Category How Apollo Flew to the Moon

Platform realignment: the LM way

The LM possessed a full guidance and navigation system similar to that in the CSM but with different names. It was the primary guidance and navigation system or just PGNS and, as often happened, the people of Apollo quickly transmogrified the pronunciation of this clumsy acronym to ‘pings’. It had its own inertial measurement unit and optical system. Upon power-up, it needed to know three things: what time it was. where it was, and which way was ‘up’. A call from the CMP in the command module allowed the commander to set the mission clock to the right time, and this information was eventually passed to the computer. Other variables were loaded into the computer to prepare it for the proper operation of the spacecraft; the LM mass, the settings for its digital autopilot and trim angles for the engine gimbals. An uplink from mission control direct to the computer’s memory provided a state vector to tell it where it was, how fast it was moving and in what direction. They also uploaded a REFSMMAT which would provide a reference for which way was ‘up’, but only when the guidance platform was aligned in accordance with it.

Although the computer at the core of the LM guidance and navigation system was essentially identical to the one in the command module, the systems connected to it were quite different. This reflected how engineering constraints altered when designing a super-light, rocket-powered Moon-lander instead of an interplanetary


Pete Conrad and Alan Bean in the LM simulator. Between them is a yellow tubular

framework that surrounds the eyepiece for the AOT. (NASA)

spacecraft that had to withstand atmospheric re-entry and parachute drop onto the surface of Earth.

For the first alignment of its guidance platform, the LM was still docked to the CSM, and procedures reflected this. First, as a starting point, the known orientation of the platform in the CSM provided an approximate alignment. Since there was no computer-to-computer connection between the two spacecraft, gimbal angles were recorded manually by the CMP and radioed through to his colleagues. A few simple calculations had to be applied to these angles to account for the different orientations of the coordinate systems of the two spacecraft, and to take into account the angle indicated in the tunnel that measured any mutual misalignment. The gimbals of the LM’s fMU were then commanded to drive the platform to this orientation. While not sufficiently accurate for precise manoeuvring, this procedure gave the platform a reasonably good idea of which way was ‘up’.

The next step was to carry out a fine alignment that used Program 52, as in the CM. However, whereas the CM sported a sophisticated motor-driven sextant and telescope, the LM had a much simpler periscope arrangement called the alignment optical telescope (AOT), which was mounted at the top of the cabin between the two crewmembers. This was a remarkably ingenious device, whose elegance was in the simplicity of its design. Its main component was a unity-power telescope with a 60- degree field of view that could be manually rotated between six fixed positions: forward, forward right, aft right, aft, aft left and forward left. It incorporated two methods of using the stars to determine the orientation of the platform. One was for in-flight use when the LM was free to rotate; the other was for use on the surface, or for when it was attached to the CSM. Despite its simplicity, the AOT allowed the commander to align the LM’s platform just as accurately as the CMP could align the platform in the command module.

Sighting the stars was done against an illuminated graticule on which were inscribed a series of patterns. A pair of cross-hairs was used when the LM was in free flight, and a pair of radial lines and spirals came into play for surface or docked alignments. In both cases, the computer was told which of the six detents the AOT was in, and which star was to be marked. In each case, it was then a two-step process.

To mark on a star during free flight, the LM was manoeuvred to make the star move across the X and Y cross-hairs, with marks being taken when it coincided with each line so that the computer could define two intersecting planes whose vertex pointed to the star. A similar pair of marks on a second star gave the two vectors the computer required to calculate the platform’s orientation.

The second method was normally used on the lunar surface, but it could also be brought into play when the LM was docked to the CSM. It was considered undesirable to try to manoeuvre the entire stack from the lightweight end so this method did not require that the LM be manoeuvred. It was also a simple two-step process once the computer knew which star was being viewed at which detent. First, the graticule was rotated until the star lay between the two radial lines. Pressing the ‘Mark X’ button yielded the shaft angle. The graticule was rotated again until the star lay between the two spirals. Pressing ‘Mark Y’ gave the reticle angle. The


The exterior aperture of the AOT on top of Spider, as photographed by David Scott from the open hatch of Apollo 9’s command module Gumdrop. (NASA)

computer could then convert this information into a vector to the star. This process was repeated using a second star. When completed, the computer could determine the platform’s actual orientation which allowed it to be accurately aligned to the required REFSMMAT, in this case, the landing site REFSMMAT.


By July of 1969, NASA had done about as much as they could to prepare for the Moon landing. On the flight of Apollo 10 two months earlier, Tom Stafford and Eugene Cernan had taken their LM Snoopy into the descent orbit but had gone no further before returning to John Young in the CSM Charlie Brown.

