A hypergolic propellant is composed of a fuel and oxidizer that ignite when they come into contact with each other. There is no need of an ignition mechanism in order to bring about combustion.
In hypergolic propellants, the fuel part normally includes:
The oxidizer is generally:
The easy start and restart capability of hypergolic propellants make them ideal for spacecraft maneuvering systems. They are also used for orbital insertion as their combustion can be easily controlled and thus allows the precise adjustments required for insertion into orbit. Hypergolic propellants are also employed for altitude control.
As we now know the properties of cryogenic and hypergolic propellants let us compare them.
Hypergolic propellants remain in liquid state at normal temperatures. They do not need the temperature-controlled storage as in case of cryogenic propellants.
As compared to cryogenic propellants, hypergolic propellants are less energetic. That is they produce less energy per unit mass. For example: in a moon bound shuttle, 75% of the onboard mass would be fuel, in case of cryogenic propellants. But in case of hypergolic propellants, the number raises to 90%.
In comparison to cryogenic propellants, hypergolic propellants are very poisonous. They react with living tissues as well cause injuries. So it is mandatory for technicians to wear full-body Self-Contained Atmospheric Protection Ensemble (SCAPE) suits.
They are corrosive therefore storage requires special containers and safety facilities. It is necessary that they be stored safely, with no possible contacts between the fuel parts.
One example of terrible outcome of improper handling is the fate of the unmanned Mars Observer in August 1993. Just three days before it would have entered orbit around Mars, an accidental mixing of mono methyl hydrazine and nitrogen tetroxide is believed to have caused uncontrollable spinning and subsequent loss of that spacecraft.
As we have seen all the three types of liquid propellants. Now let us see what are hybrid propellants and how do they differ from solid and liquid propellants.