Category How Apollo Flew to the Moon

The obvious way to rendezvous was to launch off the Moon on a trajectory that directly intercepted the CSM using a single burn of the ascent engine. This was discarded for many reasons. The timing of the launch would have had to have been extremely accurate for the LM to intercept a spacecraft passing by at 1.6 kilometres per second. Even with such split-second accuracy, engineers knew’ that the expected variations in the thrust from the ascent engine would cause the EM to miss the CSM by gross margins. Additionally, the short duration of the approach gave little time to

calculate and correct the trajectory. Furthermore, the closing speed would have been higher than the RCS thrusters could be expected to overcome and if the approach was missed, the LM would find itself in an orbit whose perilune was likely to be below the lunar surface – that is, it would climb, arc back and crash onto the Moon. Direct ascent rendezvous was dangerous in many ways, and on top of all that it would be very difficult for the CSM to rescue a stricken LM.

fn addressing these problems, engineers settled on a more elaborate technique that took a step-by-step approach to incrementally bring the LM towards the CSM in a manner that could be analysed and controlled.

Ed Mitchell, LMP on Apollo 14, once wrote. "Preparing for a burn is a serious business, and before each one. Slu [Roosa] would announce, ‘It s sweaty palms lime again, gentlemen."’ The TEI burn tvas the one where mission control sweated more than usual, and that of Apollo 8 on Christmas Eve of 1968 was viewed with greater apprehension than any other, simply because it was the first. Its CSM was only the second Block II Apollo spacecraft to have flown in space, and they had sent it and its living human cargo all the way around ihc Moon. While ihc engineers had complete confidence in the reliability of the SPS engine, there was always a deep fear that, somewhere in ihc system, human frailly would cause a problem. In the MOCR, a clock counted down to the moment w’hen, if the burn had gone well, the spacecraft should come around the limb. The Earth station at Honeysuckle Creek in Australia was mosi favoured and its 26-mcirc anienna listened carefully.

The time for acquisition of signal (AOS) arrived, and almost immediately, engineers ai Honeysuckle reported a Unified S-band radio signal coming from the spacecraft.

"Apollo 8, Houston," Capcom Ken Mattingly called out to the crew as the engineers in Australia worked to lock ihc great dish’s receivers and transmitters onto the spacecraft.

"Apollo 8. Houston. Apollo 8, Houston,” continued Mattingly.

"Apollo 8. Houston. Apollo 8, Houston.”

"Houston, Apollo 8. Over.” called Jim Lovell from the speeding spacecraft.

"Hello. Apollo 8. Loud and clear.” replied Mattingly, speaking on behalf of all at mission control, all of them relieved that they had pulled off the most daring part of the flight.

"Roger.” said Lovell. Then, with the holiday period in mind. "Please be informed, there is a Santa Claus.”

"Thai’s affirmative,” agreed Mattingly. "You are the best ones to know.”

Soon after CSM Charlie Brown appeared on its way home after TEI on Apollo 10, commander Tom Stafford, wiio tvas an enthusiastic proponent of television from Apollo, turned the spacecraft around to aim their colour TV camera at the receding Moon. One of his impressions when seeing the entire ball of the Moon in one view’ was: "It’s a good thing we came in backwards at night lime where we couldn’t see it, because if we came in from this angle, you’d really have to shut your eyes.”

When Columbia similarly reappeared on time after Apollo ll’s TEI burn. Duke was ready to quiz the crew.

"Hello Apollo 11. Houston. How did it go? Over.”

Collins cheerily replied, "Time to open up the LRL doors. Charlie.” The crew

were now officially in quarantine and were destined to spend most of the next three weeks isolated in the Lunar Receiving Laboratory in Houston.

"Roger,” said Duke. "We got you coming home. It’s well stocked.” Armstrong provided the details of the bum and then praised their trusty SPS engine. "That was a beautiful bum. They don’t come any finer.”

David Scott concurred with how well the SPS worked on Apollo 15: "What a smooth bum that one was. Just can’t beat these rocket engines for travelling.” On his mission, and all the J-missions, it was customary to adjust the spacecraft’s attitude so that the mapping camera could photograph the retreating Moon, and perhaps image more of the polar regions which had been relatively poorly covered by the Lunar Orbiters. Each succeeding exposure showed the Moon receding further and further into the darkness of space.

