water vapor mixing ratios and/or temperatures that exceed anything we have seen since observations of stratospheric water vapor and temperature were begun. Stratospheric cycles of such long periods must also have an impact on the mesosphere. 2.5 CONDENSATION AND RE-EVAPORATION IN ROCKET EXHAUSTS 2.5.1 Prefatory Comments (Bauer) The problem of condensation and re-evaporation of water vapor is significant for rocket exhausts in all altitude ranges considered here, and for a variety of applications. Two subpanels, chaired respectively by M. Mendillo and M. Pongratz (see items 2.5 and 3.4 in Table 5) addressed the problem at the workshop, and P. Bernhardt wrote a discussion afterwards. These three contributions are all presented here. There is, of course, some redundancy, but it seems preferable to present the whole of the discussion in view of the importance of the problem. Mendillo reviews the problem as a whole, Pongratz makes specific suggestions for experiments, and Bernhardt summarizes the problem as it appeared after the workshop (he participated in both Mendillo's and Pongratz' subpanels). 2.5.2 The Overall Problem (Mendillo) A. Introduction The exhaust gases from a rocket cool adiabatically on expanding through the nozzle and into the low-density ambient environment. At high altitudes, temperatures below the saturated vapor temperature of ^0 are reached and thus condensation and the formation of ice crystals are expected to occur. Ample evidence exists from laboratory experiment to show that under a wide variety of conditions condensation occurs rapidly once supersaturation is reached. However, the condensation mechanism is not always well defined or understood. Thus Wegener (H. G. Wolfhard, private communication) demonstrated that the concept of homogeneous condensation did not explain the Apollo-8 lunar injection effects, but Castleman (private communication, June 1979) points out that hydrated protons lead to the formation of clathrate structures involving some 20 H2O molecules that form nuclei for condensation. The mass fraction of the condensed phase is not well known. However, laboratory experiments indicate that under many conditions at least half the water condenses. Since condensation is enhanced by longer residence time in the rocket nozzle, there is a tendency for more condensation to occur as the size of the rocket engine increases. Condensation in the exhaust of a hydrogen-oxygen rocket was observed in the Apollo-8 lunar orbit injection burn. Molander and Wolfhard (1969) analyzed the observations made by Smithsonian Astrophysical Observatory personnel at Mt. Haleakala of a "cloud as bright as the moon" photographed during the
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