Multiple rocket launcher
A multiple rocket launcher (MRL) is a piece of artillery, which fires, simultaneously or near-simultaneously, a large number of unguided rockets. Some current systems can also fire guided missiles. With unguided rockets, they are area-effect weapons, in comparison with potentially more precise howitzers.
In the case of an MRL, the lack of guidance of its rockets often is considered an advantage rather than a disadvantage, as the most common purpose of a MRL is for covering an area, rather than a point, target. The rockets spread out to cover the area.
Another reason to use the MRL is that its lack of recoil makes it easy to mount on trucks, small boats, etc. The use of rockets, rather than a barrel that has to contain the expanding propellant of cannon, allows the MRL to be quite light in weight.
History before the 20th century
Rockets were invented by the Chinese, who used multiple rocket-propelled rockets in the 13th century, against Mongols.
They did not appear in Europe until the Napoleonic wars, when Sir William Congreve developed techniques, including a counterbalancing rod and aerodynamic fins that gave somewhat more stability to the rocket. Congreve rockets were launched both singly and in multiple launchers; they provided the "rockets' red glare" in the U.S. national anthem. Nevertheless, perhaps due to the lack of sufficiently powerful explosive warheads — they were not regarded highly until the Second World War.
WWII multiple rocket launchers
In World War II, the Germans introduced a Lightweight MRL using spin-stabilized rockets, called the Nebelwerfer (German for smoke launcher). It was originally intended as a smoke generator launcher, but soon took on additional roles, not the least of which was the psychological effect of the frightening sound of its rockets. It had six tubes for 100mm rockets with smoke, high explosive, and incendiary warheads.
The Soviets were inspired by the Nebelwerfer to start the beginning of a long and continuing series of MRLs. Their basic WWII system was a truck-mounted launcher for sixteen 130mm "Katyusha" rockets. The Katyusha had a range of about four miles. Later versions had as many as 48 tubes.
Often used in battalions of 18 launching trucks, a battalion salvo of 864 rockets covered a wide area, and was visually impressive as well as tactically effective. It fitted well with the general Soviet preference for massed artillery. As soon as the battalion fired, it would move to avoid counterfire.
Subsequent versions have gone to a ballistically improved 122mm rocket, officially the GRAD but the term Katyushka. While both types were designed for multiple launchers, they have become common weapons for guerrillas launching a single rocket, with little accuracy but a great deal of portability. Modern versions have 20 or 30 tubes.
The most common use was to put MRLs on landing craft, for final close-in saturating fire in amphibious warfare
The trend in MRLs tends to have a smaller number of larger-diameter rockets, often using cluster submunitions rather than a large number of small rockets. With the cost of electronics decreasing, there is also a trend towards using guided missiles from the same launcher.
There have been variants with antitank submunitions, which, since they have fuzing that will detonate only on large vehicles, do not present the ethical issues of dual-purpose cluster submunitions. With the U.S. built M77 bomblet, all too frequently, with estimates of 3 to 25 percent of each warhead, the bomblets landed but did not detonate. Those bomblets still could detonate with slight movement, and, especially when used in populated areas, created unacceptable and unintentional antipersonnel minefields. Israeli use of these weapons in Lebanon raised much controversy, and areas of Kuwait and Iraq remain unsafe.
U.S. M270 Multiple Launch Rocket System
The U.S. M270 Multiple Launch Rocket System (MLRS) differs from some of the WWII designs, by having a smaller number of larger rockets. The launch vehicle accepts two pods, which contain either six 227mm rockets, or one MGM-140 ATACMS short to medium range ballistic missile.
While the original M26 unguided rocket, which carried 644 dual-purpose cluster submunitions, was effective, with a devastating number of bomblet detonations, the M77 munitions had to hit the ground at an ideal angle to ensure detonation. A single launcher's salvo of 12 rockets could cover a square kilometer.
In the Gulf War, there were occasions when one or more battalions, of 18 launchers each, would fire 12 rockets per launcher, each with 644 bomblets.
Newer M28 and M30 rockets increased range, and, in the M30, added guidance. Newer cluster submunitions have improved, but not ended, the dud problem, and an alternative XM31 unitary high-explosive rocket is preferred. While it appears possible to build a safer submunition, political considerations may focus development on unitary warheads.
Russian BM-30 Smerch
Russia's BM-30 Smerch (Tornado) or 9K58 is another recent system, firing twelve 300 mm rockets.