Hydrazine

• 1. Introduction

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  Hydrazine is a colorless, flammable liquid with an ammonia-like smell, and its formula, N2H4, joins just two nitrogen atoms to four hydrogens. In the novel The Martian, which was also adapted into a feature film, the stranded main character takes surplus hydrazine fuel and uses a metal catalyst to pull hydrogen out of it. He then burns that hydrogen to make water he can drink. The idea is fiction, but it rests on real chemistry. The same simple liquid has steered spacecraft, kept fighter jets flying after engine trouble, and supplied the chemistry behind airbags and medicines. Chemists nonetheless call it highly hazardous unless it is handled dissolved in water. How did anyone first isolate such a stubborn compound? What happens inside an engine when it meets a catalyst? And why is the aerospace industry now looking for something to take its place?

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• 2. Naming and Making

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  In 1875, the chemist Emil Fischer coined the word hydrazine while trying to build organic compounds from a hydrazine core carrying a single organic group. Twelve years later, Theodor Curtius treated organic diazides with dilute sulfuric acid and obtained hydrazine sulfate. Despite repeated efforts, he never managed to get the pure liquid. That honor went to the Dutch chemist Lobry de Bruyn, who prepared pure anhydrous hydrazine in 1895. The name hides a small puzzle. The prefix hydr- points to hydrogen, and the ending that begins with -az- comes from azote, the French word for nitrogen.

  In 1907, the Olin Raschig process was announced, and it still shows what makes hydrazine hard to build. Every route has to create a single bond between two nitrogen atoms. The Raschig method does it by reacting sodium hypochlorite, the active ingredient in many bleaches, with ammonia. Monochloramine then meets more ammonia, and the nitrogen-nitrogen bond forms with hydrogen chloride as a by-product. The Lake Charles plant used this method.

  The Bayer Ketazine process came before the modern peroxide route. It also used sodium hypochlorite, so it produced an equivalent of salt for each equivalent of hydrazine. A variant that oxidizes urea instead of ammonia generates significant by-products and is mainly practised in Asia.

  The peroxide process goes by several names, including the Pechiney-Ugine-Kuhlmann process, the Atofina-PCUK cycle and the ketazine process. It combines ammonia and hydrogen peroxide with a ketone catalyst. The ketone and ammonia first form an imine, and peroxide turns that into an oxaziridine, a three-membered ring of carbon, oxygen and nitrogen. Ammonia opens the ring to give a hydrazone, which is where the nitrogen-nitrogen bond appears. A second ketone molecule produces an azine, and water splits it into hydrazine and the ketone, methyl ethyl ketone, ready for reuse. Unlike most rival methods, it makes no salt.

  Worldwide, roughly 120,000 tons of hydrazine hydrate are made each year, equal to a 64 percent solution of hydrazine in water by weight.

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• 3. Fuel With Many Names

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  During World War II, the Messerschmitt Me 163B rocket-powered fighter flew on a hydrazine mixture. The German Luftwaffe called the blend C-Stoff. It contained 30 percent hydrazine, 57 percent methanol, which they called M-Stoff, and 13 percent water. The oxidizer was another German mixture, high test peroxide, known as T-Stoff. Hydrazine on its own had a label too. It was B-Stoff, a designation that was later reused for the ethanol and water fuel of the V-2 missile.

  Pure hydrazine solidifies below 2 C, which rules it out as a general-purpose military rocket propellant. Chemists turned to relatives instead. Monomethylhydrazine, abbreviated MMH, melts at -52 C, and unsymmetrical dimethylhydrazine, abbreviated UDMH, melts at -57 C. Both usually run in two-part fuels, often paired with dinitrogen tetroxide.

  The Soviet space program leaned on UDMH rather than plain hydrazine. Fischer had discovered that compound in 1875, the same year he named hydrazine. Combined with nitric oxidizers, it earned the nickname devil's venom because it was so dangerous.

  On the American side, a 50:50 mixture of hydrazine and UDMH by weight is known as Aerozine 50. It powered the service propulsion system of the Apollo command and service module. It also drove both the ascent and descent engines of the Apollo Lunar Module, and the Titan II intercontinental ballistic missiles. These fuel and oxidizer pairs react with enormous heat and are hypergolic, meaning they ignite on contact with no spark at all.

