Multiple entry trajectory scenarios for returning from the Moon: advantages and disadvantages

摘要

When a reentry module returns from the Moon, it enters the atmosphere with near-parabolic velocity, which results in considerable thermal and inertial loads not typical for descends from low-Earth orbits. The Earth atmosphere itselfprovides a natural method for decelerating an interplanetary module: after the first atmosphere transit the orbit is an ellipse, whose apogee rapidly decreases with successive entries in the atmosphere. In order to ensure the effective realization of trajectories with multiple atmosphere dives, it is required to have the concrete combination of descending vehicle's initial velocity, its stable attack angle and the height of target perigee. If this combination is wrong, one may obtain either direct falling trajectory to the Earth without the second revolution, or a very strong bounce off from the atmosphere. While using multiple atmosphere entry scenarios one should find a compromise between the desire to rapidly decelerate the object (this may lead to a ballistic descent) and the wish to obtain a trajectory with smallest overloads. The variety of results of this problem depends on the aerodynamic parameters of descending module. However, one should also take into account the thermal processes resulting from the interaction between the module surface and the atmosphere. So, in the ballistic capsule setting, the thermal loads with direct entry in the atmosphere exceed those for the entry from the second revolution with apogee 200 km, but they are not so lengthy. As a result, a two-entry scenario becomes more advantageous from the acceleration point of view, but on the opposite side under this approach the module requires stronger thermal protection. The report is concerned with possibilities of bouncing offfrom the atmosphere in case of ballistic descending capsules and in case of gliding vehicles, such as "Soyuz" and "Apollo"-type modules. Parameters of orbits with specific attack angles and target perigee altitudes are presented. Advantag