M0 m0gg m0c68 rhf mfgg rhfc

In the turbopump the torques, powers, and shaft speeds must match. The balance of shaft speeds N can be simply written as

where a0 and af are gear ratios. If no gears are used, a0 = a/ = 1. The power balance implies that the power of turbine PT equals the power consumed by pumps and auxiliaries. The power is expressed as the product of torque L and shaft speed N:

where Pb represents the bearing, seal, friction, and transmission power losses. If there are no gears in a particular turbopump, then

The pressure balance equations for the fuel line at a point downstream of the fuel pump can be written as

(^)generator fuel system

Here the fuel pump discharge pressure (pf)d equals the fuel pump suction pressure (pf)s plus the pressure rise across the pump (A/?)pump; this in turn equals the chamber pressures p{ plus all the pressure drops in the main fuel system downstream of the pump, and this is further equal to the chamber pressure in the gas generator combustion chamber pgg augmented by all the pressure losses in the fuel piping between the generator and the downstream side of the fuel pump. The pressure drop in the main fuel system usually includes the losses in the cooling jacket and the pressure decrease in the injector. Equations 6-8 to 6-15 relate to a steady-state condition. A similar pressure balance is needed for the oxidizer flow. The transients and the dynamic change conditions are rather complex but have been analyzed using iterative procedures and digital computers.

6.8. ROCKET ENGINES FOR MANEUVERING, ORBIT ADJUSTMENTS, OR ATTITUDE CONTROL

These engines have usually a set of small thrusters, that are installed at various places in a vehicle, and a common pressurized feed system, similar to Figures

1-3, 4-13, or 6-13. They are called reaction control systems or auxiliary rockets as contrasted to higher-thrust primary or boost propulsion systems in Table 6-1. Most use storable liquid propellants, require a highly accurate repeatability of pulsing, a long life in space, and/or a long-term storage with loaded propellants in flight tanks. Figure 4-13 shows that it requires 12 thrusters for the application of pure torques about three vehicle axes. If a three-degree-of-rotation freedom is not a requrement, or if torques can be combined with some translation maneuvers, fewer thrusters will be needed. These auxiliary rocket engines are commonly used in spacecraft or missiles for the accurate control of flight trajectories, orbit adjustments, or attitude control of the vehicle. References 6-1 and 6-2 give information on several of these. Figure 6-13 shows a simplified flow diagram for a post-boost control rocket engine, with one larger rocket thrust chamber for changing the velocity vector and eight small thrusters for attitude control.

Section 4.6 describes various space trajectory correction maneuvers and satellite station-keeping maneuvers that are typically performed by these small auxiliary liquid propellant rocket engines with multiple thrusters. Attitude control can be provided both while a primary propulsion system (of a vehicle or of a stage) is operating and while its auxiliary rocket system operates by itself. For instance, this is done to point satellite's telescope into a specific orientation or to rotate a spacecraft's main thrust chamber into the desired direction for a vehicle turning maneuver.

A good method for achieving accurate velocity corrections or precise angular positions is to use pure modulation, that is, to fire some of the thrusters in a pulsing mode (for example, fire repeatedly for 0.020 sec, each time followed by a pause of perhaps 0.020 to 0.100 sec). The guidance system determines the maneuver to be undertaken and the vehicle control system sends command signals to specific thrusters for the number of pulses needed to accomplish this maneuver. Small liquid propellant engine systems are uniquely capable of these pulsing operations. Some thrusters have been tested for more than 300,000 pulses. For very short pulse durations the specific impulse is degraded by 5 to 25%, because the performance during the thrust build-up and thrust decay period (at lower chamber pressure) is inferior to operating only at the rated chamber pressure and the transient time becomes a major portion of the total pulse time.

Ballistic missile defense vehicles usually have highly maneuverable upper stages. These require substantial side forces (200 to 6000 N) during the final closing maneuvers just prior to reaching the target. In concept the system is similar to that of Fig. 6-13, except that the larger thrust chamber would be at right-angles to the vehicle axis. A similar system for terminal maneuvers, but using solid propellants, is shown in Fig. 11-28.

