Category How Apollo Flew to the Moon

Landing site REFSMMAT

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

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


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

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

"Contact light”

"Okay.” continued Aldrin. "75 feet, and it’s looking good. Down a half, six forward.” They were 23 metres up and almost hovering.

’’Sixty seconds,” called Duke as mission control continued their countdown to the land-or-abort call.

’’60 feet, down 2’A,” called Aldrin. “Two forward. Two forward. That’s good.”

Armstrong had found his spot and was taking the LM down. Like all the commanders, he wanted to land with the LM still moving gently forward so that he could always see where he was going. It was felt unwise to land going backward as it posed the risk of falling into a crater or striking a boulder that could not be seen.

“40 feet, down 2 A,” said Aldrin. “Picking up some dust.”

This was new. This was wdien the people in and around mission control realised that this was for real. No one had ever thought to mention it during the great many simulations they had run. The descent engine’s exhaust plume was blowing a substantial blanket of flying dust that wafted around the small stones scattered across the landing site. They were in a new environment and already discovering new things.

Jack Ciarman later related the phenomenal impact Aldrin’s words had. "We’d watched hundreds of landings in simulation, and they are very real.” Up to this point, and because he was used to the apparent reality of the simulations, Garman had not fully appreciated the fact that what was happening was no simulation.

Then Aldrin mentioned the dust. "And we’d never heard that before.’’ recalled Garman. "It’s one of those, ‘Oh, this is the real thing, isn’t it?’ I mean, you know it’s the real thing, but it’s going like clockwork, even with problems. We always had a problem during descent. A problem happens, you solve the problem, you go on. no sweat. Then Buzz Aldrin says, ’We’ve got dust now’.’ My god, this is the real thing. And you can’t do anything, of course. You’re just sitting down there. You’re a spectator now. Awesome. Awesome.”

On Apollo 15. as Falcon was being brought down onto the plain at Hadley. Scott thought the dust seemed completely enveloping. From his perspective, it was like flying in a fog. "At about 50 to 60 feet [15 to 18 metres], the total view’ outside was obscured by dust. It was completely IFR." Scott was comparing the experience to instrument flight rules, a mode of aircraft flying that pilots adopt when the weather closes in and restricts their visibility. He therefore had to take his attention from the view outside and use the displays in front of him.

As Young brought Orion down the final few metres on Apollo 16, Duke talked him through the dust.

“Okay, down at three [feet per second], 50 feet, down at four." They seemed to be dropping faster. "Give me one click up," advised Duke. Young operated the ROD switch and temporarily found he was hovering above a blanket of flying dust. "Come on. let her down. You levelled off," said Duke. “Let her on down. Okay, six per cent. Plenty fat." They had no problem with propellant.

“We did hover for a short period of time there,” commented Young after the flight, "at about 40 feet [12 metres] off the ground, and the [horizontal velocity] rates were practically zero and there was blowing dust. Vou could still see the rocks all the way to the ground, the surface features, even the craters, which really surprised me."

As Young brought the LM down, he just barely missed a 25-mcirc crater and landed with Orion’s rear footpad right on its edge. It was only when he and Duke got outside and could sec all around the LM that they realised how close they had come to being dangerously tilted over.

“I’m glad you weren’t 10 feet [further back], said Duke. "Whew me!"

“W’e were going forward," said Young, thankfully.

“Yeah, we were landing going forward."

Back on Apollo 11, Armstrong was only 10 metres above the surface and Aldrin was still feeding him data. “Thirty feet, 2 ‘A down." By this time, and since 70 metres altitude, Aldrin could view’ the LM’s shadow when he glanced up as it moved across the landscape. Since they always landed with a low, morning Sun behind them, the approaching shadow could be a useful tool to help to judge the final few metres. However, Armstrong could not see it because he was flying with the LM yawed to the left and the way his window was heavily recessed severely limited his field of view to the right.

“Four forward. Four forward," continued Aldrin. "Drifting to the right a little. 20 feet, down a half."

“Thirty seconds." called Duke. For all their telemetry, the flight controllers simply did not have the situational awareness that the crew’ enjoyed. With only 30 seconds remaining before the land-or-abort call, mission control was beginning to hold its collective breath.

“Drifting forward just a little bit," said Aldrin. coaching his commander down.

"That’s good."

Just then, one of the probes attached to three of the LM’s footpads struck the surface and lit an indicator in the cabin. Their footpads were less than two metres above the surface. "Contact light,” called Aldrin.

Immediately, the pair began a rehearsed series of tasks to turn Eagle from a flying machine to a home on the Moon.

‘’Shut down,” said Armstrong.

“Okay. Engine stop.” replied Aldrin.

By the lime Armstrong got the engine stopped, they had already settled onto the surface. No harm came to the engine, but he was struck by the unexpected way the lunar dust behaved in the exhaust gases in front of him. In an interview 32 years later, he talked about this surprising phenomenon: “I was absolutely dumbfounded when I shut the engine off. They just raced out over the horizon and instantaneously disappeared, just like it had been shut off for a week. That was remarkable. I’d never seen that. I’d never seen anything like that. And logic says, yes. that’s the way it ought to be there, but I hadn’t thought about it and 1 was surprised.”

