A vertical launching system (VLS) is an advanced system for holding and firing missiles on mobile naval platforms, such as surface ships and submarines. Each vertical launch system consists of a number of cells, which can hold one or more missiles ready for firing. Typically, each cell can hold a number of different types of missiles, allowing the ship flexibility to load the best set for any given mission. Further, when new missiles are developed, they are typically fitted to the existing vertical launch systems of that nation, allowing existing ships to use new types of missiles without expensive rework. When the command is given, the missile flies straight up long enough to clear the cell and the ship, and then turns on course. A VLS allows surface combatants to have a greater number of weapons ready for firing at any given time compared to older launching systems such as the Mark 13 single-arm and Mark 26 twin-arm launchers, which were fed from behind by a magazine below the main deck. In addition to greater firepower, VLS is much more damage tolerant and reliable than the previous systems, and has a lower radar cross-section (RCS). The U.S. Navy now relies exclusively on VLS for its guided missile destroyers and cruisers. The most widespread vertical launch system in the world is the Mark 41, developed by the US Navy. More than 11,000 Mark 41 VLS missile cells have been delivered, or are on order, for use on 186 ships across 19 ship classes, in 11 navies around the world. This system currently serves with the US Navy as well as the Australian, Canadian, Danish, Dutch, German, Japanese, New Zealand, Norwegian, South Korean, Spanish, and Turkish navies, while others like the Greek Navy preferred the similar Mark 48 system. The advanced Mark 57 vertical launch system is projected to be used on the new Zumwalt-class destroyer. The older Mark 13 and Mark 26 systems remain in service on ships that were sold to other countries such as Taiwan and Poland. When installed on an SSN (nuclear-powered attack submarine), a VLS allows a greater number and variety of weapons to be deployed, compared with using only torpedo tubes.