Multiple rocket launcher
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A multiple rocket launcher (MRL) or multiple launch rocket system (MLRS) is a type of rocket artillery system that contains multiple launchers which are assembled onto a same platform, and shoots its rocket ordnance in a fashion similar to a volley gun. Rockets are self-propelled in flight and have different capabilities than conventional artillery shells, such as longer effective range, lower recoil, typically considerably higher payload than a similarly sized gun artillery platform, or even carrying multiple warheads.
Unguided rocket artillery is notoriously inaccurate and slow to reload compared to gun artillery. A multiple rocket launcher helps compensate for this with its ability to launch multiple rockets in rapid succession, which, coupled with the large kill zone of each warhead, can easily deliver saturation fire over a target area. However, modern rockets can use GPS or inertial guidance to combine the advantages of rockets with the higher accuracy of precision-guided munitions.
The earliest form of a rocket was invented during the medieval Chinese Song dynasty, in which the Chinese fire lance was fixed backward on a pike or arrow and shot at enemy in 1180.  This form of a rocket evolved from the Chinese fire lance was used during the Mongol siege of Kaifeng.
Later, Chinese military created and employed multiple rocket launchers that fired up to 100 small fire-arrow rockets simultaneously. The typical powder section of such an arrow-rocket was 1/3 to 1/2 ft (10 to 15 cm) long. Bamboo arrow shafts varied from 1.5 ft (45 cm) to 2.5 ft (75 cm) long and the striking distance reached 300 to 400 paces. The Chinese also enhanced rocket tips with poison and made sure that the launchers were also mobile. The designers designed a multiple rocket launcher that could be carried and operated by a single soldier. Various forms of multiple rocket launchers evolved, including the launcher mounted on wheelbrows. 
In Joseon Dynasty of Korea, the several scientists including Yi Do and Choi Hae-san invented hwacha in 1409. The launchpad made of 100 to 200 holes containing rocket arrows was placed on a two-wheeled cart. The range of the fired arrows is estimaed to be 2000 meters. Hwacha was a key weapon used against the invading armies during the Japanese invasions of 1592–1598. The most notable example was Battle of Haengju, in which 40 hwachas were deployed to repel 30,000 Japanese soliders. 
European armies preferred relatively large single-launch rockets prior to World War II. Napoleonic armies of both sides followed British adoption of Mysorean rockets as the Congreve rocket. These were explosive steel-cased bombardment rockets with minimal launchers. European navies developed naval multiple launcher mounts with steadily improving explosive rockets for light and coastal vessels. These weapons were largely replaced by conventional light artillery during the late nineteenth century.
World War IIEdit
The first self-propelled multiple rocket launcher – and arguably the most famous – was the Soviet BM-13 Katyusha, 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. The Germans began using a towed six-tube multiple rocket launcher during World War II, the Nebelwerfer, also called the "Screaming Mimi" by the Allies. The system was developed prior to the war to skirt the limitations of the Treaty of Versailles. Later in the war, the 15 cm Nebelwerfer 41 was mounted on modified Opel Maultier "Mule" halftracks becoming the Panzerwerfer 42 4/1. Another version produced in limited numbers towards the end of the war was a conversion of sWS (Schwerer Wehrmachtschlepper) "heavy military transport" halftrack to a similar configuration as the Panzerwerfer 42 4/1 mounting the 10-barreled 15 cm Nebelwerfer.
Another German halftrack MRL system was inspired by the Russian BM-13. Keeping the Soviet 82mm rocket calibre as well as the launch and rocket stabilisation designs, it was developed into a system of 2 rows of 12 guide rails mounted to the Maultier chassis; each providing the capacity for 24 rockets (rockets mounted underslung as well as on top of the rails) for 48 rockets total. This vehicle was designated as the 8cm Raketen-Vielfachwerfer, '8 cm multiple rocket launcher'. As the launch system was inspired by and looked similar to the BM-13, which the Germans had nicknamed "Stalin-Orgel" or "Stalin-Organ", the Vielfachwerfer soon became known as the "Himmler-Orgel", or "Himmler-Organ"
There are two main types of multiple rocket launchers:
- With tubes or pipes, usually made of steel, non-removable from launcher, with options to be reloaded in on the battlefield with rockets loaded manually or semi-automatically. This was the most usual type until the 21st century. It is more convenient for battlefield usage because it does not require special tools to reload modules and test them before using them on launchers as with other types.
- With containers, pods or modules that can be removed from the launcher and quickly replaced with same or different types of rockets and calibres. They are usually reloaded at factory or within specially-equipped army workshops. These are more modern types of weapons as they are not necessarily related to just one type of rocket and give more options to commanders on field to deal with different tactical situations using different types of rockets or to quickly reload. They are also easier to upgrade for different/new types of rockets.
Like all artillery, MRLs have a reputation of devastating morale on ill-disciplined or already shaken troops. The material effect depends on circumstances, as well-covered field fortifications may provide reasonable protection.
MRLs are still unable properly to engage reverse slope positions in mountain warfare because it is more difficult to determine the trajectory compared to that of a howitzer 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 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 calibre) especially on light trucks and pickups (so-called "technicals") are often seen in civil wars, when rebels make use of captured launchers and munitions.
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 previously required such an extent of effort that making a dispersed operation of the battery was at times impractical. MRL systems with GPS can have their MRLs dispersed and fire from dispersed positions at a single target, just as previously multiple batteries were often united on one target area.
Radar may be used to track weather balloons to determine winds or to track special rockets which self-destruct in the air. The tracking allows determination of 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 at just 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 IMI ACCULAR is an example.
Fin-stabilised rockets also allow for easy course corrections using rudders or minute charges. Precision-guided munitions have been introduced to exploit this. Guidance principles such as GPS satellite navigation, inertial navigation systems and semi-active laser seekers are used for this. This improves dispersion from a CEP of hundreds of meters at dozens of kilometers range to just a few meters and largely independent of the range of the round (except for INS, as INS navigation creates a small dispersion that's about proportional to range). 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 thus pose a deconfliction challenge, as they might collide with friendly aircraft in the air.
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