Flight Maneuvers

In this section we describe different flight maneuvers and relate them to specific propulsion system types. The three categories of maneuvers are:

1. In translation maneuvers the rocket propulsion thrust vector goes through the center of gravity of the vehicle. The vehicle momentum is changed in the direction of the flight velocity. An example of several powered (trans-lational maneuvers) and unpowered (coasting) segments of a complex space flight trajectory is shown in schematic, simplified form in Fig. 4-10. To date, most maneuvers have used chemical propulsion systems.

2. In truly rotational maneuvers there is no net thrust acting on the vehicle. These are true couples that apply only torque. It requires four thrusters to be able to rotate the vehicle in either direction about any one axis (two thrusters apart, firing simultaneously, but in opposite directions). These types of maneuver are usually provided by reaction control systems. Most have used multiple liquid propellant thrusters, but in recent years many space missions have used electrical propulsion.

3. A combination of categories 1 and 2, such as a large misaligned thrust vector that does not go exactly through the center of gravity of the vehicle. The misalignment can be corrected by changing the vector direction of the main propulsion system (thrust vector control) during powered flight or by applying a simultaneous compensating torque from a separate reaction control system.

The following types of space flight maneuvers and vehicle accelerations use rocket propulsion. All propulsion operations are controlled (started, monitored, and stopped) by the vehicle's guidance and control system.

a. First stage and its upper stage propulsion systems add momentum during launch and ascent. They require rocket propulsion of high or medium thrusts and limited durations (typically 0.7 to 8 minutes). To date all have used chemical propulsion systems. They constitute the major mass of the space vehicle and are discussed further in the next section.

b. Orbit injection or transferring from one orbit to another requires accurately predetermined total impulses. It can be performed by the main propulsion system of the top stage of the launch vehicle. More often it is done by a separate propulsion system at lower thrust levels than the upper stages in item (a) above. Orbit injection can be a single thrust operation after ascent from an earth launch station. If the flight path is a Hohmann transfer ellipse (minimum energy) or a faster transfer orbit, then two thrust application periods are necessary, one at the beginning and one at the end of the transfer path. For injection into earth orbit, the thrust levels are typically between 200 and 45,000 N or 50 and 11,000 lbf, depending on the payload size transfer time, and the specific orbit. If the new orbit is higher, then the thrusts are applied in the flight direction. If the new orbit is at a lower altitude, then the thrusts must be applied in a direction opposite to the flight velocity vector. The transfer orbits can also be achieved with a very low thrust level (0.001 to 1 N) using an electric propulsion system, but the flight paths will be very different (multiloop spiral) and the transfer duration will be much longer. This is explained in Chapter 19. Similar maneuvers are also performed with lunar or interplanetary flight missions, as the planetary landing mission shown schematically in Fig. 4—10.

c. Velocity vector adjustment and minor in-flight correction maneuvers are usually performed with low thrust, short duration and intermittent (pulsing) operations, using a reaction control system with multiple small liquid propellant thrusters, both for translation and rotation. The vernier rockets on a ballistic missile are used to accurately calibrate the terminal velocity vector for improved target accuracy. The reaction control rocket systems in a space launch vehicle will allow accurate orbit injection adjustment maneuvers after it is placed into orbit by another, less accurate propulsion system. Mid-course guidance-directed correction maneuvers for the trajectories of deep space vehicles fall also into this category. Propulsion systems for orbit maintenance maneuvers, also called station keeping maneuvers (to overcome perturbing forces), keeping a spacecraft in its intended orbit and orbital position and are also considered to be part of this category.

d. Reentry and landing maneuvers can take several forms. If the landing occurs on a planet that has an atmosphere, then the drag of the atmosphere will slow down the reentering vehicle. For an elliptical orbit the drag will progressively reduce the perigee altitude and the perigee velocity on every orbit. Landing at a precise, preplanned location requires a particular velocity vector at a predetermined altitude and distance from the landing site. The vehicle has to be rotated into the right position and orientation, so as to use its heat shield correctly. The precise velocity magnitude and direction prior to entering the denser atmosphere are critical for minimizing the heat transfer (usually to the vehicle's heat shield) and to achieve touchdown at the intended landing site or, in the case of ballistic missiles, the intended target. This usually requires a relatively minor maneuver (low total impulse). If there is very little or no atmosphere (for instance, landing on the moon or Mercury), then a reverse thrust has to be applied during descent and touchdown. The rocket propulsion system usually has variable thrust to assure a soft landing and to compensate for the decrease in vehicle mass as propellant is consumed during descent. The lunar landing rocket engine, for example, had a 10 to 1 thrust variation.

