Space Trajectories. Atmospheric reentry

Some space missions include an atmospheric re-entry phase. This is particularly the case for manned flights (return of astronauts from the space station), for exploration probes landing on the surface of stars with an atmosphere (Mars, Titan) or for the recovery of reusable launcher stages. Depending on the case, the trajectory can end at sea or on land, vertically or horizontally.

Re-entry is controlled so as to dissipate the initial kinetic energy, of the order of 25 to 30 MJ/kg in orbit, and land at near-zero speed. The use of propulsive means would lead to excessive propellant masses, incompatible with the size of an orbiting spacecraft. The most economical solution is to use aerodynamic braking and dissipate the energy in the form of heat absorbed by the thermal protection. The vehicle's aerodynamic capabilities enable it to control its descent speed so as to regulate heating and stress. The trajectory followed must be compatible with the vehicle's thermal and mechanical dimensions, and must arrive at the landing site with a low residual speed. These constraints are closely linked to the conditions at the start of re-entry, which must be precisely targeted.

This article presents the dynamic equations for re-entry in a rotating earth frame. Determination of the maneuver and orbital arc preceding re-entry, together with simplified analytical solutions, make it possible to evaluate vehicle performance and the thermal and mechanical loads experienced during re-entry.