On later flights, the commander was spring-loaded to stop the engine as soon as the probes touched the surface, particularly on the. l-missions where the longer engine nozzle provided only 30 centimetres of clearance to level ground.

“АСА out of detent,” was Aldrin’s next item, vvhieh referred to the controller in Armstrong’s hand. As they touched down, the LM adopted whatever attitude the surface dictated. However, the RCS thrusters w’ere still busily firing in a futile attempt to restore their previous attitude. By moving the stick, known as the attitude control assembly (АСА) out of its central position, Armstrong made the system think that the current attitude was also the desired attitude, and thereby stopped the jets from firing.

“Out of Detent. Auto.” said Armstrong.

“Mode control, both auto. Descent engine command override, off. Engine arm, off. 413 is in.” Aldrin’s litany of checklist instructions ended with an entry into the AGS, their secondary guidance system. Aldrin was entering a number into address 413 that told the machine they had landed and that it should take note of their current attitude in case they had to abort from it. The body-mounted gyros of the AGS were prone to drift and were unlikely to provide an accurate attitude by the time an abort might be called.

“We copy you dowm. Eagle." said Duke, spokesperson for an anxious mission control and a waiting Earth. Armstrong was not yet finished with the checklist.

“Engine arm is off,” he responded to Aldrin." Then to the w orld, he announced. "Houston, Tranquillity Base here. The Eagle has landed.”

Duke tvas caught by the moment and by Armstrong’s sudden change of call sign, despite having been forewarned. “Roger. Twan… Tranquillity. We copy you on the ground. You got a bunch of guys about to turn blue. We’re breathing again. Thanks a lot."

In the view of the public, the defining moment of the event would be when a human footprint deformed the lunar dust. This would have a human dimension; there would be a personal link to the hearts of all people who left footprints on Earth and a sense of the frailty of a mere human stepping out on a hostile alien world. The moonwalk would be the pinnacle of the whole achievement, itself a supremely difficult accomplishment.

Neil Armstrong didn’t view a moonwalk as being particularly difficult not in comparison to the rigours of gelling Eagle onlo the Moon in one piece. Speaking in 2001 to NASA historians, he weighed up how difficult the landing had been. ’‘The most difficult part from my perspective, and the one that gave me the most pause, was the final descent to landing. 1 hat was far and away the most complex pari of the flight. The systems were very heavily loaded at that time. The unknowns were rampant. The systems in this mode had only been tested on Harth and never in the real environment. There were just a thousand things to worry about in the final descent. It was hardest for the system and it was hardest for the crews to complete ihai part of the flight successfully.’’

Then somewhat humourously, he tried to enumerate the difficulty of the landing as compared to an excursion outside. “Walking around on the surface, you know, on a ten scale, was one. and I thought that the lunar descent on a ten scale was probably a thirteen.”

A final and eloquent perspective on the moment of touchdown comes from Jim Scotti, a planetary scientist who once asked Gene Cernan about the sounds he could hear as he landed on the Moon. Scotti wanted to know about the noise produced by the engineering that surrounded Cernan; pumps, switches, thrusters, and that big descent engine. The answer he got was not what he expected. Jim picks up the story. "What he heard in the moments after landing was… silence! You sec, before landing, he was so engrossed in the activity that he heard Jack calling out numbers and the occasional call from Houston and everything else blended into the background because he was so focused on the task of landing. At touchdown, however, the spacecraft fell silent and mission control was staying quiet to try not to interfere with what they expected was the final moments of touchdown. And Gene added: ‘And the guy standing next to me was struck silent staring out the window looking at the surface and he sure wasn’t saying anything!’”

Science in the driving seat

Lunar exploration came of age with the J-class missions of Apollos 15, 16 and 17. Traverses on foot to single points of interest gave way to wide-ranging sorties that could visit multiple destinations. The crews were carefully coached on the skills of field geology and on the need for strict documented sampling of the rocks and soil. A powerful illustration of NASA’s move towards a science-based justification for the flights was the successful lobbying of the geology community to have a professional scientist included on the final Apollo mission.

To accommodate this expansion of Apollo’s science role, engineers made a series of improvements to both the Saturn V launch vehicle and the Apollo spacecraft. The payload of the launcher was increased by small improvements in the E-l engines and by the deletion of some of the ullage and retrorocket motors that pulled the first and second stages apart at staging. Minor improvements were made to the loading and utilisation of its propellants to ensure greater depletion at cut-off. Changes to the launch trajectory included using a more easterly launch azimuth to Lake maximum advantage of Earth’s rotation and a lower parking orbit because a rocket that did not have to lift so high could carry more weight.

Changes to the spacecraft included an extra hydrogen tank in the service module to supply more power and w’ater. Another oxygen tank had already been added in the light of the Apollo 13 incident. The LM was endowed with larger propellant tanks and additional tanks for water and oxygen along with an extra battery in the descent stage. This increased the time on the Moon from two to three days. The thrust of the LM’s descent engine was increased merely by extending the length of its nozzle to direct the expanding exhaust gases along the thrust axis before they are released to space. The limiting factor was the nozzle’s clearance from the lunar surface. The increased ability to take mass to the Moon was exploited to greatest effect by one additional piece of equipment housed in an empty bay of the descent stage; the lunar rover.

