What does hypergolic mean?

Most rocket engines need something to start the fire. A Merlin uses a pyrophoric mixture called TEA-TEB, squirted in to light the kerosene. A hypergolic pair needs none of that: bring the fuel and the oxidiser together and combustion begins immediately, every time.

That reliability is why they are chosen for jobs where failure is not survivable. Crew Dragon’s SuperDraco escape engines are hypergolic, burning monomethylhydrazine with nitrogen tetroxide, so an abort works even if everything else has already gone wrong. There is no igniter that can fail because there is no igniter.

The second advantage is patience. Cryogenic propellants boil away and cannot sit in a tank for long; hypergolics are liquid at ordinary temperatures and can wait for months without attention. That is what makes them right for a spacecraft that has to manoeuvre after six months docked at a space station, and for the small Draco thrusters Dragon uses to turn.

They also restart cleanly. An engine that lights purely by contact can be fired, stopped and fired again as many times as there is propellant for, which matters for a capsule making repeated small corrections.

The trade is that these chemicals are genuinely nasty. Nitrogen tetroxide and hydrazine compounds are toxic and corrosive, and ground crews handle them in sealed protective suits. Engineers accept that only where guaranteed ignition is worth the handling cost — which is why you find hypergolics in escape systems and thrusters, and kerosene or methane in the big engines that only ever light once, on a launch pad, with people standing well back.