""Underway replenishment"" is the raison d'etre for the navy's auxiliary vessels. Storeships, oilers, aircraft and submarine tenders, hospital ships and ammunition ships are just a few of the types of vessels that enable thenavy to sustain war operations halfway around the world from US bases. The larger Fast Combat Support Ships combine several functions by transporting fuel, ammunition and stores, but there are more than 100 types of auxiliary vessels. The auxiliaries enable the navy to maintain a global presence while providing the full range of required logistical support.