Rioz took a hesitant step forward. Space was clear, so to hell with sitting and looking at a blank, green, pipless line. He said, “What’s the Grounder been talking about?”
“History of space travel mostly. Old stuff, but he’s doing it well. He’s giving the whole works—color cartoons, trick photography, stills from old films, everything.”
As if to illustrate Long’s remarks, the bearded figure faded out of view, and a cross-sectional view of a spaceship flitted onto the screen. Hilder’s voice continued, pointing out features of interest that appeared in schematic color. The communications system of the ship outlined itself in red as he talked about it, the storerooms, the proton micropile drive, the cybernetic circuits…
Then Hilder was back on the screen. “But this is only the travel-head of the ship. What moves it? What gets it off the Earth?”
Everyone knew what moved a spaceship, but Hilder’s voice was like a drug. He made spaceship propulsion sound like the secret of the ages, like an ultimate revelation. Even Rioz felt a slight tingling of suspense, though he had spent the greater part of his life aboard ship.
Hilder went on. “Scientists call it different names. They call it the Law of Action and Reaction. Sometimes they call it Newton’s Third Law. Sometimes they call it Conservation of Momentum. But we don’t have to call it any name. We can just use our common sense. When we swim, we push water backward and move forward ourselves. When we walk, we push back against the ground and move forward. When we fly a gyroflivver, we push air backward and move forward.
“Nothing can move forward unless something else moves backward. It’s the old principle of ‘You can’t get something for nothing.’
“Now imagine a spaceship that weighs a hundred thousand tons lifting off Earth. To do that, something else must be moved downward. Since a spaceship is extremely heavy, a great deal of material must be moved downward. So much material, in fact, that there is no place to keep it all aboard ship. A special compartment must be built behind the ship to hold it.”
Again Hilder faded out and the ship returned. It shrank and a truncated cone appeared behind it. In bright yellow, words appeared within it: MATERIAL TO BE THROWN AWAY.
“But now,” said Hilder, “the total weight of the ship is much greater. You need still more propulsion and still more.”