The Space Shuttle performs its reaction control with 38 different thrusters, as shown schematically in Figs. 1-13 and 6^1; this includes several duplicate (spare or redundant) thrusters. Selected thrusters are used for different maneuvers, such as space orbit corrections, station keeping, or positioning the

outer skin

FIGURE 6-13. Schematic flow diagram of the helium-pressurized, bipropellant rocket engine system of the fourth stage of the Peacekeeper ballistic missile, which provides the terminal velocity (in direction and magnitude) to each of several warheads. It has one larger gimballed thrust chamber for trajectory translation maneuvers and eight small thrusters (with scarfed nozzles) for attitude control in pitch, yaw, and roll. (Courtesy of USAF.)

outer skin

FIGURE 6-13. Schematic flow diagram of the helium-pressurized, bipropellant rocket engine system of the fourth stage of the Peacekeeper ballistic missile, which provides the terminal velocity (in direction and magnitude) to each of several warheads. It has one larger gimballed thrust chamber for trajectory translation maneuvers and eight small thrusters (with scarfed nozzles) for attitude control in pitch, yaw, and roll. (Courtesy of USAF.)

Space Shuttle for reentry or visual observations. These small restartable rocket engines are also used for space rendezvous or docking maneuvers, where one spacecraft slowly approaches another and locks itself to the other, without causing excessive impact forces during this docking manuever. This docking operation requires rotational and translational maneuvers from a series of rocket engines.

Broadly, the application of pure torque to spacecraft can be divided into two classes, mass expulsion types (rockets) and nonmass expulsion types. Nonmass expulsion types include momentum storage, gravity gradient, solar radiation, and magnetic systems. Some space satellites are equipped with both the mass and nonmass expulsion types. Reaction wheels or flywheels, a momentum storage device, are particularly well suited to obtaining vehicle angular position control with high accuracies of less than 0.01° deviation and low vehicle angular rates of less than 10~5 degrees/sec with relatively little expenditure of energy. The vehicle angular momentum is changed by accelerating (or decelerating) the wheel. Of course, when the wheel speed reaches the maximum (or minimum) permissible, no further electrical motor torquing is possible; the wheel must be decelerated (or accelerated) to have its momentum removed (or augmented), a function usually accomplished through the simultaneous use of small attitude control rockets, which apply a torque to the vehicle in the opposite direction.

The propellants for auxiliary rockets fall into three categories: cold gas jets (also called inert gas jets), warm or heated gas jets, and chemical combustion rockets, such as bipropellant liquid propellant rockets. The specific impulse is typically 50 to 120 sec for cold gas systems and 105 to 250 sec for warm gas systems. Warm gas systems can use inert gas with an electric heater or a monopropellant which is catalytically and/or thermally decomposed. Bipropellant attitude control thrust chambers allow an Is of 220 to 325 sec and have varied from 5 to 4000 N thrust; the highest thrusts apply to large spacecraft. All basically use pressurized feed systems with multiple thrusters or thrust chambers equipped with fast-acting, positive-closing precision valves. Many systems use small, uncooled, metal-constructed supersonic exhaust nozzles strategically located on the periphery of the spacecraft. Gas jets are used typically for low thrust (up to 10N) and low total impulse (up to 4000 N-sec). They have been used on smaller satellites and often only for roll control.

Small liquid monopropellant and liquid bipropellant rocket units are common in auxiliary rocket systems for thrust levels typically above 2 N and total impulse values above 3000 N-sec. Hydrazine is the most common monopropellant used in auxiliary control rockets; nitrogen tetroxide and monomethylhydrazine is a common bipropellant combination. The next chapter contains data on all three categories of these propellants, and Chapter 10 shows diagrams of small auxiliary rocket engines and their thrusters.

Combination systems are also in use. Here a bipropellant with a relatively high value of Is, such as N204 and N2H4, is used in the larger thrusters, which consume most of the propellant; then several simple monopropellant thrusters (with a lower Is), used for attitude control pulsing, usually consume a relatively small fraction of the total fuel. Another combination system is to employ bipropellant or monopropellant thrusters for adding a velocity increment to a flight vehicle or to bleed or pulse some of the pressurizing gas, such as helium, through small nozzles controlled by electromagnetic valves to provide roll control. The specific mission requirements need to be analyzed to determine which type or combination is most advantageous for a particular application.

Special thruster designs exist which can be used in a bipropellant mode at higher thrust and also in a monopropellant mode for lower thrust. This can offer an advantage in some spacecraft applications. An example is the TRW secondary combustion augmented thruster (SCAT), which uses hydrazine and nitrogen tetroxide, is restartable, vaporizes the propellants prior to injection and therefore has very efficient combustion (over 99%), can operate over a wide range of mixture ratios, and can be throttled from 5 to 15 lbf thrust.

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