S.1 Spacecraft Propulsion Systems Chapter 1: Introduction to ...

Bipropellant Systems (Operate)
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Bipropellant systems are characterized by the combustion of two (Bi) propellants, a fuel
(e.g. MMH) and an oxidizer (e.g. N2O4) to produce thrust. The propellants are injected
separately into the bipropellant thruster combustion chamber where they react
spontaneously (hypergolic propellant) to perform high-temperature, low molecular weight
combustion products, which are the expelled through a nozzle.
The system basically consists of a pressurizing-gas system, propellant tanks (with surface
tension propellant management devices), propellant lines and thrusters. Unlike hydrazine
thrusters, bipropellant thrusters accept only a limited range of propellant inlet pressure
variation of ≤ 2. Therefore, the high-pressure gas, generally nitrogen or helium is regulated
to the desired tank pressure, e.g. 17 bar. This mode of operation is also referred to as the
pressure constant mode.
The system contains check valves upstream of the propellant tanks to prevent possible
back-flow, mixing, and combustion of the propellant vapors in the common pressuring gas
line. Relieve valves are incorporated in the system upstream of the propellant tanks to
prevent system rupture in the event of a pressure regulator failure. Filters are provided in
the propellant lines directly upstream of the thruster valves to prevent clogging of the
injector or damage of the valve seat by entrained foreign material. Finally, the system
contains pyro- or latch valves, line pressure transducers, fill and drain valves and various
test ports for system check out.