Of course, peak thrusts might be 10 times the average value. If structural strength of materials degrade in space during a 30-year life time, tensile strength might be reduced by a factor of 10. Still, a structural mass budget of 1000 kg is conservative by a factor of more than an order of magnitude. Finally, what is the magnitude of thruster linear accelerations? Assuming a 1 N force on a 600,000 kg mass, the peak linear acceleration is of the order 2 x 10’6 m/sec2. This is equivalent to about 2 X 10'7 g, where "g" is one Earth surface gravity. 8.5 Calculation of Required Gas Masses for Inflatable Sphere SPS’s We begin by first estimating the amount of gas required to be within an inflatable sphere SPS at any time, and then estimate the amount of gas necessary to counteract leakage through the SPS walls. Gas mass within SPS: We begin with the Ideal Gas Law24: The minimum gas pressure within the sphere that is required to maintain inflation is here defined as twice the amount required to compensate for the algebraic sum of maximum gravity gradient and radiation pressure stresses. We estimate the absolute value of the gravity gradient acceleration, following the formalism in Section 8.1, as 2.5 x 10’5 m/sec2. Multiplying by the tabulated values for mass for both the terrestrial material and lunar material inflatable sphere SPS, we find that the gravity gradient forces are 4.3 N for the terrestrial materials case and 16.1 N for the lunar materials case. The temperature T is taken to be 245 K and V is calculated, for the sphere radius in the above table:
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