e. Rendezvous and docking involve both rotational and translational maneuvers of small reaction control thrusters. Rendezvous and its time windows were discussed on page 123. Docking (sometimes called lock-on) is the linking up of two spacecraft and requires a gradual gentle approach (low thrust, pulsing node thrusters) so as not to damage the spacecraft.

f. A change of plane of the flight trajectory requires the application of a thrust force (through the vehicle center of gravity) in a direction normal to the original plane of the flight path. This is usually performed by a propulsion system that has been rotated (by the reaction control system) into the proper orientation. This maneuver is done to change the plane of a satellite orbit or when going to a planet, such as Mars, whose orbit is inclined to the plane of the earth's orbit.

g. Simple rotational maneuvers rotate the vehicle on command into a specific angular position so as to orient or point a telescope, instrument, solar panel, or antenna for purposes of observation, navigation, communication, or solar power reception. Such a maneuver is also used to keep the orientation of a satellite in a specific direction; for example, if an antenna needs to be continuously pointed at the center of the earth, then the satellite needs to be rotated around its own axis once every satellite revolution. Rotation is also used to point a nozzle of the primary propulsion system into its intended direction just prior to its start. It can also provide for achieving flight stability, or for correcting angular oscillations, that would otherwise increase drag or cause tumbling of the vehicle. Spinning or rolling a vehicle will improve flight stability, but will also average out the misalignment in a thrust vector. If the rotation needs to be performed quickly, then a chemical multi-thruster reaction control system is used. If the rotational changes can be done over a long period of time, then an electrical propulsion system with multiple thrusters is often preferred.

h. De-orbiting and disposal of used or spent spacecraft is required today to remove space debris. The spent spacecraft should not become a hazard to other spacecraft. A relatively small thrust will cause the vehicle to go to a low enough elliptical orbit so that atmospheric drag will cause further slowing. In the dense regions of the atmosphere the reentering, expended vehicle will typically break up or overheat (burn up).

i. Emergency or alternative mission. If there is a malfunction in a spacecraft and it is decided to abort the mission, such as a premature quick return to the earth without pursuing the originally intended mission, then some of the rocket engines can be used for an alternate mission. For example, the main rocket engine in the Apollo lunar mission service module is normally used for retroaction to attain a lunar orbit and for return from lunar orbit to the earth; it can be used for emergency separation of the payload from the launch vehicle and for unusual midcourse corrections during translunar coast, enabling an emergency earth return.

Table 4-6 lists the maneuvers that have just been described, together with some others, and shows the various types of rocket propulsion system (as mentioned in Chapter 1) that have been used for each of these maneuvers. The table omits several propulsion systems, such as solar thermal or nuclear

TABLE 4-6. Types of Rocket Propulsion System Commonly Used for Different Flight Maneuvers

TABLE 4-6. Types of Rocket Propulsion System Commonly Used for Different Flight Maneuvers

Launch vehicle booster x x x x

Strap-on motor/engine x x x x

Upper stages of launch vehicle x x x x x

Satellite orbit injection and transfer orbits x x

Flight velocity adjustments, Flight path corrections,

Orbit raising x x x

Orbit/position maintenance, rotation of spacecraft x x

Docking of two spacecraft x x

Reentry and landing,

Emergency maneuvers x x

Deorbit x x

Deep space, Sun escape x x

Tactical missiles

Strategic missiles x x x x x

Missile defense x x x

rocket propulsion, because these have not yet flown in a real space mission. The electrical propulsion systems have very high specific impulse (see Table 2-1), which makes them very attractive for deep space missions, but they can be applied only to missions with sufficiently long thrust action time for reaching the desired vehicle velocity with very small acceleration. The items with a double mark "xx" have been the preferred methods in recent years.

Reaction Control System

The functions of a reaction control system have been described in the previous section on flight maneuvers. They are used for the maneuvers identified by paragraphs c, e, and g. In some vehcle designs they are also used for tasks described in b, part of d, and f, if the thrust levels are low.

A reaction control system (RCS), often called an auxiliary rocket propulsion system, is needed to provide for trajectory corrections (small Au additions), as well as correcting the rotational or attitude position of almost all spacecraft and all major launch vehicles. If only rotational maneuvers are made, it has been called an attitude control system. The nomenclature has not been consistent throughout the industry or the literature.

