Otto fuel II is an artificial substance used to power torpedo engines, originally by the United States Navy. It was first used in the Mk 48 torpedo in the early 1960s.

By the mid 1950s, the submarine contest between the NATO allies and the Warsaw Pact - practically, the USSR - was starting to heat up. The imminent nuclearization of submarines necessitated changes in their primary armament, the torpedo, as a nuclear submarine was able to move quietly, quickly, and for long enough to have a much better chance of evading a normal torpedo. The Mk 35, the standard torpedo used in WWII-design 19" torpedo tubes, was too slow and too short-ranged to compete. As a result, the U.S. Navy's Bureau of Ordnance (BuOrd, in Navy-ese) began work on its successor, the Mk 48.

One of the most pressing problems was how this new weapon would be propelled. Originally, BuOrd had thought to use a turbine engine with its commensurately greater speed; however, tests showed that this solution caused a great deal of noise as water flowed past the now-complex pressure systems around the weapon. Although torpedoes had always been noisy, this problem had taken on a greater dimension - the Mk. 48 contained on-board hydrophones and active sonar systems in order to allow it to guide itself if it was separated from its guide wire. These systems had difficulty 'hearing' through the wall of turbine noise. In addition, as the torpedo moved to deeper and deeper depths (chasing the new submarines, which also dove deeper) the increasing pressure around the torpedo decreased the turbine's output, slowing it.

As a result of these problems, BuOrd decided to use a standard piston engine inside the Mk 48, as it did away with the high-speed water flow noise and could compensate for increased pressure by simply using more fuel for increased power. The next question was how it was to be fueled. As a submarine weapon, it would have to carry its own oxidizer or use a monopropellant. Traditionally, the fuel of choice for underwater combustion was hydrogen peroxide, which had been introduced by the Kriegsmarine as a means of propelling submarines without surface air. However, hydrogen peroxide was not only toxic, but corrosive and horribly difficult to safely work with - not something that was ideal for storing for long periods of time on a working submarine.

In 1963, a scientist working for BuOrd named Dr. Otto Reitlinger produced a new monopropellant which the Bureau (more likely the working submariners) promptly named 'Otto fuel' in his honor. 'Otto fuel II,' in fact, as it was the second iteration that was accepted into service. Otto fuel II, or just 'Otto fuel,' was a mixture of propylene glycol dinitrate (the explosive fuel), dibutyl sebacate (a desensitizing additive) and 2-nitrodiphenylamine (a stabilizer). Mixed together, they form a relatively stable (if toxic) oily substance. It does not throw off fumes, being oily (sebacate) and in liquid form is resistant to fire. However, if the fuel is vaporized, the stabilizer and desensitizer will separate from the propellant, which is then unstable enough to decompose violently, releasing energy. One of its reaction products is hydrogen cyanide, which is horribly toxic; as a result, torpedos which accidentally 'run hot' inside submarines will release highly toxic fumes into the boat which must be contained and vented.

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