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Precise, long-term stabilization of a large manned spacecraft requires an advanced or second-generation CMG system, relative to the ATM CMG system. Large angle maneuver, possible long-term earth pointing, and artificial g (spinning) operational modes require continuous, spherical, CMG, momentum vector freedom and therefore make the SIXPAC configuration (ATM) attractive. Inner and outer CMG axis slip rings are required as are improved actuator-gimbal bandwidths, minimum smooth rates, and critical-component-replacement capability. On the assumption that solar panels will provide power, DC brushless spin motors appear to offer power savings.

The state-of-the-art of the logic for the vehicle stabilization system is proportional and rate-limited with CMG torque produced by gimbal rate commands from analog steering laws. These steering laws (ref. 6) are either open loop, torque feedback, or momentum feedback as exists on ATM (ref. 7). Current research is directed toward optimal, statespace vector systems using a digital control computer.

Reference-sensor system concepts for vehicle attitude include solar reference (analog or digital solar sensors) with star

EARTH

tracker update, earth reference (horizon scanner) with solar or stellar update, stellar reference with solar update, and inertial unit during large angle, maneuver-reference transition.

Vehicle Dynamics

Large multimodule spacecraft with solar arrays characteristically have several lowfrequency (less than 1.0 hertz) flexible-body modes superimposed on large-inertia rigidbody modes. High docking-connection compliance, necessitated by docking requirements themselves, and solar array size, necessitated by large spacecraft system power demands, are the main contributors to vehicle flexibility. Variable rigid-body and flexiblebody dynamics is a second important spacecraft characteristic. The variation is a result of continuous solar array reorientation and basic configuration changes when modules or logistic vehicles are docked or undocked. State-ofthe-art vehicle stabilization systems (for example, ATM) include separate vehiclebending-mode filters that are designed to gain stabilize one configuration only. A single, adaptive, phase-stabilized, digital system is an obvious solution to variable low-frequency vehicle bending; however, considerable research effort is required before prototype hardware can be built.

Experiment Pointing Systems

Broad categories of experiments requiring low ambient acceleration, pointing control, or free-flying modules include space biology, space manufacturing, earth resources, solar astronomy, astronomy survey, high energy astronomy, and both stellar and galactic astronomy.

Low ambient acceleration can be obtained by passive systems that isolate flexible-body vibration, provided that rigidbody slewing rates are below controlled upper bounds. Pointing control can be provided by passive isolation of the experiment package or canister from vehicle vibration and by active

control, using sensors on board the experiment package and torquing against the vehicle inertia or against onboard stored momentum.

An artist's concept of the ATM vernierpointing control system (VPCS) is shown in figure 8. This two-degree-of-freedom system has passive (low torsional stiffness springs) and active (DC torquers and onboard sensors) pitch and yaw pointing control, with an accuracy of 2.5 arc seconds. Roll positioning capability is also provided. Initial analyses of the ATM VPCS with ideal sensors indicate an order of magnitude improvement in pointing accuracy is possible with this system (ref. 8).

Six-degree-of-freedom systems attached to the main vehicle may possibly provide the 0.01-arc-second accuracy required for galactic astronomy; however, complete six-degree-offreedom isolation or free-flying experiment modules may be required.

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spherical inertia ellipsoid. Aerodynamic torques result when the center of pressure and center of mass of the vehicle are not coincident. Long-term disturbances act on rigidbody modes and affect vehicle stabilization primarily. Short-term disturbances include crew-motion and onboard-machinery (for example, a centrifuge) local force and moment profiles. These disturbances are internal to the spacecraft and act primarily on vehicle and experiment-package flexible-body modes affecting vehicle and experimentpackage pointing.

Long-term (external) disturbance torques will saturate a CMG system when the time integral of the disturbance torque exceeds the initial stored momentum of the system. Gravity-gradient desaturation logic can prevent this condition by orienting the vehicle so that the time integral of the disturbance torques is bounded at a value that is below the stored momentum of the CMG system. If pointing requirements conflict with gravity-gradient desaturation orientations, desaturation must be accomplished by using reaction-jet torques; however, long-term fuel requirements and limitations on contamination of the external environment of the spacecraft may prohibit reaction-jet use. A technique is being developed at the Langley Research Center called "quiescent dump,' whereby the spacecraft performs desaturation maneuvers during periods of experimentpointing inactivity and, for given experiments, chooses vehicle orientations that minimize accumulated momentum.