Apollo 16’s view of the receding Moon taken by its mapping camera. At first, only the far side was visible, but gradually, the eastern mare came into view. (NASA)

As Alan Bean watched the stark lunar globe move away from Yankee Clipper on Apollo 12, he and his crewmates were struck by the unreality of their situation. "This Moon is just this white ball right out in the middle of a big black void, and there just doesn’t seem to be any rhyme or reason why we are here, or why it’s sitting out there. All the time we were in lunar orbit we were discussing this thing – how unreal it looked. And it is amazing to us to fly around it as it is. When you just think about going to the Moon, it is very, very unreal to be there. It’s really getting small in a hurry. It’s just sort of unreal to look outside. It is almost like a photograph moving away from you. It doesn’t seem possible it can be a whole sphere that you were orbiting a couple of hours ago.”

When the CSM left Earth, the service module’s tanks were loaded with 18.5 tonnes of propellant. By the time it was on its way back, the majority of this had

been consumed. What remained had been kept aside as a contingency in case the CSM had to make manoeuvres to rcseue a stricken LM in lunar orbit. For the Apollo 8 flight, with no LM to transport to lunar orbit, the tanks were still a quarter full after TEI. while for Apollo 11, which did have a heavy LM. only an eighth remained. Not all of this remaining propellant was usable. By Apollo 17, the planners had become more knowledgeable about the spacecraft and its capabilities and felt confident to plan the mission such that, after TEI, only four per cent of usable propellant remained in its tanks.

Much of the communication between harth and the spacecraft during a flight was carried out using the S-band communication system, either through the directional high-gain antenna soon to be discarded along with the service module, or through one of the four omnidirectional antennae around the command module’s periphery. However, during re-entry the spacecraft would be travelling at high speed, relatively close to the ground and below the horizon of the major S-band stations. Also, as it entered, it would roll regularly from side to side to steer a course towards the recovery forces.

As the directional nature of S-band communications made it impractical for use during entry, the CM reverted to the shorter range VIIF radio that had been used by earlier Earth-orbiting missions and was used for communication with the lunar module during operations around the Moon. This enabled them to talk to mission control via ARIA communications aircraft deployed along the ground track, and later directly to the recovery aircraft carrier and its associated helicopters. In preparation for this, the VIIF communication system was powered up to enable it to be tested when the spacecraft came within range.

With only 3.000 metres of altitude remaining, another barometric sw itch operated to fire mortars that deployed three pilot chutes into the smooth air stream, which in turn pulled the three main parachutes out from their bays around the tunnel. These were a welcome sight to the crews and became familiar to the public as the impressive 25-metre red-and-white canopies that featured clearly on colour television coverage of an Apollo’s return to Earth.

Both the main and drogue chutes were deployed in a reefed condition; that is. they were inhibited from inflating properly for the first 10 seconds by a line that ran around the edge of the canopy in order to reduce the mechanical shock of their deployment. A timed pyrotechnic device eventually cut the reefing line to allow the canopies to fully open.

“Going to free fall.-’ called Conrad as the drogue chutes disappeared.

“There go the mains!” yelled Gordon when he saw1 them replaced by the three glorious main parachutes.

“Hang on,” said Conrad. “We’ve got all three. A good show.-’

“They’re not dereefed yet,– warned Gordon. They couldn’t slow – enough until at least two canopies were fully inflated.

“There they go,” said Bean. "They’re dereefed.”

“A couple of them are,” said Gordon. “One of them isn’t yet. There they go,” as the last reefing cord let go. “Hello, Houston; Apollo 12,” he yelled to mission control. “Three gorgeous, beautiful chutes, and we’re at 8,000 feet on the way down in great shape.”

When things are occurring rapidly all around, events can appear to happen in slow motion. Collins was watching the deployment of the parachutes intently. “It seemed to me there was quite a bit of delay before they dereefed. All three chutes were stable and all were reefed and they kept staying that way until I was just about the point where I was getting worried about whether they were ever going to dereef; then they did.”

The fully deployed main parachutes rapidly slowed the spacecraft’s descent to just

8.5 metres per second.

