The HL-20 concept of the PLS is adaptable to several launch vehicle concepts. Titan III was an existing booster system which could have been used for uncrewed prototype launches or, with modification, as a crewed system. A future launch system option would have been the National Launch System under study by the Air Force and NASA in the 1990s. Choice of a launch system for the HL-20 PLS would depend both on the required date of initial PLS operations and the cost of booster development and launches.