"Doctor, this is outside my specialty. But everybody knows that your best ships use hydrogen as reaction mass heated by a fusion reactor. But hydrogen is expensive in Luna and any mass could be reaction mass; it just would not be as efficient. Can you visualize an enormous, brute-force space tug designed to fit Lunar conditions? It would use raw rock, vaporized, as reaction mass and would be designed to go up into parking orbit, pick up those shipments from Terra, bring them down to Luna’s surface. It would be ugly, all the fancies stripped away—might not be manned even by a Cyborg. It can be piloted from the ground, by computer."