Abort mode three

By six minutes into the mission, the S-II had worked long enough that, were it to fail, it could be jettisoned and both the S-1VB and the service module’s engine would allow the spacecraft to reach orbit. This was essentially abort mode three but the scenario was more often known as contingency orbit insertion (COl). However, had it been invoked, the spacecraft would have had insufficient propulsion remaining to take it to the Moon. Instead, it would have had to embark on a planned for. but never implemented. Earth orbit mission to make the best of a poor situation.

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 S­IC, Whereas the first stage had given them a good squeeze over a couple of minutes, the S-II’s acceleration rose smoothly over 6 ‘A minutes from less than 1 g to a little below 2 g – a peak less than half that of the first stage. Charlie Duke gave his impressions of the S-II during his Apollo 16 debrief: "I thought the S-II was very smooth and very quid. 1 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.’’

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 S-IC provided about 30 per cent of this and the S-II Look it up to 90 per cent. The final 10 per cent was provided by the S-IVB stage in the first of its Lwo 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. It Teels’ no effect from the fact that the planet is revolving below. Instead, its speed in space is quoted with respect to some wider frame of reference, usually referred to as inertial space, but more easily grasped as being with respect to

the stars. In this inertial frame of reference, Earth had itself supplied an initial 0.4 kilomcirc-pcr-second boost as a consequence of its axial rotation. In total therefore, the stack had to be travelling at 7.8 kilometres per second to maintain a useful orbit. Around the 6-minute point in the ascent, 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 help achieve orbit.