HOW NOT TO CRASH INTO THE MOON Part III
If a 24-sccond burn made around the Moon’s far side could lower the spacecraft’s near-side altitude from 300 kilometres down to only about 15 kilometres, it is easy to appreciate that an overburn of only a few seconds would so reduce the altitude that an impact with the surface could become a real possibility. The resultant precautions involved in the DOI burn are especially understandable in view of the fact that there was considerable uncertainty about the Moon’s precise shape, especially with regard to the more northerly regions that would later be overllown by two of the J-missions. Apollos 15 and 17, where some of the mountains reach four or five kilometres above the surrounding terrain.
As with all burns, the amount of delta-v was monitored by the crew via the DSKY. For a DOI burn, the typical delta-v was 210 feet per second (64 metres per second), which was the amount by which their velocity along the. v axis had to drop. One second of burn accounted for about 10 feet per second. As the burn progressed, they would see this value decrease towards zero. If the computer did not shut the engine down at the expected time, the crew had to promptly terminate the burn manually. They would then consult the DSKY to see if there was any overburn. The rules were that if they had slowed a mere 2.2 feet per second (0.67 metre per second) more than planned, they should immediately use their RCS thrusters to regain this speed. If they had overshot by as much as 10 feet per second (three metres per second), they were to turn the spacecraft around 180 degrees and regain the lost speed by firing a burp of the SPS.
Whatever the result of the DOI manoeuvre, once the crew were happy with it, they began to prepare for a possible bail-out burn. This was in case some other sign, in particular radio tracking from Earth, were to suggest they were at risk of impacting the ground. If so. they had at most an hour before the unthinkable would occur, and because they were over the far side at the time of the DOI burn, they would have no confirmation one way or the other for half of that time. The exquisite accuracy of radio tracking could only be brought to bear after AOS. with less than half an hour remaining to any theoretical impact. Therefore, w? hile the tracking stations measured their trajectory, the crew waited for a call from mission control to confirm that their orbit wasn’t going to spray them across some near-side mountain at over 5,000 kilometres per hour. This never became a real threat, and the crew’s and mission control felt confident enough with their hardware and procedures to view the bail-out burn as little more than a formality, but, in the NASA way, they were prepared for it.