Hydrogen peroxide is used as both an oxidiser and a monopropellant. Relatively high density and non-toxic, it was abandoned after early use in British rockets, but recently revived as a propellant for the Black Horse spaceplane. Hydrogen peroxide solutions are clear, astringent, colorless liquids which are slightly more viscous than water. They are described by Military Specification MIL-H-16005. High-strength hydrogen peroxide solutions are very reactive oxidising agents. Hydrogen peroxide is miscible in all proportions in water; it is soluble in a large number of organic liquids which are also soluble in water. However, many of these mixtures form explosive mixtures. Hydrogen peroxide-water solutions are normally insensitive to detonation by shock or impact. Surfaces that come in contact with hydrogen peroxide must be specially treated (passivated) before use, to prevent the decomposition of the hydrogen peroxide. Hydrogen peroxide-water solutions and their vapours are considered non-toxic, but are characterised by their ability to produce local irritation.
Hydrogen peroxide is manufactured commercially by several processes. Inorganic processes employ the electrolysis of an aqueous solution of sulphuric acid or acidic ammonium bisulphate, followed by hydrolysis of the peroxydisulfate which is formed. For reasons of economy and flexibility of plant location, organic processing methods have become important in the production of hydrogen peroxide. These include (1) the autoxidation of hydroquinone or one of its homologues in a suitable solvent system and (2) the partial gas-phase oxidation of hydrocarbons.
Dilute aqueous hydrogen peroxide is concentrated to about 90 per cent by conventional distillation. Higher-strength solutions are prepared by fractional crystallisation of 90 per cent feed stock. Estimated United States production for 1959 was 50,000 tonnes based upon 100 per cent hydrogen peroxide. In large quantities, 95 per cent hydrogen peroxide then cost approximately $1.00 per kg. In small drum lots, 98 per cent solutions cost $ 2.00 per kg. Density varies: 2.44 g/cc for 100% H2O2, 2.43 for 98%, 2.42 for 96%, 2.33 for 75%.
Unsymmetrical Dimethylhydrazine ((CH3)2NNH2) became the storable liquid fuel of choice by the mid-1950's. Development of UDMH in the Soviet Union began in 1949. It is used in virtually all storable liquid rocket engines except for some orbital manoeuvring engines in the United States, where MMH has been preferred due to a slightly higher density and performance. Unsymmetrical dimethylhydrazine (UDMH) is 98 to 99 per cent pure and is described by Military Specification MIL-D-25604. The normally expected impurities are dimethylamine and water. UDMH is a clear, hygroscopic liquid which yellows on exposure to air. It absorbs oxygen and carbon dioxide. UDMH is a toxic volatile liquid. It exhibits the sharp ammoniacal or fishy odour which is characteristic of organic amines. It is completely miscible with water, ethanol, and most petroleum fuels. It is not shock sensitive. The vapours are flammable in air over 2.5 to 95 per cent concentration range. UDMH can be produced commercially by nitrosation of dimethylamine, to N-nitro-sodimethylrtmine, followed by reduction of the intermediate to UDMH and subsequent purification. UDMH can be prepared, also, by a modification of the Raschig process (see discussion of hydrazine), in which the chloramine intermediate is with dimethylamine rather than with ammonia. The price in 1959 for tank-car quantities was under $ 1.00 per kg. Engineering studies indicated a price of $ 1.00 per kg with large scale sustained production. But due to its toxic nature, production and transport costs soared in response to environmental regulations. By the 1980's NASA was paying $ 24.00 per kg.