Rocket propellant RP-1 is a straight-run kerosene fraction, which is subjected to further treatment, i.e., acid washing, sulphur dioxide extraction. Thus, unsaturated substances which polymerise in storage are removed, as are sulphur-containing hydrocarbons. Furthermore, in order to meet specification requirements of density, heat of combustion, and aromatic content, the kerosene must be obtained from crudes with a high naphthene content. RP-1 is an excellent solvent for many organic materials. The flash point is above 43 deg C. Above that temperature RP-1 will form explosive mixtures with air. The temperature range for explosive mixtures (rich limit) is 79 to 85 deg C. RP-1 is not so toxic as the JP series of fuels because of its lower aromatic content. In the United States, suitable kerosene fractions in 1960 were limited almost exclusively to the West Coast. The estimated 1956 United States production was 7700 tonnes, and the price was $0.05 per kg. By the 1980's it was typically $ 0.20 per kg. Russian formulations have typical densities of 0.82 to 0.85 g/cc, and even higher densities were achieved in the N1 and Soyuz 11A511U rockets by superchilling the fuel prior to loading.