While the service module had been attached, spacecraft communications on the VHF system had used two scimitar antennae mounted in semicircular housings on either side of that module. For VHF communication with the recovery forces, two small antennae stored beneath the apex cover popped up automatically soon after the main parachutes had been deployed. To use them, the crew had to manually switch the output of the VHF electronics across to the ‘Recovery’ position.

Engineers wisely allowed a generous margin by designing the main parachutes to enable the CM to land safely with only two inflated canopies. This precaution was

justified when one of the canopies that should have been lowering Endeavour. the Apollo 15 CM to the ocean, failed and uselessly streamed beside its two functioning counterparts. The impact speed only rose from 8.5 to just less than 10 metres per second. Apollo 15‘s CMP Л1 Worden noted that all three chutes had inflated properly when first deployed so blame was put on the crew s next task, their propellant dump.

The propellant tanks for the RCS thrusters still contained much highly noxious propellant, especially hydrazine fuel. As such hazardous substances could not be on board when swimmers were clambering all over the spacecraft after splashdown, the excess was dumped by firing all their thrusters until the tanks were depleted as the spacecraft descended on its three main parachutes. Before doing so. the crew – closed the cabin pressure relief valve to prevent RCS fumes from entering the cabin, and instead, released fresh oxygen from the surge tank into the cabin. When Endeavour’s thrusters fired, its oxidiser tanks had emptied before its fuel tanks so that for a few seconds, unburnt hydrazine was leaving the engines. As hydrazine can burn in air, it has been blamed for damaging the parachute. On subsequent flights, engineers biased the propellant load towards the oxidiser and altered the liming of the burn to try to avoid the problem.

The timing of Apollo 8’s arrival meant that it re-entered just before dawn over the recovery site, so when the RCS tanks started emptying as the spacecraft descended on its main parachutes, the crew were treated to a sight which, though spectacular, was somewhat worrying. ‘The ride on the mains was very smooth,’’ said Borman afterwards, "and we could not. of course, see the mains because of the darkness until we started dumping the fuel. When we dumped the fuel, we got a good chute check, but there was so much fire and brimstone around those risers that we were really glad to see the fuel dump stop.”

Once the RCS propellant tanks had been emptied, the system’s plumbing was purged with helium gas to drive out as much trace propellant as possible.

At 1,000 metres altitude, with the RCS dump completed, the cabin pressure relief valve was reset to its dump position, which allowed the cabin’s air pressure to fully equalise wfith the outside atmosphere. It was finally closed 250 metres up, to prevent water entering the cabin at impact. For a short Lime, the spacecraft would be partially submerged when it hit the water and there was a good chance that it might be upside-down for a few minutes. The parachutes suspended the command module at an angle of 27.5 degrees to the horizontal with the main hatch facing upwards. This caused the hull to hit the water ‘toe first’, in a fashion that spread the final deceleration over the longest possible time. Also, the periphery of the CM structure was formed by shaped ribs. Those opposite the hatch, where the spacecraft would contact the water first, were designed to be crushable to help to reduce the force of impaet. They were primarily intended for the undesirable contingency of a land impact but could deform to help to reduce the shock of a conventional sea landing.

Whether they had entered the descent orbit or w’ere in the circular orbit that was a characteristic of the earlier expeditions, the crew had reached their quarry and. in most cases, could relax a little before the exertions of the next day: undocking, separation, descent and landing, along with, perhaps, a trip on the lunar surface. This was time to get out a meal, look after the housekeeping of the CSM and take photographs lots and lots of photographs.

How’ever, for the crew of Apollo 8 there was no time to relax. Once they had completed their LOI-2 burn, Frank Borman, Jim Lovell and Bill Anders had eight orbits and 16 hours remaining in the Moon’s vicinity. Their Lime was precious, and had been carefully rationed. Borman took care of actually flying the ship – not in the sense of sweeping over hills and dow:n valleys; orbital mechanics was the arbiter of their flight path. Instead, his job was to make sure that the spacecraft was aimed in

Two era-defining craters seen from Apollo 17. Top. Copernicus, about 900 million years old, still sports a clear ray system. Below. Eratosthenes is similar to Copernicus in structure but is old enough, about three billion years, to have lost its rays.

