Monte Carlo scattering of sunlight by high altitude rocket plumes

A comprehensive model for the scattering of sunlight by high altitude rocket plumes is presented. The model consists of a Monte Carlo simulation of the multiple absorption-emission of solar photons by plume molecules. The physical and optical properties of high altitude plumes are discussed. It is shown that the expansion of rocket exhaust gases into a vacuum produces a highly nonequilibrium distribution of quantum state populations. As an example, the terminal translational and rotational temperatures for CO₂, for a 1 kN thrust engine, are predicted to be 10 and 15 K, respectively. Because these very low rotational temperatures result in a molecular absorption spectrum which consists of only several rotational lines, the optical opacity of both i.r. and u.v. transitions can be of order unity at large distances from the nozzle exit. For a 1 kN thrust engine, the centerline optical diameter equals unity at axial distances of approx. 10 m for CO₂ (4.3 μm) and 100m for CO (2,0) A → X. Detailed Monte Carlo calculations are presented for i.r. CO₂(4.3 μm), H₂O (2.7 μm), and CO (4.6 μm)] and u.v. H Lyα and CO A → X, B → X, C → X] transitions which demonstrate the dependence of scattered solar power on engine thrust, plume length and solar illumination angle. In general, plumes characterized by thrusts greater than 10 kN and lengths less than 0.1 km are optically opaque for i.r. transitions. However, u.v. transitions for H Lyα are quite optically thick even for a thrust as small as 1 kN and a plume length of 1 km. The u.v. transitions for CO can be optically opaque for 1 kN thrust and a plume length less than 1 km.