Category How Apollo Flew to the Moon

The Moon has little protection from the Sun’s constant output of x-rays that wash over its day-lit surface and strike whatever gets in their way. When they strike certain elements, particularly those at the lighter end of the Periodic ‘fable, they cause the atoms to re-emit or fluoresce x-rays in certain well-defined energies. Therefore, by comparing the spectral make-up of x-rays from the Sun with x-rays from the sunlit lunar surface, scientists could identify some of the elements in the topmost layer. This was a particularly powerful technique because it could sense those elements that formed the bulk of rocky planets, namely oxygen, silicon, aluminium, magnesium and iron. Apollo’s x-ray spectrometer therefore consisted of two detectors, one of which w’as built into the SIM bay to receive x-rays from the Moon. The second was on the opposite side of the service module where it measured the x-ray flux from the Sun.

It wasn’t long after the SIM bay w-as used for the first time on Apollo 15, that a comparison was made of the signals from the x-ray spectrometer with altitudes from the laser altimeter revealing an important clue to the Moon’s history. Scientists had noticed that a graph from the laser altimeter showing the surface elevation beneath the CSM bore a strong resemblance to another from the x-ray spectrometer showing the concentration of aluminium along the same path. The aluminium concentration declined in low-lying terrain. The significance of this lies in the fact that aluminium is a relatively lightweight element. The discovery that its concentration was greater in the highlands strongly implied that, at one time, the Moon must have been largely molten to allow that element to rise to the top. This ran counter to one of the two popular theories about the Moon’s genesis that were vigorously debated at that time.

One school, dubbed the ‘cold Mooners". believed that the Moon accreted from the solar nebula without generating significant internal heat, that the large basins on its surface w’ere made by impacts and that the maria were splashes of melted rock w’hich pooled in low-lying areas. The ‘hot Mooners’ believed that the interior of the Moon was sufficiently hot for thermal differentiation into a core and a mantle, and that it had later undergone substantial surface volcanism. creating the maria.

Both schools had grasped elements of the truth. Current theories contend that upon its formation, the Moon was so hot that its mantle was completely molten in what is descriptively called a magma ocean. Within this fluid mass, gravity allowed the various constituents of the magma to migrate either up or down according to their weight, such that the fresh crust tended to have a high concentration of aluminium. The fact that strong evidence of this chemical differentiation is still extant today is testament to the extraordinary antiquity of the lunar surface when compared to that of Earth.

Mass spectrometer

Mourned on the end of a boom to place ii clear of the spacecraft was the mass spectrometer. It was designed to characterise an)’ lunar atmosphere by measuring the atomic weight of the atoms and molecules that entered an aperture on one side of the instrument. They were promptly electrically charged, or ionised, by electrons from a filament source. Л magnet then diverted the path of the resultant ion stream towards a pair of detectors. Simply stated, the heavier an atom or molecule, the more resistant is its motion to change by an applied magnetic field. By measuring the deflection of the particle stream, the masses of its constituent parts could be determined.

When it was deployed out of the SIM bay, its inlet aperture faced away from the bulk of the CSM and in the same direction as the engine bell in an attempt to shield it from the gases that emanated from the spacecraft. During its time in lunar orbit, the instrument was flown with the inlet either facing the direction of travel or facing backwards. The hope was that differences between the two modes of operation would allow scientists to discriminate between atoms that were genuinely part of the Moon’s atmosphere (which should Lend not to enter when the inlet was facing backwards) and those that were coming from the spacecraft (which would enter from either direction).

In practice, little difference was detected whichever way the inlet faced, implying that most of what was being detected was essentially pollution from the spacecraft. This supported, on a global scale, the same results that researchers were finding from ALSEP experiments placed by Apollos 12, 14. 15 and 17. These were deluged with contaminants from the Apollo spacecraft, which made it very difficult to extract natural data from their results. This was hardly surprising considering that estimates for the total mass of the natural lunar atmosphere were around 10 tonnes – a figure very similar to the quantity of gases released during each Apollo mission, mostly from operation of the descent and ascent engines. Essentially, each Apollo flight temporarily doubled the mass of the entire lunar atmosphere.

