Modeling of the Lightning Plasma Channel Stroke to a Spacecraft during Ascent

Lightning protection is an important aspect of modern aerospace design: the increased use of composite materials (\textit{vs.} metals) reduces the shielding and robustness of the conducting path that the outer shell of a vehicle can present to a lightning discharge. A spacecraft during ascent becomes vulnerable to lightning strokes immediately after leaving the launch pad: in addition to natural lightning conditions, there is the possibility of triggered lightning events, caused by a perturbation of the atmospheric electric field [1]. The purpose of this study, in support of the NASA Constellation program, is to determine the evolution of the plasma current and its distribution on the spacecraft surface. Following earlier ``gas dynamic'' approaches [2], the model considers a plasma channel attached to the ascending spacecraft after a return stroke is established. The conductive exhaust plume [3] is an integral part of the model. The \textit{NIMROD} 3D plasma fluid code [4] is used to model the plasma channel, reproducing the full transient due to the self-consistent magnetic field and the possibility of sweeping of the attachment point along the moving structure [5]. \textbf{References}: [1] M.A. Uman, Proc. IEEE, 76, 1548 (1988). [2] V. A. Rakov, M. A. Uman, IEEE Trans. EMC, EMC-2940, 403 (1998). [3] J. D. Norgard, G.S. Smith, IEEE Trans. EMC, EMC-29, 157 (1987) [4] C. R. Sovinec \textit{et al.}, J. Comput. Phys. 195, 355 (2004). [5] Larsson \textit{et al}., J. Phys. D, 33, 1876 (2000)