Off to the Moon

One second prior to ignition, the central light in the indicator cluster came on to tell the crew that the J-2 was about to spring into life. When it did. the crew felt an acceleration of about 0.5 g that gently rose to about 1.5 g over the duration of the burn as the tanks in the third stage emptied. Translunar injection typically lasted just under six minutes and it increased their speed from 7.8 to 10.8 kilometres per second. As soon as the burn began, the position at which it occurred became the perigee of the stack’s new elliptical orbit. Then as the burn proceeded and as they continued to orbit around Earth. their height began to rise; slowly at first, but at an increasing rate as the apogee of their orbit was drawn out. Although the crew monitored their instruments in case the S-fVB showed signs of trouble, they were not averse to taking a look out of the window and enjoying the view. As Eugene Cernan described on Apollo 17, "As the S-IVB manoeuvred, we flew through a sunrise during TLI. which in itself was very interesting, very spectacular.’’

Like the burn of the S-II stage, the S-IVB’s engine changed its mixture ratio during this burn. However, the effect of the change and the strategy behind it were very different. For the S-II. the change lowered the thrust and increased the stage’s efficiency to ensure simultaneous depletion of propellant and maximum possible impulse from the stage. For the S-IVB‘s second burn, the change brought an increase in thrust. Maximum use of the propellant was still desirable but of greater importance was that the shutdown should occur as soon as a precise velocity had been attained. Moreover, the loading of propellants had to take account of the constant boil-off of hydrogen fuel in Earth orbit and of the fact that there was a contingency for TLI to be made on the third rather than the second orbit, by which time a substantial quantity of hydrogen would have been lost. Therefore hydrogen had to be held in reserve for a delayed TLI. This meant that if the burn occurred at the first opportunity there would be an excess of the stuff. So for approximately 100 seconds of an on-time TLI the engine burned with a fuel-rich mixture to use up this excess and then the remainder was burned using a normal, more efficient mixture ratio. A delayed TLI was never performed, but if it had then the time required for the fuel-rich burn would have been very short, and if the boil-off of hydrogen had been greater than expected the engine would have moved into a fuel – lean regime.

к was common for commentators of the day to say that the I’Ll burn accelerated Apollo to Earth escape velocity. This statement implied that when the S-IVB finished its work, the slack was travelling so fast that it would never return to Earth’s vicinity without intervention, whether by the Moon or a rocket engine. Strictly speaking, this was not true, as the stack’s long elliptical orbit around Earth would have eventually returned them to perigee if the Moon had not intervened. Nevertheless, it was well within the capability of the S-IVB to add the few extra metres per second in order to escape into solar orbit.

With respect to the ground, the stack’s new trajectory took it less and less parallel to Earth’s surface, and instead more and more perpendicular to it as it pulled away from the planet. As it did so, its horizontal speed across the ground diminished; so much so. in fact, that the rotation of the planet began to catch up with the spacecraft, with the result that the ground track, which had been towards the cast, slowed, halted and began to travel towards the west, which kept the spacecraft in view of the United States for a few hours.

For a few minutes, as they raced away at about 10 kilometres per second, the crew passed through the van Allen belts, where they received a small dose of radiation. The Apollo flights represent the only example of human spaceflight through and beyond the van Allen radiation belts into interplanetary space. These belts consist of diffuse toroidal volumes around Earth’s equator within which radiation levels arc elevated by the planet’s magnetic field trapping energetic particles from the Sun. There is an inner torus populated by energetic protons, which the spacecraft passed through in a matter of minutes, and against which the spacecraft’s skin was an effective shield. The spacecraft took about an hour and a half to traverse the more extensive outer torus, but this region has mainly low – cncrgy electrons and so was less of a worry to mission planners. Also, the inclination of the trajectory, being in the plane of the Moon’s orbit, avoided the strongest regions of the belts near the equator.

Over a complete mission, including exposure to very energetic particles encountered in the solar wind environment beyond Earth’s magnetosphere and relativistic cosmic-ray particles, crews w-erc believed to have sustained a dose of a similar magnitude to that allowed annually for workers in the nuclear industry. There were additional dangers from occasional explosive events on the Sun when huge quantities of radiation were spewed out in coronal mass ejections and flares, but the Apollo programme simply ran the gauntlet of these events, accepting such risks along with the many other risks already inherent in an Apollo mission. Astronauts came from the test-pilot milieu where danger was a given and risks had to be weighed against the gains of mission success.