Editing Torpedo (section)

== Propulsion ==
[[File: US Navy inactive torpedo launch.jpg|thumb|right|{{USS|Mustin|DDG-89|6}} launches a dummy torpedo during exercises.]]

===Compressed air===
The Whitehead torpedo of 1866, the first successful self-propelled torpedo, used [[Pneumatics|compressed air]] as its energy source. The air was stored at pressures of up to {{convert|2.55|MPa|abbr=on}} and fed to a [[piston engine]] that turned a single [[propeller]] at about 100 [[Revolutions per minute|rpm]]. It could travel about {{convert|180|m|yard}} at an average speed of {{convert|6.5|kn|km/h|lk=in}}. The speed and range of later models were improved by increasing the pressure of the stored air. In 1906 Whitehead built torpedoes that could cover nearly {{convert|1000|m|yard}} at an average speed of {{convert|35|kn|km/h}}.

At higher pressures the [[Adiabatic process#Adiabatic heating and cooling|adiabatic cooling]] experienced by the air as it expanded in the engine caused icing problems. This drawback was remedied by heating the air with seawater before it was fed to the engine, which increased engine performance further because the air expanded even more after heating. This was the principle used by the Brotherhood engine.

===Heated torpedoes===
Torpedoes propelled by compressed air encountered a significant problem when attempts were made to increase their range and speed. The cold compressed air, upon entering the expansion phase in the piston chambers of the torpedo's engine, caused a rapid drop in temperature. This could freeze the engine solid, by jamming the piston heads inside the cylinders. This led to the idea of injecting a liquid fuel, like [[kerosene]], into the compressed air and igniting it inside a separate expansion chamber. In this manner, the air is heated more and expands even further, and the burned propellant adds more gas to drive the engine. The earliest form was the "Elswick" heater as patented by [[Armstrong Whitworth]] in 1904. The device was demonstrated in an {{Convert|18|in|adj=on}} Fiume Mark III torpedo at [[Bincleaves Groyne|Bincleaves]] in 1905 before an audience of British and Japanese experts. The weapon speed was {{cvt|9|kn|kph}} more than for the otherwise-identical unheated version. Construction of such ''heated'' torpedoes started the same year by Whitehead's company.<ref>{{cite book|first = Geoff|last = Kirby|title = JRNSS Volume 27, No.1 - A History of the Torpedo, The Early Days|publisher = Journal of the Royal Navy Scientific Service|date = January 1972|url=https://archive.org/details/DTIC_AD0595766|pages=43–44}}</ref>

==== Dry heater ====

The earliest production version of the heated torpedo propulsion system, which became known as the Whitehead heater system, mixed the fuel and compressed air after the pressure regulator. Combustion took place in a specialized expansion chamber, with hot combustion products driving the pistons of a reciprocating engine. This had the disadvantage of badly sooting the air vessel with combustion byproducts, and could also engage in a [[thermal runaway]], jamming the engine—not from low temperature as observed with compressed air, but from excess heat. The "dry heater" distinction was only made after wet heater torpedoes were developed, prior to which all heated torpedoes were of the dry heater type—originally simply called "heated".

==== Wet heater ====
{{Anchor|Wet heater|Wet-heater}}A further improvement was the use of water to wash and cool the [[combustion chamber]] of the fuel-burning torpedo. Water would be injected into the combustion chamber, at a rate commensurate with the fuel supply rate. This water would flash to steam, with stray condensate carrying the soot combustion byproducts out through the engine. An early example was the wet heater system developed by Lieutenant Sydney Hardcastle at the [[Royal Arsenal|Royal Gun Factory]], in 1908.<ref>{{cite book|chapter=The Officers of the Royal Navy Before 1918|chapter-url=https://link.springer.com/content/pdf/10.1057/9781137481962_2.pdf|website=Springer Link|date=2015 |doi=10.1057/9781137481962_2 |access-date=4 October 2024|chapter-url-access=subscription |last1=Farquharson-Roberts |first1=Mike |title=Royal Naval Officers from War to War, 1918–1939 |pages=8–23 |isbn=978-1-349-57163-5 }}</ref> The compressed air bottle was partially filled with water, with an outlet at the bottom leading into the combustion chamber. This would guarantee that compressed air and water would be injected into the combustion chamber at the same pressure. The system not only solved heating problems so more fuel could be burned 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''. Most torpedoes used in World War I and World War II were wet heaters.

==== Increased oxidant ====
The amount of fuel that can be burned by a torpedo engine (i.e. wet engine) is limited by the amount of [[oxygen]] it can carry. Since compressed [[Atmosphere of Earth#Composition|air contains only about 21% oxygen]], engineers in Japan developed the [[Type 93 torpedo|Type 93]] (nicknamed "Long Lance" postwar)<ref name="Morison, Samuel Eliot 1950 p.195" /> for destroyers and cruisers in the 1930s. It used pure compressed oxygen instead of compressed air and had performance unmatched by any contemporary torpedo in service, through the end of World War II. However, oxygen systems posed a danger to ships carrying such torpedoes under normal operation, and more so under attack; Japan lost several cruisers partly due to catastrophic secondary explosions of Type&nbsp;93s.

