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carbons it has highly reproducible properties. With liquid oxygen it is a candidate propellant combination for launch vehicle booster rocket engines and also reaction engines control when oxygen is available from the main engines). Experimental oxygen-methane engines have been tested, but they have not yet flown.

Liquid Hydrogen (H2)

Liquid hydrogen, when burned with liquid fluorine or liquid oxygen, gives a high performance, as shown in Table 5-5. It also is an excellent regenerative coolant. With oxygen it burns with a colorless flame; however, the shock waves in the plume may be visible. Of all known fuels, liquid hydrogen is the lightest and the coldest, having a specific gravity of 0.07 and a boiling point of about 20 K. The very low fuel density requires bulky fuel tanks, which necessitate very large vehicle volumes. The extremely low temperature makes the problem of choosing suitable tank and piping materials difficult, because many metals become brittle at low temperatures.

Because of its low temperature, liquid hydrogen tanks and lines have to be well insulated to minimize the evaporation of hydrogen or the condensation of moisture or air on the outside with the subsequent formation of liquid or solid air or ice. A vacuum jacket often has been used in addition to insulating materials. All common liquids and gases solidify in liquid hydrogen. These solid particles in turn plug orifices and valves. Therefore, care must be taken to scavenge all lines and tanks of air and moisture (flush with helium or pull vacuum) before introducing the propellant. Mixture of liquid hydrogen and solid oxygen or solid air can be explosive.

Liquid hydrogen has two species, namely, orthohydrogen and parahydro-gen, which differ in their nuclear spin state. As hydrogen is liquefied, the relative equilibrium composition of ortho- and parahydrogen changes. The transformation from one species to another is accompanied by a transfer of energy. Liquid hydrogen is manufactured from gaseous hydrogen by successive compression, cooling, and expansion processes.

Hydrogen gas, when mixed with air, is highly flammable and explosive over a wide range of mixture ratios. To avoid this danger, hydrogen gas leakage (a tank vent line) is often intentionally ignited and burned in the air. Liquid hydrogen is used with liquid oxygen in the Centaur upper stage, the Space Shuttle main engine, and upper stage space engines developed in Japan, Russia, Europe, and China.

Hydrogen burning with oxygen forms a nontoxic exhaust gas. This propellant combination has been applied successfully to space launch vehicles because of its high specific impulse. Here the payload capability usually increases greatly for relatively small increases in specific impulse. However, the low density of hydrogen makes for a large vehicle and a relatively high drag.

One method to increase the density of hydrogen is to use a subcooled mixture of liquid hydrogen and suspended frozen small particles of solid hydrogen, which is denser than the liquid. Experiments and studies on this "slush" hydrogen have been performed; it is difficult to produce and maintain a uniform mixture. It has not yet been used in a flight vehicle.

Some studies have shown that, when burned with liquid oxygen, a hydrocarbon (such as methane or RP-1) can give a small advantage in space launch vehicle first stages. Here the higher average propellant density allows a smaller vehicle with lower drag, which compensates for the lower specific impulse of the hydrocarbon when compared to a hydrogen fuel. Also, there are some concepts for operating the booster-stage rocket engine initially with hydrocarbon fuel and then switching during flight to hydrogen fuel. As yet, engines using two fuels, namely methane (or hydrocarbon) and hydrogen, have not yet been fully developed or flown. Some work on an experimental engine was done in Russia.

Hydrazine (N2H4)

Reference 7-8 gives a good discussion of this propellant, which is used as a bipropellant fuel as well as a monopropellant. Hydrazine and its related liquid organic compounds, monomethylhydrazine (MMH) and unsymmetrical dimethylhydrazine (UDMH), all have similar physical and thermochemical properties. Hydrazine is a toxic, colorless liquid with a high freezing point (274.3 K). Hydrazine has a short ignition delay and is spontaneously ignitable with nitric acid and nitrogen tetroxide.

Its vapors may form explosive mixtures with air. If hydrazine is spilled on a surface or a cloth, a spontaneous ignition with air can occur.

Pure anhydrous hydrazine is a stable liquid; it has been safely heated above 530 K. It has been stored in sealed tanks for over 15 years. With impurities or at higher temperatures it decomposes and releases energy. Under pressure shock (blast wave) it decomposes at temperatures as low as 367 K. Under some conditions this decomposition can be a violent detonation, and this has caused problems in cooling passages of experimental injectors and thrust chambers. Harmful effects to personnel may result from ingestion, inhalation of vapors, or prolonged contact with skin. The OSHA 8-hr personnel exposure limit is 0.1 ppm or 0.13 mg/m3. Hydrazine is a known carcinogen.

Hydrazine reacts with many materials, and care must be exercised to avoid storage contact with materials that cause a decomposition (see Ref 7-9). Tanks, pipes, or valves must be cleaned and free of impurities. Compatible materials include stainless steels (303, 304, 321, or 347), nickel, and 1100 and 3003 series of aluminum. Iron, copper and its alloys (such as brass or bronze), monel, magnesium, zinc, and some types of aluminum alloy must be avoided.

Unsymmetrical Dimethylhydrazine [(CH3)2NNH2]

A derivative of hydrazine, namely, unsymmetrical dimethylhydrazine (UDMH), is often used instead of or in mixtures with hydrazine because it forms a more stable liquid, particularly at higher temperatures. Furthermore, it has a lower freezing point (215.9 K) and a higher boiling point (336.5 K) than a hydrazine. When UDMH is burned with an oxidizer it gives only slightly lower values of Is than pure hydrazine. UDMH is often used when mixed with 30 to 50% hydrazine. This fuel is used in the Titan missile and launch vehicle and spacecraft engines in 50% mixtures and has been used in the lunar landing and take-off engines. UDMH is used in Russian and Chinese rocket engines.

Freezing does not affect UDMH, MMH, or hydrazine, but freezing of a 50:50 mixture of UDMH and hydrazine causes a separation into two distinct layers; a special remixing operation is necessary for reblending if freezing occurs in a space vehicle. The OSHA 8-hr personnel exposure limit for vapor is 0.5 ppm, and UDMH is a carcinogen.

Monomethylhydrazine (CH3NHNH2)

Monomethylhydrazine (MMH) has been used extensively as a fuel in spacecraft rocket engines, particularly in small attitude control engines, usually with N204 as the oxidizer. It has a better shock resistance to blast waves, better heat transfer properties, and a better liquid temperature range than pure hydrazine. Like hydrazine, its vapors are easily ignited in air; the flammability limits are from 2.5 to 98% by volume at atmospheric sea level pressure and ambient temperature. The materials compatible with hydrazine are also compatible with MMH. The specific impulse with storable oxidizers usually is 1 or 2% lower with MMH than with N2H4.

Both MMH an UDMH are soluble in many hydrocarbons; hydrazine is not. All hydrazines are toxic materials, but MMH is the most toxic when inhaled, and UDMH the least toxic. Atmospheric concentrations of all hydrazines should be kept below 0.1 ppm for long periods of exposure.

Monomethylhydrazine, when added in relatively small quantities of 3 to 15% to hydrazine, has a substantial quenching effect on the explosive decomposition of hydrazine. Monomethylhydrazine decomposes at 491 K, whereas hydrazine explodes at 369 K when subjected to pressure shocks of identical intensity. MMH is a suspected carcinogen and the OSHA personnel 8-hour exposure limit is 0.2 ppm.

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