The LM approached the CSM with its windows facing its quarry. When originally envisaged, the LM was to have had two docking ports, the second being at the forward hatch. But in the drive to cut weight from the spacecraft, the heavy docking collar was replaced by a simple square hatch through which a suited crewman and his back pack could crawl on his way to the lunar surface. With the docking port at the top of the ascent stage, a small window was installed above the commander’s head to enable him to view through the roof of the LM. Having lined up in front of the CSM, he had to pitch down until the docking apparatus of both spacecraft faced each other, essentially lining up their v axes. The LM was then rotated 60 degrees to

line up the docking aids between the two spacecraft. From here, the CMP took over m bring the spacecraft together and dock. The commander could carry out the docking, but to do so would have meant craning his head backwards uncomfortably he was still fully suited. It was much easier for the commander to hold the LM steady while the CMP, who was comfortably viewing through a rendezvous window, brought the CSM up to the small spacecraft.

When Armstrong was manoeuvring Eagle for docking, he decided to change the procedure, but soon wished he hadn’t. He realised that if he were to pitch down at that point, the Sun would shine straight into his eyes. Therefore, he chose to line up 60 degrees further around before he pitched down, thereby avoiding the Sun. Having done so, Collins asked him to rotate a little further to get the docking aids properly aligned.

“We complied and promptly manoeuvred the vehicle directly in the gimbal lock.” related Armstrong in the debrief. Having gone into gimbal lock, the PGNS was no longer able to hold the vehicle stable. "I wasn’t aware of it because I was looking out the top window. No doubt, we were firmly ensconced in gimbal lock. We had all the lights on.” Bui they had a backup system. “We just pul il in AGS and completed the docking in AGS. This was just a goof on our part. We never should have arrived at the conclusion from any series of manoeuvres. However, that’s how ii happened. Ii wasn’t significant in this ease, bui il certainly is never a desirable thing to do.”

Unfortunately, holding the LM siable using the AGS had a side effect that caught Collins unaware once he had soft docked and began to retract the docking probe for a hard dock. As the probe began to pull the light ascent stage towards the much heavier CSM, ihe AGS delected a change in altitude and furiously tried to compensate for it by firing the LM’s thrusters. Collins didn’t realise this and began his own aitcmpi to correct ihe motions of the spacecraft relative to one another, but as the two spacecraft were flexibly joined at the capture latches, their motions were somewhat complex. "That was a funny one.” he remarked immediately afterwards. “I thought things were pretty steady. I went to ‘retract’ there, and that’s when all hell broke loose. [The LM was] jerking around quite a bit during the retract cycle.” In eight seconds the time it took the probe to retract the problem disappeared as the two spacecraft became one. But this incident worried engineers who looked into the dynamical issues of having two vehicles joined by a flexible probe.

Dick Gordon and Pete Conrad had no such worries bringing CSM Yankee Clipper and LM Intrepid together on Apollo 12. “It was a simple, easy task to perform,” said Gordon post-flight. “It could have been done in darkness as well as daylight with just as much ease. I don’t think that the vehicles moved hardly at all at contact. There was certainly no noticeable motion, anyway.”

Conrad concurred: “We came right in and stopped. I pitched over and did the yaw manoeuvre after Dick did his roll. We did the docking just the way we stated before. Dick came in and docked; I maintained attitude hold – tight dead band. As soon as he got his top latches barber poled, we went to Free. Neither spacecraft so much as moved a muscle, and we got a complete, good lock. He straightened out

attitude with his translations thrusters and went to hard dock, and it pulled us right in there without either spacecraft deviating; bango! We had 12 latches.”

When Eugene Cernan brought Apollo 17’s ascent stage. Challenger, to rendezvous with the CSM America he thought the spacecraft was a fine sight to see.

