For large deployabie modular antenna reflectors, it is necessary to clarify which module configuration satisfies all electrical and mechanical requirernents. This paper describes a method of finding module size and of module number considering the electrical characteristics and the mechanical characteristics. At first, for different module numbers, the module size is determined under the restraint of constant area gain. The tube radii of the modules are determined so as to satisfy the rigidity requirement. The masses are calculated for each module configuration. The results, calculated for aperture diameters from 5 to 20m, show that module number increases with aperture diameter (for the least mass configuration), and the mass of the reflector decreases as the aperture efflciency increases.

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Laser output power characteristics of the combustion-driven CO₂ Gasdynamic Laser (CO₂GDL) are presented. Laser extraction is carried out with supersonic nozzle of circular-circular (CC) and shock free (SF), which has common area-ratio (A/A*=20) and various expansion length (CC nozzle with expansion length of 30mm and SF nozzle of 45, 33, 20mm). In laser output power measurements, maximum power of 24.5W is obtained at CC 20 nazzle of expansion length of 30mm, which has maximum small-signal gain of 0.5m^-1. However, at SF nozzle which has small-signal gain of 0.2-0.3 m^-1, laser output power can not be obtained. Investigation of the laser excitation condition suggests that the system requires the threshold value for small-signal gain over 0.23m^-1. It is concluded that, with these results a feasible laser excitation system by the CO₂GDL with CC nozzle has been established. Increase of small-signal gain, extension of cavity width and selection of most suitable laser light resonator will provide a radiative heating simulator for thermal protection research.

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The excitation discharge in discharge-pumped excimer lasers causes various instabilities in subsequent discharge which collapse the highly-repetitive operation. The double pulse discharge has been demonstrated in a stable gas and a flow with velocity of 200m/s where a gas mixture of He/Ar is used to simulate the excimer laser gas. The shock waves and the density perturbatian between the two successive discharges have also been visualized by the shadowgraph. The second discharge of the double pulse discharge in a stable gas becomes an arc when an equivalent pulse repetition rate (f) is higher than 30Hz. In a flow, the second discharge at f=10kHz becornes an arc through the heated column which is produced by the first discharge and still remains between the electrodes. At f=5kHz, the glow discharge can be produced between the electrodes, however, an arc is also produced through the heated column which locates somewhat downstream of the electrodes. Only glow discharge is succeeded at f =3 kHz. It is found that the heated column tends to induce an arc in successive discharge.

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In the regime of low power DC arcjet, a research how to stabilize the operation modes was conducted in the Institute of Space and Astronautical Science (ISAS) using a 2-dimension-like DC arcjet for internal visualization, designated as ISAS-VAJ. As the results, an intermediate-voltage operation mode was newly found between the conventional low-voltage and the high-voltage mode. In the new intermediate-voltage mode, it was observed that the electron density and the ionization features were locally enhanced in the upstream region of the nozzle. It was also found that these localized features turned to be relaxed just after the formation, and the operation mode transferred to a high-voltage mode. Applying Kuriki model to this phenomenon, the operation mode and the transition were understood in view of the conditions for arc column formation. A proposal to stabilize the operation mode was made to be applied to ISAS-VAJ by adding a second-cathode in the upstream region of the nozzle. Initiating high voltage pulsed discharges on it, the operation mode was successfully transferred from a low-voltage to a stable high-voltage mode.

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Time-optimal control for flexible space structures is studied in this paper both numerically and experimentally with taking air damping and system parameter robustness into account and applied to large angle rest-to-rest slew maneuver. A simple model of flexible space structures is a rigid main body equipped with a flexible appendage and is treated to slew in a single-axis rotation. Control histories are obtained to be bang-bang control with multiple switches in each control variable. Robustness of the designed control is examined and two situations are studied including one model as an undamped system in space and the other model damped system on ground with air drag. The trajectory of timeoptimal control is realized for the undamped system by employing tracking control and the robust time-optimal control is realized for the damped system with air drag by employing open-loop control.

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Spacecraft, Attitude Control, Flexible Structure, Unsymmetric Structure, Torque Arm Abstract: Unsymmetric flexible spacecrafts have particular dynamic characteristics; when the attitude control actuator such as RCS thruster is used, the imbalance between negative torque and positive torque is caused. And even worse, an inertia matrix and a coupling matrix are changed depending on the displacement of the rotational paddle and the mobile sensor. In this paper, an attitude control system with consideration of these characteristics is proposed. Location of thrusters is designed to satisfy that the nominal control torque is larger than the imbalance torque. And control method includes PD, filter and variable gain, which maintain high quality of closed loop system. The validity of the proposed method is verified through the numerical simulation results.

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