Category How Apollo Flew to the Moon

The right time to return from the Moon was dependent on the mission, the consumables available to the crew, the propellant available to the engine, and the status of the flight; that is, whether an emergency forced an early departure. As soon as they arrived in lunar orbit, and throughout their stay, the crews of all the missions were given abort PADs at regular intervals, lists of numbers giving instructions for a TEI manoeuvre that would allow them to make an early independent return to Earth. None of the missions ever needed to use these PADs.

The first flight to enter lunar orbit, Apollo 8. did not stay for long because, as a pioneering flight, it was noi one of intense exploration. Rather, it was more of a ‘grab and run’ affair, orbiting for only 10 revolutions and 20 hours in the second Apollo CSM to fly, and proving that it and its crew – could achieve lunar orbit and still return home safely, with a little reconnaissance thrown in for good measure. Prior to loss of signal on each orbit, Frank Borman insisted that mission control give him an explicit Go to continue orbiting, otherwise he intended to use the contingency TEI data to fire up the SPS engine and send the spacecraft back to Earth. In the event, Apollo 8’s CSM worked like a charm and there was no reason to come home early. Borman and his crew made a successful burn at the end of the tenth orbit around the far side to begin their long fall to Earth as planned.

Similarly, the lunar missions immediately following Apollo 8 did not stay around the Moon for long. Once the LM crew had returned from their exploration of the surface, the crews either headed for home soon after the lander’s ascent stage had been jettisoned, or took a single night’s rest in lunar orbit. This changed with the introduction of the J-missions. Having spent significant sums to extend the capability of the CSM and to pack a suite of scientific instruments into the side of the service module. NASA decided that the spacecraft should remain in orbit around the Moon for a full day after the LM had been jettisoned. This additional time was of particular benefit to Apollos 15 and 17. ‘fheir northerly landing sites required the CSM’s orbit to be significantly tilted with respect to the lunar equator. This meant that the Moon’s rotation brought new terrain into the realms of the sensors and cameras and allowed the sunrise terminator to crawl across the surface for another day, another 12 degrees of longitude, thereby bringing more landscape into view?. The near-equatorial orbit of Apollo 16 offered little benefit from an extended stay and, in the event, the problem with Casper’s SPS engine gimbals led mission control to forego the extra day. The crews of the other two J-missions reported that extra time in lunar orbit gave them a chance to wind down and rest after w hat had been an arduous expedition to the surface.

The first generation of manned spacecraft for example, the American Mercury and the Soviet Vostok ships – were designed to re-enter the atmosphere in a purely ballistic fashion. Once they were set on their Earthward trajectory, they had no ability to change their flight path and steer towards a landing site. Later generations of spacecraft like Gemini and Apollo, and the Soyuz, could fly in a controlled manner even without wings.

Although the Apollo command module had a symmetrical shape, its internal weight distribution was deliberately offset to place its centre of mass towards the crew;’s feet. This made it adopt an aerodynamically stable attitude that leaned to one side as it ploughed through the atmosphere because the lighter side of the spacecraft tended to succumb to the atmospheric drag to a greater extent. Such a lopsided presentation to the hypersonic airflow turned the stubby spacecraft into a crude
wing, giving it the ability to generate lift in a direction towards the crew’s feet. Therefore, simply by performing a roll manoeuvre, the spacecraft could aim this lift vector in any direction perpendicular to the flight path which allowed the re-entry to be flown in a controlled manner, usually by the computer.

This term, ‘lift vector’ can be confusing as it is borrowed from the aeronautical world where it is applied to a wing’s ability to provide a lifting force. But just like an aerobatic aircraft that rolls and loops, that force can be in any direction perpendicular to the airflow. It is perfectly possible for the direction of the so – called ‘lift’ to be downwards, towards Earth. If the spacecraft was a little high in the re-entry corridor and was going to overshoot the landing site, the roll thrusters could fire to turn the spacecraft around to a heads-up attitude and aim the lift vector towards Earth. This would force it into a lower flight path where the thicker atmosphere would reduce its speed further. The meagre lift that such a poor wing could generate was amplified by the huge speed of re-entry to the extent that, for a few minutes, the spacecraft would typically fly at about a constant 60-kilometres altitude and, in some cases, even manage to rise away from Earth.

