A hypergolic propellant is a rocket propellant combination used in a rocket engine, whose components spontaneously ignite when they come into contact with each other.
The attendant wears a full hazmat suit due to the hazards of the hypergolic fuel hydrazine, here being loaded onto the MESSENGER space probe
An early hypergolic-propellant rocket engine, the Walter 109-509A of 1942–45.
Rocket propellant is the reaction mass of a rocket. This reaction mass is ejected at the highest achievable velocity from a rocket engine to produce thrust. The energy required can either come from the propellants themselves, as with a chemical rocket, or from an external source, as with ion engines.
A Delta IV Heavy during liftoff. The rocket is entirely fuelled with liquid hydrogen and liquid oxygen cryogenic propellants.