Jeffrey, your idea falls into the general category of momentum energy banks. Usually, concepts are based on tethers:

http://en.wikipedia.org/wiki/Space_tether#Rotovators

Conceivably, you could have spinning tethers in orbit around various planets, essentially "throwing" spacecraft back and forth. Donald Kingsburry wrote a story ("The Moon Goddess and the Son") with an orbital coilgun design, though.

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