Hypergolic Capable of spontaneous inflammation on contact; hypergolic propellants are useful for spacecraft propulsion. An example is the mixture of dinitrogen tetroxide, N2O4, and unsymmetrical dimethylhydrazine, (CH3)2NNH2, which was used by Apollo astronauts to leave the Moon’s surface.

The New Penguin Dictionary of Science, M.J. Clugston, 1998

I certainly didn’t conceive of a fuel/oxidizer combination that would spontaneously ignite, but what a great idea! The folks planning the lunar landings couldn’t have felt comfortable with the astronauts on the surface relying on a single engine to get them back to the command module. If anything went wrong with that engine, the astronauts on the surface would stay there for the rest of their very short lives. The lunar module did end up using only one engine, but it’s dead-simple. Ignition requires only opening the valves to the oxidizer and fuel tanks. The two liquids flow to the rocket engine, come into contact, and spontaneously ignite. There’s no ignition system to fail. It’s elegant in its simplicity.

On the downside, this particular bipropellant combination is very toxic with dinitrogen tetroxide being corrosive and unsymmetrical dimethylhydrazine a carcinogen that the skin easily absorbs. Every rose has its prickles.