“Okay. I’ve got you right out the overhead, Ron." called Cernan to his CMP Ron Evans. He had pitched over to face the LMs drogue at the CSM’s docking probe and the mirror-like surface of the command module looked resplendent in the sunshine. “Now I’m going to yaw.”

“Okay, yaw her around,” replied Evans as Cernan began to line up the two craft to get the docking aids aligned.

“Okay, here we go.” Cernan was enjoying the responsive spacecraft he had under his fingertips. “What a super flying machine!”

“Still looks kind of tinny to me.” mocked Evans.

“Command module looks just as good as the day they pul it on the pad,” said Cernan.

“And, you know-, so does Challenger, by gosh.” said Evans. “You’re missing some of the pieces.” The last time he had seen Challenger, it had a descent stage attached, one that still sits quietly on the Moon.

“Yes, one big piece we left behind.”

When Evans tried to dock with Challenger, he found the lightness of the LM took a bil of getting used to. His approach speed was only two or three millimetres per second. “Coming in nice and slow; no problems,” he informed his commander.

“Okay, you’re looking good, babe.” said Cernan encouragingly. “I got you on my COAS right up in the middle of the window. Looking good. Must be a couple of feet away.”

“Stand by,” warned Evans as he neared the point where 16.3 tonnes of CSM would impact 2.3 tonnes of LM ascent stage whose tanks were nearly empty. But the capture latches failed to engage with the inside lip of the drogue. Evans had brought the CSM in too slowly.

“Okay; I didn’t get it. Let me plus-.v it.”

“Okay. You didn’t get it," confirmed Cernan.

“Might have been a little bit slow. Stand by.” Evans went to have another go but this time driving the probe home by firing his thrusters to give the CSM a positive push in the plus-.v direction.

“You got it! Capture!” Cernan had heard the three latches at the tip of the probe engage.

“Barber pole,” called Evans, as he saw the ’talkback’ indicators on his instrument panel change to show that they had latched. "Capture, go Free.”

Evans’s call for Cernan to ’go Free’ meant that he wanted the LM to stop trying to hold its attitude. The lightweight spacecraft was at the end of the probe and any motion it had would be damped out by the probe’s articulated tip.

“Crazy thing,” muttered Evans as he waited for the CSM and LM to line up on the end of the probe.

Cernan wondered what the problem was: "Say again?”

"I get the right…” Evans laughed at the jittery LM. “And then it goes around the other way. I think you’re bouneing around up there, too, you know.”

”J know it. I’m just swinging free,” replied Cernan.

“You’re bouncing around more on the probe.” said Evans. “See, I’m not moving at all.”

To try to stabilise the situation. Evans suggested that Cernan allow the LM to hold its attitude again. Thruster jets began to pull the small spacecraft onto the attitude it had been programmed to keep. As it did so. it also applied a small torque to the CSM via the probe.

“Okay. I’m stable now,’’ called Cernan once the LM’s motions had damped out. “Okay. Now let me come up to you."

“Okay, when you’re happy. I’ll go free. Looking good now.”

“Looking good, yes.” confirmed Evans. “See that’s what we needed. Okay. Why don’t you go to Free, and we’ll go to retract?"

“Okay,” said Cernan. “I’m Free.” Cernan once more stopped the LM from controlling its attitude.

“Okay, retract. Here you come.” The struts of the probe mechanism began to fold, in the process drawing the two halves of the docking tunnel together. “Bang! I got two barber poles.”

“You got what?” quizzed Cernan.

“Okay,” laughed Evans. lie had got the sense of the talkback indicators the wrong way round. "Two greys, I mean.”

Cernan shared the humour. "That’s better. Sounded good in here.’’

“Yes, sounded good in here." confirmed Evans.

“Okay, Houston.” announced Cernan. "We’re hard docked.”