If the entry plan required the spacecraft to make a skip, perhaps to extend the flight path as happened on Apollo 11, the computer advanced to P65 which controlled the ascending part of the skip-out trajectory. Whether the skip caused the spacecraft to briefly leave the atmosphere depended on how far the flight path was being extended from entry interface. Nevertheless, there was a program, P66, ready to revert attitude control to the RCS thrusters should P65 sense that deceleration had dropped sufficiently low. If the spacecraft did not rise out of the atmosphere, as was the case with Apollo 1 l’s skip, then P66 was not used and P65 handed over to P67. If there was a second re-entry, it would be P66 that passed control to P67. In most cases however, re-entry did not include a skip-out phase so P64 handed directly to P67.

In the case of Apollo 11. as they approached Earth on their three-day coast, the weather in the prime recovery area looked increasingly poor so the decision was taken to maintain their trajectory and revise the re-entry flight path to include a skip – out. thereby extending their flight through the atmosphere from 2,200 kilometres to nearly 2,800 kilometres. “1 wasn’t very happy with that,” said Collins at his debrief, "because the great majority of our practice and simulator work had been done on a 1.187 [2.200-km] target point. The few Limes we fooled around with long-range targets, the computer’s performance and the ground’s parameters seemed to be in disagreement. So. when they said 1,500 miles [2.800 kilometres], both Neil and I thought. ‘Oh God. we’re going to end up having a big argument about whether the computer is Go or No-Go for a 1,500-mile entry.’ Plus 1,500 miles is not nearly as compatible. It doesn’t look quite the same on the EMS trace. If you had to take over, you’d be hard-pressed to come anywhere near the ship. For these reasons. I wasn’t too happy about going 1.500 miles, but I cannot quarrel with the decision. The system is built that way and. if the weather is bad in the recovery area. 1 think it’s probably advantageous to go 1.500 miles than to come down through a thunder­storm.”

Apollo missions were always timed to arrive at their planned landing sites soon after sunrise and therefore close to the terminator the line that divides night from day. If the mission had been planned to set down on the eastern side of the Moon’s disk then, from Earth, the Moon would appear as a crescent at the time of landing because the sunrise tenninator would also be to the east. Л western landing site called for a western terminator, at which time the Moon would appear gibbous, or approaching full. In all cases, the spacecraft flew’ over a night-time Moon soon after it lost contact with Earth in the run up to LOI.

Apollo 8 pioneered human travel to the Moon. Its crew’ did not intend to land, but were to reconnoitre an easterly landing site in Mare Tranquillitatis under planned lighting conditions. The flight was limed to have the sunrise terminator near their target which meant that most of the illuminated surface was on the far side. Having approached through ihc lunar night, they could hope to cross the sunset terminator and fly back into daylight around the far side barely five minutes before they were to fire the LOI burn. Frank Borman peered out of a rear-lacing spacecraft, hoping to see some sign of a lunar horizon in order to crosscheck his attitude, even if only by seeing a part of the sky that lacked stars.

“On that horizon, boy. I can’t see squat out there."

Bill Anders suggested that they turn some lights off to help him to see some trace of the lunar surface. As they w’ere flying heads-dowm. their large windows were looking off to the side and below and even though they were fogged, a sunlit lunar surface ought to have been visible. Then Lovell, looking through the hatch window’, piped up: “Hey. I got ihc Moon."

For the first time in the mission, the crew could see shafts of sunlight obliquely illuminating the lunar surface.

"Do you?" asked Anders.

"Right below’ us."

When Anders managed to catch his first view of the forbidding, harsh landscape, he expressed his astonishment. "Oh, my God!"

His commander, wlio was focused on the preparations for the burn, was brought up short by this most uncharacteristic of utterances for a test pilot.

