Nerva sounds very interesting. Hmm, I wonder if spacex would have use for a Nerva LEO to anywhere tug.... So launch a geosynchronous payload with a minimal upper stage to get to LEO, get a tug to dock with the payload and boost it to wherever.... Every so often an f9 or FH would put up a fuel tanker into orbit to refuel the tug... Are there any other options to nuclear propulsion for the tug? Solar? Lasers?
For longer distances like earth to mars I don't think it really makes sense. The tug will be out orbiting the sun for quite a while before it is useful again. So you might as well leave it attached to the cargo and use it for a return journey. But then isn't really a tug any more.
Maybe could be useful for moving things between LEO and Lagrange. Then the tug could be used again quickly.
