The SF-22 was ready to take off and the crowds were drawing back into the obscurity beyond the huge circle of blinding light. One after another her twelve engines sputtered into life, and ponderously she moved over the field, gathering speed as the staccato barking of the exhausts gradually blended into a smooth though deafening purr. The tail of the great vessel came up, then the wheels, and she was off into the night. * * * * * Hart Jones sat at a bewildering array of instruments that covered almost the entire forward partition of the cabin. He pressed a button and the starting motor whined for a moment. Then the single engine of the _Pioneer_ coughed and roared. Slowly we taxied in the direction taken by the SF-22, whose lights were now vanishing in the darkness. I saw George open a valve on the wall and Hart stretched the fingers of his left hand to what appeared to be the keyboard of a typewriter set into the instrument board. He pressed several of the keys and pulled back his stick. There was a whistling scream from astern and I was thrown back in my seat with painful force. With that, the motor roared into full speed and we had left the airport far behind. "What on earth?" I gasped. "Rocket propulsion," laughed Hart. "I should have warned you. Those tubes you saw outside at the tail and along the leading edge of the wings. Only used three of them, but that was sufficient for the take-off." "But I thought this rocket business was not feasible on account of the wastage of fuel due to its low efficiency," I objected. "We should worry about fuel," said Hart. I looked about me and saw that there was very little space for the storage of this essential commodity. "Why?" I inquired. "What fuel do you use?" "Make our own," he replied shortly. He was busy at the moment, maneuvering the _Pioneer_ into a position above and behind the SF-22 and her convoy. "You make your own fuel enroute?" I asked in astonishment. "Yes. That sphere you saw on top. It is the collecting end of an electrical system for extracting nitrogen and other elements, from the air. This extraction goes on constantly while we are in the atmosphere and my fuel is an extremely powerful explosive of which nitrates are the base. The supply is replenished continuously, so we have no fear of running short even in the upper levels." * * * * *