Power-Efficient Algorithm of Controlling the Engine Thrust at the Final Phase of Soft Moon Landing

The  theme  is topical  because  of the  ongoing Russian  lunar  program.  Four  more  launches  of Luna-25 to Luna-28 are planned  for the next several years. Only  in 8 out of 14 cases, the soft Moon  landing was provided.  This  fact prompts researchers to seek after new ways of solving this problem. The  article deals with a power-efficient  control algorithm that controls landing  engine thrust at the final phase of spacecraft landing  from a given hovering point to the point of contact with the Moon’s  surface.  Initially not supposed to be used for solution of terminal  control tasks, the power-efficient  algorithm can be applied here, which can be explained  by availability  of an auxiliary  system in it that provides reaching specified motion parameters within a specified period. At the final phase of soft Moon landing,  the proposed algorithm treats the spacecraft as a material  point that moves by the attractive force of the Moon  and the opposite vertical force of the landing engine  thrust.  It is supposed  that  to form  the  thrust  the  vertical  velocity  is measured  by a Doppler velocimeter  and  the altitude  by a multibeam  vertical-building radio altimeter, throughout the whole motion interval.  W hen calculating  the parameters of spacecraft motion under the conditions of the Moon’s gravitational field at the final phase of spacecraft contact with the Moon’s  surface by the mentioned  algorithm,  there is a possibility of some overcorrection,  which  is inadmissible. To exclude  it, a well-known approach is used when the motion  is considered  on the assumption  of absence of the Moon’s gravitational  field.  In this  case  the  control  will be implemented  without  overcorrection, but  to obtain  the  actual  engine thrust it is necessary to add the force opposite to the direction of the Moon’s attraction force, acting upon the spacecraft, to the algorithm-generated signal at every control step. The  author has mathematically modeled  the algorithm. The  results of modeling proved the appropriateness of the problem statement and allow finding the boundary  of the algorithm applicability: to exclude  reversing of the landing  engine thrust the initial  spacecraft  hovering altitude  should  be less than  647  m.  The algorithm can also be used for controlling automatic  landing of vertical takeoff  aircraft.

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