"What’s wrong?" he demanded.

"Look at that!" Anders exclaimed again. But his commander w as more concerned that, at this critical phase of the mission, his crew mates had become distracted by the unreal scenery passing below’.

"Well, come on," said Borman, working to bring the crew back onto the task in hand. "Let’s – What’s, what’s the…" Lovell immediately got into line, calling out the mission time.

"69:06"

"Stand by." commanded Borman. "We’re all set."

For the next minute, the crew’s concentration returned to the cheeks and calls defined in their checklists. Yet with three minutes remaining, Anders’s attention returned to the scenery below’. "Look at that fantastic!"

"Yes," confirmed Lovell.

"See it?" continued Anders. The curious scientist in him was dominating.

"Fantastic, but you know. I still have trouble telling the holes from the bumps."

Borman, whose main responsibility was to ‘keep the troops focused’, had to gently chide his crcwmatcs to keep their eyes inside the cabin.

‘‘All right, all right, come on. You’re going to look at that for a long time."

And they did, while circling the Moon ten times over a period of 20 full and tiring hours prior to the crucial burn that would get them home.

The Apollo 10 crew experienced the same dynamic when LMP Eugene Cernan had opportunities to see out w hile his commander Tom Stafford and the CMP John Young exchanged checklist calls.

"Look at the size of..he exclaimed in the middle of his colleagues’ dry technical checks. "God, that Moon is beautiful; we’re right on top of it…”

"Oh shit!" wfas Stafford’s reaction. Cernan continued. "God dang. We’re right on top of it. I can see it.’’

Stafford began telling Young what was out of his window on the left. "Oh. shit; John! It looks like a big plaster-of-paris cast.”

With less than two minutes to their LOI burn. Stafford’s sense of responsibility reasserted itself. "Ok. let’s get busy," he called to his crew.

For some time, their procedures took precedence until Cernan’s curiosity bubbled up again. "My God. that’s incredible." he said in w’onderment as the very rough, obliquely-lit landscape of the far side slid below.

"It looks like we’re close.” said Stafford.

"That’s incredible,’’ interrupted Cernan.

"It does look like we’re – well, we’re about 60 [nautical] miles, I guess.”

And they w’ere. With only a minute to go. the spacecraft was at its perilune of 110 kilometres and Stafford and Cernan were again getting caught up in the view’.

"Shit, baby; we have arrived – It’s a big grey plaster-of-paris thing…”

"Oh, my God, that’s incredible," Cernan interjected.

"Okay, let’s keep going; we’ve got to watch this bear here,” said Stafford, referring to the strength of the engine about to keep them in the Moon’s arms. Eventually, it w’as the ever-cool Young w’ho reeled the moonstruck Cernan back in.

"Put your head back in the cockpit. Genc-o.”

"Look at that!” was the final spurt of wonder that came from Cernan before he settled dow’n to monitor the health of the engine on which their lives depended.

Each crew’ reacted differently to their initial view’ of the Moon. Apollo ll’s crew’ were very focused during their preparations for LOI, making little comment on anything but the health of their ship until the last few seconds before the burn. Then Mike Collins, the most gregarious of the three, threw in an observation: "Yes, the Moon is there, boy, in all its splendour.’’

Neil Armstrong started into conversation. "Man, it’s a…,” before Collins interrupted, "Plaster-of-paris grey to me.’’

Buzz Aldrin felt moved to speak. "Man, look at it,” he said before Armstrong, maintaining the mantle of his command, advised, "Don’t look at it; here we come up to Tig [time of ignition].”

Apollo 11 began its entry into lunar orbit and the crew’ chatted about tank pressures, propellant utilisation and how much their engine w’as moving from side to side as it controlled its aim which prompted them to wonder whether there might be a problem with it. After the burn’s successful conclusion, they concentrated on the follow-up activities; power-down, backing out of the armed status of the SPS engine and setting up the spacecraft for coasting flight again. Only then did any of them relax enough to take in the scene. Armstrong was first to comment; "That was a beautiful burn.’’

