The third release of interest is the QMS engine burn during the Skylab II mission. This engine produced exhaust at a temperature of 921 K with a density of 0.0028 kg/m^. The low density of the exhaust indicates that the degree of condensation will be even less than during the Apollo 8 (or the Skylab I) mission. Our calculations indicate that no more than 6% of the exhaust should condense. Our modeling of condensation assumes nonequilibrium, steady-state nucleation theory. As such, it tends to overestimate the amount of condensation. Time-dependent (nonsteady-state) homogeneous nucleation has been investigated by Draine and Salpeter (1975). We are attempting to use their theory for the rocket-plume and explosive-release condensation problem. Besides theoretical calculations and experiments in space, laboratory measurements of rocket exhaust condensation should be considered. Measurements of condensation in a vacuum chamber, however, may be prohibitive. Chamber measurements have been made of the composition of a rocket plume by McCay, Powell, and Busby (1970). Condensation measurements were not feasible because their chamber size was too small by at Least a factor of two. The condensation from the Saturn IVB was calculated to start at 41 to 55 meters from the nozzle (Wu, 1975). In summary, from the condensation calculations we conclude that nucleation is important for Lagopedo-type releases but may not be significant for rocket-plume releases. This is because of the low initial density of the vapor leaving the rocket nozzles. We are left inquiring: Does condensation seem to be necessary to explain the Skylab I ionospheric depletions (e.g., Zinn et al., 1979)? 2.6 SPREADING OF ROCKET EXHAUST CLOUDS: LOCAL, REGIONAL, ZONAL, AND GLOBAL EFFECTS (Bernhardt) Domain A lies generally below the ”turbopause” (which normally lies at 110 + 10 km) so that turbulent diffusion dominates over molecular diffusion. In this region the mean free path is one meter or less and the rocket exhaust relaxes to collisional equilibrium with the background atmosphere. The effective atmospheric diffusion coefficient may be as much as a factor of ten greater than the molecular diffusivity at the lower altitudes. Mesospheric winds may contain shears as large as 120 m/sec per km in altitude, which elongate and so disperse the exhaust cloud. Table 7 sketches cloud dispersion in the mesosphere, which indicates that 1-10 days after release the cloud from a point injection has spread to regional dimensions (300-3000 km). Since HLLV launches average approximately one per day, one would expect to find a zonal band of 1000-3000 km width (south to north) spread around the globe near the latitude of launch (28°N). The various chemical and photochemical reactions have different time constants, so that it is possible that rather different phenomena may be observed on a local scale, as in a point release or rocket plume, than on a regional, zonal, or global scale. The numerical values of cloud width shown in Table 7 are schematic only, being based on the very limited data set shown in Fig. 3. At higher
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