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providing the propellant isolation valves DO NOT leak. (If there is a leak, the lunar spacecraft will probably be totally destroyed by a violent explosion. With the demise of clean machine shops with dust and oils contamination controls that existed for the Mercury, Gemini, and Apollo programs, the potential for contaminated surfaces and leaking hypergolic isolation valves remains a concern today). The 112.3-km lunar orbit has a 2-hour period and makes a good lunar holding orbit if a rendezvous in lunar orbit is required. The mass ratio to descend to the surface, with some margin, is about two. A mass ratio of 3.5 is sufficient for the escape maneuver. The spacecraft essentially falls toward Earth once it clears the lunar sphere of influence. As the spacecraft approaches Earth it can be traveling at a speed greater than the lunar injection speed and greater than escape speed, so it is necessary to have braking rocket propulsion or aerodynamic breaking in the upper atmosphere to slow the spacecraft speed so it can be captured in an Earth orbit. In the case of a braking rocket, the returning spacecraft must have available a mass ratio similar to that in Table 6.2. In the case of a spacecraft braking aerodynamically in the upper atmosphere, the attitude is one for maximum drag; and if a lifting body configuration, it may roll upside-down and lift-down to increase the energy dissipated and decrease the heating intensity, as the heating pulse is spread out over a longer time in the upper atmosphere. The actual mission mass ratio will depend on trajectory and configuration specifics, but these tables give the reader an estimate of the propulsion and propellant requirements. From a LEO the round trip to the Moon can require less mass ratio than an out and back mission to GSO.

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