thrust vector coincides with the orbital motion and solar cells generate the normal power. Orbital Operation The orbital operation of the SFU platform is different from that of the SEEL because of difference in the orbital altitude. The altitude difference affects atmospheric density and plasma conditions. At the altitude of the space station and SEEL, the deceleration due to air drag is typically as low as 10~6g. It is a concern that the air drag is large compared with the acceleration by the electrical propulsion at the altitude where the SFU platform can be deployed. Figure 7 shows an example of a trajectory of motion of the SFU platform at an altitude of 400 km (2). The trajectory is drawn on the orbital plane. The vertical axis is the local vertical, and the horizontal axis is the direction of orbital motion. The origin of the coordinate is fixed to a drag-free object representing the orbiter. The figure shows that the SFU platform leaves the orbiter being accelerated by electrical propulsion and goes upward and behind the orbiter. When electrical propulsion can work, the SFU platform is decelerated to come back to the orbiter. For a plasma experiment, the phenomena of interest will occur more strongly at the lower altitude, because the plasma density is higher. To study magnetized plasma, the alignment of the orbiter and the SFU platform is required to obtain a certain angle relative to the geomagnetic field line, as shown by Fig. 8 (2). Such a maneuver can be made by the STS orbiter more easily than in the case of a space station. 7. CONCLUSION The SFU is a universal type of space platform which is carried to and deployed in orbit by the STS orbiter. Since the platform is designed to be used for various purposes, the resources of the SFU such as power supply and heat rejection capability are not sufficient for those experiments for the SEEL (space energetics and environment laboratory). However, additional resources can be carried as a payload of the platform. If a high-voltage solar array is carried as an experiment item to be tested, the electrical power can be used for electrical propulsion. The STS orbiter will play a similar role to that of the space station in the case of SEEL operation. REFERENCES 1. K. Kuriki, M. Nagatomo and T. Obayashi, Space Energetics and Environment Laboratory, Space Solar Power Review 5(2), 1985. 2. Small Space Platform Working Group of ISAS and Space Station Task Team, Advanced Technology Experiment on board Space Flyer Unit (SFU), MS-SS-0376-02, Institute of Space and Astronautical Science (ISAS), March f985.
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