1.2 CHEMICAL STAGE 1.2.1 PARAMETRIC DATA The low-thrust chemical stage uses O2/H2 propellants. The propellant selection was based on considerations of cost, availability, technology base, and environmental effects of exhaust gases. Stage weight trends for the chemical stage were developed using a computerized vehicle synthesis program known as SWOP (Stage Weight Optimization Program). The model used was one developed for a LO2/LH2 space tug several years ago during the course of the JSC in-house Space Tug Study. The results from this model are shown in figure VI-D-1-2 as stage mass fraction (impulse propellant/stage weight) versus impulse propellant mass. Also indicated on figure VI-D-1-2 are data points representing point designs of LO2/LH2 COTV's done by the Boeing Company for the FSTSA study. All these data points are for a system with a relatively high T/W in the range of 0.2 and a mission duration of approximately 5 days. Some cursory hand calculations indicated that the reduced T/W requirement would reduce main propulsion system weight so that the stage mass fraction would be .01-.02 higher; however, the longer mission duration will require additional insulation and power, and will result in increased boiloff losses so that, as an initial approximation, the same curve was used for sizing studies for both the high and low thrust chemical stages. Performance values for the O2/H2 propellant combination are well-known and documented; using an expander cycle engine of 25,000 Ibf thrust at a chamber pressure of 600 psia and an area ratio of 400:1 , the expected nominal Isp is 460 seconds. Figure VI-D-1-3 illustrates the payload capability of a chemical stage operating in an expendable mode. The ratio of propellant weight to payload weight (WP/WPL) is shown as a function of the stage mass fraction. The two curves shown for a T/W of 10“J are applicable to this vehicle. The shaded band of each curve illustrates the effect of an Isp variation from 450 to 470 seconds. The AV of 19,000 ft/sec corresponds to a KSC launch, while the 15,000 ft/sec aV is the orbit transfer requirement if an equatorial launch site is used. From figure VI-D-1-3, it is seen that, at a mass fraction of .93 and an Isp of 460, launching from KSC results in a ratio of propellant weight to payload weight of 3.3; from an equatorial launch site, the value of WP/WPL is approximately 2. Thus, the choice of launch site is seen to be extremely important for a low Isp COTV such as the O2/H2 stage. The effects of staging are illustrated by figure VI-D-1-4. The upper and lower sets of curves are for the KSC and equatorial launch sites respectively, as discussed above. The "single stage" curves are reproduced from figure VI-D-1-3 for comparison. The two-stage curves illustrate the performance of two equally-sized stages. With this ground rule, the AV of first stage is approximately 6500 ft/sec for the KSC launch case and 5500 ft/sec for the equatorial launch case.
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