Collins agreed, "God damn. I guess."

"Whoo!" exhaled Aldrin. before making an initial window observation. "Well, I have to vote with the [Apollo] 10 crew. That thing is brown."

There had been some debate and contradiction between the first two flights about what colour the Moon appeared to be close up. The Apollo 8 crew had reported nothing but grey, whereas the Apollo 10 crew thought that tans and browns were common. Armstrong and Collins agreed with them but took their observations further.

"Looks tan to me," observed the commander before Aldrin qualified himself.

"But when I first saw it, at the other sun angle.. .”

"It looked grey." interjected Collins.

".. .it really looked grey," Aldrin concurred.

The last word goes to John Young when he arrived at the Moon for the second time on Apollo 16. Soon after their first AOS, Young described his crewmates’ reaction to the scenery: "It’s like three guys, they’ve each got a window, and we’re staring at the ground. Boy. this has got to be the neatest way to make a living anybody’s ever invented.”

Immediately after undocking, the CMP executed a short burn to put some distance between the two spacecraft to avoid a collision. This was a translation manoeuvre in the CSM’s minus-.v direction to back it away from the LM at 0.3 metre (1 foot) per second. Both crews filmed each other and, especially on the early flights, the CSM would station-keep to allow the LM to be visually inspected and its landing gear verified.

Now flying as separate spacecraft, two important checks had to be made before the LM would be allowed to fly away from the safety afforded by its mothership. The first check looked at the main engine that would control the descent to the Moon, including the control system that throttled its thrust. The second check exercised the rendezvous radar that would be crucial to navigate a return to the CSM. ~

Six minutes into Apollo ll’s descent. Duke came up with a time for the crew. "Six plus 25. throttle dowm.”

“Roger. Copy,” said Aldrin.

‘’Six plus 25.’’ reiterated Armstrong.

Houston had calculated that they could expect the engine to start to throttle 6 minutes 25 seconds into the descent. It was part of a clever strategy that engineers had come up with to w’ork around the engine’s forbidden throttle settings in its high thrust range. They arranged that P63 should compute a course to a spot 4.5 kilometres short of the landing site. In some cases, strangely enough, this point could be below’ the surface but this didn’t matter since P63 never took the LM anywhere near it. This profile had been chosen to achieve two goals. First, it protected the engine from the erosion that would result from the forbidden throttle settings. Second, it yielded nearly optimal efficiency. The profile called for an initial thrust level that w as higher than the engine could achieve, to which the engine responded with its constant high thrust setting that is, 92.5 per cent of maximum, referred to as the fixed throttle position. For about 6.5 minutes of the burn, the spacecraft continued to lose speed and gently arc towards the surface while the engine continued in its high thrust setting. Hvenlually, the thrust required to achieve the profile fell below 57 per cent of maximum whereupon the program’s logic permitted the engine to move into its allowable throttle range which lay between 65 and 10 per cent of maximum. For the remaining 2.5 minutes of P63’s work, the computer could control the throttle and adjust it as necessary to compensate for any errors and drive the spacecraft’s trajectory towards an optimal flight path.

"Wow! Throttle down," called Aldrin. joyfully.

“Throttle down on time,” said Armstrong.

“Roger." said Duke. “We copy throttle down."

“You can feel it in here when it throttles down," noted Aldrin. "Better than the simulator." The crews had intensively simulated the descent, but the one thing the simulators could not provide was the g-force provided by the engine.

Navigation on the lunar surface was of no concern to Armstrong and Aldrin. They never ventured more than 60 metres from the LM. But as the crews of Apollos 12 and 14 made their H-mission traverses it became increasingly difficult to tell where they were as they roamed further on foot to locate features of scientific interest that geologists had identified from aerial photographs. At close quarters, the lunar surface is remarkably homogenous, especially on the plains. Huge areas consist of little more than large, time-worn craters pockmarked with smaller craters, all covered with a ubiquitous layer of grey dust and shattered rock.

