Musk hasn't discussed the exact details of his plan, but it seems like the unmanned missions in 2018, 2020, 2022, and possibly 2024 (if there's no manned missio

Musk hasn't discussed the exact details of his plan, but it seems like the unmanned missions in 2018, 2020, 2022, and possibly 2024 (if there's no manned mission that year) are meant to establish a rudimentary fuel production infrastructure and start producing fuel in anticipation of a manned mission. NASA, I believe, is currently trying to solve the problem with more efficient engines for interplanetary travel. Ion drives seem to be the preferred method, but there is still some research going on in nuclear thermal rockets.