During World War II, Germany experimented with [[hydrogen peroxide]] for the same purpose.

The British approached the problem of providing additional oxygen for the torpedo engine by the use of oxygen-enriched air rather than pure oxygen: up to 57% instead of the 21% of normal atmospheric compressed air. This significantly increased the range of the torpedo, the {{Convert|24.5|in|adj=on}} Mk 1 having a range of {{convert|15000|yard|m}} at {{convert|35|kn|km/h}} or {{convert|20000|yard|m}} at {{convert|30|kn|km/h}} with a {{convert|750|lb|kg|adj=on}} warhead. There was a general nervousness about the oxygen enrichment equipment, known for reasons of secrecy as "No 1 Air Compressor Room" on board ships, and development shifted to the highly efficient Brotherhood Burner Cycle engine that used un-enriched air.<ref name="ReferenceA"/>

==== Burner cycle engine ====
After the First World War, [[Peter Brotherhood]] developed a four-cylinder burner cycle engine which was roughly twice as powerful as the older wet heater engine. It was first used in the British Mk VIII torpedoes, which were still in service in 1982. It used a modified diesel cycle, using a small amount of paraffin to heat the incoming air, which was then compressed and further heated by the piston, and then more fuel was injected. It produced about {{Convert|322|hp}} when introduced, but by the end of WW2 was at {{Convert|465|hp}}, and there was a proposal to fuel it with nitric acid, in which it was projected to develop {{Convert|750|hp}}.<ref>Torpedo: The Complete History of the World's Most Revolutionary Naval Weapon
By Roger Branfill-Cook</ref>

===Wire driven===
[[File:TorpedoMotor.JPG|right|thumb|U.S. World War II [[PT boat]] torpedo<!--Mk 8? Or Mk13?--> on display]]
The [[Brennan torpedo]] had two wires wound around internal drums joined to the propellers. Shore-based steam [[winch]]es pulled the wires, which spun the internal drums and drove the propellers. An operator controlled the relative speeds of the winches, providing guidance. Such systems were used for [[Coastal defense and fortification|coastal defense]] of the British homeland and colonies from 1887 to 1903 and were purchased by, and under the control of, the Army as opposed to the Navy. Speed was about {{convert|25|kn|km/h|0}} for over {{Convert|2400|m}}.

===Flywheel===
The [[Howell torpedo]] used by the [[US Navy]] in the late 19th century featured a heavy [[flywheel]] that had to be spun up before launch. It was able to travel about {{convert|400|yd|m}} at {{convert|25|kn|km/h}}. The Howell had the advantage of not leaving a trail of bubbles behind it, unlike compressed air torpedoes. This gave the target vessel less chance to detect and evade the torpedo and avoided giving away the attacker's position. Additionally, it ran at a constant depth, unlike Whitehead models.

===Electric batteries===
[[File:Torpedo model Z-13 mg 5165.jpg|thumb|Electric batteries of a French [[Z13 torpedo]]]]

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, because [[Battery (electricity)|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 a shorter range than the conventional [[G7a]], but was wakeless and much cheaper. Its [[lead–acid battery|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 cell]]s.

The United States had an electric design, the [[Mark 18 torpedo|Mark 18]], largely copied from the German torpedo (although with improved batteries), as well as [[Mark 24 Mine|FIDO]], an air-dropped acoustic homing torpedo for anti-submarine use.

Modern electric torpedoes such as the [[Mark 24 Tigerfish]], the [[Black Shark torpedo|Black Shark]] or DM2 series commonly use [[Silver-oxide battery|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{{When|date=March 2025}} revived in Russian and German torpedoes, as it is especially suitable for [[supercavitating]] devices.<ref>[http://www.geoffkirby.co.uk/rocket-torpedoes.pdf "The Development of Rocket-propelled Torpedoes"], by Geoff Kirby (2000)</ref>

===Modern energy sources===
Modern torpedoes use a variety of propellants, including electric batteries (as with the French [[F21 (torpedo)|F21 torpedo]] or Italian [[Black Shark torpedo|Black Shark]]), [[monopropellant]]s (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]]).

=== Propeller ===
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|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-jet]]s.

=== Supercavitation ===
Some torpedoes—like the Russian [[VA-111 Shkval]], Iranian [[Hoot (missile)|Hoot]], and German [[Superkavitierender Unterwasserlaufkörper|Unterwasserlaufkörper/ Barracuda]]—use [[supercavitation]] to increase speed to over {{convert|200|kn|km/h}}. Torpedoes that do not use supercavitation, such as the American [[Mark 48 torpedo|Mark 48]] and British [[Spearfish torpedo|Spearfish]], are limited to under {{convert|100|kn|mph km/h|abbr=on}}, though manufacturers and the military do not always release exact figures.