Where Snoopy had feared to wander, Eagle swooped in. Although the first landing attempt, flown by Neil Armstrong and Buzz Aldrin, would be ultimately successful, it was by no means a straightforward descent. Landing on the Moon was a 12-minute rocket ride from orbit with a starting speed of nearly 6,000 kilometres per hour leading to a gentle touchdown on a terrain where no prepared ground awaited the LM. In that short time, a plethora of problems were served up to the crew of Eagle that would have curled the toes of everyone involved had it merely been a simulation. The fact that they all occurred on the actual landing attempt in full view of the world, yet were successfully handled by the mission control team and the crew, is testament to their professionalism, and to the power of exhaustive simulation as a means of properly preparing people for the challenges they may face.

Programs and phases

Planners broke the descent into three parts with each controlled by a dedicated program in the computer. The first was the braking phase, when most of the spacecraft’s orbital speed was countered by the thrust of the descent engine. This was the domain of Program 63 which began 10 minutes before the powered descent. It included the engine’s ignition and continued for the first nine minutes or so of the nominally 12-minute burn while the computer worked to take the crew to a point in space known as high gate, typically 2,200 metres in altitude and about seven kilometres from the landing site. At the start of the braking phase, the LM flew with its engine pointing against the direction of travel. Then as the burn

W. D. Woods, How Apollo Flew to the Moon, Springer Praxis Books,

DOI 10.1007/978-1-4419-7179-1 10. © Springer Science+Business Media. LLC 2011

Подпись: Perilune & PDI. 500 km and 11.5 minutes to landing Landing site Diagram to show how Program 63 began nearly 1,000 kilometres before PDI.

progressed, the spacecraft gradually tilted a little more upright. At high gate, P64 took over.

P64 handled the approach phase of the descent. When the program assumed control, its first action was to pitch the LM further towards an upright attitude in order to enable the crew to see the landscape ahead. The point to which the computer was taking them was just on the near side of the horizon. They then flew in a manner roughly similar to a helicopter, but with the LM carefully balanced on top of the engine’s exhaust with the computer still in full control of where it was going. P64 included a method of informing the commander of where the computer was taking them, but if he deemed this to be unsuitable, then with a nudge of his controls he could instruct the computer to move the aim point. P64 was targeted to take the LM to a point about 30 metres above the surface and about five metres from the landing site. Prior to reaching this point, the crew would reach low gate, about 200 metres altitude and 600 metres short of landing.

As low gate approached, the commander was faced with a range of options. If he was completely satisfied with the job the computer was doing, he could allow it to automatically move on to P65, which could complete the landing. No commander ever allowed that, although it is said that Jim Lovell had intended to if Apollo 13 had reached this point. These competitive ex-test pilots, many of them experienced at landing on aircraft carriers, were happier to have some degree of control and steer the LM, and they all selected P66 before reaching low gate. P66 continuously throttled the engine to control their rate of descent, and the commander could adjust this rate as conditions warranted. At the same time, he assumed manual control of the LM’s attitude, which allowed him to steer the ship to a site of his own choosing. One other program, P67, was available to the commander, which gave him full manual control of the spacecraft, both the attitude and the throttle setting, but this

“Go for the pro”: the landing begins 287

option was never used. Both P65 and P67 were dropped from later versions of the LM software.

A barely stable throttle

As if Apollo ll’s descent wasn’t exciting enough, later analysis demonstrated that Eagle’s software had harboured a problem that could have forced an abort. In fact, Apollo 12 flew with the same problem coded into its software. Alhough it was most prominent during the final few metres before touchdown, when the commander was flying in P66, it could have afflicted any phase of the descent.

The story of the problem, as told by Don Eyles, came to light when an engineer, Clini Tillman, ran a simulation of a descent. Tillman worked for the LM’s manufacturer, Grumman. This gave him the ability to run his simulation using real hardware rather than having to mathematically model the operation of the engine. As a result, faulty assumptions in the model were avoided.

Tillman noticed that the commanded thrust was varying in an apparently random, stepped fashion that came to be known as throttle castellation, after the similarity of its waveform to castle battlements. The variation was only slight but when Tillman dug into the stored telemetry readings from Apollo 11 and 12. he discovered that during P66 these variations were not only present, they were unduly large and hinted at some intrinsic instability in the system as a whole. One aspect of the problem was uncovered by Eyles’s colleague. Allan Klumpp. Know’n as IMU bob. it came about because the accelerometers at the centre of the IMU did not reside at the centre of mass of the LM. So when the LM underwent significant rotation, the 1MIJ sensed a component of that rotation as being vehicle velocity, which it was not. When this fed into the thrust calculations, it caused a small degree of instability.

