The U.S. Department of Defense has bought 31,800 more GMLRS (GPS guided MLRS) rockets. These are used in containers (pods) of six each. This order contains some shorter range (and cheaper) training rockets as well as some refurbished rockets. This order also includes thousands of rockets for foreign customers. But the U.S. Army and Marine Corps will get most of them. Deliveries will be made over the next three years.
The 680 pound GMLRS (guided multiple launch rocket system) missile is a GPS guided 227mm rocket that entered service six years ago. It was designed to have a range of 70 kilometers and the ability to land within meters of its intended target, at any range. This is possible because it uses GPS (plus a back up inertial guidance system) to find its target. Two years ago, the army tested GMLRS at max range (about 85 kilometers) and found that it worked fine. This enables one MLRS/HIMARS vehicle to provide support over a frontage of 170 kilometers, or, in places like Afghanistan, where the fighting can be anywhere, an area of over 20,000 square kilometers. This is a huge footprint for a single weapon (an individual MLRS/HIMARS vehicle), and fundamentally changes the way you deploy artillery in combat. By way of comparison, Excalibur (GPS guided 155mm shell) has a max range of 37 kilometers, and 120mm mortars about 7.5 kilometers.
Another edge GMLRS has is the HIMARS rocket launcher. Only costing about $3 million each, these smaller, truck mounted MLRS (HIMARS) rocket launcher systems have become very popular. HIMARS carries only one, six MLRS rocket, container (instead of two in the original MLRS vehicle). But the 12 ton truck can fit into a C-130 transport (unlike the 22 ton tracked MLRS) and is much cheaper to operate. The first HIMARS entered service in 2005, about a year after GPS guided rockets did.
Most of the GMLRS rockets are fitted with a 89 kg (196 pound) high explosive warhead. About half of that is actual explosives. That's twice as much explosives as the air force 130 kg/285 pound Small Diameter Bomb. A 155mm artillery shell has 6.6 kg of explosives, and the 500 pound (227 kg) bomb has 127 kg of explosives, which produced an excessive blast for many urban combat situations. The GMLRS seemed to be just right most of the time.
GMLRS has been used with great success in Iraq and Afghanistan, where over a thousand have been fired so far. The guided rocket is much more effective than the older, unguided, version, and is replacing it in most cases. No more of the unguided rockets are being purchased by the U.S. The accuracy of GMLRS means that one rocket does the job that previously required a dozen or more of the unguided ones. That's why HIMARS is so popular. While HIMARS only carries six rockets, that's often enough to last for days, even when there's a lot of combat.
Meanwhile, the army has found that GPS guided shells were more successful than anticipated. So they are reducing orders for these weapons. The 155mm Excalibur shells are used less frequently than expected, largely because other precision munitions often take out targets before Excalibur gets a chance to. And the army uses a lot of Hellfire missiles, fired from AH-64 helicopter gunships. In addition to the reduction in Excalibur production, the army has cut the number of GPS guided 120mm shells, which are to enter service within a year.
Meanwhile, there is still demand for unguided 155mm and 120mm shells. There are times when you need firepower over a large area (several hundred meters by several hundred meters), and for this, unguided shells do the job best, and cheapest.