An Independent Degree-eight Mars Gravity Field Model and the Expected Results from the Tianwen-1 Mission

Tianwen-1 is China's first independent interplanetary exploration mission,targeting Mars,and includes orbiting,landing,and rover phases.Similar to previous Mars missions,the Tianwen-1 orbiter was designed for polar orbits during the scientific mission period but has an exceptional eccentricity of approximately 0.59.We provide the first independent eight-degree Martian gravity field model in this paper,which was developed exclusively by a team working in China with our independent software as well,based on about two months of radiometric Doppler and range data from only the Tianwen-1 mission.This model is independent from the models created by the groups at NASA Jet Propulsion Laboratory and Goddard Space Flight Center in the United States,as well as the Centre National d'Etudes Spatiales in France.Furthermore,in order to optimize the engineering and scientific benefits,we proposed a number of potential orbits for the extended Tianwen-1 mission.In order to solve a higher-degree independent Mars gravity field model,the viability of modifying the perigee height was investigated,with the priority considerations of fuel savings and implementation hazards being controlled.

飞雁式异形钢箱拱制作线形控制关键技术研究

钢结构桥梁采用大节段分段施工,工厂化预制,板块线形的精确放样是钢结构制作线形精准控制的关键因素。飞雁式异形钢结构拱肋曲线复杂,拱肋截面高度随拱轴线线性变化,给钢结构制作加工带来很大困难。桥梁拱肋设计成桥线形和预拱度线形的理论求解可为后续解决钢结构拱肋顶、底板加工制作,板块精确放样提供理论支持,为成桥后钢结构拱肋受力稳定提供保障。预制钢箱拱拱肋理论制作线形是确保偏态抛物线异形变截面钢箱拱桥顺利安装和桥梁服役阶段受力合理的重要技术指标。通过设计计算软件MIDAS建立桥梁理论计算模型,采用正装分析计算的方法计算出张拉预应力系杆、拆除拱肋安装支架、张拉吊杆、卸除主梁桥下支架、二次张拉吊杆等施工工序转换对拱肋成桥线形的影响,得到钢拱肋在施工、架设期间体系转换的理论预拱度值。再结合设计图纸给出的异形钢箱拱肋拱轴线计算方程式,通过坐标转换的方法,建立拱轴线上任一点O点的坐标系1和拱轴线上无限接近O点的相邻点B的坐标系2,以及全桥原始坐标系三者之间的联系。通过原始坐标系中O点位置坐标(x 1,z 1),B点位置坐标(x 2,z 2)以及O点处的截面高度H之间的关系,结合坐标系1和坐标系2确定出三者之间的三角形相似关联,从而求得成桥状态下异形钢箱拱肋制作的顶、底板,横隔板及吊杆与拱轴线计算方程式之间的几何关系,确定出钢箱拱拱肋顶、底板独立的成桥线形的计算方程式。运用此计算式求出的拱肋成桥曲线与设计院给出的设计线形吻合良好,当x 2-x 1=0.5 m时计算线形与成桥线形的最大偏差为2 mm,证明了此种方式的正确性。再通过成桥状态下的计算式,结合MIDAS模型计算出的拱肋曲线各点的预拱度值,求解出钢结构拱肋无应力状态下顶、底板,横隔板;吊杆的计算方程式。此计算方程式确定出的曲线状态就是钢拱肋无应力状态下线形,运用此计算结果求得的无应力状态曲线比成桥设计线形长出80 mm。此计算方法有效地解决了异形钢箱拱肋工厂预制过程中无应力状态板块线形精确放样的难题。