In a similar manner the external hydrogen tank drag is based on the shuttle external tank shape scaled to contain the required volume of liquid hydrogen. The reference area for the external tank is 55.56 square meters (598 square feet) and the reference volume is 2,220.9 cubic meters (78,431 cubic feet). The power on base force was calculated from the power on base pressure using the total area of the base of the lifting body minus the area of the engine bells. As engines were turned off, the force was reduced by the percentage of engines not operating. The power on based pressure data given in table VI-B-2-4 were obtained from the shuttle phase B study reports. An analysis was made to determine the best expansion ratio for the SSME. Simulations were run using 15 SSME's for expansion ratios of 77.5, 60, 50, and 40. Payload as a function of expansion ratio is shown in figure VI-B-2-3. It is observed that the lower expansion ratio results in the largest payload. The increased thrust at sea level from the lower expansion ratio results in more propellant being lifted off the launch pad. The added propellant increases the payload more than the loss in specific impulse as the lower expansion ratio decreases the payload. This is an important result since the size of the lifting body is limited by the number of engines that can be fitted on the base, the volume of the lifting body increases as the cube of the scale, whereas, the base area increases only as the square and therefore, for each type engine, there is a maximum vehicle size. The smaller expansion ratios result in a higher thrust density and a larger maximum size. Payload as a function of the number of main engines for 40.1 expansion ratio is shown in figure VI-B-2-4 for both the SSME and the uprated SSME. As should be expected, the payload is linearly related to the number of main propulsion engines. III. VEHICLE SIZE The baseline heavy lift launch vehicle was sized to carry the modified orbiter glider with 45,000 Kg (100,000 pounds) payload in the payload bay. The gross weight of the modified orbiter from table VI-B-2-8 is 129,000 Kg (283,800 pounds). From figure VI-B-2-4 it is seen that the heavy lift launch vehicle required 19 standard SSME's or 15 uprated SSME's to lift the modified orbiter into orbit. Since it may not be possible to mount 19 standard engines on the base, the baseline launch vehicle has 15 uprated SSME's and can carry a 324,000 pound payload into 100 n.mi. circular orbit. A typical launch trajectory for the 15 engine vehicle is shown in table VI-B-2-5. A weight summary for the baseline vehicle is given at the bottom of table VI-B-2-5.
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