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• 4. Inside the Thruster

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  Inside a hydrazine monopropellant engine, the liquid is pushed over a catalyst such as iridium metal on high-surface-area alumina. It splits in three possible ways. In the first, N2H4 breaks into nitrogen and hydrogen. In the second, three hydrazine molecules become four of ammonia plus one of nitrogen. In the third, ammonia reacts with more hydrazine to give nitrogen and hydrogen.

  The first two reactions release a great deal of heat. The catalyst chamber can reach 800 C within milliseconds, and a small volume of liquid becomes a large volume of hot gas. That gives a vacuum specific impulse of about 220 seconds, a respectable figure for a thruster propellant. The second reaction is the hottest but makes fewer gas molecules. The third absorbs heat and cracks the ammonia back down, so the gas ends up cooler but more plentiful. The structure of the catalyst controls how much ammonia breaks apart. Thrusters want the higher temperature, while gas generators want the larger molecule count.

  The Viking landers of the 1970s used engines of this kind for the final descent to Mars. So did Phoenix in May 2008, Curiosity in August 2012 and Perseverance in February 2021. Hydrazine also serves as a low-power fuel for spacecraft maneuvering thrusters.

  Its storability is the other draw. The Dawn mission to Ceres and Vesta carried hydrazine because it can sit in a tank for a very long time.

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• 5. Power on Demand

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  A pilot of the single-engine General Dynamics F-16 Fighting Falcon rarely thinks about hydrazine until the engine flames out. Then the Emergency Power Unit starts, either automatically or by the pilot's command. It supplies emergency electrical and hydraulic power when hydraulic pressure or electrical power is lost, which keeps the flight controls working. The Lockheed U-2 reconnaissance aircraft, nicknamed the Dragon Lady, uses hydrazine differently. Its Emergency Starting System offers a highly reliable way to relight the engine in flight after a stall. The United States Air Force regularly uses H-70 for both aircraft, a mixture of 70 percent hydrazine and 30 percent water.

  The Space Shuttle and its solid rocket boosters used hydrazine in their auxiliary power units. Those units produced hydraulic power for steering the engines and moving aerodynamic surfaces.

  In January to June 1991, the U.S. Army Research Laboratory reviewed early bulk-loaded liquid propellant gun programs. The standard experimental propellant for such artillery was 63 percent hydrazine, 32 percent hydrazine nitrate and 5 percent water. It was among the most predictable and stable mixtures, with a flat pressure profile during firing. Trouble came from misfires caused by weak ignition. The shell's movement then opened a large bubble, gas production surged, and pressure could climb until the tube failed catastrophically.

  In the 1960s, fuel cells built by Allis-Chalmers Corp. ran on hydrazine and supplied electric power in some space satellites. The Italian catalyst maker Acta has since proposed hydrazine as a hydrogen alternative. It could deliver over 200 mW/cm2 more than a comparable hydrogen cell, and without expensive platinum catalysts. Hydrazine's electromotive force is 1.56 V against 1.23 V for hydrogen. Because the fuel is liquid at room temperature, storage is easier. Soaking it in a tank of carbonyl turns it into a safe solid hydrazone, and warm water later frees the liquid hydrazine hydrate.

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• 6. Workhorse Chemistry

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  Between 100 and 200 mL of gas per gram: that is what the blowing agents azodicarbonamide and azobisisobutyronitrile release. Both are made from hydrazine, and polymer foams are its largest use. Airbags also depend on it. Their gas-forming agent, sodium azide, is made by reacting hydrazine with sodium nitrite.

  Hydrazine is also a precursor to pharmaceuticals and pesticides, often by way of heterocyclic rings such as pyrazoles and pyridazines. The commercial drug derivatives include cefazolin, rizatriptan, anastrozole and fluconazole. Pesticide examples include metazachlor, metamitron, metribuzin, propiconazole and triadimefon. Hydrazine compounds can work as insecticides, fungicides, herbicides, antiviral agents and plant growth regulators, among other roles. One salt, hydrazinium hydrogensulfate, was tested against cancer-induced cachexia and proved ineffective.

