Analysis of the orbit lifetime of CubeSats in low Earth orbits including periodic variation in drag due to attitude motion

It is estimated that more than 22,300 human-made objects are in orbit around the Earth, with a total mass above 8,400,000 kg. Around 89% of these objects are non-operational and without control, which makes them to be considered orbital debris. These numbers consider only objects with dimensions larger than 10 cm. Besides those numbers, there are also about 2000 operational satellites in orbit nowadays. The space debris represents a hazard to operational satellites and to the space operations. A major concern is that this number is growing, due to new launches and particles generated by collisions. Another important point is that the development of CubeSats has increased exponentially in the last years, increasing the number of objects in space, mainly in the Low Earth Orbits (LEO). Due to the short operational time, CubeSats boost the debris population. One of the requirements for space debris mitigation in LEO is the limitation of the orbital lifetime of the satellites, which needs to be lower than 25 years. However, there are space debris with longer estimated decay time. In LEÓs, the influence of the atmospheric drag is the main orbital perturbation, and is used in maneuvers to increment the losses in the satellite orbital energy, to locate satellites in constellations and to accelerate the decay.The goal of the present research is to study the influence of aerodynamic rotational maneuver in the CubeSat?s orbital lifetime. The rotational axis is orthogonal to the orbital plane of the CubeSat, which generates variations in the ballistic coefficient along the trajectory. The maneuver is proposed to accelerate the decay and to mitigate orbital debris generated by non-operational CubeSats. The panel method is selected to determine the drag coefficient as a function of the flow incident angle and the spinning rate. The pressure distribution is integrated from the satellite faces at hypersonic rarefied flow to calculate the drag coefficient. The mathematical model considers the gravitational potential of the Earth and the deceleration due to drag. To analyze the effects of the rotation during the decay, multiple trajectories were propagated, comparing the results obtained assuming a constant drag coefficient with trajectories where the drag coefficient changes periodically. The initial perigees selected were lower than 400 km of altitude with eccentricities ranging from 0.00 to 0.02. Six values for the angular velocity were applied in the maneuver. The technique of rotating the spacecraft is an interesting solution to increase the orbit decay of a CubeSat without implementing additional de-orbit devices. Significant changes in the decay time are presented due to the increase of the mean drag coefficient calculated by the panel method, when the maneuver is applied, reducing the orbital lifetime, however the results are independent of the angular velocity of the satellite.     

乘波构型的钝化方法及其对性能影响研究

前缘钝化是解决乘波构型飞行器气动热问题的有效方法之一.按照乘波构型的设计特点,对已有的两种钝化方法分别进行了改进.采用CFD方法分析了前缘钝化及不同钝化半径对乘波构型性能的影响,得出了乘波构型气动力和气动热性能参数随钝化半径的变化规律.计算结果表明:在相同的钝化半径下,按改进的Tincher方法钝化后的乘波构型与按改进的Takashima方法钝化后的乘波构型相比:升阻比大、总的表面积小、最大热流密度基本一样,非驻点区乘波构型前缘的热流密度峰值较大.因而按改进的Tincher方法钝化后的乘波构型气动性能明显好于按改进的Takashima方法钝化后的乘波构型,而气动热性能则略差于后者.分析表明:钝化后的乘波构型性能不仅与钝化半径有关,而且受钝化方法的影响也很大.在对高超声速乘波飞行器进行布局设计时,应针对乘波构型的设计特点,采用合适的钝化方法,综合考虑钝化方法和钝化半径对气动力和气动热性能的影响效应,寻找最佳的钝化方案.研究结论可为高超声速乘波飞行器的外形设计提供一定的依据.     

固体火箭发动机气动喉部非定常过程

利用流体计算软件FLUENT,通过UDF编程对气动喉部调节固体火箭发动机推力的典型工作过程进行了数值模拟。对推力调节过程中的瞬态轴对称燃烧室流场进行了分析,得到了压强-时间曲线。分析了发动机空腔容积、二次流喷射角度对推力响应特性的影响,并得到了典型的推力-二次流总压调节特性曲线。其研究结果可为利用气动喉部技术的变推力发动机设计提供一定依据。     

飞航导弹飞行中影响因素分析

为了预先准确地确定飞航导弹的实际飞行轨迹，保证战术指标的实现，从自然环境，弹上设备，特征参数三方面入手，分析了干扰源对导弹飞行性能的影响，得出了有益的结论。     

一种运载火箭减载控制工程方法

为降低运载火箭飞行中的气动载荷，在改进弹道模型的基础上，提出了一种用实时补偿对实时气流攻角和侧滑进行反馈控制的方法。研究了工程上用实时视加速度与姿态角偏差近似获得实时攻角和侧滑角的方法，并给出了控制模型。仿真结果表明，该方法能有效降低火箭的qα值，且对关机点参数的影响不大。     

提高航空发动机性能的弯扭导向器技术

给出了采用弯扭导向器提高发动机性能的设计原则：在全三维弯扭设计时，必须在保证涡轮前总温、总压和燃气流量的前提下，合理调整尾喷管面积，提高涡轮后背压，保证在设计转速下涡轮输出功与原型涡轮级相同，从而保证了压气机的设计点和喘振裕度，同时提高了发动机推力、降低油耗。     

宽速域乘波构型设计方法研究综述

乘波体是一种典型的高速气动构型,由于高升阻比和均匀的下表面流动等特性使其成为机身/进气道一体化设计的理想构型。随着对乘波体设计方法的不断研究,提高乘波体在非设计条件下的气动性能,实现乘波体的宽速域飞行成为乘波体实用化的一个重要研究方向。将目前的宽速域乘波体设计方法分成变马赫数、多级组合和涡波结合3种类型,并详细介绍了这些方法的设计过程,分析了设计方法的优缺点。     

压气机效率的正确变比热计算

随着实际压气机压机压比不断增高，按定比热等熵指数ｋ＝１．４的算法，误差越来越大。文中按照给定或实验测定压比，初温，终温的命题，给出计算效率的正确变化比热算法，并控压比５至３０分档作出算例，同时检验出，如简单地按初温冬温求ｋ算效率误差很大，几乎与定比热法同一量级。工程中常年用定比热已久，如何给出简便校正方法。尚待研究。     

便携式多用途导弹方案设想

提出了一种便携式多用途导弹方案的设想。根据确定的设计理念,给出了刚性（捷联）制导系统中的目标探测装置、控制系统,引战系统中的引信、战斗部,以及电源系统和动力系统。讨论了制导舱、引战舱、动力舱等各舱段以及部位安排。介绍了导弹气动外形、战技指标、作战过程和关键技术。     

A faster optimization method based on support vector regression for aerodynamic problems

In this paper, a new strategy for optimal design of complex aerodynamic configuration with a reasonable low computational effort is proposed. In order to solve the formulated aerodynamic optimization problem with heavy computation complexity, two steps are taken: (1) a sequential approximation method based on support vector regression (SVR) and hybrid cross validation strategy, is proposed to predict aerodynamic coefficients, and thus approximates the objective function and constraint conditions of the originally formulated optimization problem with given limited sample points; (2) a sequential optimization algorithm is proposed to ensure the obtained optimal solution by solving the approximation optimization problem in step (1) is very close to the optimal solution of the originally formulated optimization problem. In the end, we adopt a complex aerodynamic design problem, that is optimal aerodynamic design of a flight vehicle with grid fins, to demonstrate our proposed optimization methods, and numerical results show that better results can be obtained with a significantly lower computational effort than using classical optimization techniques.