Integrated Power/Attitude Control System (IPACS)

An integrated power and attitude control system (IPACS) for spacecraft is described. The system utilizes energy wheels for electrical energy storage as well as attitude control. Results from the feasibility studies of this concept are summarized and indicate potential weight and cost savings up to 30% over conventional power and control systems. The IP ACS advantage is particularly significant for the longer duration missions which have a large number of energy charge-discharge cycles and higher power requirements. A system for a shuttle-launched Research and Applications Module (RAM) free-flying observatory spacecraft is described. The system utilizes three gimbaled, control, and energy-momentum gyros in a planar array. Each gyro unit is rated at 2.4 kw and delivers 1095 w-hr of energy while maintaining control angular momentum above 1115 N-m-sec. Dynamic response of combined power and control functions was evaluated by digital simulations which included significant nonlinearities and a symmetrical energy distribution law. Simulation data indicate that spacecraft attitude control response is similar to that achieved without the superposition of energy wheel speed changes and is essentially uncoupled from that of the faster power control loop. Both power and control dynamics are well regulated. A NUMBER of spacecraft designs have been develxmoped for the missions of the shuttle era. Most of these require subsystems with lifetimes of 5-7 yr to meet cost effectiveness goals. Pointing requirements below 0.25° are common, with specific scientific missions requiring experiment pointing to 1 arc sec. Momentum storage devices normally are used to provide control torques for long-life missions where control thruster propellant weights and valve life test costs prove excessive. The choice of momentum storage is reinforced, or even required, in several missions where mass expulsion contaminants are prohibited by experiment viewing requirements or where fine pointing stability and slewing is required. The significant impact of the long-life requirement on the electrical power system design is in the sizing of components rather than in the type of system selected. This is because nearly all systems postulated utilize solar arrays for electrical power generation and secondary batteries for electrochemical energy storage. The batteries prove to be the heaviest components of advanced spacecraft solar power systems. The weight of the batteries is determined by the rated energy densities and their inherent characteristic of decreasing life with increased depth-of-discharge and charge-discharge rate. Thus, for a specific energy storage requirement, the designer's major option for increasing battery life is that of increasing the size or number of battery cells thereby decreasing the depth of discharge. As a result, batteries and their controllers commonly constitute 30-40% of an electrical power system weight. Developments of recent years1'2 have shown that flywheels designed to store energy can provide higher energy densities than can be expected from several conventional spacecraft electrochemical devices. In spacecraft applications, parity in energy density between the energy wheel and battery subsystems may result in significant advantage to the energy wheel system. This is because many spacecraft designs currently employ flywheels in momentum storage attitude control systems which approximate the weight of energy wheels.