The view from orbit. Top left, Herodotus and Aristarchus among lava channels. Top right, Rimae Prinz and a lava-formed depression. Centre, Mons Riimker is a large, low volcanic mound. Bottom left, craters Stratton (foreground) and Keeler on the beat-up far side. Bottom right, central peak of Tsiolkovsky. (NASA)

whichever direction was required to satisfy the tasks of his colleagues. This became particularly important in view of their main windows having become fogged. To gain a clear view of the surface, they were left with only the two small forward-pointing rendezvous windows, whose narrow field of view was never intended to facilitate general photography.

Each orbital circuit was split into four by the geometry of the Sun and Earth with respect to the Moon, and this defined their tasks. Any task that involved working with mission control could only occur during a near-side pass. Anders’s prime responsibility was a programme of photographic reconnaissance of the Moon, and most of this work could only occur over the sunlit lunar hemisphere. Therefore, when they were over the night-time portion of the near side, he was free to check over the spacecraft’s systems and write down abort PADs from mission control. For about half an hour of each orbit, soon after AOS. the crew became especially busy as they approached Mare Tranquillitatis. As well as chatting to mission control, Lovell and Borman w orked together to view’ and photograph one of the planned landing sites, looking for visual cues that could be used by a landing crew’ and for obstacles that might pose a danger to a future lunar module.

Part of the reason for Apollo 8 going to the Moon, beyond the political act of getting one over on the Soviets, was to gain its much experience of lunar operations as possible before the landing missions were finalised. One of the techniques pioneered by this crew’ was the first use of the spacecraft’s guidance and navigation system, along with visual sightings of landmarks passing below-, to help to determine their orbit more accurately. Prior to the mission, a number of landmarks were selected for Lovell to view – through the spacecraft’s sextant. A mark was taken by pressing a button when a landmark was perfectly centred in the optics. From repeated marks and careful tracking from Earth, they were able to improve knowledge of the precise shape of the Moon, and also prove the techniques of lunar orbit navigation for future missions.

In addition to performing for the first time the unique tasks associated with flying next to another world, this crew continued to care for the spacecraft that was keeping them alive. Lovell occasionally took over the steering of the spacecraft while he looked for stars with which to realign the guidance platform. Anders looked after the environmental and propulsive systems, taking time out for a series of systems checks. All of their tasks were swapped around, allowing them, in turns, to gel some rest during this frenetic period as they tried to nurse their own exhausted metabolisms on a mission that, so far, had denied them adequate rest. Catching sleep when the other members of the crew were busy had proved impracticable on the way to the Moon. Trying to do so. as laid out in the flight plan, during the climax of humanity’s furthest adventure, proved even more difficult.

They had arrived tired and none of them could rest as they shared the excitement of seeing the Moon close up for the first time. By the seventh orbit. Borman began to notice that he was making mistakes. Worse, Lovell was having finger trouble with the computer. Aware that in a little over six hours they had to make a TEI burn to get themselves home, that they had a historic TV broadcast to make during the near-

side pass prior to that bum and that they had all been awake for at least 18 hours, Borman took control. “I’m going to scrub all the other experiments, the converging stereo or other photography. As we are a little bit tired, I want to use that last bit to really make sure we’re right for TEI.”

To make sure that mission control understood what he intended, he specifically referred to the CMP tasks: “I want to scrub these control point sightings on this next rev too, and let Jim take a rest.”

Despite the can-do spirit of his colleagues, Borman stuck to his guns. “You’re too tired,” he admonished. “You need some sleep, and I want everybody sharp for TEI; that’s just like a retro.”

He was comparing the TEI bum to the retro bum used to get out of Earth orbit and return to the ground. In many ways the two types of bum had similar dire implications if they were to fail, except that, lor the latter, there might be a remote possibility of a rescue mission around the Earth. Anders realised that his commander wasn’t fooling and suggested a way of getting more science done wBile they rested: "Hey. Frank, how about on this next pass you just point it down to the ground and turn the goddamn cameras on; let them run automatically?"

"Yes, we can do that."

Mission control were used to having things done as prescribed, but understood the crew’s need for rest. Still, Capcom Mike Collins had to relay a request for exactly what was being cancelled. "We would like to clarify whether you intend to scrub control points 1, 2 and 3 only, and do the pseudo-landing site; or whether you also intend to scrub the pseudo-landing site marks. Over."

