For this mission, a trip time of 200 days was chosen. This requires just 70 m/s more delta-v, and its sufficiently short that the worst of the long-term effects of exposure to microgravity will be minimized. (Note: Actual mission delta-v's can vary by as much as 20%, depending on trajectory used, optimality of planetary positions, etc. The numbers used in this paper are average values). The second stage must provide 1400 m/s worth of delta-v. Of this, 540 m/s is for the HEEO—>MTO burn, 200 m/s is allowed for course corrections during the trip, and 660 m/s to match orbits with Deimos after aerobraking. Once matched with Deimos the ship uses small thrusters to land/dock. Then operations begin. Vehicle Design The first choice to be made here is what type of propulsion system the ship will use. Because of the desire to limit ourselves to off the shelf technology, chemical propulsion is the only choice. LOx/hydrogen is the most obvious, due to its high specific impulse. LOx/hydrogen engines with specific impulses greater than 480 seconds have been built and tested in the lab. The best engine currently in use is the SSME (Space Shuttle Main Engine), with a specific impulse of 460 seconds. However, neither of these are available on the open market, and both would be very expensive if they were. The Pratt & Whitney RL-10 engine, used on the Centaur upper stage, is commercially available, and has a specific impulse of 446.4 seconds [6]. Seventeen engines will be used. (The number of engines used is a function of the thrust required to minimize gravity losses.)
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