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Short-term disturbances excite flexiblebody modes. The energy associated with these modes passes from the vehicle, through the passive isolation system of the experiment package, to the experimental apparatus itself and may ruin a given experiment. Controlsystem active damping (phase stabilization) of the lower bending modes reduces vehicle energy level. Phase stabilization requires CMG bandwidths well above the frequencies of any modes to be actively damped. CMG optimization work at the Langley Research Center and the Marshall Space Flight Center includes effort to improve CMG gimbal drive

response by using low backlash and antibacklash mechanical transmissions. Direct-drive hydraulic torquers are also being considered for future spacecraft having hydraulic systems on board.

Integrated System Control Functions

Spacecraft performance can be evaluated by simulating the control functions associated with spacecraft operational modes. A performance index that considers the performance of each simulated control function can provide a quantitative system rating. This performance index is an initial requirement prior to any system development. The following is a list of spacecraft pointing and stabilization control functions:

Long-term vehicle stability
Long-term experiment pointing
Large angle spacecraft maneuvers
Simultaneous target tracking
Solar array pointing
Short-term vehicle damping
Short-term experiment pointing
Centrifuge operation
Crew operations

Module or logistic vehicle docking
Free-fly module control
Spacecraft transition to spin
Spinning spacecraft control

These control functions will be examined at the Langley Research Center by using a spacestation, digital-computer-hardware simulation, including man-in-the-loop.

Simulation Description

The following is a description of the proposed Langley Research Center space station simulation. The simulation is intended as a general-purpose engineering tool to aid and to help direct effort on space-stationvehicle and experiment-pointing systems research. It is a logical extension of initial theoretical analyses and nonreal-time computer simulation and will serve as a focal point for integrated research on total system

capability. The proposed simulation is illustrated in figures 9 and 10.

Central control is provided by a program control station (fig. 11). This station allows control over the Langley Research Center CDC 6600 computer and input/output hardware. The station includes a simulation console for data entry and control, a display console for post-operation data display, a typewriter for data exit and operator comment, recorders, X-Y plotters, and site communications. The computer system allows memory up to 130,000 60-bit words, 192 digital-to-analog converters (to sites), 80 analog-to-digital converters (from sites), and 960 discretes each from sites and to sites. The application time per iteration for simulation calculation is shown in figure 12. The refined Langley Research Center ATM simulation presently uses 6.5 milliseconds operating with a frame time of 1/32 of a second, while dedicated machine availability is 27 milliseconds at this frame time. This allows an increase in simulation complexity of over 400 percent. Computer main routines will include vehicle rigid-body and flexible-body dynamics, CMG steering laws, CMG gravitygradient desaturation logic, vehicle-pointingcontrol logic, external and internal disturbance profiles, vehicle and experiment attitude and update logic, monitor and diagnostic logic, and the pointing-control logic and hardware simulation for earth, solar, and stellar experiments.

A second-generation CMG system (modified SIXPAC, fig. 5) mounted in torque measurement fixtures (similar to the fixture shown in fig. 4), which, in turn, is mounted on the inner axis of a three-axis-control flight test bed (fig. 6), is housed at the second. simulation site. CMG electronics and simulation interface electronics are housed in an adjacent control room (fig. 13). Two, 14-channel, FM tape recorders in this control room allow continuous, prerecorded, astronaut-force and moment-disturbance profiles to be fed to the computer.

The third simulation site is the FreeBody Dynamics Facility (fig. 14). This 60-foot sphere will serve as a sensor location

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Figure 9. Simulation configuration; full-scale, control-system test concept.

and will allow actual sensor characteristics and their limitations to be included in the real-time simulation. The sensors will be mounted on a small, high-accuracy, computerdriven, servo table (fig. 15) positioned at the center of the sphere. Planet, solar, and stellar radiation simulators (figs. 16, 17, and 18) housed in the sphere will be rotated or positioned to simulate the position and attitude of the space station in orbit.

An integrated vehicle-and-experimentpointing-systems control console (shown as a cardboard model in fig. 19 and in initial layout form in fig. 20) will be housed at the fourth simulation site. This console will allow

an evaluation of man's computer-aided capability both to control the vehicle-andexperiment pointing systems and to monitor and diagnose system status, failure, and redundant component switching.

The earth, solar, and stellar pointing subsystems comprise the final simulation sites. Specific simulation software and hardware at these sites is not defined at the present time. The concept of experiment-pointing-system evaluation, coupled with control-system hardware and computer software, has been defined. Figure 21 shows an example, the ATM solar-experiment test setup at the Marshall Space Flight Center.

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