Required Capability: The air-breathing engines must be able to meet the performance requirements of an SPS HTO-SSTO vehicle. In particular, they must support launch to low earth orbit with good turnaround and low cost. Versatility should be enhanced by the ability to ferry the vehicle with the airbreathing engines. Development Plans: Existing studies should be extended to include today's technology for materials and turbofans. Prototype hardware should be fabricated and subjected to ground and flight testing over a wide Mach and altitude range to verify performance. 5.3.7 Attitude Control and Stationkeeping Item — Control of Large Flexible Structures Cricicality — Critical to program success The first natural frequency of the SPS vehicle is estimated to lie between 1 and 10 cycles per hour. The primary disturbances occur at frequencies corresponding to half orbit period and orbit period. A simple control system capable of countering environmental disturbances may have a bandwidth encompassing the first one or more structural frequencies leading to the possibility of structural resonance. Careful control system design will be required to meet system requirements. Present Capability: Existing flexible spacecraft consist of a central rigid body with attached flexible appendages such as antennae and solar cell panels. First natural frequencies are typically of the order of a few tens of Hertz or higher. Resonance is avoided by limiting the bandwidth of the control system, providing notch filtering at the resonant frequency, providing high damping of the mode, or applying appropriate phase shifting. These methods may be inadequate for SPS because of small allowable deflections, large amplitude disturbances, small separation between structural vibration and disturbance frequencies, and a totally flexible vehicle. Required Capabilities: Control system design and analysis techniques need to be developed for very large space structures which are generally flexible throughout and not restricted to flexible appendages. It needs the capability to determine number, type, and location of actuators and sensors for distributed control systems. Modern control techniques will be needed to process data and generate control commands. Development Plan: The first step is to define analytical techniques for simulating flexible systems and defining control system requirements. These are then used to synthesize control systems. The analytical models need to be implemented in digital computer programs that can be used for actual control system design. Earth based experiments can be used to verify adequacy of the techniques. An actual control system for a very flexible vehicle should be demonstrated in a space based experiment or on a pilot SPS.
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