This paper presents an attitude control law for astronomy or earth-observation satellites, which require highly stable attitude-pointing for observation and large-angle attitude maneuverability between successive observations. In the control law, magnetic bearing wheels (MBWs) are used instead of conventional ball bearing wheels (BBWs). MBWs, whose rotors are magnetically suspended and thus have no mechanical contact, are low “microvibration” actuators for spacecraft attitude control systems. “All-axes-actively-controlled” MBWs, just as in a control-moment gyro (CMG), provide the capability of tilting the rotational axis besides the rotor-speed control, whose allowable tilt angle, however, is small (typically less than 3 degrees or so). In the proposed control law, multiple MBWs (which represent at least three for three axes control and preferably four for increased performance and hardware redundancy) of this type are adopted as actuators of attitude control. The capability of rotor tilting is applied for broadening control bandwidth to improve the pointing performances while maintaining stability of the control system. The rotational control of the wheels are used for the purpose of 1) accommodating for the excessive angular momentum (=rotor-tilt-angle increments) that may otherwise result in too much tilting of the rotor to cause rotor touchdown, and also 2) large-angle maneuvers of spacecraft attitude. Moreover, the increased degrees of control freedom of MBWs are advantageously used for a further decrement of rotor-tilt angle. The mathematical formulation of our proposed control law is presented, and the results of the numerical simulation on the control performance are also shown.
We adopted optimal preview control methodology to design a terrain-following controller for a cruise missile. In this methodology, tracking errors and control increments are both considered in a quadratic penalty function. An augmented error system that involves future command inputs is built. Thus the preview control problem can be formulated as an optimal regulator problem. Integrating the general optimal servo system with preview feed-forward compensations that respectively feed forward future command inputs and future disturbances produces an optimal preview servo system. In the terrain-following system, the flight altitude of a cruise missile is the command input, and its future information can be known a priori. The wind is viewed as the disturbance in the system and is not previewable. Thus we designed a terrain-following controller with a basic state feedback and a feed-forward compensation for future altitude information, regarding the wind as a constant signal. Simulation results show that the performance of the terrain-following system with such an optimal preview controller is improved dramatically.
This paper presents the application of the moving horizon states estimation (MHSE) method to estimate the states of nonlinear aircraft equation of motion from a dynamic maneuver’s flight test data. To determine the optimum solution of minimizing the performance index, a Quasi-Newton or gradient method is used. The present method also uses the Armijo’s line search gradient to guarantee the solution/estimation to converge faster to the global optimum estimate. The MHSE method is applied to flight test data of N250 PA-1 aircraft for parameter identification and flight path reconstruction. The result of estimation is also used to evaluate the accuracies of the measurement systems.
The first stage of the H-2 rocket used a 110-ton thrust liquid oxygen, liquid hydrogen, pump-fed engine, the LE-7. This engine required high-pressure and high-power liquid oxygen and liquid hydrogen turbopumps to achieve the two-stage combustion cycle in which the combustion pressure is around 13 MPa. Furthermore, it was very important to operate both turbopumps at higher rotational speeds to obtain a smaller, lighter-weight engine because the LE-7 had not low-speed, low-pressure pumps ahead of both the main pumps. The present paper shows the design, test results, and modifications that had been performed until a flight-type liquid oxygen turbopump for the LE-7 engine was completed. The liquid oxygen turbopump had been developed by the use of three models, that is, research, prototype, and flight models.
The studies of low-frequency discharge current oscillation phenomena in the 20 kHz range causing a deterioration of propulsive efficiency and operational instability are important to improve Hall thruster performance. A possibility that the amplitude of 20 kHz-range oscillation could be controlled by changing ionization-zone length was predicted in our previous work, based on experiments and theoretical analyses. In this paper, the physical mechanisms of the ionization process in the acceleration channel are studied by using an unsteady one-dimensional numerical analysis; the methods of controlling oscillation amplitude are concretely discussed from the viewpoint of ionization-zone length. The numerical code generated in the present study would be an effective tool for the best design of a Hall thruster.
Flow simulation around a moving body is a challenging problem, especially when there is more than one body moving relative to each other. To tackle this problem, a Cartesian grid is used in this paper, where its non-body-fitted property allows the grid to stay stationary while the bodies move across it. As a result, it requires only local grid modification in the vicinity of body surface, which may lead to a significant saving of computational cost. On the other hand, the body-fitted grids such as structured and tetrahedral-based unstructured grids move with the body; thus global grid modification is necessary during the movement. In the present study we devised a new implementation procedure for an existing two-dimensional cell-merging method to overcome the problem of conservation regarding gas dynamic properties, a problem caused by body movement across a grid. The present method may have a better potential for extension to 3D. It is based on our previous algorithm developed for a 3D unstructured Cartesian grid and was successfully applied to several test cases in this study. In particular, the efficiency of the method is greatly improved by employing a tree-based data structure to reduce the time to find body panels during computation of the cell’s geometrical properties.