The top five or six levels of scaffolding had already been removed, and more of the girders and pipes were coming down in bundles on lines from giraffelike cranes. There were some new-type trucks in view, too, giants of the kind that carry ready-mixed concrete through city streets. They were pouring a doughy white paste into huge buckets that carried it aloft, where it vanished into the mouths of tubes that seemed to replace the scaffolding along the Platform's sides. "Lining the rockets," said Sally in a subdued voice. Joe watched. He knew about this, too. It had been controversial for a time. After the pushpots and their jatos had served as the first two stages of a multiple-rocket aggregation, the Platform carried rocket fuel as the third stage. But the Platform was a highly special ballistic problem. It would take off almost horizontally--a great advantage in fueling matters. This was practical simply because the Platform could be lifted far beyond effective air resistance, and already have considerable speed before its own rockets flared. Moreover, it was not a space ship in the sense of needing rockets for landing purposes. It wouldn't land. Not ever. And again there was the fact that men would be riding in it. That ruled out the use of eight- and ten- and fifteen-gravity acceleration. It had to make use of a long period of relatively slow acceleration rather than a brief terrific surge of power. So its very special rockets had been designed as the answer. They were solid-fuel rockets, though solid fuels had been long abandoned for long-range missiles. But they were entirely unlike other solid-fuel drives. The pasty white compound being hauled aloft was a self-setting refractory compound with which the rocket tubes would be lined, with the solid fuel filling the center. The tubes themselves were thin steel--absurdly thin--but wound with wire under tension to provide strength against bursting, like old-fashioned rifle cannon. When the fuel was fired, it would be at the muzzle end of the rocket tube, and the fuel would burn forward at so many inches per second. The refractory lining would resist the rocket blast for a certain time and then crumble away. Crumbling, the refractory particles would be hurled astern and so serve as reaction mass. When the steel outer tubes were exposed, they would melt and be additional reaction mass.