Multiple rocket launcher
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A multiple rocket launcher (MRL) is a type of unguided rocket artillery system. Like other rocket artillery, multiple rocket launchers are less accurate and have a much lower (sustained) rate of fire than batteries of traditional artillery guns. However, they have the capability of simultaneously dropping many hundreds of kilograms of explosive, with devastating effect.
The beginning model of multiple rocket launcher was created in Korea which used black gunpowder to accelerate the projectiles that were basically arrows with small fuel tanks. The hwacha, considered the world's first multiple rocket launcher, was a technological breakthrough  consisting of a two-wheeled launch pad capable of firing up to 200 singijeon (a type of fire arrow rocket), used during the Joseon Dynasty (1392–1897).
The first self-propelled multiple rocket launchers — and arguably the most famous — were the Soviet BM-13 Katyushas, first used during World War II and exported to Soviet allies afterwards. They were simple systems in which a rack of launch rails was mounted on the back of a truck. This set the template for modern multiple rocket launchers. The Americans mounted tubular launchers atop M4 Sherman tanks to create the T34 Calliope rocket launching tank, only used in small numbers, as their closest equivalent to the Katyusha.
Simple MRL types still have a reputation of having a devastating morale effect on ill-disciplined or already shaken troops. The material effect depends on circumstances, as well-covered field fortification provide much protection.
MRL are still rarely able to properly shoot at rear slope positions in mountain warfare because the MRL crew cannot determine the trajectory as well as a howitzer's crew can do by adding or removing propellant increments. Simple MRL rocket types have a rather long minimum firing range for the same reason. An approach to lessen this limit is the addition of drag rings to the rocket nose. The increased drag slows the rocket down relative to a clean configuration and creates a less flat trajectory. Pre-packaged MRL munitions such as with MLRS do not offer this option, but some MRL types with individually-loaded rockets do.
Improvised multiple rocket launchers based on helicopter- or aircraft-mounted rocket pods (typically of 57–80 mm caliber) on light trucks and pickups (so-called "technicals") are especially often sighted in civil wars, when rebels make use of captured launchers and munitions.
The Convention on Cluster Munitions has led to the destruction and cessation of production of most cluster munitions for MRLs. Large submunitions, smoke agents and unitary high explosive warheads are still permitted under the Convention.
Some of the not banned submunitions are relatively large Explosively Formed Projectile (EFP) warheads with sensors (millimetre wave radar, infrared) which search the ground for tanks to attack in a spiral pattern during their descent (retarded by parachutes). Western examples are SMArt 155, 155 Bonus and SADARM.
Modern MRL systems can use modern land navigation (especially satellite navigation such as GPS) for quick and accurate positioning. The accurate determination of the battery position was previously enough effort to make a dispersed operation of the battery impractical. MRL systems with GPS can have their MRLs dispersed and shoot from dispersed positions at a single target, just as previously multiple batteries were often united on one target area.
Radars are in use to track weather balloons to determine winds or to track special rockets which self-destruct in the air. The tracking allows to determine the influence of winds and propellant temperatures on the rocket's flight path. These observations can then be factored into the firing solution for the rocket salvo for effect.
Such tracking radars can also be used to predict the range error of individual rockets. Trajectory correcting munitions may then benefit from this, as a directional radio may send a coded message to the rocket to deploy air brakes just at the right time to correct most of the range error. This requires that the rockets were originally aimed too far, as the range can only be shortened by the air brakes, not extended. A more sophisticated system makes use of radar data and one-way radio datalink to initiate a two dimensional (range and azimuth) correction of the rocket's flight path with steering by fins or nose thrusters. The latter is more common with systems which can be used to upgrade old rockets and the Israeli Accular is an example.
Fin-stabilised rockets also allow for easy course corrections using rudders or minute charges. Guided munitions have been introduced to exploit this. Guidance principles such as GPS satellite navigation, inertial navigation system and semi-active laser seekers are used for this. This improves dispersion from a CEP of hundreds of metres at dozens of kilometers range to just a few meters, and (except for INS, as INS navigation creates a small dispersion that's about proportional to range) largely independent of the range of the shot. This in turn made great increases of rocket (or missile) ranges useful; previously dispersion had made rockets too inefficient and often too dangerous to friendly troops at long ranges. Long range MRL missiles often fly a higher quasiballistic trajectory than shorter ranged rockets and pose thus a deconfliction challenge, as they might collide with friendly aircraft in the air.
- List of rocket artillery
- List of U.S. Army Rocket Launchers By Model Number
- Nebelwerfer family of multiple rocket launchers, World War II Wehrmacht
- Kim, Myung Oak; Jaffe, Sam (2010). The new Korea: an inside look at South Korea's economic rise. AMACOM Div American Mgmt Assn. p. 149. ISBN 978-0-8144-1489-7. Retrieved 2012-05-30.
- Michael E. Haskew, Christer Joregensen, Eric Niderost, Chris McNab (2008). Fighting techniques of the Oriental world, AD 1200–1860: equipment, combat skills, and tactics. Macmillan. p. 201. ISBN 978-0-312-38696-2. Retrieved 2012-05-30.
- Jane's Armour and Artillery 2011-2012 data sheets