NASA Principal Technologist lays out strategy to develop
advanced space propulsion over the next 25 years:
"For optimal outcome, it is recommended that the future development
of advanced propulsion capability proceed along parallel technology
development pathways, one near-term path for proven but immature
technologies (TRL ≥ 3) based on proven processes and a second
far-term path for unproven but high capability concepts (TRL < 3) that
could completely revolutionize mission possibilities.
The near-term pathway would ideally have a four-prong emphasis:
(1) advanced chemical propulsion,
(2) electric/plasma propulsion,
(3) nuclear thermal propulsion, and
(4) hybrid nuclear thermal/electric propulsion."
See:
http://nextbigfuture.com/2016/03/nas...-lays-out.html