A more profound cause of throttle castellation was related to the time taken for the descent engine to respond to commands to change thrust so called ‘throttle lag’. As described by Lyles, the paperwork for the engine stated that its lag time was 0.3 seconds and it was his task to compensate for this in software. To determine the best compensation value to use. Hyles carried out simulations to model the performance of the engine. lie saw how unstable the throttle command was with no compensation, then how compensating for 0.1 seconds helped a lot and how compensation for 0.2 seconds of lag essentially eliminated the instability. He therefore programmed the flight software with compensation for 0.2 seconds of throttle lag instead of 0.3 seconds.

Now it turned out that the compensation was trying to hit a sweet spot and that overcompensation could also induce throttle instability. It also transpired that the documentation for the engine was wrong. The engine was still evolving and by the time it was installed in the LM. it could react to throttle commands in a mere 0.075 seconds. It was later demonstrated that if Hyles had followed the paperwork and programmed for 0.3 seconds compensation, the throttle would have been wildly unstable. Any further source of instability; for example the I MU bob problem and the mission would have had to have been aborted. As it was. the first two flights to the Moon landed with throttles that were barely stable.


The rocks and soil returned by the Apollo 11 crew quickly revealed that not only was Mare Tranquillitatis a basalt plain, it was astonishingly old in comparison to typical terrestrial rocks. 1’he Standing Stones at Calanais happen to be made of gneiss which is among Earth’s oldest rocks at around two to three billion years old. Compare this to 3.6 billion years for the Apollo 11 site. Also of surprise to the geologists was the presence of particles of anorthosite, a mineral rich in aluminium, among the soil samples. Later missions would reveal the importance of this type of rock in decoding the Moon’s history.

Some scientists were none too impressed when Apollo 12 was sent to another mare site merely to prove they could land near a defunct probe. It seemed there was little to distinguish it from the Apollo 11 site but when the samples were returned, its basalts were found to be a half billion years younger, showing that lunar volcanism had been active over an extended period.

Both Apollos 12 and 14, especially the latter, returned samples that came to be described as being KREEPy (K. is the chemical symbol for potassium, P for phosphorus and REE means rare earth elements, and the V makes it an adjective). The importance of KREEP lies in the fact that these elements are not easily incorporated into the crystal lattice of a solidifying rock. Therefore in a large body of magma that is slowly solidifying, the last rock to harden will be rich in KREEP and this clue would become significant as later missions added further evidence to our evolving knowledge of the Moon’s early history.

Alpha-particle spectrometer

Although the Moon appeared to be a very dead world to anyone who looked at it. scientists wondered if some traces of volcanism were still spluttering in some corner of the globe. Tantalisingly, some telescopic observers had reported seeing ’emissions’ in the form of brief glows and hazes, which kept alive hopes of finding extant activity. The alpha-particle spectrometer was designed to look for indications of such activity.

Lunar rock samples from earlier missions were found to contain traces of uranium and thorium, two elements which, through their radioactivity, decay to form gaseous radon-222 and radon-220 among other elements. The alpha-particle spectrometer could detect these substances from lunar orbit by their emission of alpha-particle radiation essentially the nuclei of helium atoms as they further decayed and. by inference, locate areas of possible volcanism or other features that might cause the concentration of uranium and thorium to vary. Any emissions from the Moon of gases such as carbon dioxide and water vapour would also be
detectable as they would be expected to include a small amount of decay­ing radon gas.

Подпись:The major result to come from this instrument was that there is a small degree of outgassing of radon at various locations on the Moon, especially in the vicinity of the prominent crater Aristarchus – a result confirmed a generation later by the Lunar Prospector probe. Interestingly, Aristarchus, which is also one of the brightest places on the Moon, was the locale for some of the reported emanations seen by telescopic observers. These tentative indications of possible ongoing lunar activity should be seen in the light of studies of a crater, Lichtenberg, on the western side of Oceanus Procel – larum. This crater exhibits a ray system that is believed to be just less than a billion years old, which is quite young by lunar standards. Yet, on a world where most of the basalt is much older, a distinctive dark lava flow has obliterated much of its southern ray system. From this evidence, and as far as is known, the final gasps of lunar volcanism occurred about 800 million years ago. To put this into a terrestrial context, this is 300 million years before complex multicellular life appeared on Earth.

