Modeling of spacecraft explosions during re-entry

Spacecraft and rocket bodies in low altitude orbits re-enter the Earth's atmosphere some time after the end of their mission. During re-entry they are exposed to high thermal and mechanical loads, which result finally in the disintegration of the body and the generation of a number of fragments. There exist a number of numerical tools to calculate the fragmentation process in more or less detail. A break-up mode considered by none of these tools yet is the explosion of on-board tanks. While not probable for the majority of re-entries it has to be considered for special cases where the on-board fuel tanks may not be fully vented due to a contingency situation. There exist models, derived from observations and ground tests, for explosions happening in orbit. They have not yet been applied to explosions during re-entry. For in-orbit explosions the evolution with time of the debris cloud and its resulting contribution to the orbital debris environment is of main interest. For re-entry calculations the resulting ground risk is the major concern. The present paper will discuss the ideas how to combine deterministic re-entry software with probabilistic fragmentation models. The latter models give correlations between the fragments' area-to-mass ratio, size, and velocity. They give no information about the shape and material of the fragments. For a deterministic re-entry analysis such information is needed. In addition, for an explosion during re-entry the explosion event itself has to be modeled. Therefore the possible reasons, the likelihood and the consequences of an explosion are discussed in detail. For massive spacecraft with small tanks (explosion sources) a complete disintegyation into a fragment cloud appears unlikely, meaning that a large part of the original spacecraft may keep its integrity. In this case it seems appropriate to split the explosion fragments into a fragment cloud with statistical properties and some big debris parts with well-defined geometry.