Electric batteries[edit]
Electric propulsion systems avoided tell-tale bubbles. John Ericsson invented an electrically propelled torpedo in 1873; it was powered by a cable from an external power source, as batteries of the time had insufficient capacity. The Sims-Edison torpedo was similarly powered. The Nordfelt torpedo was also electrically powered and was steered by impulses down a trailing wire.
Germany introduced its first battery-powered torpedo shortly before World War II, the G7e. It was slower and had 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 G7ep, 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 or DM2 series commonly use silver oxide batteries that need no maintenance, so torpedoes can be stored for years without losing performance.
Wet-heater[edit]
A further improvement was the use of water to cool the combustion chamber of the fuel-burning torpedo. This not only solved heating problems so more fuel could be burnt but also allowed additional power to be generated by feeding the resulting steam into the engine together with the combustion products. Torpedoes with such a propulsion system became known as wet heaters, while heated torpedoes without steam generation were retrospectively called dry heaters. A simpler system was introduced by the British Royal Gun factory in 1908. Most torpedoes used in World War I and World War II were wet-heaters.
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