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

Monopropellant Systems (Operate)
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The hydrazine propellant is decomposed in a thruster by a catalyst and the resulting hot
gas is expelled through a nozzle, thus generating thrust force on the spacecraft.
A typical monopropellant system uses nitrogen or helium gas to expel the propellant from
a diaphragm tank into the chamber catalyst beds of the thrusters. Since the pressuring gas
is stored (at a pre-selected but relatively low pressure, e.g. 22 bar) in the propellant tank,
the propellant pressure varies with propellant usage.
A typical selection of the ullage volume of 25% filled with pressuring gas (thus containing
75% propellant) will results in a propellant feed pressure decay, and thus in a thrust decay
of 4:1.
This mode of operation is also referred to as the blow-down mode, in contrast to the
pressure constant mode, which requires the storage of a high-pressure gas in a tank
external to the propellant tank (see bipropellant systems).
In a hydrazine gas generator system, the hydrazine decomposition gases are exhausted
into a gas storage tank for later gas expulsion.
The catalytic thruster and gas generator systems have identical propellant feed systems
consisting typically of propellant tank(s) with a diaphragm expulsion device(s), propellant
and gas fill valves, eventually latch valves (start valves), line pressure transducers and
filters.