Section 3.8 mentions the equations related to this topic. One of the advantages of liquid propellant rocket engines is the ability to throttle or to randomly vary the thrust over a wide range. Deep throttling over a thrust range of more than 10:1 is required for relatively few applications. Moon landing, interceptor missiles, and gas generators with variable power output are examples. Moderate throttling (a thrust range of up to perhaps 2.5:1) is needed for trajectory velocity control (as in some tactical missiles), space maneuvers, or temporarily limiting the vehicle velocity (to avoid excessive aerodynamic heating during the ascent through the atmosphere), as in the Space Shuttle main engine.
Throttling is accomplished by reducing the propellant flow supply to the thrust chamber and thus reducing the chamber pressure. The pressure drop in the injector is related to the injection velocity by Eq. 8-5. The accompanying reduction of the injector pressure drop can lead to a very low liquid injection velocity and, thus, to poor propellant mixing, improper stream impingement patterns, and poor atomization, which in turn can lead to lower combustion efficiency and thus lower performance and sometimes unstable combustion.
The variation of flow through a given set of injection orifices and of thrust by this method is limited.
There are several throttling methods whereby the injection pressure drop is not decreased unduly. This permits a change in chamber pressure without a major decrease in injector pressure drop. A moving sleeve- mechanism for adjusting the fuel and the oxidizer injection circular sheet spray areas is shown in Fig. 8-3.
One way of preventing unstable operation and a drop-off in performance is to use multiple thrust chambers or multiple rocket engines, each of which operates always at or near rated conditions. The thrust is varied by turning individual thrust chambers on or off and by throttling all of them over a relatively narrow range.
For small reaction control thrusters the average thrust is usually reduced by pulsing. It is accomplished by controlling the number of cycles or pulses (each has one short fixed-duration thrust pulse plus a short fixed-duration zero-thrust pause), by modulating the duration of individual pulses (with short pauses between pulses), or alternatively by lengthening the pause between pulses.
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