With the undergoing and planned implementations of mega constellations of thousands of Low Earth Orbiting(LEO)satellites,space will become even more congested for satellite operations.The enduring effects on the long-...With the undergoing and planned implementations of mega constellations of thousands of Low Earth Orbiting(LEO)satellites,space will become even more congested for satellite operations.The enduring effects on the long-term space environment have been investigated by various researchers using debris environment models.This paper is focused on the imminent short-term effects of LEO mega constellations on the space operation environment concerned by satellite owners and operators.The effects are measured in terms of the Close Approaches(CAs)and overall collision probability.Instead of using debris environment models,the CAs are determined from integrated orbit positions,and the collision probability is computed for each CA considering the sizes and position covariance of the involving objects.The obtained results thus present a clearer picture of the space operation safety environment when LEO mega constellations are deployed.Many mega constellations are simulated,including a Starlink-like constellation of 1584 satellites,four possible generic constellations at altitudes between 1110 km and 1325 km,and three constellations of 1584 satellites each at the altitudes of 650 km,800 km,and 950 km,respectively,where the Resident Space Object(RSO)spatial density is the highest.The increases in the number of CAs and overall collision probability caused by them are really alarming.The results suggest that highly frequent orbital maneuvers are required to avoid collisions between existing RSOs and constellation satellites,and between satellites from two constellations at a close altitude,as such the constellation operation burden would be very heavy.The study is not only useful for satellite operators but a powerful signal for various stakeholders to pay serious attention to the development of LEO mega constellations.展开更多
A new extrapolation approach was proposed to calculate the strain energy release rates of complex cracks. The point_by_point closed method was used to calculate the closed energy, thus the disadvantage of self_inconsi...A new extrapolation approach was proposed to calculate the strain energy release rates of complex cracks. The point_by_point closed method was used to calculate the closed energy, thus the disadvantage of self_inconsistency in some published papers can be avoided. The disadvantage is that the closed energy is repeatedly calculated: when closed nodal number along radial direction is more than two, the displacement of nodes behind the crack tip that is multiplied by nodal forces, the closed energy has been calculated and the crack surfaces have been closed, and that closed energy of middle point is calculated repeatedly. A DCB (double cantilever beam) specimen was calculated and compared with other theoretical results, it is shown that a better coincidence is obtained. In addition the same results are also obtained for compact tension specimen, three point bend specimen and single edge cracked specimen. In comparison with theoretical results,the error can be limited within 1 per cent. This method can be extended to analyze the fracture of composite laminates with various delamination cracks.展开更多
基金the National Natural Science Foundation of China[Grant No.41874035]the National Natural Science Foundation of China[Grant No.12103035]+1 种基金the Natural Science Foundation of Hubei province,China[Grant No.2020CFB396]the Fundamental Research Funds for the Central Universities,China[Grant No.2042021kf0001].
文摘With the undergoing and planned implementations of mega constellations of thousands of Low Earth Orbiting(LEO)satellites,space will become even more congested for satellite operations.The enduring effects on the long-term space environment have been investigated by various researchers using debris environment models.This paper is focused on the imminent short-term effects of LEO mega constellations on the space operation environment concerned by satellite owners and operators.The effects are measured in terms of the Close Approaches(CAs)and overall collision probability.Instead of using debris environment models,the CAs are determined from integrated orbit positions,and the collision probability is computed for each CA considering the sizes and position covariance of the involving objects.The obtained results thus present a clearer picture of the space operation safety environment when LEO mega constellations are deployed.Many mega constellations are simulated,including a Starlink-like constellation of 1584 satellites,four possible generic constellations at altitudes between 1110 km and 1325 km,and three constellations of 1584 satellites each at the altitudes of 650 km,800 km,and 950 km,respectively,where the Resident Space Object(RSO)spatial density is the highest.The increases in the number of CAs and overall collision probability caused by them are really alarming.The results suggest that highly frequent orbital maneuvers are required to avoid collisions between existing RSOs and constellation satellites,and between satellites from two constellations at a close altitude,as such the constellation operation burden would be very heavy.The study is not only useful for satellite operators but a powerful signal for various stakeholders to pay serious attention to the development of LEO mega constellations.
文摘A new extrapolation approach was proposed to calculate the strain energy release rates of complex cracks. The point_by_point closed method was used to calculate the closed energy, thus the disadvantage of self_inconsistency in some published papers can be avoided. The disadvantage is that the closed energy is repeatedly calculated: when closed nodal number along radial direction is more than two, the displacement of nodes behind the crack tip that is multiplied by nodal forces, the closed energy has been calculated and the crack surfaces have been closed, and that closed energy of middle point is calculated repeatedly. A DCB (double cantilever beam) specimen was calculated and compared with other theoretical results, it is shown that a better coincidence is obtained. In addition the same results are also obtained for compact tension specimen, three point bend specimen and single edge cracked specimen. In comparison with theoretical results,the error can be limited within 1 per cent. This method can be extended to analyze the fracture of composite laminates with various delamination cracks.