The detection of radon gas, particularly at Aristarchus, is best explained by the effect of the huge impact that formed the Imbrium Basin, within which Mare Imbrium now lies. The current magma ocean theory of the Moon’s early evolution not only explains the richness of aluminium in the upland regions of the Moon, but also predicts that, as the magma ocean cooled, the last vestiges of lava to solidify would have been rich in the KREEP elements that would have found it difficult to become part of the rock’s crystal lattice. Geologists now believe that the violence of the Imbrium impact event nearly four billion years ago was enough to punch through the crust and excavate KREEPy rocks to the surface. A lot of this slightly radioactive rock was covered by the lava flows that drowned the western portion of the Imbrium Basin over three billion years ago. Then half a billion years ago the impact that formed Aristarchus drilled through the layers of basalt to re-excavate KREEPy material.


For their pioneering journey to the surface of the Moon, Armstrong and Aldrin made only a single foray onto the surface before attempting to get some sleep in the uncomfortable confines of the LM. The rendezvous and docking next day were therefore carried out by a crew that were hopefully rested to some extent. As each successive flight became more ambitious and the LM was trusted with a crew for longer periods, the rendezvous and docking day grew increasingly packed. At first, moonwalks of four-hour, and eventually nearly 6-hour duration were shoe-horned into that day. Then by the time two hours had been added for getting into a suit in the morning, plus time to prepare for lift-off, meals and the rendezvous itself, the day became especially long and intense. And it was not as if docking marked the end of the working day.

For mission control, the excessive length of the crew’s day became an issue when Scott and Irwin returned from their highly successful stay at Hadley Base near the eastern rim of the mighty Imbrium Basin. This was one of the very few times when the crew in an Apollo spacecraft and the people in mission control managed to get out of sync with one another, probably because managers in the mission operations control room (MOCR) had a perception of the crew’s tiredness and, in the wake of the Soyuz 11 tragedy only a month earlier, they tvere overly worried about it.

Lights in the eyes

On leaving the protection of Earth’s magnetic field, many crews, beginning with

Apollo 11, mentioned occasional brief flashes that would appear in their vision irrespective of whether their eyes were open or closed. On Apollo 12 Conrad noted the same thing. The Apollo 14 crew made a basic study of the phenomenon after the cancellation of a mid-course correction manoeuvre that left them with time on their hands. The Apollo 15 crew had some time set aside specifically to further investigate the phenomenon whereby the crew would sit in various positions in the cabin wearing blindfolds for an hour.

“I would say 90 per cent were of what I’d call a point source of light,” explained Scott at the end of their first experimental period. "And to give you an analogy, you might picture yourself sitting high in the stands of a darkened arena, and you look across at the other side and somebody shoots a flashbulb or something, and that would be what I’d call a typical flash of intensity five on a scale one to five.”

Worden added to Scott’s description: "Most of the light flashes seem to be of the order of flashcubes or maybe starbursts that you’ve seen in the summertime. I saw very few streaks or radial paths of light. They all seem to be just point sources of light.”

The next two missions took the study further by having one crewman wear a film- based particle detector, the Apollo light flash moving emulsion detector (ALFMED) while he described the flashes that he saw. Though it was attributed to cosmic rays passing through the head and interacting with the human visual system, the results of these small-scale experiments were inconclusive. Long-term dedicated experiments

The ALFMED equipment in use during Apollo 17. (NASA)

on board the space stations Mir and the ISS showed that they could also be detected in Barth orbit.

The crews carried out other experiments, both scientific and technological, during their coast home. The sensors built into the SIM bays of the J-mission CSMs could no longer look at the Moon but opportunities were taken to aim them at celestial objects. For example, just prior to Apollo 15. the Uhuru x-ray astronomy satellite had discovered a strong x-ray source called Cygnus X-l. The x – ray spectrometer in the SIM bay was therefore brought to bear on it to help to characterise its emissions.

Spacecraft condensation

Prior to re-entry, the crews noticed how the area around the forward hatch up in the CM’s apex tended to cool and attract condensation from the cabin’s atmosphere.

‘"You know, I bet when we splash down out there.’’ said Tom Stafford, “this cold water runs all out in that…”

“Bet you’re right,” interrupted John Young. “That’s probably where all the water comes from.”

“I bet there’ll be water galore," said Stafford.

“Well, a lot of it’s condensing up the hatch, too," said Young. “That’s a good place for it; there ain’t no wires up here. I don’t give a shit if we get ice up here as long as there ain’t no wiring up there. As long as we don’t have to live up there.’’

