative speeds reduce) the wind direction and speed can change dramatically throughout the flight.
The change in wind speed/direction over altitude and range is probably the hardest part of wind compensation as you never know all the relevant information and have to assume or extrapolate from known data.
They aren't the simplest to use from scratch...try this one which has some explaination on it although I think it is an artillery one it might give you a good start until you can find a mortar one....it's also american I think.
I'm not sure how 'hardware' you are going but i would suggest the ideal would be a combined unit with windspeed and direction sensor, GPS and an input pad....that way you have current location and windspeed/dir already input and updated regularly all you need to do is into target loc and bomb type and you are sorted.
p.s. I forgot if you are being really flashy a temp and humidity sensor as well.
Don't forget shell weight. Artillery shells were (are?) measured in squares, the standard being two square. Haven't a clue if this is the same with mortar rounds.
A first round fired at a target, is the quickest and most accurate met data you will get for that F.M.
S_R, thanks for your earlier info. I remember when GPS and Self-Orienteering Theodolites wre just coming into service just after GW1 and it seemed to have a fetish on China co-ords!
Ballistics calculations are not simple if you require any degree of accuracy, not only do you need to take wind into consideration but also things like air temperature, pressure, the temperature of the mortar propellant and the fact that all the time the mortar is in flight the Earth is turning underneath it. If you can get access to it try to get your hands on something called the NABK, the NATO Armaments Ballistic Kernel. It's basically an electronic firing table, you pump in the relevant data, your position, target position, Met data, mortar type etc. and it does all the calculations and spits out bearing and QE. Last time I used it was a few years ago now and it wasn't perfect but it was still good enough for us to do the calculations that enabled us to hit moving naval targets with an AS90.
First obvious question is why rediscover the wheel by 'inventing' such a device? In UK service the first such calculator was the MFDC (Mortar Fire Control Calculator not to be confused with Missile Fire Direction Centre), introduced circa 1983, this was a re-packaged HP41C programmable calculator, the software was written by Zengrange, a Leeds company. It used polynomials to represent firing table data, and required data about current meteor conditions to be entered, this data being the standard NATO ballistic meteor message. Lots of people on this list will be familiar with its arty twin AFDC.
However, NATO standardised (a few decades ago) on the modified point mass model, this uses 4 degrees of freedom (the MLRS experts will now leap around waving their hands saying please sir please sir MLRS used to use 6 degrees of freedom). More recently (well starting in the 1990s) the 'NATO Armaments Ballistic Kernel' (NABK) was developed, this software package implements the modified point mass model, is written in Ada 95, is fully tested and meets safety critical software requirements (mandatory for such softeware in reputable countries like UK), this avoids the need for each nation in NATO to create a calculation program and is a good example of responsible use of the taxpayers money. The UK Fire Control Application (which is in service and can handle guns and mortars) and FC-BISA, both developed by Logical-CMG at the taxpayers expense, use the NABK. The NABK has weapon specific aeroballistic data files and takes data of the moment for wind, air temp and air density from the standard meteor message, with charge temp from local measurement.
Petardier old chap read the thread and you'll notice the bloke is doing an electronics course and has to produce an item to demonstrate his skills, he has been told that something military related would be a good slant and so has chosen a range calculator for mortar firing. He's not trying to re-invent the wheel.
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