The issue came to a head as A1 Shepard and Ed Mitchell set off to reach the rim of Cone Crater, their primary scientific target. Seen from above, Cone was a majestic 1,000-metre-wide hole in the ground but it was surrounded by a landscape that, at a human scale, w7as unrelentingly undulating. Worse, they had to climb a rough slope to reach it and since from their perspective atop the ridge the far rim of the crater was lower than its near rim, it would remain invisible until they were almost at its edge. It had been thought that the crater would have a raised rim that would make it easy to locate.

“Okay. We’re really going up a pretty steep slope here,” puffed Mitchell as his heart rate peaked at 128 beats per minute.

“Yeah. We kind of figured that from listening to you,” said Fred Haise in Houston. The heart rate of Shepard, a much older man at 47, had just risen to 150 as the effort of their climb and the frustration of their inability to locate the rim of Cone began to tell.

As the two laboured up the ridge, they pulled a little hand cart, the modular equipment transporter (MET) which was the engineers’ answer to the shortcomings of the hand tool carrier. It not only provided a place for the tool carrier, it allowed a much larger range of tools to be taken on a walking traverse as well as cameras, film magazines and sample bags; and it permitted a greater weight of rock to be returned to the LM. The MET rolled along on two rubber tyres pressurised with nitrogen. It was of some surprise to the crew that the wheels did not throw up rooster-tails of dust, but rather formed smooth ruts in the soil that reflected the Sun when viewed into the light. Included with the MET was a magnetometer to measure the local magnetic field at points along the way to Cone. Its sensors were mounted on a tripod which was deployed on the end of a 15-metre cable. After allowing time for the unit to settle, readings were gathered from a unit on the MET itself.

"I thought the MET worked very well." said Shepard alter the flight. "It enabled us to operate more efficiently than we would have otherwise.”

Mitchell agreed. "’We would have been in real trouble trying to move all that stuff out with just a hand tool carrier, and still get the same amount of work done.”

Thick soil and an increasing incline were adding to their difficulties. "The grade is getting pretty steep,” warned Mitchell, clearly breathing heavily. "And the soil here is a bit firmer, 1 think, than we’ve been on before. We’re not sinking in as deep."

"‘That should help you with the climb there.” said Haise encouragingly.

"’Yeah. It helps a little bit.”

Shepard had another way of making the climb easier.

"’Al’s got the back of the MET now, and we’re carrying it up. I think it seems easier.”

"Left, right, left, right,” prompted Shepard as he tried to quicken their progress up to Cone.

The backup crew of Gene Cernan and Joe Engle were listening in next to Haise. "There’s two guys sitting next to me here that kind of figured you’d end up carrying it up."

“Well, it’ll roll along here,’’ explained Mitchell, "except we just move faster carrying it.”

Always mindful of the need to return before their consumables ran out, and having already awarded a 30-minute extension, mission control enquired of their progress.

"Л1 and Ed. do you have the rim in sight at this Lime?”

"’Oh, yeah,” confirmed Mitchell.

"’It’s affirmative.” said Shepard. "It’s down in the valley.”

But they had misheard ’rim’ as ‘LM‘.

"’Em sorry,” said liaise. ""You misunderstood the question. I meant the rim of Cone Crater.”

"’Oh, the rim. That is negative,” said Shepard. “Wc haven’t found that yet.”

The problem they faced was that they were being fooled by the terrain that surrounded the crater and. by heading for the highest ground, they were being taken south, to one side of their goal. They were not lost in the strict sense of the word. The LM was easily visible from their elevated route and there would be no difficulty in finding their way back. On behalf of mission control, Haise called a halt to the quest. "Ed and Al. we’ve already eaten in our 30-minute extension and we’re past that now. I think we’d better proceed with the sampling and continue with the EVA.”

It was a bitter moment for the two explorers. They had come 400.000 kilometres to the Moon and had laboured uphill for over a kilometre to sample the deepest ejecta from the rim of Cone Crater. The superficial metric of their success would have been to take possibly spectacular pictures that looked across the crater to its far rim.

