Most toxic rocket fuel

The most poisonous substance currently used as a rocket engine propellant is hydrazine (N2H4) and its variants (the most commonly used being Unsymmetrical Dimethylhydrazine, or UDMH). Animal studies have put the median lethal dose for hydrazine at around 60 mg/kg (meaning that an average adult could be killed by just 4.2 grams), and the CDC classify it as "Immediately Dangerous to Life or Health" at gas concentrations of anything higher than 15 parts per million.

Hydrazine can be absorbed into the body through skin contact, ingestion or inhalation. Exposure to hydrazine vapour causes skin burns, irritation to the eyes and mouth, vomiting, bleeding in the lungs and sometimes seizures. In the long term, exposure has been linked to liver and kidney damage, nerve damage (including tremors) and several different cancers.

Unsurprisingly for rocket fuel, hydrazine is also extremely flammable. It is what is called a "hypergolic" fuel, meaning that it ignites on contact with its oxidizer (typically dinitrogen tetroxide, but also sometimes the almost-as-toxic red-fuming nitric acid). It also ignites on contact with most acidic substances, many kinds of metal, and porous materials such as cloth or wood. Its flames are extremely hot, but totally transparent, and therefore invisible to the human eye.

The first vehicle to use hydrazine was the German Me-163 rocket-powered fighter aircraft. While the rocket provided excellent performance, fuel leaks and accidental explosions reportedly killed more pilots than enemy action. The combination of UDMH and RFNA was referred to as "devil venom" by Soviet engineers. It apparently smells of fish, but but getting close enough to smell it is extremely inadvisable.

Despite its many risks, hydrazine and its derivatives are still used because they are generally stable over a wide range of temperatures when stored correctly, burn cleanly and can function as monopropellants (fuels that don't require an oxidizer) when catalysed with iridium. Today hydrazine is most commonly used in monopropellant manoeuvring thrusters on spacecraft.

Although unpleasant, hydrazine and its derivatives are not exceptionally toxic relative to other proposed propellants. In the 1960s and 1970s, for example, engineers in search of performance improvements tried fuelling rockets with a mixture of the highly toxic chlorine trifluoride and hydrogen, a combination that created an exhaust plume of hydrofluoric acid. Other ideas that have been tried include injecting mercury into the combustion chamber as an additive.