Borman was uncompromising. Only the success of the mission was important to him. If he sensed that their reconnaissance task was jeopardising their chances of getting home, he had no hesitation in dropping it. "We’re scrubbing everything. I’ll stay up and point – keep the spacecraft vertical and take some automatic pictures, but I want Jim and Bill to get some rest."

Mission control relented. Anders, being a typical driven perfectionist, tried again to continue with his tasks: “I’m willing to try it," he offered.

"You try it. and then we’ll make another mistake, like "Entering’ instead of "Proceeding’ [on the computer] or screwing up somewhere like I did."

When Lovell spoke up. Borman stood his ground. “I want you to get your ass in bed! Right now! No, get to bed! Go to bed! Hurry up! I’m not kidding you, get to bed!"

Despite their tiredness, the crew completed their 10 orbits around the Moon over Christmas and fired their engine for a safe THI and return home.

The lunar module carried two important radar systems that were tested prior to landing. The first checkout was for the rendezvous radar while the CSM was still nearby. This radar worked in conjunction with a transponder on the CSM to give the crew’ and the LM computer information about how far away the CSM was, how fast it was approaching and in what direction it was located. Although there were backup methods for the spacecraft to rendezvous, this radar was an important primary component for bringing the two spacecraft together. Its dish antenna wras attached to a 2-axis mount that permitted pan and tilt movement. When it started operating, it sw ept the view in front of the LM, looking for the CSM until a return signal from the transponder was found. The receiving horn was split into four so that if the dish were not exactly borcsighied on the CSM, the received signal would be stronger in one of the horns. The electronics could then operate to aim the antenna until all four horns received an equal strength signal. The angle of the dish then represented the direction to the CSM. The information from the radar wras factored in along with knowledge of the LM’s state vector and orbit to derive all the necessary information needed by the crew to make appropriate rendezvous manoeuvres.

Whereas the first three landings had been on open, if rugged sites, the approach taken by Falcon on Apollo 15 took the LM between a pair of mountains. This made the experience of landing somewhat different, especially during P64’s regime.

“Falcon, Houston,” said Ed Mitchell in mission control. “We expect you may be a little south of the site; 3,000 feet.” By that, he meant that their flight path, travelling

east to west, seemed to be taking them to a point one kilometre south of their intended target. When they started P64, Scott would have to steer to the right to get them back on track.

“Okay. Coming up on 8,000,” said Irwin as they passed 2,500 metres altitude. Even before P64, he had begun to concentrate on keeping his commander up to date. This was one of two events that biased Scott’s estimation of where they were going to land. The other concerned what he and Irwin saw out to the left prior to P64. “I looked out the window, and I could see Mount Hadley Delta. We seemed to be floating across Hadley Delta and my impression at the time was that we were way long because I could see the mountain out the window and we were still probably

10,0 to 11,000 feet high.” Scott then approached pitchover thinking he was going to land long and south, which was worrying because several kilometres to the west of the intended landing point was Hadley Rille, and he didn’t want to come down in its canyon. Actually, they were at about 2,750 metres, and the 3,350-metre mountain was towering 600 metres above them.

In later years, Irwin discussed the moment of pitchover. “We’re not looking down as we come over the mountains,” he chuckled. “We’re looking straight up until we get down to around 6,000 feet [1,800 metres] and we pitch forward about 30 degrees and, at that point, we could look forward and see where we were. We could see the mountains. 1 was startled because, out the window, 1 could see Mount Hadley Delta which towered about 6.000 or 7,000 feet above us. And we never had that type of presentation in the simulator." Landing simulations had used a small TV camera flying over a plaster model of the surface to present the crew with the view they would get out of their window’s. It included the impressive rille that would be in front of them, but not the mountains to either side of the approach track.

"When we pitched over." continued Irw’in, "I could see the mountain that tow’ered above us out Dave’s window. I’m sure it startled Dave, too, because we wanted to know, you know, were we coming in to the right place? Fortunately, the rille was there and it was such a beautiful landmark that we knew’ we were coming in to the right area. But we’d never had that side view in any of our simulations. It was just the front view, a level plain with the canyon. And it would have been very impressive to be able to look out as we were skimming over the mountains with about 6,000-foot terrain clearance. At that speed it would have been really spectacular, like a low-level pass as we came over the mountains down into the valley.’’

"7.000 feet. P64!" called Irwin as they passed 2.150 metres altitude.