"It was terribly, terribly frustrating,” remembered Mitchell twenty years later, on how it felt to have seemed to have failed. "Coming up over that ridge that wc were

going up, and thinking, finally, that was it; and it wasn’t – suddenly recognizing that, really, you just don’t know where the hell you are. You know you’re close. You can’t be very far away. You know you got to quit and go back. It was probably one of the most frustrating periods I’ve ever experienced. There’s no feeling of being lost. I mean, the LM is there; we can get back to the LM. It’s not reaching and looking down into that bloody crater. It’s terribly frustrating.”

Like most of the Apollo astronauts, Mitchell came from a high-achieving military background and was used to reaching goals that had been set. This particular goal was also the pinnacle of a crewman’s career and the apparent failure was a blow to a pilot’s pride. But in truth, Apollo 14 admirably fulfilled the scientific task it had been set. Their objective was to gain deep samples from the rim of Cone and since their closest sampling point was later established to be only about 30 metres from the true edge of the 1,000-metre crater, they had, without realising it, been successful.

Scientists had always been vocal that Apollo crews must place high-quality scientific instruments on the lunar surface. After all, many in the scientific community saw manned spaceflight as a sink for funds that ought to go to unmanned craft. If man on the Moon was being shoved down their throats, then at least something useful ought to come from it. Arrangements were made to develop a system of instruments that would work off a common pow’er source and radio system. It was known as the Apollo lunar surface experiments package (ALSHP) and would be deployed on the surface with sufficient radioactively-sourced pow’er to last years. Across 1965 and 1966. principal investigators were recruited by NASA to design the instruments which Bendix would develop and supply. Early plans assumed that the first landing would to have tw’o moomvalks; one to deploy the ALSEP and the other for a geological traverse. However, as the planning for the initial landings continued, it became clear that an ALSEP would not be ready in time for the first crew. There would be a single short moonwalk which w’ould combine the history and ceremony of the moment with a small amount of sample gathering and photography. ALSEP would have to wait for subsequent missions.

When the entire Apollo stack of LM and CSM arrived at the Moon, it was placed in an orbit that would pass over the landing site at the time of landing. After the LM had set off for the surface, the CSM returned to a 110-kilometre orbit if it wasn’t already there. While the surface crew carried out their exploration, the Moon continued to rotate on its axis and. in most cases. Look the landing site away from the orbital plane of the CSM. The exception was Apollo 11. for which the landing site and the CSM’s orbit were more or less aligned with the equator, with the result that the landing site did not significantly stray from the CSM’s ground track. On all the other landings there was sufficient tilt in the CSM’s orbit to require a plane change manoeuvre, and given the LM’s minimal fuel reserves, it was most efficient for the CSM to make it. Therefore, at some point while his colleagues were on the surface, the CMP in orbit executed a plane change manoeuvre.

Changing the plane of the orbit required a burn of the SPS engine of between 10 and 20 seconds. Unlike height adjusting burns that added or subtracted energy from the orbit by firing along the spacecraft’s direction of motion, a plane change burn was usually made at righi-angles to the orbital plane, often near the point where it crossed the Moon’s equator. Preparations for this burn w ere just the same as for any other SPS burn, except that it was usually made using only one of the two engine control systems in view of its short duration and, similar to the circularisation burn, everything had to be done by the CMP alone. In mission control, FIDO calculated ihc details of a burn that would achieve the objectives with minimum use of propellant. This information was written on a PAD and read up to the lone crewman, ready to be entered into the computer under Program 30.

Richard Cordon was the first CMP to fire the spacecraft’s big engine alone in lunar orbit. Rather than make the burn just before the LM returned, he carried it out the previous evening, at the end of the day they landed. "I realised at this time that it had been a real long day and I was tired and more prone to make mistakes. I certainly didn’t w-ant to be making mistakes during an SPS burn.’’ Normally for any SPS burn, two of the crewmen worked together through a checklist using the ‘challenge and response’ technique designed to ensure that no step w:as missed a luxury the solo CMP did not have. Gordon’s solution was to have mission control listen to him as he went through each step. Fortunately, unlike most SPS burns, the plane change was made while in communication with Harlh and he had 14 minutes between acquisition of signal (AOS) and the actual burn.

