This page no longer updated from 31 October 2001. Latest version can be found at LF2/LH2

Oxidiser: LF2. Oxidiser Density: 1.51 g/cc. Oxidiser Freezing Point: -219.00 deg C. Oxidiser Boiling Point: -188.00 deg C.

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.

Fuel: LH2. Fuel Density: 0.07 g/cc. Fuel Freezing Point: -259.00 deg C. Fuel Boiling Point: -253.00 deg C.

Liquid hydrogen was identified by all the leading rocket visionaries as the theoretically ideal rocket fuel. It had big drawbacks, however - it was highly cryogenic, and it had a very low density, making for large tanks. The United States mastered hydrogen technology for the highly classified Lockheed CL-400 Suntan reconnaissance aircraft in the mid-1950's. The technology was transferred to the Centaur rocket stage program, and by the mid-1960's the United States was flying the Centaur and Saturn upper stages using the fuel. It was adopted for the core of the space shuttle, and Centaur stages still fly today.

In Russia hydrogen fuelled upper stages were designed and developed by the mid-1970's, but the Russians never seem to have found the extra performance to be worth the extra cost. Europe and China developed liquid oxygen/liquid hydrogen engines for upper stages of the Ariane and Long March launch vehicles.

The equilibrium composition of liquid hydrogen is 99.79 per cent parahydrogen and 0.21 per cent orthohydrogen. The boiling point of this composition is -253 deg C. Liquid hydrogen is transparent and without a characteristic odour. Gaseous hydrogen is colourless. Hydrogen is not toxic but is an extremely flammable material. The flammable limits of gaseous hydrogen in air are 4.0 to 75 volume percent.

Hydrogen is produced from by-product hydrogen from petroleum refining and the partial oxidation of fuel oil. The gaseous hydrogen is purified to 99.999+ per cent, and then liquefied in the presence of paramagnetic metallic oxides. The metallic oxides catalyse the ortho-para transformation of freshly liquefied hydrogen. Freshly liquefied hydrogen which has not been catalysed consists of a 3:1 ortho-para mixture and cannot be stored for any length of time because of the exothermic heat of conversion. The delivered cost of liquid hydrogen in 1960 was approximately $ 2.60 per kg. Large-scale production was expected to reduce the cost to $ 1.00 per kg. In the 1980's NASA was actually paying $ 3.60 per kg.

Engines Using LF2/LH2

Eng-engineslink Thrust(vac)-kgf Thrust(vac)-kN Isp-sec Isp (sea level)-sec Designed for Status
X-430 34,000 334.00     Upper Stages Developed 1960's
RD-350 150,000 98.00 464   Upper Stages Design concept 1963-

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Last update 3 May 2001.
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© Mark Wade, 2001 .