Hypergolic propellant

Fuel and oxidiser that ignite on contact, with no ignition system at all.

Hydrazine and nitrogen tetroxide catch fire the instant they touch, which means an engine with no igniter, no start sequence and almost nothing to fail. That reliability is why they fly on spacecraft that must restart after months in the cold: the Apollo lunar module ascent engine, most satellite thrusters, and the Soyuz abort system. The cost is that both are appallingly toxic and corrosive — ground crews handle them in sealed suits — and that is the reason newer designs move to methane or non-toxic alternatives wherever the mission allows.