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Several folks have commented on kick stages and tugs for BFS cargo. Some stuff from reddit and NSF : https://www.reddit.com/r/SpaceXLounge/comments/7553za/wildass_speculation_thread_20_3b_whoops_i_dun/ https://www.reddit.com/r/spacex/comments/9hys4y/bfr_gto_trajectory_ideas_with_falcon_9like_kick/ https://forum.nasaspaceflight.com/index.php?topic=46412.0 I am interested in reusable methalox kickstage designs that can deliver a 7 tonne sat out to GTO (2500m/s), then 16 hrs later at perigee burn back into LEO (another 2500m/s sans payload) for a later pickup and return to surface. cluster while waiting for pickup in LEO can be carried in a carousel on bfs cargo Why return to surface? Refuel in orbit might be doable...but payload transfer from BFS to kick would be tricky. Would need an extra berthing system. Keep it simple. Just do integration and fueling on the ground and wear the price of lifting 5t extra dry-mass on your 120t lifter. For example 12t of sub-cooled methalox (tanks are ~2m long by 3.0m diameter or another combination with gives ~14 cubic meters of tank space), 1 tonne dry mass (~7.7%), 10t raptor (100kN, sea-level for simplicity - 356s in vacuum, mass ~50kg) Solar / batteries / apu for 16hrs of operation and maybe a few weeks of dormant operation while waiting for pickup. Boil-off management up-to 90 minutes for GTO burn fuel(~11t) and ~16hrs for LEO return fuel(~1.1t), maybe 150hrs for clustering fuel. various attitude and ullage thrusters payload up to 7t Total package ~20t. 2018 BFS cargo (120t LEO) could lift a carousel of 5 or 6 of these. Dump them in LEO, let them GTO burn when they are in position, pickup a cluster of spent kicks from the last mission, and return home for dinner. Ideas/Comments?
