离轨帆弹性桅杆力学性能分析

采用有限元分析软件ABAQUS分析弹性桅杆几何参数对其力学性能的影响,为离轨帆弹性桅杆设计提供一定的理论依据。结果表明:增加壳体厚度、保持弧长不变,减小曲率半径以及增大壳体截面圆心角都能有效提高离轨帆自动展开能力及弹性桅杆支撑刚度;弹性桅杆的弯曲内部应力只与材料属性、曲率半径、缠绕半径以及壳体厚度有关;曲率半径减小、壳体厚度增加都会增大弹性桅杆的弯曲内部应力,有可能会导致弹性桅杆在缠绕时发生塑性变形。因此,在对弹性桅杆设计时,需综合考虑弹性桅杆几何参数对其展开能力以及缠绕性能的影响。     

Novel orbit-attitude combination mode for solar power satellites to reduce mass and fuel

Solar power satellite receives great attention because it can release the energy crisis and environmental problems in the future. However, the launch and maintenance costs are tremendous due to the large system mass and large fuel consumption to counteract space perturbations. To reduce mass and fuel, a novel quasi-Sun-pointing attitude in Sun-frozen orbit is proposed. The Sun-frozen orbit has a nonzero eccentricity vector that always points towards the Sun. The quasi-Sun-pointing attitude is a periodic solution of the Sun-pointing attitude angle. Although about 3 % electricity must be given up because of the variation of Sun-pointing attitude angle, little control action is required to deal with the solar radiation pressure and gravity-gradient torque. The algorithm to obtain initial conditions is proposed. The influences of system parameters and structural flexibilities are studied. Simulation results reveal that the quasi-Sun-pointing attitude in Sun-frozen orbit dramatically reduce fuel consumption, the dry mass, and complexity of the control system. In addition, structural vibration is hardly induced by the gravity-gradient torque. Thus, the bending stiffness as well as the mass of the supporting structure can be reduced.     

无强波构型理论发展现状研究

本文引入了一种由Busemann超声速双翼理论发展而来的无强激波构型,该构型可明显消弱超声速飞行时带来的波阻和声爆。文中分析了无强波构型的机理和气动特性,介绍了该构型理论研究和应用研究进展,并探讨了该构型在超声速飞行器设计应用中的关键性问题。该理论为未来低声爆超声速飞机的设计提供了一个全新的思路,是未来超声速/高超声速飞行器发展必要的技术基础。     

Velocity distribution of elliptical galaxies in the framework of Non-local Gravity model

We investigate the velocity distribution of elliptical galaxies in the framework of Non-local Gravity. According to this approach, it is possible to recover the fundamental plane of elliptical galaxies without the dark matter hypothesis. Specifically, we compare theoretical predictions for circular velocity in Non-local Gravity context with the corresponding values coming from a large sample of observed elliptical galaxies. We adopt the surface brightness, effective radius and velocity dispersion as structural parameters for the fundamental plane. As final result, it is possible to show that non-local gravity effects can reproduce the stellar dynamics in elliptical galaxies and fit consistently observational data.     

Far-field sonic boom prediction considering atmospheric turbulence effects: An improved approach

Accurate prediction of sonic boom is one of key challenges for the design of a low-boom supersonic aircraft. For most of available far-field prediction methods, the effect of atmospheric turbulence appearing in the planetary boundary layer cannot be considered, which results in remarkable inaccuracy of predicting ground-level sonic boom waveform. Although some efforts have been made to overcome the shortcoming, the turbulence effects are not yet well described so far. This article proposes an improved method by extending the two-dimensional Heterogeneous One-Way Approximation for the Resolution of Diffraction (HOWARD) equation to account for the axial and transverse convections of wind fluctuation as well as the effect of temperature fluctuation. The proposed method is validated by comparing the predictions with the flight-test data of JAXA D-SEND#1 LBM, which shows that the result of the proposed method is in better agreement with the flight-test data than that of the method without considering atmospheric turbulence effects. Then, distortion mechanism of sonic boom waveforms caused by atmospheric turbulence is analyzed by using the proposed method. It is indicated that the effect of turbulent convection makes uniform sonic-boom wavefronts irregular, which creates the condition of diffraction effect to perturb waveforms. Finally, the proposed method is applied to investigate the behavior of two types of waveforms given by the sonic boom minimization theory. Results show that a far-field waveform with a weaker initial shock is more beneficial for low-boom design of a supersonic aircraft.     

