Spruance class destroyers operate in support of carrier battle groups, surface action groups, amphibious groups and replenishment groups. The Spruance class destroyers primarily perform antisubmarine warfare duty while guided missile destroyers like the Arleigh Burke are multi-mission (ASW, anti-air and anti-surface warfare) surface combatants. The addition of the Mk-41 Vertical Launch System to the destroyers has greatly expanded their role in strike warfare. The Spruance class destroyers were the first large U.S. Navy warships to employ gas turbine engines as their main propulsion system and are undergoing extensive modernizing. The upgrade program includes addition of vertical launchers for advanced missiles on 24 ships of this class, in addition to an advanced ASW system and upgrading of its helicopter capability. Spruance class destroyers are expected to remain a major part of the Navy's surface combatant force into the 21st century.