Advantages
Hypergolic rockets do not need an ignition system, so they tend to be inherently simple and reliable. While the larger hypergolic engines used in some launch vehicles use turbopumps, most hypergolic engines are pressure fed. A gas, usually helium, is fed to the propellant tanks under pressure through a series of check and safety valves. In turn, the propellants flow through control valves into the combustion chamber. They ignite instantly on contact, without any risk that a mixture of unreacted propellants might build up and ignite in a potentially catastrophic hard start.
The most common hypergolic fuels, hydrazine, monomethylhydrazine and unsymmetrical dimethylhydrazine, and oxidizer, nitrogen tetroxide, are all liquid at ordinary temperatures and pressures. Thus they are sometimes referred to as storable liquid propellants. They are suitable for use in spacecraft missions lasting for years. In contrast, liquid hydrogen and liquid oxygen are both cryogens whose practical use is limited to space launch vehicles where they need be stored for only a short time.
Because hypergolic rockets do not need an ignition system, they can be fired any number of times by simply opening and closing the propellant valves until the propellants are exhausted. This makes them uniquely suited for spacecraft maneuvering. They are also well suited, though not uniquely so, as upper stages of space launchers such as the Delta II and Ariane 5 that must perform more than one burn. Restartable cryogenic (oxygen/hydrogen) rocket engines do exist, notably the RL-10 on the Centaur and the J-2 on the Saturn V.
Read more about this topic: Hypergolic Propellant
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