After a spaceflight, the planetary entry (a.k.a. reentry for entry into Earth's atmosphere) of a spacecraft is subject to the same aerodynamic and physical laws and equations (see Eqs. (6.3.6) and (6.3.7)) as ascent. One might therefore infer that the circumstances of both situations are the same. But they actually differ vastly due to the initial and boundary conditions: At launch we have h ¼ 0; v ¼ 0 at flight path angle c ¼ 90 and full thrust during ascent, while at reentry it is exactly the other way round, h % 350 km; v % 7:9 km/s; c % 0 ; and no thrust. Owing to these converse initial conditions the S/C in LEO prior to reentry possesses a high amount of potential and kinetic energy of approximately 33 MJ kg À1. This energy has to be annihilated during reentry in a controlled way and in a relatively short period of time, while the structural load on the vehicle needs to be kept within limits. In face of this problem there are four critical parameters to be considered when designing a vehicle for atmospheric reentry to avoid damage to the S/C and the crew: • Peak heat flux • Heat load • Peak deceleration • Peak dynamic pressure Peak heat flux (heat per unit area and unit time = heat flow density) selects the thermal protection material, while heat load selects the thickness of the protection material stack. Peak deceleration is of major importance for the crew and should not exceed 8 g. Dynamic pressure causes aerodynamic stress load to the vehicle and is significant in particular for winged bodies: The Shuttle was designed for 2.5 g load while Apollo for a 12 g load. In total, these constraints impose boundaries on the reentry trajectory, which are depictured for a Shuttle reentry in Fig. 10.1.
Walter, U. (2018). Planetary Entry. In Astronautics (pp. 427–492). Springer International Publishing. https://doi.org/10.1007/978-3-319-74373-8_10