wire. Germany introduced its first battery-powered torpedo shortly before World War II, the G7e. It was slower and had a shorter range than the conventional G7a, but was wakeless and much cheaper. Its lead-acid rechargeable battery was sensitive to shock, required frequent maintenance before use, and required preheating for best performance. The experimental G7es, an enhancement of the G7e, used primary cells. 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, the Black Shark or DM2 series commonly use silver oxide batteries that need no maintenance, so torpedoes can be stored for years without losing performance. Rockets Several experimental rocket-propelled torpedoes were tried soon after Whitehead's invention but were not successful. Rocket propulsion has been implemented successfully by the Soviet Union, for example in the VA-111 Shkval—and has been recently revived in Russian and German torpedoes, as it is especially suitable for supercavitating devices. Modern energy sources Modern torpedoes use a variety of propellants, including electric batteries (as with the French F21 torpedo or Italian Black Shark), monopropellants (e.g., Otto fuel II as with the US Mark 48 torpedo), and bipropellants (e.g., hydrogen peroxide plus kerosene as with the Swedish Torped 62, sulfur hexafluoride plus lithium as with the US Mark 50 torpedo, or Otto fuel II plus hydroxyl ammonium perchlorate as with the British Spearfish torpedo). Propulsion The first of Whitehead's torpedoes had a single propeller and needed a large vane to stop it spinning about its longitudinal axis. Not long afterward the idea of contra-rotating propellers was introduced, to avoid the need for the vane. The three-bladed propeller came in 1893 and the four-bladed one in 1897. To minimize noise, today's torpedoes often use pump-jets. Some torpedoes—like the Russian VA-111 Shkval, Iranian Hoot, and German Unterwasserlaufkörper/ Barracuda—use supercavitation to increase speed to over . Torpedoes that don't use supercavitation, such as the American Mark 48 and British Spearfish, are limited to under , though manufacturers and the military don't always release exact figures. Guidance Torpedoes may be aimed at the target and fired unguided, similarly to a traditional artillery shell, or they may be guided onto the target. They may be guided automatically towards the target by some procedure, e.g., sound (homing), or by the operator,