Computational analysis of automated transfer vehicle reentry flow and explosion assessment

At the end of its mission to the International Space Station, during its reentry into Earth atmosphere, the automated transfer vehicle is subject to high heat fluxes leading to structural heating and fragmentation of the vehicle. It has been concluded that, depending on the mode of release, onboard residual hypergolic propellants may ignite and explode upon exposure to the hot and reactive flow environment. Because an earlier explosion of the vehicle would change drastically the impact footprint of its fragments onto the Earth surface, this study proposes a reassessment of the explosion potential. From the trajectory analysis, several points of the reentry path have been computed using a Navier-Stokes solver accounting for nonequilibrium effects. Numerical simulations have been performed with and without perforation of the structure. In parallel, a comprehensive literature survey on ignition of monomethyl hydrazine and dimethyl hydrazine vapors with pure air or air mixed with nitrogen tetroxide has been performed to assess the autoignition potential of the mixture. Finally, the results of the computational fluid dynamics computations have been used to estimate the explosion risk in the presence of a propellant leakage. Analysis confirms the risk of a destruction of the automated transfer vehicle at higher altitude, which could induce a different footprint of the fragments on the ground.