Present launch plans, designed in part to minimize F-layer depletion effects, call for ignition of the second-stage engines at 75-km altitude and engine shutdown with orbital insertion at 108 km, after descent from an intermediate apogee at 123 km. This launch trajectory (5) leads to an eccentric initial orbit with an apogee at about 480 km. Upon arrival at apogee, the vehicle executes a circularization maneuver, involving the burning of 211 of propellant and emission of 9x 1029 exhaust molecules. Upon completion of its mission in LEO, the HLLV returns to earth. The deorbit maneuver requires 11 t of propellant and emission of 5x 1029 exhaust molecules. It is interesting, if not altogether relevant, to note that the number of exhaust molecules from a single HLLV, about 1032, is similar to the total number of electronion pairs in the global ionosphere, again about 1032. Since each H2O molecule is capable, under proper circumstances, of destroying two ion pairs, the effect of one HLLV is potentially large. Fortunately, those circumstances are seldom met. This paper examines the conditions that determine the “chemical efficiencies” of exhaust molecules for destroying F-layer ions. It describes two-dimensional computer model studies of an HLLV launch and a circularization maneuver, and a model simulation of the Skylab I launch (see also Zinn et al. (6)). 2. BACKGROUND Observations of the creation of a large ionospheric hole by the launch of Skylab I (Saturn V rocket launch, 1230 EST, 14 May, 1973) were reported by Mendillo et al. (1,2). The ionospheric electron column density was observed to be reduced by 50% or more over a period commencing within 10 min after the launch and persisting for about 4 h. The depletion extended over a region of approximately 1000-km radius. The observations were made in the course of routine Faraday-rotation measurements of the VHF signals from geostationary satellites ATS-3 and ATS-5. The ionospheric depletion is attributed to injection of rocket exhaust products, primarily H2O and H2, which lead to enhancement of the effective electron-ion recombination rate through the substitution of polyatomic ions H2O+, H3O+, and OH+, etc., in place of the normally occurring O+. The effect is confined to the F-layer above 200-km altitude where O+ is the dominant positive ion. The main reactions are
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