> Sails don't do that. The propulsive force is not as controllable as it is for a low-thrust rocket, true. But Heinlein was not claiming an exact comparison.

> Sails don't do that. The propulsive force is not as controllable as it is for a low-thrust rocket, true. But Heinlein was not claiming an exact comparison. > they're exactly the same as pole-less rafts. No, they're not. Pole-less rafts don't have sails. > Ever heard of being becalmed? Ever hear of missing the point? Yes, sailing ships can be becalmed. But they can also not be becalmed. And sailing ships that were not becalmed created a worldwide system of trade. Pole-less rafts would not have been capable of doing that.