“When I came around this lime and had AOS. I chose to go to VOX operation and read the checklist as I performed it. to the ground so they could monitor exactly where I was, exactly what I was doing, and would be abreast of the status of the spacecraft at all times.” VOX meant that his transmissions were controlled by a voice-operated switch. Each time the CMP spoke, his words were transmitted to Earth, and there was no need to operate a push-lo-lalk button. "It gave me the assurance that I was reading the checklist correctly, not leaving anything out. Now, 1 would think that the ground probably appreciated this. They knew exactly where I was in the checklist, what I was doing, and if I was behind and if I was ahead, so if any particular problem came up. they knew that I was with it or behind it.” Without the weight of his two crewmales and their lunar module, the SPS burn felt much more sporty, as Gordon noted post-flight: ”’l’he acceleration, of course, is much more noticeable than with the LM docked.”

CSM to the rescue

Given a normal mission, the role of the CMP might seem to be minimal in the upcoming orbital ballet of rendezvous, but NASA’s defence-in-depth policy ensured that he would have plenty to keep him occupied. It is true that the LM was always the active participant, as it was its responsibility to get off the Moon, into lunar orbit, then find, track and pull alongside the CSM. But the CMP had the role of rescuer in case the LM failed to execute the rendezvous. For this possibility, he had practised a wide range of scenarios where the CSM would become the active spacecraft and w’ould hunt down an ailing LM.

In an effort to get around the terribly short period of time that an Apollo CSM was permitted to orbit the Moon, barely a w? eek at most, scientists added a small, 35.6- kilogram subsatellite to the SIM bays of Apollos 15 and 16. This was ejected just before the crew? headed home. Its function was to investigate the various particles and fields in the lunar environment. ‘Particles and fields’ is an expression used within the planetary science community for the investigation of planets and their environments whereby, rather than taking pictures of a planetary body, measure-

ments are taken of the force fields, molecules and radiations that surround and interact with it. In the late 1990s, this work was continued by the Lunar Prospector probe.

The subsatellites added an extra complication to the missions’ flight plans because the scientists did not want them to be placed into the CSM’s normal orbit. Orbits around the Moon are inherently unstable. Given enough time, the influence of the mascons beneath the lunar surface and the tug of Earth’s gravity will cause an orbiting body to hit the surface. The subsatellite had no means of propulsion with which to compensate for these perturbations and if it were to be deployed from the CSM’s normal orbit, its lifetime would have been measured in weeks. But by manoeuvring the CSM prior to deployment it would be possible to extend its life towards a year.

“I have the Shape SPS/G&N PAD, when you’re ready for that,” said Joe Allen. He was ready to read up the details of the bum that would shape Apollo 15’s orbit in preparation for the subsatellite launch.

Jim Irwin usually took on the task of copying down the pads for this mission: "Okay, Joe. I’m ready on the Shape PAD.” Occurring only two and half hours before TEI, the manoeuvre required only a З-second bum of the SPS engine to raise their orbit’s apolune and perilune from 121.1 by 96.7-kilometre values to 140.9 by 100.6 kilometres respectively.

The shaping burn was made successfully just before Endeavour went behind the Moon for its penultimate lime. Then, around the far side. Worden executed ‘Verb 49′ in the computer, w’hich instructed it to bring the spacecraft’s attitude around to one that would place the long axis of the subsatellite perpendicular to the ecliptic and therefore perpendicular to the Sun. The launching mechanism was designed to spin the subsatellite as it was ejected from its receptacle in the SIM bay. This stabilised the small craft as it drifted aw-ay from the CSM. allowing the solar panels around its body to receive the sunlight required to power it. When they came back around the near side, the crew’ armed the pyrotechnics of the ejection mechanism while mission control monitored the spacecraft’s telemetry. An hour and 20 minutes before 1 HI, Allen piped up: "Endeavour. W’e verify your SIM pvro bus arm, and your rates look good to us down here. Over.”