Finite time non-singular sliding mode control for spinning tether system with prescribed performance under artificial gravity mission

In this paper, the spinning tether system (STS) is applied to produce the artificial gravity overload with a proposed spin-up scheme in elliptical transfer orbits. To improve the comfort of the astronauts during spinning, the combined control scheme with tension and thrust is designed for spin-up. Then, based on the novel performance function and sliding mode technology, a finite time prescribed performance controller is proposed to track angular velocity for the desired artificial gravity overload. Compared with the existing works, the tracking error is controlled within the preset transient performance and converged to the origin within finite time even in the presence of unknown external disturbance. The stability of closed-loop tracking system is proved by Lyapunov theorem. Finally, the numerical simulations are given to demonstrate the effectiveness and robustness of the proposed controllers in the artificial gravity mission.     

An improved altimeter-derived gravity anomaly from shipborne gravity based on the mean sea surface height constraint factors method

Coastal marine gravity modeling faces challenges due to the degradation of the quality and poor coverage of altimeter data in coastal regions. The effective fusion of shipborne gravity data and altimeter-derived marine gravity data can make shipborne gravity data more useful for the accurate estimation of altimeter-derived coastal marine gravity. A mean sea surface height constraint factor (MSSHCF) method based on the ordinary kriging method and the remove-restore technique is proposed to fuse altimeter-derived gravity model with shipborne gravity data. In this method, all data are standardized during the interpolation process to reduce the error and mean sea surface as a vertical variable is added to the semi-variance function in ordinary kriging to obtain the residual shipborne gravity as corrected data source. The coastal marine gravity models V2.1 and V3.1 which fused altimeter-derived gravity data with shipborne gravity data and V1.1 without shipborne gravity data at a spatial resolution of 1′×1′ can be obtained. Validation experiments show that the accuracy of the gravity model V3.1 obtained by the MSSHCF method more closely agrees with the validated gravity model DTU17 and SS V31 than the model V2.1 obtained by the ordinary kriging interpolation method and the V1.1 model. Our results were validated against shipborne gravity data; the accuracy of model V3.1 was 4.95 % higher than the model V1.1 in South China Sea area A and 2.48 % higher in South China Sea area B. Meanwhile, the accuracy of model V3.1 was 2.07 % higher than model V2.1 in South China Sea area A and 2.42 % higher in South China Sea area B. The effects of distance from the coast and sea depth on the marine gravity model were also evaluated. The results show that the gravity model V3.1 has higher accuracy with the change in ocean distance and depth than the V2.1 and V1.1 gravity models. Thus, our study shows that the MSSHCF method effectively refines coastal altimeter-derived gravity using shipborne gravity data.     

Autonomous navigation for lunar final approach based on gravity gradient measurements

Lunar final approach navigation is critical for pin-point lunar landing in future missions. This study investigates the use of lunar gravity gradient measurements for autonomous navigation of a lunar probe during the final approach phase. As the spacecraft approaches the Moon, the strength of gravity gradient signals improves. A spaceborne gravity gradiometer can precisely measure local gravity gradients, and the latest lunar gravity model GL1500E is used to provide reference values. The employed truncation degree and order of the gravity model are increased stepwise considering the decreasing altitude of the spacecraft in order to reach a compromise between computational costs and model accuracy. An iterative Kalman filter is developed for coupled orbit and attitude estimation using gravity gradient measurements and attitude quaternions obtained from star sensors. A simulated spacecraft with a gradiometer noise level of 0.01 E is considered. Simulation results show that the spacecraft’s position converges rapidly and achieves an accuracy of less than 100 m at the last epoch.