Liquid Fluorine is the highest performance oxidiser and in the early 1960's it seemed in both American and Russia that a new generation of higher performance engines would emerge. However although test engines were built, fluorine was found to be just too toxic and reactive to be safely used as a propellant. Liquid fluorine is essentially 100 per cent pure, containing only traces of oxygen, nitrogen, and hydrogen fluoride. The liquid exhibits a clear; yellow colour, while the gas has a pale greenish-yellow colour at ambient temperature. Fluorine is the strongest oxidising agent known. Under proper conditions, fluorine reacts with practically every element or compound except the inert gases. It is stable to shock, heat, and electric spark. It is non-flammable with air. Fluorine is highly toxic and irritating to all tissues. It has a characteristic pungent halogen odour and is irritating to the respiratory tract. Fluorine is produced from a molten mixture of HF and KF by an electrolytic procedure. The 1959 United. States production of liquid fluorine was estimated at 40,000 tonnes/year. Liquid fluorine's cost, ex-works, was then $ 6.00 per kg. In the Soviet Union Glushko founded a plant for production of fluorine propellants 23 km from Leningrad at Karlelsko. The population eventually reached 120,000, but although one engine was ready for production, this was never authorised due to the safety problems in case of a launch vehicle failure.
In January 1953 Rocketdyne commenced the REAP program to develop a number of improvements to the engines being developed for the Navaho and Atlas missiles. Among these was development of a special grade of kerosene suitable for rocket engines. Prior to that any number of rocket propellants derived from petroleum had been used. Goddard had begun with gasoline, and there were experimental engines powered by kerosene, diesel oil, paint thinner, or jet fuel kerosene JP-4 or JP-5. The wide variance in physical properties among fuels of the same class led to the identification of narrow-range petroleum fractions, embodied in 1954 in the standard US kerosene rocket fuel RP-1, covered by Military Specification MIL-R-25576. In Russia, similar specifications were developed for kerosene under the specifications T-1 and RG-1. The Russians also developed a compound of unknown formulation in the 1980's known as 'Sintin', or synthetic kerosene. 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.