"Okay,” replied David Scott. "We’ll go Free.”

As it w’as desirable for the spacecraft to be as still as possible for the deployment, time had been allowed for its rate of rotation to settle down within the half-degree dead band around the ideal launch attitude. Then, rather than risk the thrusters firing just as the subsalelliie departed, the control mode for attitude was switched to Free essentially disengaging the autopilot and allowing the spacecraft to drift. This w’as the first time that such a satellite ejection had occurred on a NASA spacecraft. "And wn know one of you will be watching out the window,” reminded Allen. "We’re particularly interested if the spin of the satellite is sweeping out a cone or if it seems to be a fairly flat spin as it comes out.” What Allen meant was that the satellite should be spinning around its long axis. It w’as important to the long-term future of the little spacecraft that this rotation be as even as possible as it departed.

However, there was still enough rotation in the CSM to take it slightly out of the desired attitude. ”Endeavour. we’re requesting you go back to Auto and do another ’Verb 49’. please. We see you drifted off about a degree.”

"In work," obliged Worden.

A suggestion then came from someone in mission control that the CSM should constantly correct its attitude until just before the launch. "Okay. Endeavour called Allen. “We’re recommending that you go back to Free at launch minus one minute."

"Okay; Free at launch minus one minute." confirmed Irwin. Mission control was still considering this one. Allen came on the air/ground a minute later with a revised procedure: "Endeavour, we’ve got a new update for the last instructions. Go Free at launch, please.”

Scott took a turn to reply: "Roger; Free at launch.”

By minimising the time spent Free they would reduce the scope for drifting off attitude.

Scott counted down the moments to launch: "Three, tw’O, one. Launch. We have a barber pole."

The subsatellite and its deployment mechanism moved along a track, opening a door in the process. It then engaged a switch that Tired the pyrotechnics to release it. allowing a spring to push it away from the spacecraft. A pin engaged in a curving
groove in a cylinder and imparted a rotation to the subsatellite which was stabi­lised by the deployment of three antennae. Scott saw a talkback indicator go to its ‘barber pole’ state. Once launch was complete, the deployment mechanism was retracted and this placed a grey flag in the indicator.

“And a grey,” confirmed Scott. “Tally Ho!”

“Can you see much?” asked Allen.

“Oh, looks like it might be oscillating maybe 10 degrees at the most,” said Scott as the long, hexagonal satellite drifted away, its three long, thin antennae sweeping out arcs in the sunlight. “A very pretty satellite out there. We get about two flashes per rev off each boom, and it seems to be rotating quite well. Very stable.”

The Apollo 15 subsatellite worked well for seven months before its telemetry failed. Apollo 16’s fared less well because mission control had decided to save the iffy SPS engine for the TEI manoeuvre and therefore cancelled the burn to shape their orbit. The subsatellite operated perfectly for 34 days before the changes in its orbit caused it to impact somewhere on the far side. The main result was a greater understanding of how the solar wind interacts with the Moon. In particular, the magnetometers on board each subsatellite also provided detailed information of the remanent magnetic field that some areas of the Moon exhibited in the form of miniature magnetospheres which warded off the solar wind.

Other tasks that had to be completed prior to the TEI bum on a J-mission included the retraction of instruments and paraphernalia that projected from the SIM bay. As the mapping camera was operated while extended out along a track to give the stellar camera a view to the side, the entire device was supposed to be retracted. However, this mechanism failed on Apollo 15. On Apollos 15 and 16, two instruments were operated on the end of 7-metre-long booms that could not withstand the load of an SPS engine bum. Although these booms were excluded from Apollo 17, it had two long antennae that projected out to either side of the service module, and these had also to be retracted. If any of these protuberances failed to retract, the crew had the option of jettisoning them, as was done when Apollo 16’s mass spectrometer boom failed.