2.5.3 Experimental Studies, Mainly in Domain B (Pongratz) Our current understanding of the phenomenology of the water in rocket exhausts indicates that a fraction ranging between 0.2 and 1.0 condenses and freezes into ice crystals. The characteristic ice crystal dimensions are generally considered to be in the range of 0.01-0.3 pm. Indeed, the size distribution may be dominated by large numbers of very small particles. While a reasonable consensus exists regarding condensation, the evaporation picture changes "by the hour." Early calculations by Bernhardt indicated that in the absence of sunlight only a small amount of sublimation occurs. Sublimation in sunlight depends on whether or not the ice crystals can absorb sunlight. Bernhard (1976), Zinn et al., (1979), and Wolfhard (1969) have made calculations assuming that sunlight is absorbed, which give sublimation time constraints of about minutes. The LAGOPEDO UNO experiment probably represents the only quantitative unclassified data on sublimation. Two instruments measured the spectrum of the reflected sunlight, which can be used to determine a characteristic size for the ice crystals. Two other cameras recorded images of the ice cloud. One imaging camera used EKIR film (3 layers, 500-900 nm) and the other camera was electrostatically intensified and filtered at 455 nm. These imaging data can give ice crystal inventories as a function of time. Preliminary data indicate that a large fraction of the HoO froze and gave a sublimation time constant of about 10 seconds. Experiments should be conducted to verify codes used to predict the fraction of the water vapor that condenses, based upon its specific enthalpy. Experiments are preferred that closely match the temperature, pressure, and density values anticipated for SPS rockets; however, a verification of the condensation codes using initial values of temperature, pressure, and density is a necessary minimum first step. Because of the disparate ideas regarding sublimation, experimental input is vital. Because of the rapid expansion of the ice cloud in Domain B, optical techniques using scattered sunlight can probably detect sublimation if the relevant time constant does not exceed 500 seconds; otherwise the cloud will become tenuous and the signal will fall below background before significant sublimation can be observed. Lest it be omitted elsewhere, the issue of heterogeneous chemistry involving small ice crystals with a relatively large surface area should be raised and that of ions and electrons attaching themselves to these crystals (see Castleman, 1979). Such reactions would produce significantly different neutral chemistry and airglow compared to the charge exchange and dissociative recombination reactions. Unfortunately, the upcoming Atlas-Centaur launch will not be useful for studying condensation and sublimation in Domain B because of the local time of the launch. The launch occurs several hours before sunrise and the ice crystals will disperse and fall long before sunlight can hit them. The Canadian National Research Council (Dr. Brian Whalen) and Los Alamos Scientific Labora- ory (Dr. Gordon Smith) are proposing a LAGOPEDO-type release called Project
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