Otto fuel II

Otto fuel II is a monopropellant used to drive torpedoes and other weapon systems. It is not related to the Otto cycle.

Properties
This distinct-smelling, reddish-orange, oily liquid 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 ideal for use in the constrained environment of a submarine. Although the fuel can be made to explode, this requires extreme conditions (such that it can be regarded as practically stable). The vapour pressure of the fuel is low (i.e., it is not volatile), minimising toxic hazards. Finally, the fuel's energy density far surpasses the capacity of the electric battery used in other torpedoes, maximising range.

Major ingredients
Named after its inventor, Dr 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 U.S. Navy as a stabilizer in the manufacture of Otto Fuel II, 2-nitrodiphenylamine is employed for similar purposes by the U.S. Army in the manufacture of double base solid propellants. It also has civilian applications as a solvent dye.

Dibutyl sebacate is a desensitizer in Otto Fuel II. However, its major use is as a plasticizer in production of plastics, namely cellulose acetate butyrate, cellulose acetate propionate, polyvinyl butyral, polystyrene, and many synthetic rubbers. It can be used for plastics in use in the food packaging industry. It is also used as a lubricating ingredient in shaving lotions, and a flavoring additive in non-alcoholic beverages, ice cream, ices, candy, and baked goods.

Toxicity
Otto fuel II is a toxic substance found in EPA's National Priorities List. Ingestion of contaminated food or direct exposure at worksites can cause headaches, poor eye–hand coordination, eye irritation, congested noses, nausea, dizziness, and difficulty breathing. Fresh air and black coffee are used to relieve exposure symptoms. No fatal cases of overexposure have yet been reported.

Used in

 * Mark 48 torpedo
 * Mark 46 torpedo
 * Spearfish torpedo
 * Yu-6 torpedo