  Power plants use it as an oxygen scavenger. In both nuclear and conventional steam cycles, it lowers dissolved oxygen to reduce corrosion, and large industrial boilers use it to control pH as well. The reason is neat: its by-products are usually just nitrogen gas and water. It also reduces salts of metals such as copper, silver, lead, platinum and palladium straight to the element. That enables electroless nickel plating and plutonium extraction from nuclear reactor waste. Some color photographic processes use a weak solution as a stabilizing wash. It is also the most common and effective agent for converting graphene oxide into reduced graphene oxide.

  In the laboratory, the Wolff-Kishner reduction turns a ketone's carbonyl group into a methylene bridge through a hydrazone intermediate. With 2,4-pentanedione, hydrazine condenses to give 3,5-dimethylpyrazole. It also cleaves N-alkylated phthalimide derivatives, which makes the Gabriel synthesis of amines possible.

  Nature makes hydrazine derivatives too. Hydrazine is an intermediate in anammox, the anaerobic oxidation of ammonia, and the open-ocean bacterium Brocadia anammoxidans produces it. The false morel makes a poison, gyromitrin, that the body converts to monomethylhydrazine. Even the popular button mushroom, Agaricus bisporus, produces hydrazine derivatives, including agaritine.

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• 7. Exposure and Limits

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  Hydrazine can enter the body through skin, eyes, lungs or swallowing. Reported effects range from skin irritation and burning eyes to nausea, headache, dizziness, temporary blindness, seizures and coma. It can damage the liver, kidneys and central nervous system. It is a strong skin sensitizer, so a first exposure can raise sensitivity to related compounds. Tobacco smoke carries small amounts of it as well.

  The cancer verdicts differ from agency to agency. NIOSH lists it as a potential occupational carcinogen. The National Toxicology Program says it is reasonably anticipated to be a human carcinogen. The ACGIH grades it A3, a confirmed animal carcinogen with unknown relevance to humans, while the U.S. Environmental Protection Agency uses B2, a probable human carcinogen. The International Agency for Research on Cancer rates it 2A, probably carcinogenic to humans, and links exposure to lung cancer. A National Academies committee found suggestive evidence for lung cancer only. The European Commission's SCOEL calls it a genotoxic carcinogen, citing a reaction with endogenous formaldehyde that forms a DNA-methylating agent.

  The numbers are small. The NIOSH recommended limit is 0.03 ppm as a 2-hour ceiling. OSHA's permissible limit is 1 ppm over eight hours, and the ACGIH threshold is 0.01 ppm over the same period. The nose is a poor guard, because the odor threshold is 3.7 ppm, far above those limits. If a worker smells it, they are likely already over the limit. For unplanned public exposures near aerospace work, the Air Force uses a guideline of 2 ppm for one hour and 0.08 ppm for 24 hours.

  In an emergency, NIOSH advises removing contaminated clothing at once, washing skin with soap and water, and flushing eyes for at least 15 minutes. Treatment is symptomatic, with attention to the lungs and liver, and past cases have responded to pyridoxine, vitamin B6. The preferred controls are process enclosure and local exhaust ventilation. Protective gear includes non-permeable gloves and clothing, splash goggles and a face shield. Respirators are the last resort. Air Force Occupational Safety and Health Standard 48-8, Attachment 8 requires emergency showers and eyewash stations and a process for decontaminating protective clothing.

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• 8. Finding a Replacement

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  A possible ban across the European Union is pushing the aerospace industry to look past hydrazine. Nitrous oxide-based propellants are one promising path, led by the commercial companies Dawn Aerospace, Impulse Space and Launcher. In 2021, D-Orbit flew the first nitrous oxide-based system ever to operate in space. It sat aboard the ION Satellite Carrier and used six Dawn Aerospace B20 thrusters.

  A second path keeps hydrazine but makes it safer to handle. Aerojet Rocketdyne developed a blend called HPB-G28, with a vapor pressure 150 times lower, which cuts the inhalation hazard. It matches the specific impulse of neat hydrazine and has a density specific impulse that is 35 percent higher. Its freezing point is -55 C, so propellant lines need no heating. The mix is 65 percent hydrazine by mole, 27 percent hydroxyethylhydrazinium nitrate and 8 percent hydrazinium nitrate. Because it works with the same thrusters and catalysts as hydrazine, an operator could adopt it without redesigning the engine.

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