“Good place to pul your feel up,’’ suggested Stafford.

“If I was designing the spacecraft,” continued Young, ever the hardened engineer, ”I’d make the bastard get the water out of it before it ever starts; but once it’s designed, that’s probably as good a place to have a water separator as anywhere.”

“Did the other spacecraft notice water under there?” asked Stafford.

“I don’t know if they ever noticed ice or not. We’ve got a lot of water up there now, a lot, a lot. Let me get my rag and go up in there and clean it out.”

Small amounts of water were not a problem in the cabin’s electrical system, partly as a result of the Apollo 1 fire. One of the changes made to the spacecraft was that all the electrics had to be hermetically sealed. When Odyssey, the Apollo 13 CM, re­entered, its wiring had been chilled for four days and had gathered condensation that covered every surface. Upon re-entry, large quantities of water rained down on the crew.

Keeping cool

Over the final hour of a mission, as the crew prepared for re-entry, most of the systems in the command module were powered up. Throughout the mission the heat generated by these systems had been absorbed by a water glycol solution not unlike that found in the radiator of an automobile, and then shed to space by the two large radiators on the side of the service module or. if required, the primary and secondary water evaporators in the command module.

However, by design and a mere 15 minutes before re-entry, most of the elaborate systems for dissipating the spacecraft’s excess heat were about to be cast away along with the rest of the discarded service module, so a special provision had to be made to manage the heat generated within the command module during the half hour between separation and splashdown. Shortly before separation, a ‘chill-down’ process was begun, where both radiators and the primary and secondary water evaporators were used to cool the vvater/glycol to around 5 C. This didn’t cool the cabin, which remained at about 24 C, but it prepared the coolant to absorb large amounts of heat from the electronics. This took advantage of the fact that water has by far the highest heat capacity of the common liquids. Although the total amount of heat that could be absorbed by the coolant was still quite limited, it was sufficient to last from entry to splashdown. The water/glycol within the command module was only used to cool the spacecraft’s electronics. No attempt was made to actively cool the exterior during the fiery plunge through the atmosphere, the heatshield being more than adequate to protect the structure.

One system that did not require to be cooled, but to be heated, was the command module reaction control system and its thrusters. These RCS thrusters had been exposed to the cold of space or the heat of the Sun for up to 12 days. Heaters ensured that they were all warm enough before they were operated for the first time.

Landing site REFSMMAT

The landing site REFSMMAT was another of the many frames of reference used during an Apollo flight. It was carefully chosen to aid a landing crew by having their attitude displays, the FDAIs. or 8-balls, give readings that would make sense to a pilot as he approached the lunar surface. This frame of reference was defined as being the orientation of the landing site with respect to the stars at the predicted time of landing. The actual orientation of the landing site, of course, continuously changed as the Moon rotated on its axis and only matched the landing site REFSMMAT at one moment in time. This coincidence of the two was known as the ’REFSMMAT 00 Lime’ and therefore this time represented the intended moment of landing.

When properly aligned to this REFSMMA T, the platform’s, v axis would be parallel to a vertical line running from the centre of the Moon out through the landing site position. Its 2 axis would be tangential to the landing site yet parallel with the GSM’s orbital plane and thus with the LM’s approach path, pointed in the direction of flight. Use of this frame of reference meant that if the LM landed at the planned time and place, w-as in a fully upright attitude and was pointed forward, then its FDAI display should show 0 degrees in all axes.


Apollo ll’s troubles began as they came around the Moon’s eastern limb. There had been a major change to the configuration of the lunar module since Apollo 10 had rehearsed the descent orbit. Plume deflectors had been added around the descent stage to protect it from the blast of hot gas from the RCS thrusters and these were now interfering with the radiation pattern of the steerable high-gain antenna. Worse, Armstrong was flying with the windows facing the Moon to gain timings relating to his orbit. This meant that the steerable antenna had to peer past the LM structure. The diagrams that indicated the resultant restrictions and which angles the steerable antenna could use were in error. At acquisition of signal after Eagle had entered the descent orbit, mission control found that not only did this interference make voice communication with the crew difficult, it interrupted the engineering telemetry with which flight controllers would soon make a decision on whether to proceed with the landing.

To try and alleviate the problem, Charlie Duke in mission control passed on a recommendation from Pete Conrad, who was sitting close by, that they yaw the LM right by 10 degrees. Enough data did get through for the Go/no-Go decision to be made positively, though in the event, it had to be relayed via Mike Collins in the command module.