A hypergolic propellant is a type of rocket fuel which spontaneously ignites when its two components come into contact with each other. While the fuel is difficult to handle, the hypergolic engine is easy to control and very reliable.
The engine can be precisely controlled with only two valves, one for each fuel component. This simplifies the control system and eliminates points of failure. With no complex starting procedure the thrust is predictable i.e. the direction and velocity of the rocket will closely match calculations. It can also be reliably restarted in flight.
Some common hypergolic fuel combinations are:
* Hydrazine-nitric acid (toxic but stable) * Aniline-nitric acid (unstable, explosive) * Hydrogen peroxide-aniline (dust-sensitive, explosive) * UDMH-nitrogen tetroxide (by far the most common hypergolic fuel, less reactive than others) * MMH-nitrogen tetroxide used in the end stage of the European Ariane 5 rocket * T-Stoff and C-Stoff used in the Messerschmitt Me 163 rocket fighter plane * Hydrazine and nitrogen tetroxide