"Not much, sir. Melted ice could be collected near the catapult head and thus be the most massy part of what will be shipped to Luna—quite a saving. Also the steel canisters would be re-used to ship grain to Earth, thus stopping another drain that Luna can’t take. No reason why a canister should not make the trip hundreds of times. At Luna it would be much the way barges are now landed off Bombay, solid-charge retrorockets programmed by ground control—except that it would be much cheaper, two and a half kilometer-seconds change of motion versus eleven-plus, a squared factor of about twenty—but actually even more favorable, as retros are parasitic weight and the payload improves accordingly. There is even a way to improve that."