After the flight, Evans discussed how this docking differed from earlier in the mission when the LM was still attached to the S-IVB. “One of the noticeable differences between this docking and the docking with the S-IVB is the fact that the ascent stage did dance a lot more than the S-IVB did. The S-IVB is steady as a rock. The LM dead band would change attitude, and you’d try to follow it.”

In view of the difficulties that Apollo 14 had experienced w hen Stu Roosa tried to dock with the LM An tares while it was still on the S-IVB. his post-rendezvous docking was approached with some apprehension by mission control. Before the lunar landing. Bruce McCandless at the Capcom console gave the crew’ a change to the procedures. “With respect to docking, again we anticipate normal operation. However, we’d like to add to the normal procedures a LM plus-.v thrust of 10 seconds, four-jet RCS. to facilitate or to give us just a little more of a warm feeling on the docking.”

By having the LM thrust towards the CSM at the same Lime as Roosa was docking was evidently something neither Shepard nor Roosa was happy with. “We mutually agreed that it would be better to give it one go at least using the normal technique with no thrusting.” explained Shepard after the flight.

“We really didn’t see any advantage to that LM thrusting," added Roosa. "I didn’t like that idea of the LM coming on with thrust. We didn’t see where we had anything to lose by trying the normal docking method. If it didn’t capture, then we’d try it." In the event, Roosa’s docking went smoothly, without any hint of the troubles that had beset their earlier docking.

Towards the end of the Apollo programme, crews were handed increasingly large programmes of scientific investigation to be carried out during the coast between the worlds, most of which Look place on the homeward leg. Some of this was a precursor to more extensive experimentation that would be carried out on Skylab. the space station that NASA was to launch after the Moon programme had been wound down.

Apollo 14’s heavy schedule of in-flight experiments included: demonstrations of electrophoresis, heal flow and convection in a weightless environment; how liquids behaved as they transferred between tanks; and a demonstration of the casting of composite materials in space. However, one of the experiments carried out on board Kitty Hawk was not to be found in the flight plan.

Some observers have noted how. across the six crews who went to the surface, there seemed to be a difference of personality between the guys who occupied the left station of the lunar module and the guys on the right. The commanders were all businesslike, supremely focused and driven and these traits tended to be carried on into their time after Apollo. The guys on the right, while equally as competent and capable, tended to lead more varied lives once the mission was over. It is unclear whether the astronaut selection process managed to tease apart two different types of
pilots from what was a very homo­geneous pool, or whether the experi­ence of the mission itself set the course of their future lives. The left – hand crewmen were all mission commanders, and after Apollo they generally went into business, management or similar professions.

Among the right-hand crewmen,

Schmitt, who was already a scientist, became a politician; Irwin and Duke had a calling in Christianity; Bean took up a career as an artist; and Buzz Aldrin struggled with depres­sion and alcoholism in the years directly after Apollo, then became an excellent ambassador for Apollo through his many TV appearances and public lectures.

The one remaining LMP in this list, Ed Mitchell, later professed a deep interest in states of being that were outside the physical – con­sciousness, spirituality and the para­normal. On the way home from the Moon, he experienced a "grand epiphany” that he later described as "nothing short of an overwhelming sense of universal connectedness”.

During the weeks leading up to the launch, Mitchell arranged a clandestine experiment with a few like-minded friends whereby they would test the ability of ‘psychic forces’ to operate over long distances. On four occasions, twice during each leg of the journey, he concentrated on a sequence of Zener cards[5] while his crewmates were settling down to sleep. The participants on Earth had to determine the sequence. The press had a field day with the story when it was revealed a week after their return. Perhaps they sniffed a chink in NASA’s reputation for having a scientific or engineering-driven approach to everything. However, Mitchell believes his study produced statistically significant results. After his flight, he founded the Institute of Noetic Sciences to continue research into the scientific investigation of the paranormal.

The crew’’s next task was to put the entry monitor system (EMS) through a series of tests to verify that it could be trusted in its role, which was. as the name implied, to monitor the progress of the re-entry.

