Artillery fire heard on Baghdad's outskirts
And a basic cannon primer

A report on cable news said that incoming artillery has been heard today detonating on the outskirts of Baghdad. That places US forces very close to the city. The American self-propelled howitzer, the M109A6 Paladin (155mm shell), has a maximum range of fire of 22 kilometers (13.6 miles). There is supposed to be a rocket-assisted projectile that will range to 30 km, but I have never known these to be used. Firing this howitzer to full range is very tough on the recoil mechanism and the automotive section of the weapon, although both were upgraded from its predecessor. The M109 series dates from the 1960s, and there is only so far you can take it.

155mm artillery rounds are called "separate loading" ammunition, because the projectile and the propellant are issued and loaded separately. Tank rounds look like a giant bullet - the round and the propellant casing are all one piece. But 155mm guns achieve lesser or greater ranges by a combination of varying the elevation of the cannon tube and varying the amount of propellant used. The cannister of propellant has eight bags of propellant stacked inside it, wrapped in light cloth. Each bag is numbered and is called a "charge." Firing data sent to the guns includes the maximum charge to be used. "Charge six," for example, means use charges one through six and discard seven and eight, and they must be placed in the breech in the right order. The unused charges are taken by a cannoneer to a powder pit 50 feet behind the gun. They are periodically carefully burned along with paper or cardboard packing materials (excellent marshmallow roasting!)

There is a separate, flat green bag that is placed on top of the maximum charge used. This bag is filled with a flash powder that helps burn the last of the propellant before it leaves the tube. This reduces light signature of the gun which enhances the unit's survival. Flash ranging of enemy artillery has long been used to locate it.

The bottom of charge one has a flat red bag filled with an igniter compound. When the number one cannoneer places the propellant bags into the breech, he announces in a loud voice, "I see red!" to tell the section chief (the NCO in charge of the weapon) that the bags are in the right order. The red bag is ignited when the gun is fired; it basically explodes and evenly ignites the propellant. If the charges are placed in reverse order (known to happen in the fatigue of battle) all manner of problems result, such as a hung round when the projectile gets halfway down the tube and stops. This can cause the tube to explode because those burning gases have to go somewhere.

The last thing the cannoneer uses is a firing cartridge, basically like a big, blank pistol round. It is inserted into the center of the breech. When the gunner fires the cannon, the cartridge is fired into the red igniter bag, and things proceed rapidly apace from there.

Using all the propellant gives a higher muzzle velocity than using less. This enables either a greater range with the tube raised higher, or a lower, flatter trajectory with the tube raised not so high. Different terrain and the threat of enemy counter-artillery fire figure into this consideration. But higher charges also wear out the bore of the cannon tubes quicker, until eventually the propellant gases can "blow by" the round as it travels down the tube. Not only is this dangerous, it wrecks range and accuracy.

Therefore, the cannon tubes are replaced when they have fired a certain number of "effective fired charges." EFCs are computed based on each round's propellant charges used. Each charge has a fractional EFC assigned to it, and one job of the NCO in charge of each howitzer is to make sure that every round is appropriately recorded for his weapon.

When I served in a 155mm unit in Germany in 1983-1985, we used the M109A3 model. We had a propellant charge called "Charge 9 Super." I recall it was used to achieve the 22km range. It was a large bag and was just brutal on the carriage and hydraulics of the weapon. Section chief hated firing it because it condemned them and their soldiers to hours of frustrating maintenance. The weapon was never designed to handle that kind of recoil.

More about artillery tactics and ammunition later.

Update: I checked out the current propellant charges for the 155mm gun. There are four kinds of charges. Green bag charges range out to nine km but are not used for DPICM (bomblet) rounds. White bag charges range to 14 km and fire all rounds. There are two red bag charges, one ranging to 18 km, the other, Charge 8 S (Super?) ranging to 30 km with rocket-assisted projectiles.

by Donald Sensing, 3/31/2003 08:56:36 AM. Permalink |