 The United States had an electric design, the Mark 18, largely copied from the German torpedo (although with improved batteries), as well as FIDO, an air-dropped acoustic homing torpedo for anti-submarine use.Modern electric torpedoes such as the Mark 24 Tigerfish or DM2 series commonly use silver oxide batteries that need no maintenance, so torpedoes can be stored for years without losing performance.A number of experimental rocket-propelled torpedoes were tried soon after Whitehead's invention but were not successful.