Figure 12-13. Electrically powered concepts comparison. 12. 2. 4. 2 GEO ASSEMBLY/INDEPENDENTLY POWERED SYSTEMS The nature of the orbit transfer function changes when assembly of the large lightweight SPS is in GEO. The acceleration limit is orders of magnitude greater. Consideration for orbit transfer propulsion in this option is given to both high thrust (chemical and nuclear) and low thrust (nuclear electric with MPD thrusters). Chemical (LOX/LH2) stages are state-of-art systems and low cost per unit hardware. Since the specific impulse of the LOX/LH2 systems is relatively low, staging either of tanks or of total stages (reusable) is mandatory. The solid core nuclear stage is an option that offers almost double the specific impulse (825 s) of the LOX/LH2 system, but the mass of the nuclear (solid core reactor) engine degrades most of the specific impulse gain. The solid core nuclear engine is a mature technology. A more speculative nuclear engine that offers substantially greater specific impulse than either LOX/LH2 or the solid core nuclear system uses a gaseous core reactor. The specific impulse of this system is estimated to range from 1800 to more than 5000 s, and thrust levels on the order of 400 000 N are achievable. These high thrust options have been evaluated and orbit transfer performance/cost comparisons are shown in Figure 12-14 (including the nuclear electric system with MPD thrusters). While the chemical system (the range covers staging options and expendable systems) and solid core nuclear system are existing or mature technologies, the gaseous core nuclear system and the nuclear electric system offer substantial performance/cost margins. Once again, trip times from LEO
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