water and carbon dioxide, two substances which already exist in abundance in the atmosphere, together with much smaller amounts of other materials, including nitric oxide (NO). Second, the emissions are injected directly into the stratosphere and mesosphere simultaneously at all altitudes below approximately 120 km. Primarily because of these differences, it is not possible at this time to make reliable, quantitative estimates of the effect of SPS- related rocket emissions on the chemical composition of the stratosphere and mesosphere. While the frontiers of scientific knowledge concerning the lower stratosphere have been greatly expanded in recent years, as documented in Refs. 3.17 through 3.30, the state of knowledge concerning the upper stratosphere and mesosphere is not as advanced. An initial evaluation of the potential for causing significant perturbations in stratospheric composition may be made by examining the relative magnitudes of the injection rates of the various exhaust components compared to rough estimates of ’’turnover rates” of the same materials at the altitudes of interest. The ’’turnover rate” is defined here as the total mass of the given substance within a one-kilometer deep layer, centered on the altitude of interest, divided by the residence time of that substance at the given altitude. The ratio of the emission or injection rate per kilometer to the turnover rate is a measure of the potential significance of the perturbation and will be termed the ’’perturbation ratio.” In order to compute perturbation ratios for the various exhaust components, estimates of their emission rates at various altitudes must be available. Table 3.1 shows the distribution of exhaust products for two alternate HLLV designs considered by the Boeing Aerospace Co. The numbers in the table represent emission rates at the mouth of the rocket motor and do not include the effects of afterburning in the rocket exhaust plume. It is assumed here that afterburning converts all carbon monoxide (CO) to carbon dioxide (CO2) and all hydrogen (H2) to water (H2O). For purposes of estimating perturbation ratios, the two-staged winged design is used since the emissions are somewhat higher than for the two-staged ballistic vehicle. The former is also preferred by NASA for engineering reasons.
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