In effect, as the rocket fuel was exhausted, the tubes that contained it dissolved into their own blast and added to the accelerating thrust, even as they diminished the amount of mass to be accelerated. Then the quantity of fuel burned could diminish--the tubes could grow smaller--so the rate of speed gain would remain constant. Under the highly special conditions of this particular occasion, there was a notable gain in efficiency over a liquid-fuel rocket design. For one item, the Platform would certainly have no use for fuel pumps and fuel tanks once it was in its orbit. In this way, it wouldn't have them. Their equivalent in mass would have been used to gain velocity. And when the Platform finally rode in space, it would have expended every ounce of the driving apparatus used to get it there. Now the rocket tubes were being lined and loaded. The time to take-off was growing short indeed. Joe watched a while and turned away. He felt very good because he'd finished his job and lived up to the responsibility he'd had. But he felt very bad because he'd had an outside chance to be one of the first men ever to make a real space journey--and now it was gone. He couldn't resent the decision against him. If it had been put up to him, he'd probably have made the same hard decision himself. But it hurt to have had even a crazy hope taken away. Sally said, trying hard to interest him, "These rockets hold an awful lot of fuel, Joe! And it's better than scientists thought a chemical fuel could ever be!" "Yes," said Joe. "Fluorine-beryllium," said Sally urgently. "It fits in with the pushpots' having pressurized cockpits. Rockets like that couldn't be used on the ground! The fumes would be poisonous!" But Joe only nodded in agreement. He was apathetic. He was uninterested. He was still thinking of that lost trip in space. He realized that Sally was watching his face. "Joe," she said unhappily, "I wish you wouldn't look like that!" "I'm all right," he told her. "You act as if you didn't care about anything," she protested, "and you do!" "I'm all right," he repeated. "I'd like to go outside somewhere," she said abruptly, "but after what happened up at the lake, I mustn't. Would you like to go up to the top of the Shed?" "If you want to," he agreed without enthusiasm. He followed when she went to a doorway--with a security guard beside it--in the sidewall. She flashed her pass and the guard let them through. They began to walk up an inclined, endless, curving ramp. It was between the inner and outer skins of the Shed. There had to be two skins because the Shed was too big to be ventilated properly, and the hot desert sunshine on one side would have made "weather" inside. There'd have been a convection-current motion of the air in the enclosed space, and minor whirlwinds, and there could even be miniature thunderclouds and lightning. Joe remembered reading that such things had happened in a shed built for Zeppelins before he was born.