"Okay." said Scott.

"We have LPD." said Irwin as an angle appeared on the DSKY.

Scott finally saw’ wiiere he was going. Ilis impression of landing long had been wrong, but mission control’s southerly estimation was correct. "LPD. Coming right," he said as he began a series of redesignations to move the computer’s targeting north to where they had planned to touch down.

To deal with ihc mountainous terrain around the Hadley landing site, planners had made a change to the approach phase of the descent. Instead of making a shallow approach of only about a 12-dcgree angle to the ground as the previous missions had done. Falcon came in on a much steeper 25-degree approach path.

"Four-zero." called Irwin as he began feeding Scott with constant updates of the LPD angle on the window and their altitude.

"5.000 feet. 39. 39. 38. 39."

"4.000 feet. 40. 41. 45. 47. 52."

"3.000 feet. 52. 52. 51. 50. 47. 47."

"2.000 feet. 42."

"Okay. 1 got a good spot." said Scott once he had decided where he was going to set it down.

LUNA COGNITA

The Moon – that inconstant orb; a glorious bright light in our night sky; an ancient vehicle for human myths and deities; and now a world become known. For thousands of years before the rise of the scientific method, humans gazed at Earth’s one natural satellite, wondered at its nature and worked it into their stories as they struggled to understand their universe and its impact upon them. It was only with the invention of the telescope that the true nature of our satellite world began to be revealed.

Luna’s face

When casually viewed from Earth, the Moon exhibits a mottling of dark grey patches against a lighter grey landscape. What was not realised until Apollo’s rocks were returned was that these features are a window into the earliest era of the solar system. We now know that the light grey areas, rough and heavily cratered, form an extremely ancient highland terrain that dates beyond four billion years ago. The dark patches, called mare (pronounced hnaa-ray’, plural maria), are great plains of basalt that solidified from immense effusions of the Moon’s particularly runny lava that flowed more than three billion years ago. In many cases, these outpourings of molten rock filled large circular basins that had been excavated some time previously by cataclysmic impacts. Peppered across its face are bright sprays of material that emanate from some of the craters. These are rays of ejecta – shocked and pulverised rock thrown out from more recent high-speed collisions by somewhat smaller bodies. Given immense time, these rays will fade and darken to match their surroundings.

Apollo 13 saw a serious push to use the CSM as an orbiting science platform from which to reconnoitre the lunar surface. The primary tool for this was the Hycon lunar topographic camera: a monster instrument modified from an aerial reconnaissance camera, whose 467-millimetre-focal-length lens peered out through the round hatch window and exposed square 114-millimetre negatives. The Hycon had an unhappy career in Apollo. It lay unused on board the Apollo 13 command module while the crew struggled to reach home after their mission was aborted. It was sent once more on Apollo 14. Once alone in his domain, and having made the circularisation burn to take the Apollo 14 CSM Kitty Hawk into a 110-kilometre orbit, Stu Roosa began a photographic pass that was to have included the Descartes region where scientists were considering sending a future mission. After about 200 exposures, the camera failed, never to work again despite the best troubleshooting efforts of Roosa and mission control.

The SI. VI bay

Lunar science from orbit really got into its stride with the J-missions. just as surface exploration had. In particular, one of the six sectors in the cylindrical service module which had been largely empty on previous missions, gained a bay of instruments and cameras that could be trained on the lunar surface for the five or so days that the CSM spent in orbit. This scientific instrument module, or SIM bay, was operated by the CMP from the time the spacecraft entered orbit until the service module was jettisoned shortly before re-entry.

Each example of the SIM bay that flew carried two cameras: a mapping camera and a panoramic camera, both of which were heavily derived from aerial and space reconnaissance cameras that were classified at the time. Each bay also included a suite of sensors, some of which were deployed out on the ends of retractable booms. These could divine the mineral composition of the lunar soil by sampling its various emissions. On Apollos 15 and 16. a tiny satellite was ejected just before the crew left to come home. It monitored the particles and fields mound the Moon for up to a year.

