Section 4.3.9.
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All About Hydrazine


Hydrazine, or diamine, in the form of propellant for thrusters, is by far the most common means of spacecraft propulsion and attitude control. Hydrazine has excellent handling characteristics, relative stability under normal conditions, and clean decomposition products. Hydrazine is not very safe above 150 C, but is technically stable to about 250 C.

Hydrazine monopropellant thrusters consist simply of an electric solenoid valve, which allows pressurised hydrazine to enter the engine as a spray, a catalytist bed of alumina pellets impregnated with iridium (such as Shell Oil Company's Shell 405), and an expansion nozzle. These engines typically have twice the thrust-to-weight ratio of MMH/N2O4 bipropellant thrusters, although monopropellant thrusters range from three times lighter to being of equal weight for a given thrust (the thrusters listed in the 100-150 N range were of equal T:W).

Hydrazine may be useful as a power source. Hydrazine-oxygen fuel cells are considered superior to ammonia and methanol fuel cells, and next to hydrogen units in specific power (see S.S. Tomter and A.P. Anthony, The Hydrazine Fuel Cell System in Fuel Cells, American Institute of Chemical engineers, New York (1963), pg 22-31). Hydrazine is reactive and highly soluble in the electrolyte, yielding high current densities. As of the late 1960s, hydrazine fuel cells awaited a significant cost reduction to see their widespread application. Hydrazine monopropellant systems have also been used as auxiliary power units on aircraft.

While all flavours of hydrazine typically used as rocket propellants have very similar properties, and the information here pertains to all of them, it is in physical properties such as boiling point and vapour pressure that the methyl hydrazines differ most importantly. In addition to a lower specific impulse, methyl hydrazines have a lower density, and have a lower melting point with a higher vapour pressure, the lower melting point being the most attractive feature.

Physical Properties

Hydrazine (N2H4) is a colourless, oily, flammable liquid from 2.0 C to 113.5 C. It is miscible in water, has a log octanol/water partition coefficient (log Kow) of 0.08, and a molecular weight of 32.05 g/mol.

As a result of the high melting point, aerospace applications commonly use methyl-hydrazines, such as monomethyl-hydrazine (MMH) or unsymmetrical dimethyl-hydrazine (UDMH) or N,N-dimethylhydrazine, sometimes as a mixture with plain-vanilla hydrazine, which has a slightly high exhaust velocity. Aerozine 50 is 50% UMDH and 50% N2H4, and is used in the second stage of the Delta II and in the Titan family of launchers.

Its liquid vapour presure is given by the following equation:

log10 Pmm = 7.80687 - 1.680745t + 227.74

Pressure is in mm Hg, t in degrees kelvin, and the equation applies from 239 K to 387 K. As a liquid its vapour pressure at 20 C is 12.38 mm Hg or 100 mm Hg at 62 C, and as a solid at 0 C its vapour pressure is 2.60 mm Hg (760 mm Hg = 1 atm). Monomethyl hydrazine has a higher vapour pressure; at 20 C it is 36 mm Hg.

Hydrazine's density can be found using the following equation:

density = 1.0253 * (1 - 0.00085t)

Where t is the temperature in degrees kelvin, in a range from 275 to 386 K. At 20 C, hydrazine has a density of 1.00 g/cm3, and at 50 C density is 0.982 g/cm2.

Methyl hydrazine (CH3NHNH2) has a density of 0.866 g/ml, a molecular weight of 46.07 g/mol, and is liquid between -52.4 C and 87.5 C. Dimethyl hydrazine ([CH3]2NNH2) has a density of 0.791 g/ml, a molecular weight of 60.10 g/mol, and is liquid between -58 C and 63.9 C. Its usual oxidiser, nitrogen tetroxide (N2O4), has a density of 1.45 g/ml, a molecular weight of 92.01 g/mol, and is liquid between -9.3 C and 21.15 C.

Hydrazine's specific heat capacity is 25.23 g-cal/mol-deg (compared to water at 18), its specific heat of fusion 3.025 kcal/mol, and of vapourisation 10.2 kcal/mol, both much lower than water.

Hydrazine fumes in air with vapours are 4% heavier than air, and enough hydrazine evaporates from the liquid to allow an explosion at 38 C (the flash point). The flash point of monomethly hydrazine is -8 C.

Chemical Properties

Hydrazine is a powerful reducing agent. It is attractive as a reducing agent due to its high hydrogen content, and friendly by-product of nitrogen. It will reduce a number of important metal salts to the element, including silver and nickel.

Producing 148.6 kcal/mol in its oxidation reaction, hydrazine has an impressive affininty for oxygen:

N2H4 + O2 = N2 + 2 H2O

It is used in this capacity to remove oxygen from boiler systems, and as an additive to many substances to prevent oxidative deterioration.

Hydrazine is a weak base (Kb = 8.5 x 10^-7 at 25 C), more reactive than ammonium sulphide and calcium oxide, but less alkali than sodium hydride, calcium hydride, potasium hydroxide, and sodium hydroxide. Hydrazine is incompatable with acids.

Hydrazine is a reasonable solvent, completely dissociating alkali-metal halides, tetraalkylammonium halides, and many carboylic acids. However, a number of oxides, sulphates, and carbonates are practically insoluble. Since ammonia's proton affinity is so similar, it is usually used rather than hydrazine as a solvent, being safer and more readily available.

