The return flight
The moon lift-off, by another burst of rocket power, is again made just that much easier than from earth owing to the reduction in the mass of the spaceship which has thrown off the multi-stage rockets, owing too to the lesser force of gravity, the lack of air resistance, and the lower speed required for orbit; all this is just as well because the rockets that developed the tremendous thrust for lift-off from earth are no longer available, and by now much of the reserve fuel and power has been expended. Even so, a high degree of accuracy is again necessary to ensure that the lunar module gets into orbit close to the command module (though again small adjustments can be made by bursts of power), so that they can again be linked up into one spacecraft. Once they have been re-united and men, films and other souvenirs have been transferred to the command module, the lunar module can itself be discarded, and left to orbit or to hit the moon – this time probably at speed!
The next stage is a further and considerable boost to put what remains of the spacecraft out of moon orbit and on the return path to earth – once more really an elongated elliptical orbit with the apogee this time near the earth. Although this has been described as a considerable boost, the thrust required is nothing like so great as was needed to start the craft on its journey to the moon because the neutral point is now comparatively near, and once this has been passed the earth’s attraction will all the time be increasing, as will the speed of the space-ship until it reaches something of the order of 10.46 km/s (more than 37 000 km/h), the speed with which it started on its journey.
Now another reverse burst is needed to slow the craft down to approximately 7.5 km/s for a similar circular orbit, or partial orbit, and in the same direction too as that used after launch.