The full capabilities of the SIM bay were never brought to bear on the whole lunar surface, largely because the CSM’s orbit was defined by the position of the landing site. This limited the reach of the cameras and sensors to a narrow swathe near the equator. At one time, planners had envisioned an I-mission that would have placed a CSM in a polar orbit for a full month, from which its cameras could have imaged the entire surface at a consistent lighting angle, and its remote-sensing instruments could similarly have sampled the entire Moon. With Apollo in its declining years, funds for such a mission were not forthcoming and the scientists had to wait a generation for comprehensive coverage from advanced probes sent by a number of interested nations.

For the early Apollo missions, NASA settled on the coelliptic rendezvous to carefully reunite the two spacecraft. To best grasp the various elements of this technique, it may help to work backwards from the rendezvous itself and see how its demands determined the orbital dance around the Moon that led up to it.

NASA standardised how it would fly the final part of the rendezvous – what it termed the terminal phase – to control the approach speed of the LM. They wanted to limit the speed to a value that could be dealt with by the RCS thrusters of either spacecraft, ft is worth remembering that by this point in its mission the LM was a very light spacecraft and its thrusters were very responsive. The CSM, on the other hand, was heavy and still had a substantial load of propellant on board. Its thusters were much less effective, should it be called upon to fulfil the rendezvous. Another issue was illumination. Theoretical studies and Gemini experience had demon­strated that the terminal phase should be flown over 130 degrees of the CSM’s orbit, with appropriate approach speeds set throughout this period to maintain control of the situation. Therefore, planners could choose where in the CSM’s orbit they wanted the rendezvous to occur, and with lighting taken into account, work back 130 degrees to define the point where the terminal phase ought to begin. This point would be where the LM would execute the terminal phase initiation (TPI) burn. One huge advantage of this approach was that as the LM rose to meet the CSM. the latter would appear to be stationary against the background stars. In one sense, the two craft would be on intersecting orbits hurtling around the Moon at nearly 6,000 kilometres per hour. But in another, inertial sense that ignored the Moon below, they would be approaching along a straight line that could be drawn out to the stars. This arrangement would allow the crew7 to visually check their progress. Because of the need to be able to see the stars clearly, most of the terminal phase was arranged to occur in the Moon’s shadow7, but the final approach would be made in sunlight for improved visibility. Additionally, opportunities were included during the approach for course corrections, based on data from their radar.

Continuing to work backwards, planners arranged for the LM to spend about 40 minutes in an orbit that was a constant 28 kilometres below7 the CSM’s orbit. An important point to note is that this constant difference in height had to be maintained even if the CSM’s orbit was elliptical. NASA used the term constant delta height (CDII) for this part of the rendezvous trajectory, and as the LM crew7 had to make a burn to shape their orbit to meet this condition, the manoeuvre was obviously known as the CDII burn. The purpose of this part of the flight was to give the crew time to track the CSM and calculate the burn that would be needed at the start of the terminal phase of the rendezvous. If the orbits of both spacecraft leading up to the CDII burn were nearly circular and the errors were small, then it was possible to dispense with this manoeuvre.

The trajectory leading up to the CDII burn was essentially a circular orbit of 84 kilometres altitude which was entered by the coelliptie sequence initiation (CSI) burn. This burn w-as made half an orbit back from where the CDH burn would occur.

As we continue to work backwards, the only section that remains is the time from launch to the CSI burn. Around the time that the CSM passed over the landing site, the LM ascent stage lifted off from the discarded descent stage. The ascent engine burned for about seven minutes, ideally to achieve an orbit with a perilune of 17 kilometres and an apolune of 84 kilometres. Half an orbit after insertion, the spacecraft had coasted to its apolune. at which point the CSI burn would circularise the orbit and the coelliptie rendezvous would begin. Except for the initial ascent, all the burns would be made with the RCS thrusters.

To summarise this sequence chronologically:

• After launch, the LM entered a 17 by 84-kilometre orbit.

• It coasted for half an orbit until it reached apolune. The crew then made the CSI burn to circularise the orbit and then began to track the CSM.

• After another half orbit, they made the CDH burn, if required, to reshape the orbit and have the LM fly 28 kilometres below the CSM’s orbit.

• During a 40-minute coast, further tracking determined the details of the burn that would begin the terminal phase.

• The TPI burn placed the LM on an intercept trajectory that was, at least for a short period, essentially a transfer orbit. It raised the orbit’s apolune slightly higher than the CSM’s altitude so that it would intercept its target over 130 degrees of orbital travel.

