Otto fuel II is a monopropellant used to drive torpedoes and other weapon systems. It is not related to the Otto cycle. Otto fuel II was developed by US Navy in the 1960s for use as fuel in torpedoes. Otto fuel II was invented by Otto Reitlinger in 1963 (although tests with the substance had taken place before, for example in 1960); it is named after Reitlinger and for being the second iteration of the fuel (sometimes it is known simply as Otto fuel). The first torpedo to use it was the Mark 48 torpedo in the 1960s. Properties Otto fuel II is a distinct-smelling (described by submariners as being similar in smell to wintergreen oil; i.e. sweet, fruity and minty), reddish-orange, oily liquid that is a mixture of three synthetic substances: propylene glycol dinitrate (the major component), 2-nitrodiphenylamine, and dibutyl sebacate. It does not need exposure to any oxidant to ignite and release energy, as its three components will react among themselves whenever vaporised and heated. Needing no oxidants and being a stable substance makes Otto fuel II ideal for use in the constrained environment of a submarine. Although the fuel can be made to explode, this requires such extreme conditions that it can be regarded as practically stable. The vapour pressure of the fuel is low (i.e., it is not volatile), minimizing toxic hazards. Finally, the fuel's energy density far surpasses the capacity of the electric battery used in other torpedoes, maximizing range. Major ingredients Named after its inventor, Otto Reitlinger, Otto fuel II consists of the nitrated ester explosive propellant propylene glycol dinitrate (PGDN), to which a desensitizer (dibutyl sebacate) and a stabilizer (2-nitrodiphenylamine) have been added. The chief component, propylene glycol dinitrate, accounts for approximately 76% of the mixture, while dibutyl sebacate and 2-nitrodiphenylamine account for approximately 22.5% and 1.5% (by weight), respectively. The principal current use of propylene glycol dinitrate is as a propellant in Otto fuel II. Nitrates of polyhydric alcohols such as this have been used in medicine for the treatment of angina pectoris, and as explosives since the mid-nineteenth century. In addition to its use by the United States Navy as a stabilizer in the manufacture of Otto fuel II, 2-nitrodiphenylamine is employed for similar purposes by the United States Army in the manufacture of double base solid propellants. It also has civilian applications as a solvent dye. Dibutyl sebacate is a