FIGURE 88 ADVANCED SHUTTLE REQUIREMENTS DEFINITION b. LEO-to-Synchronous Orbit Transportation The LEO-to-synchronous orbit transportation system under consideration by NASA includes reusable space tugs using cryogenic propellants or storable propellants. None of these chemically powered upper-stage systems has sufficiently high performance for an SSPS. For an SSPS it presently appears necessary to develop advanced high-performance propulsion systems for exclusive operation in the space environment. Ion propulsion systems are the most likely choice for LEO-to-synchronous orbit transportation systems. A comparison of current ion system technology and that required for an SSPS follows:
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