Underactuated control offers fault-tolerance for satellite systems, which not only enables the position and attitude control of a satellite with fewer thrusters, but also can reduce the number of thrusters equipped on the satellite even when considering the need for backups. Due to having fewer thrusters, the coupling effect between the translational motion and rotational motion of the satellite cannot be avoided, and the coupled motion must be considered in control procedures. This paper presents a global trajectory design procedure required for the position and attitude control of an underactuated satellite. The satellite has four thrusters with constant thrust magnitudes on one plane of the satellite body. Then, an analytical solution for coupled motion between the rotation and translation of the satellite is obtained using three-step maneuvers of attitude control. The trajectory design based on the analytical solution is shown for the control of translational and rotational motion in three dimensions. Finally, a numerical simulation is performed to verify the effectiveness of the proposed design procedure.
This paper presents the effect of dihedral stators on flow behavior in a transonic axial compressor. A commercial flow solver was used to calculate the performance and flow characteristics of a transonic axial compressor with different shapes of stators modified by changing the shape of the stacking line. In a stator with a straight stacking line, large corner separation occurred between the suction surface and the shroud endwall and caused a large total pressure loss under high loading. In this study, the dihedral stator induced a significant pressure loss on the blade at the peak efficiency point owing to the increased corner separation area. The shroud dihedral generated a radial pressure gradient and caused a low-momentum flow to migrate from the shroud endwall toward the midspan, consequently decreasing the diffusion factor near the shroud endwall. Although the hub dihedral generates unexpected hub-corner separation causing a large total pressure loss over the entire operating range, the loss region makes the high-momentum flow near the hub divert toward the upper part of the passage. Therefore, the amplitude of the low-frequency term according to the shroud-corner separation also decreased, and the stall limit of the compressor was improved with the hub dihedral stator.
In the present study, aeroelastic stability and the response of high-aspect ratio wings are numerically investigated using a coupled CFD-CSD method. The wing aerodynamic loads are calculated using a CFD flow solver based on unstructured meshes. The elastic deformation is evaluated using a FEM-based CSD solver employing a nonlinear flap-lag-torsion beam theory. The CSD solver also includes a built-in aerodynamics module based on a two-dimensional strip theory, coupled with a dynamic stall model, which is used for comparison with the coupled CFD-CSD method. Coupling of the CFD and CSD solvers is accomplished by adopting a conventional serial staggered method. At first, validation of the present coupled CFD-CSD method is made for an NACA0012 wing, and the predicted static deformation and dynamic response are compared with other predictions. The coupled method is then applied to an electric aerial vehicle wing, and the dynamic aeroelastic stability and response are investigated. It is found that the geometrical nonlinearity of the structure is responsible for degrading the dynamic stability of the wings. It is also found that the aeroelastic behaviors obtained using the coupled CFD-CSD method show higher aerodynamic damping than those of the strip theory-based analyses.
Characteristics of co-flowing jets at subsonic and correctly expanded sonic Mach numbers were investigated numerically for three different lip thicknesses namely, 0.2Dp, 1.0Dp and 1.5Dp (where Dp is the exit diameter of the primary nozzle which is 10 mm). Comparisons of numerical flow-field characteristics were made with experimental data. Lip thickness is defined as the thickness of the wall separating primary jet and the secondary jet. It has been found that co-flow with 0.2Dp lip thickness retards the mixing of the primary jet, leading to potential core elongation. For 1.0Dp and 1.5Dp lip thickness, the presence of lip thickness creates a recirculation zone between the primary jet and the secondary jet, which increases turbulence intensity in the near-field region of the co-flowing jet thereby influencing the properties in the near-field, such as potential core length reduction, static pressure rise, etc. Variation in Mach number has less significance in the flow-field characteristics of co-flowing jet.
In this paper, a tensor product model-based gain scheduling technique is utilized to design a pitch-axis autopilot for an air-to-air missile. Firstly, a recent technique for tensor product (TP) model transformation provides a convenient way to transform missile pitch-axis linear-parameter-varying (LPV) model into a convex parameter-varying weighted combination of linear-time-invariant (LTI) systems. This polytopic model is beneficial for convex hull manipulation, so that a large number of linear matrix inequality (LMI) optimization techniques can be applied controller development. This paper presents an alternative LMI technique based on a TP polytopic model, which can optimize the H∞ performance of a closed-loop system with LMI pole constraints. Then, the proposed approach is applied in the design of a simple four-loop gain-scheduled autopilot. Final simulation results indicate the missile autopilot presented has good overall performance and strong robustness, which validates effectiveness of the proposed TP model-based method.
Today, the primary identification methods in time domain for spacecraft are based on singular value decomposition (SVD), such as the eigensystem realization algorithm (ERA) or stochastic subspace identification (SSI), which requires significant computation time. However, some control problems, such as self-adaptive control, need the latest modal parameters to update controller parameters online. To improve computational efficiency, the fast approximated power iteration (FAPI) recursive algorithm, which avoids SVD, is applied as an alternative method to identify the time-varying frequencies of large flexible satellites. Moreover, an improved recursive form is also proposed to obtain the time-varying input matrix in a state-space model by rewriting the relation of the input and output data. In numerical simulations, the time-varying model of the Engineering Test Satellite-VIII (ETS-VIII) is established. The results illustrate that this recursive algorithm can implement time-varying parameter online identification and it has a better computational efficiency than the SVD-based methods.
Conceptual design procedures and design models of HATS are revised and renewed. The results calculated using the revised method are compared with the operating conditions of HATS at the JAXA Kakuda Space Center. The previous physical method of test chamber pressure and that of ejector suction are adopted in the present model. The suction model adopts the inviscid momentum exchange mechanism. The deceleration process in the supersonic diffuser is revised using the pseudo-shock model. Physical and thermodynamic models are constructed for condensation and steam saturated flow conditions. The results calculated are in reasonable agreement with the measured values (e.g., pressure of secondary flow at the ejector section and pressure changes during engine shut down). The effect of ejector steam condensation on the operating conditions of HATS is quantitatively presented.
A method to localize a space rover on a planetary body using round-trip propagation delay between the rover and its mother spacecraft has previously been proposed. The approach can provide localization with meter-order accuracy. However, the method is based on the assumption that the rover is stationary on the surface of the asteroid during the localization. In this study, this method is expanded for application to the rover's hopping motion. The proposed method is based on the use of the extended Kalman filter (EKF). Multiple motions of the rover are modeled for the time-update steps in the EKF. The localization accuracy is demonstrated through numerical simulations that assume a hopping rover on a small planetary body with the size of the asteroid Itokawa.
A nonlinear relative motion dynamics model in the presence of disturbances and parametric uncertainties is presented for the high precision relative motion of a spacecraft. The disturbances include the Earth’s oblateness, atmospheric drag, and thrust error. The parametric uncertainties in the atmospheric drag coefficients and thrust alignments are considered. To minimize fuel cost ΔV while keeping the desired relative orbit, a relative J2-invariant dynamics model is also designed. For spacecraft relative motion tracking maneuver, an adaptive backstepping sliding mode control law under limited low thrust is developed. This control law combines the advantages of adaptive backstepping and sliding mode control, where knowledge of the upper bounds of parametric uncertainties and disturbances are not required. Within the Lyapunov framework, the proposed control law is proved to guarantee global asymptotic convergence to the desired states. Numerical simulation results show the effectiveness of the nonlinear relative motion dynamics model and proposed control law.