This bit of kit on the main display console came into its own in the final minutes of the mission, but its weight wasn’t carried for two weeks without it having to work for its passage. NASA’s engineers saved weight by having systems share their components wiiere possible, so throughout the flight, the single accelerometer within the EMS provided backup to those in the guidance platform whenever the effect of
engine burns had to be confirmed. Its digital display was regularly pressed into providing the crew with extra information, but the full cap­abilities of its systems were utilised on re-entry.

It was not a single display; instead, it was a specialised gui­dance and display system to present critical entry parameters to the crew, and it occupied a prominent position on the massive instrument panel directly in front of the left couch. In normal use, it allowed them to monitor the progress of an automatic re-entry through its in­dependent measurement of velocity and g-forces. But if the main guidance system were to fail (which happily never happened), it would have yielded enough information to allow them to steer the spacecraft manually to an accurate and safe landing.

Among the switches and knobs on the EMS panel were three displays that covered various aspects of re-entry. The largest was a window behind which was a scrolling graph on a long Mylar tape, 8.75 centimetres wide. A scribe drew a line across the tape to show the progress of re-entry. The axes of this graph were deceleration and velocity: in other words, the rate at which velocity was being lost – the deceleration – was plotted against how much still had to be lost. As their velocity reduced, the scroll moved to the left and its scribe left a trace to enable the CMP to visualise the trends. If, during the early stages of re-entry, the onset of g – force was too rapid, then the spacecraft was coming in too steeply and needed to roll around and try to fly higher in order to avoid flying through the thicker air for any length of time. Conversely, a low g-force while still at a high velocity could mean that the spacecraft was failing to lose enough energy and would need to dive more deeply into the atmosphere or risk sailing out into space on a long lethal orbit. The graph made these decisions apparent. The scroll was inscribed with lines that represented limits of g-force and the distance that could be achieved during the entry to aid interpretation.

Below the scroll was a digital display that was discussed in earlier chapters owing to its use in previous stages of the flight. During re-entry, it displayed either the range in nautical miles to the splash point, or the current velocity. The initial values for both were entered into the EMS prior to re-entry.

The third display, the roll stability indicator (RSI), looked a little like a simplified artificial horizon, and that is essentially what it was. It consisted of a pointer on a circular display that told the CMP the direction in which the spacecraft’s lift vector was aimed with respect to the Earth below, and was driven by the GDC and its gyros. Since the lift vector was always in the direction of the crew’s feet, it essentially

indicated whether they were flying feel-down or feel-up. Two indicator lamps were located ai ihe lop and bottom of Ihe display which told the CMP which way round the lift vector should be aimed to achieve the correct re-entry conditions. If he had to fly the re-entry manually, he would roll the spacecraft to aim the pointer at whichever lamp was lit.

The panel had two other lamps: one to indicate when the SPS engine had been commanded to fire, and the other to indicate the onset of 0.05 g at the start of re­entry. A knob allowed selection of the instrument’s various functions, including access to live built-in tests with which to verify the unit’s correct operation. These tests ensured that the lamps would trigger at the correct point, that the digital counter was operating, and that the scroll and its scribe were working properly. Two different patterns were printed on the scroll: one for a conventional re-entry, and the other for an entry that included some degree of skip away from Earth for a time. The start of the scroll included several patterns for testing its operation, whereby the scribe was expected to make a predefined scries of motions.

APOLLO IN RETROSPECT

As soon as Eugene Cernan and his crew stepped off their recovery helicopter, Apollo’s lunar programme began its recession into history. Many commentators have tried to weigh up its historical position with arguments that range from Apollo being a shallow political stunt hatched from the hubris of America’s political establishment, to it being the first step in the movement of our species off this planet into the wider cosmos. Perhaps it depends on whether the glass is seen as being half empty or half full.

Journalist William Hines of the Chicago Sun Times took the glass-half-empty approach when he characterised Apollo’s quest for the Moon as being like the quest of a little dog he once watched as it stalked after his car, caught up with it when he stopped, then marked it territorially before walking away. To Hines, Apollo was the same. "We caught the Moon, we peed on it and we left.”

