We have already discussed the operation of small thrusters on spacecraft equipped with unified liquid bi-propellant systems. However, spacecraft equipped with solid propellant primary engines, have a need for an independent thruster arrangement, and this is usually a mono-propellant hydrazine system. "Mono" implies that this involves a single liquid fuel, as opposed to the fuel/oxidizer combination in the bi-propellant system; this single fuel is fed under gas pressure to the thrusters. To fire a particular thruster, the fuel inlet valve is opened, and the mono-propellant hydrazine is squirted through a bed of chemicals in the thruster, causing an exothermic (heat producing) chemical reaction, which breaks down the liquid fuel into hydrogen, nitrogen, and ammonia gases. These hot gases are then exited through the thruster nozzle to produce thrust. Figure 9.5 shows a typical mono-propellant hydrazine thruster, which fits comfortably in the palm of a hand.
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