What does hypergolic mean?
Hypergolic propellants catch fire the instant they touch each other. No spark or igniter is needed, which makes them extremely reliable for an engine that must work first time.
Most rocket engines need something to start the fire. A hypergolic pair does not: bring the fuel and the oxidiser together and combustion begins immediately.
That reliability is why they are chosen for jobs where failure is not survivable. Crew Dragon’s SuperDraco escape engines are hypergolic, so an abort works even if everything else has already gone wrong.
The trade is that these chemicals are toxic and corrosive, and have to be handled in protective suits. Engineers accept that only where the guaranteed ignition is worth it.
See it happen
Living in SpaceHome among the starsSource: Dragon 2 (Crew Dragon)