Vertical launching system
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A vertical launching system (VLS) is a type of missile-firing system used aboard the submarines and surface vessels of several navies around the world.
Derived from the launch systems developed for ballistic missiles aboard SSBNs, a VLS forms a scaled-down equivalent for launching cruise missiles such as the Tomahawk and surface-to-air missiles (SAMs) such as the Standard missile. The system enables SSNs (nuclear-powered attack submarines) to carry more weapons in addition to their torpedo tubes. More significantly, VLSs allow both submarines and surface combatants to have more weapons ready for firing at a given time than with other launching systems such as the Mk-13 single-arm and Mk-26 twin-arm launchers, which were fed from behind by a magazine below the main deck on older guided-missile cruisers, destroyers, and frigates, all of which have been removed from service by the USN, or else had their surface-to-air missiles removed (Oliver Hazard Perry-class guided-missile frigates.) The Mk-13 and the Mk-26 remain in service on ships that were sold to other countries such as, Taiwan, Greece, and Poland.
[edit] Hot launch and cold launch
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Western VLSs have the missile cells arranged in a grid with one lid per cell and are "hot launch" systems, i.e. the engine ignites within the cell during the launch, and thus it requires exhaust piping for the missile flames and gasses, while Russia produces both grid systems and a revolver design with more than one missile per lid, and the People's Republic of China uses a circular "cold launch" system that ejects the missile from the launch tube before igniting the engine. Russia also uses a cold launch system for some of its VLS missile systems (eg. Tor).
The advantage the hot-launch system has over the cold-launch system is that the missile propels itself out of the launching cell using its own engine. This reduces the additional weight, size, maintenance requirement, and initial production cost for the additional power sources the cold-launch system uses to propel the missile out. The missiles of the hot-launch system also come out of their tubes immediately ready to start seeking their targets.
The advantage of the cold-launch system is in its safety: should the missile engine malfunction while the warhead is armed to detonate during firing, the hot-launch system could be destroyed, but the cold-launch system can still eject the missile out of the cell and eliminate or reduce the threat. For this reason, Russian VLS is often designed with a slanted angle instead of being perpendicular so that when the malfunctioned missile is ejected, it would fall into the water instead of landing on the deck. Another advantage of the cold-launch system is in its low life-cycle cost of the launching tubes: since the engine starts within the tube in a hot-launch system during launches, the tubes of the hot-launch system can only sustain a limited number of launches - after which the tube must be replaced (just like in the large naval guns of obsolete and retired ships). A cold-launch system, in contrast, can last much longer because the tubes are not subject to the extreme heat blasts compared to the hot-launch system's.