As the LM rose to meet the CSM during the terminal phase, the rendezvous radar on the LM continually worked with the transponder on the CSM to determine their separation and their rate of closure. The crew monitored these values using the PGNS and the AGS, searching for any hint that they might be deviating from their preferred trajectory – which was a straight line in terms of inertial space, this being implied by the fact that they held the target fixed against the stars. There were two opportunities for mid-course corrections, then the commander made a series of braking burns using his thrusters to bring their closing speed to zero as the LM pulled up alongside the CSM, thereby never reaching the notional apolune of the orbit of which the start of the terminal phase was a short arc.

On Apollo 12, Pete Conrad, like all the other Apollo commanders, did most of the actual flying as he monitored Intrepids return to orbit by watching the PGNS and making burns based on its results. Meantime, to his right, A1 Bean was never really given the chance to pilot anything, which was normal on an Apollo mission. Despite being the lunar module pilot, his role was more that of a flight cngincer/co – pilot, although in extreme situations he could take over using the controls that were provided at his station. His chief task was to operate the AGS in case this backup guidance system had to be brought in to control the spacecraft. Like the PGNS. it generated numbers that reflected its determination of their trajectory, which he compared to the answers coming from the primary system. At any point in the rendezvous, usually after they completed an important step, the LMP could update the AGS’s knowledge of where they w’ere, with that from the PGNS assuming, that is, that everyone w-as happy with the performance of the PGNS. The AGS w’ould then continue to determine its independent trajectory from that point onwards.

As a test in Apollo 12. Bean w’as to try and operate the AGS entirely separately from the PGNS to see if it were possible. Unfortunately, a misskey on the run up to the CSI burn meant that he had to make one update from the PGNS after which he resumed the test. Bean found it to be quite exhausting: “After CSI, we realigned the AGS to the PGNS. Then I made all the AGS marks after that just as we’d planned to do, and got solutions that all compared very favourably. This show’s that the AGS w’ould do the job, would get solutions, w’hich we. of course, suspected anyhow-. But the whole point is that you dona want to use the AGS as the normal rendezvous mode. It requires that every two or three minutes, you make a lot of entries in the AGS. It requires that you point the spacecraft exactly at the command module, which takes time and effort. The LMP is working continually and isn’t able to sit back and think through exactly what’s going on in the rest of the spacecraft.”

Bean wished his role in operating the AGS could have been less manual. Of all the LM crewmen, this man. who w-as to become an accomplished artist, perhaps deserved more than others a little time to absorb the experience. During his debrief, he related how Conrad had been sensitive to his needs: "I continued to work to input the data into the AGS until the second mid-course [in the terminal phase] when Pete said, Tley. why don’t you quit working and sit back and enjoy the flight?’ I got to thinking about it later and that was the first Lime I’d really looked out to see what was going on. The rest of the time I’d just been w’orking my fanny off trying to get all those marks into the AGS. and that’s not the way you want to fly a spacecraft.”

Years later, he told how Conrad had offered him the controls over the far side of the Moon. Out of earshot of mission control. Bean experienced how: the light spacecraft, with its main tanks nearly empty, responded keenly to every impulse from the thrusters. At last, a lunar module pilot had been allowed to pilot a lunar module.

The step-by-step coelliptic rendezvous took nearly two orbits to fulfil but it gave crew’s plenty of Lime to Lake optical and radar measurements of the angle and distance to their quarry, to evaluate their progress and to calculate burns that minimised the risk of errors placing them into a dangerous orbit. It also permitted greater flexibility if the CSM had to perform a rescue. For this possibility, and as a

backup, the CMP in the passive CSM was kept busy making his own determinations of their orbit in permanent readiness for a LM abort.

The coelliptic rendezvous used for Apollos 10, 11 and 12 took nearly four hours to execute, which was a significant amount of time in a spacecraft whose total working life was itself measured in mere hours. When the advanced missions added a six-hour moonwalk to the last surface day, such a rendezvous would keep the crew awake for nearly 24 hours. In the hiatus imposed by the Apollo 13 incident, Scott and Irwin experimented in the simulators to see whether they could shorten the rendezvous, and the arrangement that they devised, which removed an entire orbit lasting two hours, was successfully demonstrated by the Apollo 14 mission.