Hydrazine, N2H4, is a commonly used rocket propellant. The hydrazine molecule is basically what you get when you couple two ammonia molecules, by removing a hydrogen from each one. It is a reactive substance, and quite toxic. Hydrazine is a colorless liquid at room temperature. Its melting point is about 1Â° C, and its boiling point is 114Â° C. In that sense, it is liquid over about the same range of temperatures as water. However, hydrazine is much more reactive than water, and it has a flash point of about 38Â° C. It is a highly reactive reducing agent. There are several industrial and medical uses for hydrazine, but this is primarily a space oriented blog, so I’ll focus on its uses as a rocket propellant.
Hydrazine is considered a monopropellant, meaning that all you need is hydrazine and not some other chemical to mix with it. That is really quite useful. Most rocket fuels require an oxidizer. That means that you have to keep track of two chemicals and they have to be mixed in the rocket engine in just the right way to provide the desired thrust. Secondly, hydrazine does not need an ignition source. Again, most rocket fuels need to be ignited in order to burn. Igniting an engine in space is tough. Only a few rocket engines are designed to be stopped and restarted. Most fire until they are done, and then they are turned off, never to be turned on again. Using hydrazine as a propellant, you can fire the engine as needed, and then when you cut off the supply of hydrazine, the engine stops. If you turn on the hydrazine again, the engine starts. More hydrazine gives more thrust. Less hydrazine gives less thrust. That makes hydrazine a nice propellant for thrusters and maneuvering jets.
Interestingly, hydrazine has been used for rocket propulsion since the 1940s. Hydrazine hydrate (mixed with water and alcohol) was used in the German’s Messerschmitt 163B rocket interceptor. Today, it is used fairly regularly in satellite maneuvering and station keeping thrusters, the Space Shuttle, the Soyuz spacecraft, and in the Auxiliary Power Units aboard the International Space Station and the Space Shuttle.
So, how come we don’t see hydrazine used even more in spacecraft? Well, there are several reasons. One, the stuff is pretty toxic. Breathing hydrazine can cause coughing and irritation of the throat and lungs. Those effects can occur at fairly low concentrations. At higher concentrations, hydrazine can trigger tremors or convulsions. Ingesting hydrazine can cause tremors, nausea, neurological problems, and drowsiness. Hydrazine is very water soluble, and it can be absorbed through the skin. Prolonged exposure can cause liver and kidney damage, as well as damage to reproductive organs. Hydrazine has caused tumors in animal studies, and it is listed as a carcinogen. When the Space Shuttle lands, the astronauts don’t go piling out of it as soon as it comes to a stop. Instead, special trucks roll out to it to “safe” the vehicle. This involves removing any unused hydrazine and “sniffing” the air around the Shuttle to see if any hydrazine vapors are present. One concern is that traces of hydrazine may remain in the thruster exhaust ports and pose a hazard to the astronauts. It is better for them to remain on board the spacecraft than to be exposed to possible health problems. Only when the all clear is given do the astronauts emerge from the Shuttle. It was in part due to the risk posed by the hydrazine that the public was warned to stay away from pieces of debris from the Space Shuttle Columbia that broke apart and showered much of east Texas with debris. The hydrazine contamination issue was also cited as a reason to shoot down USA-193, an errant American reconnaissance satellite rather than letting it crash to Earth of its own accord in some random location.
Personally, I agree with some public warnings about hydrazine contamination with space debris, but hydrazine is so reactive, volatile, and with such a low flash point, that I think that most of the debris is probably cleansed in passing through the high heat of burn up in the atmosphere. But, apparently at least some of the hydrazine tanks from the Columbia survived passage through Earth’s atmosphere during reentry, so it is quite likely that the same could happen for USA-193. That would most likely be the biggest hydrazine danger, rather than just random pieces of debris being contaminated. Nonetheless, I would not go out and handle any pieces of the satellite without taking protective measures, just in case.
Hydrazine is also corrosive and reactive. It breaks down rather readily in the environment, so long term environmental damage from small spills is unlikely. But, these chemical properties mean that it is not something that can be stored for extended periods of time. But, the instability turns out to be something very useful for rocket propulsion.
Because hydrazine is unstable, it can be easily broken down. All it takes is a catalyst, such as iridium, to cause the hydrazine to break down into ammonia, nitrogen, and hydrogen gas. The process is extremely exothermic (meaning that it releases a lot of heat when hydrazine breaks down this way). With a sufficient flow of hydrazine through a chamber containing the catalyst, the temperature in the chamber can very rapidly reach extremely high temperatures. That means that the gases produced in the reaction become extremely hot. Gas takes up nearly a thousand times the volume of liquids to start with. Then, if you heat the gas, it expands even more. That is how rocket and jet propulsion works. A solid or liquid is burned, and the combustion byproducts are hot expanding gases. These gases are then expelled at high velocity from the rocket engine (or jet engine). This hot rapidly expelled exhaust provides the engine’s thrust.
But, normally, a rocket or jet engine needs a fuel and an oxidizer. For a jet engine, the oxidizer is simply oxygen from air intake into the engine. For a rocket, though, the oxidizer is carried on board. Sometimes it is liquid oxygen, and sometimes a very reactive oxidizing chemical. Liquid oxygen is a cryogenic fluid that requires special handling. Oxidizing liquids also are exceeding corrosive and also require special handling. But, without the need for an oxidizer to burn the hydrazine to produce the hot gas, the engine can be simpler. A quick squirt of hydrazine can produce a quick burst of gas expelled. That makes hydrazine a very attractive propellant for attitude thrusters or station keeping thrusters.
But, even though hydrazine can be used as a monopropellant, it can also also be used with an oxidizer such as nitric acid to produce an even more powerful thrust. Hydrazine is so reactive that when used with a suitable oxidizer, there is no need for any sort of ignition system. The fuel and oxidizer just spontaneously ignite and burn. Propellants like this are called hypergolic propellants. An even more powerful derivative of just straight hydrazine is monomethyl hydrazine (CH3NHNH2), which is used with nitrogen textroxide as a hypergolic propellant. (This is the form used in the Space Shuttle’s thrusters).
I am not a chemist, but I thought that I’d look up some information on hydrazine because of all the news recently about the decision to shoot down USA-193. Possible injuries to people on the ground from hydrazine released in an uncontrolled reentry of that satellite was cited in many of the news reports as a major source of concern in reaching that decision. What I have written here is not to be taken as the authoritative expert fact sheet on hydrazine, but I hope that I’ve explained it sufficiently for people to get an idea of what the substance is, why it is used in spacecraft, and what concerns exist when spacecraft crash back to Earth with hydrazine aboard.