9. 8 The overall system Tis dependent on the oxidizer-fuel mixture ratio n,as expressed by: T = For the space shuttle main engine, n is equal to 6 and Tis equal to .0292. The tank commonality mass, MT, represents the basic installation mass of a propellant tank. It is equal to 370 kg per liquid hydrogen tank, and 320 kilograms per liquid oxygen tank (9.5). Fixed structural mass MF consists of the sum of the component masses. The primary propulsion system of the OTV is one space shuttle main engine, with a mass of 3000 kg (9.3). Thrust structure weight is .25% of engine thrust (9.5), which for the SSME is 2.09 million newtons. This results in a thrust structure mass of 500 kg. The reaction control system mass is assumed to be 500 kg, and the avionics and miscellaneous fixed equipment mass is estimated at 1000 kg. The total fixed structural mass MF is the sum of these factors, or 5 metric tons. The only remaining sizing parameter is the payload m~ss. For simplicity, an integer multiple of the payload mass of a launch vehicle is assumed,since payload can then be directly transshipped from the shuttle/DOL to the OTV. A tradeoff exists between number of orbital transfer vehicles, which increases with smaller payloads, and vehicle size, which increases with larger payloads. It is desirable to maintain thrust to weight ratios of the order of on,, in order to avoid the need for low-thrust corrections to the transfer 6V calculations. For this reason, the payload is set at four space shuttle payloads, which corresponds to a payload mass for the OTV of 110 tons. With the design factors now known, it is possible to solve the derived equation by matrix techniques for the OTV propellant and tank masses. Since the number of fuel and oxidizer tanks are not yet known, the first trial is made with a value of MT equal to 3000 kg. Since the principal mission of the OTV is to transport payloads
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