An RCS can be incorporated into the payload stage and each of the stages of a multiple stage vehicle. In some missions and designs the RCS is built into only the uppermost stage; it operates throughout the flight and provides the control torques and forces for all the stages. Liquid propellant rocket engines with multiple thrusters have been used for almost all launch vehicles and the majority of all spacecraft. Cold gas systems were used with early spacecraft design. In the last decade an increasing number of electrical propulsion systems have been used, primarily on spacecraft, as described in Chapter 19. The life of an RCS may be short (when used on an individual vehicle stage), or it may see use throughout the mission duration (perhaps 10 years) when part of an orbiting spacecraft.

The vehicle attitude has to be controlled about three mutually perpendicular axes, each with two degrees of freedom (clockwise and counterclockwise rotation), giving a total of six degrees of rotational freedom. Pitch control raises or lowers the nose of the vehicle, yaw torques induce a motion to the right or the left side, and roll torques will rotate the vehicle about its axis, either clockwise or counterclockwise. In order to apply a true torque it is necessary to use two thrust chambers of exactly equal thrust and equal start and stop times, placed an equal distance from the center of mass. Figure 4—13 shows a simple spherical spacecraft attitude control system; thrusters x — x or x — x apply torques that rotate about the X-axis. There is a minimum of 12 thrusters in this system, but some spacecraft with geometrical or other limitations on the placement of these nozzles or with provisions for redundancy may actually have more than 12. The same system can, by operating a different set of nozzles, also provide translation forces; for example, if one each of the thrust units x and x were operated simultaneously, the resulting forces would propel the vehicle in the direction of the y-axis. With clever design it is possible to use fewer thrusters.

An RCS usually contains the following major subsystems: (1) sensing devices for determining the attitude, velocity, and position of the vehicle with respect to a reference direction at any one time, such as provided by gyroscopes, star-trackers, or radio beacons; (2) a control-command system that compares the actual space and rotary position with the desired or programmed position and issues command signals to change the vehicle position within a desired time period; and (3) devices for changing the angular position, such as a set of high-speed gyroscopic wheels and a set of attitude control thrust-providing devices. See Refs. 4-12 and 4-14.

FIGURE 4-13. Simplified attitude control system diagram for spacecraft. It requires 12 thrusters (identified as x, y, z) to allow the application of pure torques about three perpendicular axes. The four unlabeled thrusters are needed for translation maneuvers along the z axis. They are shown here in four clusters.

FIGURE 4-13. Simplified attitude control system diagram for spacecraft. It requires 12 thrusters (identified as x, y, z) to allow the application of pure torques about three perpendicular axes. The four unlabeled thrusters are needed for translation maneuvers along the z axis. They are shown here in four clusters.

A precise attitude angular correction can also be achieved by the use of an inertial or high-speed rotating reaction wheel, which applies torque when its rotational speed is increased or decreased. While these wheels are quite simple and effective, the total angular momentum change they can supply is generally small. By using a pair of supplementary attitude control thrust rocket units it is possible to unload or respin each wheel so it can continue to supply small angular position corrections as needed.

The torque T of a pair of thrust chambers of thrust F and a separation distance / is applied to give the vehicle with an angular or rotational moment of inertia Ma an angular acceleration of magnitude a:

For a cylinder of equally distributed mass Ma = ¿mr2 and for a homogeneous sphere it is Ma = \mr2. The largest possible practical value of moment arm / will minimize the thrust and propellant requirements. If the angular acceleration is constant over a time period t, the vehicle will move at an angular speed co and through a displacement angle 0, namely a> = at and 6 = iat2

Commonly a control system senses a small angular disturbance and then commands an appropriate correction. For this detection of an angular position change by an accurate sensor it is actually necessary for the vehicle to undergo a slight angular displacement. Care must be taken to avoid overcorrection and hunting of the vehicle position or the control system. For this reason many spacecraft require extremely short multiple pulses (0.010 to 0.030 sec) and low thrust (0.01 to 100 N) (see Refs. 4-13 and 4-14).

Reaction control systems can be characterized by the magnitude of the total impulse, the number, thrust level, and direction of the thrusters, and by their duty cycles. The duty cycle refers to the number of thrust pulses, their operating times, the times between thrust applications, and the timing of these short operations during the mission operating period. For a particular thruster, a 30% duty cycle means an average active cumulative thrust period of 30% during the propulsion system's flight duration. These propulsion parameters can be determined from the mission, the guidance and control approach, the desired accuracy, flight stability, the likely thrust misalignments of the main propulsion systems, the three-dimensional flight path variations, the perturbations to the trajectory, and several other factors. Some of these parameters are often difficult to determine.

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