This is one of the variants of the interplanetary tug - with 4 nuclear reactors 15 MW each, using Brayton cycle, liquid droplet radiators and Xenon fueled Ion Engines. Total mass of the tug - 700 mT, fuel mass - 580 mT.
More renders of this variant, and little info about other dicussed variants here:
http://nickd.freehostia.com/OrbiterVault/mars.html
More renders of this variant, and little info about other dicussed variants here:
http://nickd.freehostia.com/OrbiterVault/mars.html