Haulin the mail

Pogo problems notwithstanding, crews generally found that if the S-II wasn't buzzing and rattling, it gave them a smooth ride after the thrash and fury of the S-

IC. Whereas the first stage had given them a good squeeze over a couple of minutes, the acceleration from this stage smoothly rose over 6% minutes from less than 1 g to a little below 2 g - less than half that of the first stage. Charlie Duke gave his impressions of the Apollo 16 S-II during his debrief: "I thought the S-II was very smooth and very quiet. I had the sensation of very low acceleration or g's and no noise at all that I could tell. I felt like we were almost floating at that time.''

Also around the 6-minute point, the stack had more or less reached its orbital altitude. From then on, the S-II's main task was to add additional horizontal velocity to get them into orbit. To reach a valid orbit, the space vehicle had to achieve a speed of 7.4 kilometres per second with respect to the Earth below. The SIC provided about 30 per cent of this, and the S-II took it up to 90 per cent. The final 10 per cent was provided by the S-IVB stage in the first of its two burns.

However, once a rocket has left Earth's atmosphere there is no longer a need to quote its speed with respect to the surface of the home planet. In space, the rocket's physics is only dictated by the gravitational pull of Earth. As it 'feels' no effect from the revolving planet below, its speed in space is quoted with respect to some wider frame of reference, usually referred to as inertial space, but more often with respect to the stars.

In this inertial frame of reference, Earth itself supplied an initial 0.4 kilometre per second by virtue of its rotation. In total therefore, the stack had to be travelling at 7.8 kilometres per second to maintain a useful orbit.

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