HOW NOT TO CRASH INTO THE MOON Part I

For Apollo to enter lunar orbit the crew simply burned the SPS engine at perilune to slow down sufficiently to ensure that the two joined spacecraft did not have enough momentum to escape the Moon’s vicinity. If the burn was timed right, they would instead enter a close orbit around it. If the burn failed to occur at all. the crew were left in the fail-safe scenario of returning by default to the vicinity of Earth, with only a tweak of their trajectory by the RCS thrusters required to bring them to a safe splashdown. However, in between the two scenarios of’no burn’ and ‘a burn of the required duration’, there were a range of possible outcomes that depended on exactly how much the engine had managed to slow the spacecraft prior to some kind of failure, and some of these were potentially lethal. The flight plan included notes on how these should be handled.

In the scenario of a very short burn, the stack would come around the Moon and begin to head towards Earth. However, its trajectory would be far from ideal and would require a major engine burn, perhaps from the LM’s descent engine, to restore a successful interception with the atmosphere.

A somewhat longer burn could leave the spacecraft with insufficient momentum to leave the Moon’s gravity. This scenario was inherently unstable as it would leave the spacecraft languishing in a region above the Moon’s near side for some time and return to the Moon’s vicinity. However, owing to perturbations from Earth’s gravity, and the fact that the Moon was still travelling in its orbit, the stack could either pass the Moon’s trailing edge to be slung out into the depths of the solar system, or it could impact the Moon’s surface. In this case, the crew would wait until

the spacecraft was high above the near side and then burn the descent engine to effect a return to Earth.

If the LOI burn lasted long enough prior to SPS failure, the stack would enter an elliptical lunar orbit with a perilune of about 110 kilometres over the far side, and an apolune over the near side whose altitude depended on the length of the burn. A longer burn resulted in lower altitude at apolune. Л typical get-out from this predicament would have been to complete one orbit and burn the descent engine to return to Earth. Additional power was available from the LM’s ascent engine if required.

In truth, if the SPS engine managed to start, there was very little likelihood that it would stop until commanded. What w:as of far greater concern was the possibility that it might continue to burn after the required shut-down Lime – another lethal scenario.

Part II

With a sufficiently long burn at LOI. the altitude of the resulting orbit’s apolune would match that of perilune, 110 kilometres, and the orbit would therefore be circular. For Apollo, the burn to achieve circular lunar orbit was between five and 6 /2 minutes long, depending on the mass of the spacecraft and the precise thrust of the SPS engine. However, to make an LOI manoeuvre for a circular orbit at the first attempt raised great dangers for an Apollo crew. If the engine were to slow them down Loo much, either by over-performance or perhaps through failure of the control equipment, the altitude of their orbit over the near side could drop so low as to become a negative value. Put less euphemistically, the spacecraft would descend until it augured into the lunar surface at great speed. Given the imprecise knowledge of the Moon’s shape at the time of the early missions, this was considered to be a very real danger.

The situation was even more extreme for the crews from Apollo 14 onwards. Rather than targeting for a circular parking orbit, they deliberately brought the altitude of their final orbit over the near side right down to only about 17 kilometres, with the low point conveniently located to enable the lander to subsequently descend to the surface. To put this into perspective, consider that for Apollo 15. every second that the SPS engine burned dropped the near-side altitude by about 11 kilometres. An overburn of only two seconds would wipe out their near-side altitude and have them impact the surface.

To avoid the possibility of impact, the Apollo SPS took two bites at the task. An initial huge burn was made that was slightly shorter in duration than that which would be expected to produce a circular orbit. This first burn was called LOI-1 on the early missions, or just LOI on the later missions. It placed the spacecraft in an elliptical orbit with a perilune of 110 km around the far side, and an apolune over the near side that was typically around 300 km altitude. After two orbits, which was more than adequate time for the shape of the orbit to be precisely measured by radio tracking from Earth, an additional short burn was made at perilune in order to lower the near-side altitude to the required height. This was called the LOI-2 burn on the early missions. Its name was changed to DOI on the later missions for descent orbit insertion.

When the crew monitored these long burns, duration was not the only value that interested them. Although the thrust from the SPS was accurately calibrated, there were always small variations in its power that made the length of the burn less reliable as an indicator. What was more important was delta-v, the change in velocity brought about by the engine. Throughout a burn, this value was measured by the guidance system and displayed in front of the crew’. If all was proceeding normally, the burn would be stopped automatically by the computer once it had achieved the required delta-v. If that w’ere to fail, there was a backup system that independently measured and displayed delta-v and which could also shut the engine down at the right time, f inally, there w ere three pairs of eyes that eagerly looked at both delta-v displays, their owners ready to reach out and manually cut the SPS engine if it seemed to be burning for too long.