Alkali metal hydrazindes, formed by dissolving the metals into hydrazine, are one of the few substances which behave as bases. These tend to be pyrophoric with air, and sodium hydrazide (NaN2H3), for example, explodes violently when heated above 100 C. As a result, alkali metals are incompatable with hydrazine. Alkali metals are common to reduce melting temperatures in some glasses-glass composites, for example.

Platinum-group metal catalysis such as platinum black and Raney nickel are incompatible with hydrazine, causing exothermic decomposition. A hydrazine-oxygen mixture in the presence of platinum ignites at a mere 30 C, while hydrazine-air mixtures on an iron rust surface ignite at 24 C and on a metal burner ignite at 130 C. On glass surfaces, auto-ignition temperature is 270 C. Contact with many porous substances such as rusty surfaces, earth, asbestos, wood, or cloth can cause oxidation or explosion.

Hydrazine spontaneously explodes upon contact with calcium oxide, barium oxide, iron oxides, copper oxide, chromate salts, and many others. Calcium is a possible metal tank alloy ingredient, the oxide of which forms on tank surfaces. Metallic manganese, lead, and copper alloys also spontaneously participate in violent reactions. It is incompatable with ammonia and with organic compounds containing easily reduced functional groups. Hazardous polymerisation does not occur, however.

All flammable liquids, inclusing hydrazine, are incompatable with chemicals such as nitrates, permanganates, chromic acid, hydrogen peroxide, nitric acid, sodium peroxide, halogens, or halogenated chemicals such as phosphorous tetrachloride, oxidizing materials, and highly oxygenated or halogenated solvents.

Hydrazine is sometimes listed as incompatable with metals and combustable materials, as its presence greatly increases burning rates in an oxygenated environment such as Earth.

Dry chemical, alcohol-resistant foams, or CO2 fire extinguishers all are effective against hydrazine reactions. Water may be ineffective against hydrazine fires, although it's useful in keeping affected materials cool. As the vapour is heavier than air, it may travel along the ground or with ventilation to an ignition source, and "flash back" much like hydrocarbon vapours. Unless fires can be fought from a distance with hose-holders and such, it is best just to let it burn itself out.

Hydrazine is listed among shock-sensitive chemicals, as a chemical prone to rapidly decompose or explode when struck, vibrated, or otherwise agitated. It is flammable in mixtures with air from 4.7% to 100% hydrazine.

The presence of hydrogen increases thermodynamic stability by promoting decomposition into ammonia, rather than to nitrogen and hydrogen. Ammonia dissociation has a reaction energy of -44.8 kcal/mol, rather than -22.8 for decomposition into the gasses.

Butyl rubber and polyvinyl chloride provide good to excellent protection, and should be considered for plumbing materials.

Biological Effects

Hydrazine is a dangerous, although non-cumulative poison, is corrosive to eyes, skin, and mucuous membranes, and is a probable human carcinogen. Its reducing effects make it extremely destructive to tissues, and it has a variety of adverse systemic effects. Monomethyl hydrazine has a half-life of 2 days in the environment, and does not bioaccumulate.

Caustic burns to the skin are the immediate result of contact with the liquid. Hydrazine dissolves hair, and causes caustic-like burns on skin.

The vapour is extremely irritating to the eyes and temporary blindness can result, and eye contact with the liquid causes burns and possibly permanent damage. Flushing with large quantities of water for 15 minutes is the recommended treatment.

Vapours cause irritation to the nose, throat, and respiratory tract. High concentrations cause spasm, inflammation, chemical pneumonitis, and pulmonary edema, and coma in humans, being fatal to half of rats exposed for four hours at 570 ppm and half of mice at 252 ppm. Other symptoms of exposure may include a burning sensation, coughing, wheezing, laryngitis, shortness of breath, headache, nausea, and vomiting. Symptoms from chronic, nonacute exposure include lethargy, vomiting, itching and burning sensations, tremors, conjunctivitis, and contact dermititis.

Readily absorbed through the skin, it can inhibit certain enzyme systems causing reduced body metabolism. Other systemic effects may include damage to liver, kidneys, nervous system, and red blood cells. For skin contact with rabbits, its LD-50 is 91 mg/kg, and with guinea pigs it is 190 mg/kg. For acute oral doses with rats, its LD-50 is 60 mg/kg, and with mice it is 59 mg/kg.

Hydrazine has a fishy or ammonia-like odor detectable at a mean of 3.7 ppm. The normal 8-hour OSHA permissible exposure limit (PEL) is 1 ppm, and the immediately dangerous to life and health (IDLH) value is 50 ppm. Hydrazine can be detected in the blood of exposed individuals.

Recommended safe levels for hydrazine's carcinogenic properties are far lower than those of biological effects stemming from chemical properties. EPA Cancer Risk Level (1-in-a-million excess lifetime risk) is 2.0 x 10-73. The EPA suggests that air containing 0.02 mg/m3 of hydrazine directly increases cancer risks by 1 part in ten thousand.

The chemical is not considered to have adequate warning properties. Hydrazine detectors, similar to ammonia detectors, are used where the chemical is handled, and detector cards are used which discolour in the presence of hydrazine.

Selected Bibliography

These are great resources worthy of checking out.


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