the 8-12 cm units. About 100 watts of power per milipound of thrust is required for the large units. Small ACS systems, providing ISP of 3000 secs at 180 watts per milipound, have been flown. Life testing of components and systems to date has been limited to life spans of one to three years. Required Capability: Electric thrusters for use in transportation and vehicle control have the following requirements: 1. Life requirement of up to 30 years will require life testing of power conditioning units and thruster components. 2. The large propellant requirement for the SPS system dictates use of the highest performance systems available. The use of an argon system with state-of-the-art 5100 sec impulse is an attractive option with potential of up to 13,000 sec if the power can be made available. The demonstrated performance of 13,000 sec would permit a dramatic reduction in propellant (and, hence, hardware) requirements. 3. The use of the larger thruster for all propulsion requirements appears to be a feasible approach for reducing the complexity of the SPS system. Development Plan: Failure mechanisms that presently limit thruster life must be fully understood. Materials and designs that permit longer life must be developed and life tested. Thruster designs that produce high thrust without compromising specific impulse, efficiency, or life must be developed and tested. 5.3.8 Reflector Technology Item — Reflectors Criticality — Critical to program success SPS concepts published to date employ coated plastic films to concentrate the solar energy. Failure of the reflector will degrade system performance to the extent that replacement or refurbishment will be necessary. Present Capability: Existing plastic films are deteriorated in a space environment by ultraviolet radiation. After sufficient exposure they lose their structural properties and are subject to failure. Existing materials can last for five to ten years. Reflective coatings are degraded in a space environment by erosion from low energy protons and dust. Their expected life is five to ten years. Required Capability: The reflectors should last for the design life of the vehicle without requiring replacement or refurbishment to maintain system performance. Development Plan: New materials and designs for reflectors should be analyzed for their suitability to SPS applications. Ground and in-orbit tests should verify their performance.
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