Futurist and science-fiction writer Ray Bradbury had higher aspirations for the lunar programme’s long-term meaning. In 1994 he said, “I’m willing to predict to you that 10,000 years from now, the people of the future will look back and say July 1969 was the greatest month and the greatest day in the history of mankind. It will never change because on that day, mankind freed itself from gravity. We’ve been clinging here on this planet for millions of years and hoping someday to reach the Moon. We dreamt about it when we were living in caves. And finally we broke free and the spirit of mankind soared into space on that night and it will never stop soaring.”

As for myself, I am with Bradbury and his glass-half-full notions. In my perhaps naive optimism, I cannot help but see Apollo as having been a strange, mad but ultimately satisfying adventure of the human spirit. Whatever basal national posturing created it, or filthy pork-barrel politics spread its wealth around, I see in the people of Apollo a generation who rose above such narrow concerns to take one of the most powerful nations on Earth to the Moon and realise a dream that had haunted us since culture existed, and do it in a way that was laid bare to the world – mistakes included.

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

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

On the penultimate day of the Apollo 11 mission, the crew held a press conference from their spacecraft. During the proceedings. Buzz Aldrin pul Apollo’s optimism in these terms on behalf of his crewmates. “We’ve come to the conclusion that this has been lar more than three men on a voyage to the Moon. More, still, than the efforts of a government and industry team. More, even, than the efforts of one nation. We feel that this stands as a symbol of the insatiable curiosity of all mankind to explore the unknown.”

As I studied Apollo, I was always impressed at the monumental dedication of the people associated with the programme; people who would gladly work 16- to 18- hour days, 6 or 7 days a week in buildings where the 5 o’clock rush home was unknown, and without the weight of totalitarian dictatorship bearing down on them; people who felt in their bones that Apollo contained a historic significance that transcended its genesis; people who thought that going to the Moon was just the greatest, coolest thing to have ever been involved with. I count myself as having been hugely blessed to have lived in an age when dedication could achieve such a difficult goal. 1 rejoice that 1 grew to see a new form of bravery in the men who rode the rockets. This kind of heroism did not require that they engage in the slaughter of fellow men and women for the sake of an ideal, but instead it required trust in the brilliance, hard work and imagination of hundreds of thousands of people who placed them at the top of a fantastic machine that could easily kill them, but thankfully on most occasions, did not. Rather, it took them on a voyage of momentous discovery.

Just prior to the first moonlanding, NASA commissioned film director Theo Kamecke to make a reflective documentary about Apollo ll’s journey. One section pondered on the dawn of the mission’s launch day. Over shots of sunrise, the narrator asked, “In what age of man will the meaning of this morning be understood?” The short answer is: Probably not in this age. Detractors who ask why Apollo should have consumed so much of America’s resources while poverty, disease and hatred exist in the world, are asking the wrong question. It is not in the nature of our species to resolve every problem before embarking on something creative, otherwise we would never have had impressionist art, theories of relativity or Egyptian pyramids.

Perhaps Apollo was really about going and seeing what is out there; and who. as a child, did not want to do that?

[1] Hypergolics are a family of chemicals that ignite spontaneously when mixed.

[2] The concept of the orbit is explained in the next chapter.

[3] See Chapter 6 for a fuller explanation of the RHFSMM AT, a defined orientation in space.

[4] The EMS or entry monitor system was discussed in Chapter 5 where we saw how its ability to measure velocity change could be used for manoeuvres to retrieve the LM from the S – IVB. It will be discussed further in Chapter 15. where we shall sec how it is used for its prime purpose, monitoring re-entry into Earth’s atmosphere.

[5] Zener cards are familiar tools for paranormal researchers. Each card has one of five

symbols; a circle, square, cross, star and wavy line.

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

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

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

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.

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.

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.

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.