"I don't know," said McCauley. "Anyhow it's done." Furness stared at him. "What else was there to do?" "I wouldn't know," said McCauley. "The ship can't be test-flown, of course--not in any ordinary sense of the word. You can't test-fly a hydrazine rocket, and among other things that's what this ship is. You just have to take it up. But--hm--how were the tests on the rocket motor?" "They gave four per cent over the maximum expected thrust," said Furness, exuberant again. "Nothing wrong there!" "They were cut in and out frequently?" asked McCauley. That was one of the tricky items. A rocket motor is cut off, in a ballistic rocket, and cut in again after a pause in its firing. It isn't a sensible thing to do ordinarily, but it would be necessary in flying the X-21. It was a point about which McCauley had certain reservations. A rocket motor is very nearly a device for producing a continuous explosion, the recoil from the explosion constituting the thrust. Rocket motor design is pretty well worked out, but there are occasional failures, as in any high-precision apparatus. And the motor of the X-21 would need to cut in and out, often. It would burn fuel at the rate of more than two thousand gallons per minute. It would have to start instantly, with full pressure and full flow of two dissimilar liquids, and they would have to meet at exactly the proper spot in the rocket motor cavity and burn completely on contact. When the rocket was cut off, the fuel would have to stop flowing instantly, without the fraction of a fraction of one per cent of either liquid left unburned, or there would be trouble when the motor started again. The bare fact that the X-21's motor would have to fire and stop and fire again meant that absolute perfection was needed in all sorts of auxiliary equipment. The pumps. The fuel flow lines. There was the possibility of hydraulic hammer. There could be turbulence in the tanks because of intermittent flow. Decidedly the motor should be tested intensively for flaws in cut-in and cut-out operation, and it should be tested in the ship and not merely in a static-thrust frame. Furness frowned. "I don't know what the tests were," he said with a trace of impatience. "They tested everything. They say everything's all right. I'm no reaction motor technician! I'm a pilot! They give me a ship and I fly it! I leave the other stuff to the slide-rule boys!" "Who are plenty good," agreed McCauley, "and since the take-off's scheduled, that's that. We take off at 1200 hours tomorrow."