Wheels on the Moon

The extra mobility afforded by the lunar roving vehicle (LRV) had a profound effect on the scientific harvest that was gained from the Apollo J-missions and there were many reasons for this. It could take crews to diverse sites for study and sampling. It gave them a little physical rest as they drove between planned stops. It could carry a substantial load of tools, cameras and ultimately rock samples. And also by virtue of a remotely-controlled TV camera, each geology stop could be supported by the eyes and knowledge of the scientists and engineers on Earth.

The rover was an ingenious device that managed to fulfil a wide range of tasks within extremely narrow constraints. It had to be light in weight and foldable in order to be carried aboard the LM. It had to withstand the rigours of launch and the passage across cislunar space as well as being able to operate in the extremes of dust, vacuum and temperature on the Moon’s surface. Beyond the basic task of


John Young works at Apollo 16’s rover. (NASA)


David Scott works with engineers on a checkout of the deployment of Apollo 15’s rover from the side of LM Falcon. (NASA)

Motor controller


Diagram of the layout of the lunar roving vehicle. (Redrawn from NASA source.)

transporting two crewmen and their tools across the Moon, the rover had to help them navigate while out of sight of the LM and it had to support a demanding suite of communications functions between the Moon and Earth, including live television. This unusual and highly successful contraption was designed, built and tested within 18 months of being given the go-ahead.

Л large square panel formed the central part of the chassis upon which were mounted simple foldable seats, an instrument panel and a T-shaped control stick. Two smaller chassis panels were attached at the ends, with hinges so the)’ could be folded against the central panel during flight. Each end panel held a pair of wheels which themselves were folded over the central chassis to allow the whole contraption to fit within one of the wedge-shaped bays of the descent stage. To deploy it. a crewman would pull on a lanyard to lower the rover from its bay. First the rear, then the forward chassis panels came free, both spring-loaded to fold out and engage in place. Likewise, the wheels w:ere spring-loaded to swing into their correct position and lock. Once the rover was on the ground, it was straightforward for the crewmen to lift it off its deployment hinge and begin to load it up with the tools, cameras and other equipment they would need for their traverses. The front chassis carried a pair of batteries that were installed in a fully charged state by technicians on the launch pad. In total, these could supply over eight kilowatt-hours of electrical power. Electronic packages were mounted nearby and these used the batteries as heatsinks. Engineers then arranged that when the rover came to a stop, the crew would lift dust covers to expose a series of radiators to deep space in order to release the accumulated heat.

The problem of surface navigation was solved by use of a directional gyro and by the measurement of distance based on pulses that marked the rotation of each wheel. The system’s logic was clever enough to select the third-fastest wheel so that slipping wheels would be ignored. By processing this information, the bearing and distance to an initialisation point could be displayed on the instrument panel. Normally, of course, this initialisation was done near the LM at the start of each day’s drive and it included an alignment of the gyro.

’’Okay, Bob, let me give you some numbers." said Gene Cernan to Robert Parker in Houston after he and. lack Schmitt had deployed their science station at Taurus – Littrow. They were ready to go on their first drive, but first Cernan had to initialise the rover’s nav system. He Look readings from a tilt meter that gave pitch and roll angles for the vehicle. The yaw angle could be worked out from a foldout sundial that indicated the angle of its centreline relative to the Sun.

"Sun shadow is zero. I am rolled right four degrees. I am pitch zero. I can’t be rolled right four degrees. That indicator can’t be right. I question that.’’ Perhaps the low’er gravity and the alien landscape were playing tricks with his sense of orientation. "I might be rolled left a couple of degrees. Are you happy with that. Bob? Roll indicator is indicating… Make it three degrees right.’’

"Okay, and I copy."

Almost instantly, a flight controller turned the attitude angles into a heading with respect to north w’hich Parker relayed to the Moon. "Okay, torque to 279." Cernan then slewed a heading indicator to show 279. The rover was aimed slightly north of due w’est. As they moved across the surface, the indicator would display their heading, and their route back would be shown by two numerical displays for bearing and distance.

Each wheel had its own 180-watt electric motor that was sealed into a pressurised unit along with gearing and a brake. A clever arrangement called a harmonic gear
stepped the motor’s rotation down by a ratio of 80:1 by having the motor turn an elliptical rotor inside a flexible cylinder. The cylinder had gear teeth on its outside which meshed with gear teeth on the inside of an outer ring, but only at the two ends of the ellipse. This arrangement meant that a complete turn on the inner rotor would move the cylinder by only a small number of teeth. The braking system was quite conven­tional, being operated by cable linkages which actuated drum brakes. All the control functions of the rover; steering, forward, reverse and braking; were brought into a centrally mounted T-handle that was accessible to both crewman though no LMP ever got to drive a rover on the Moon. Each wheel had a fender and these had pull-out extensions which were deployed to contain dust raised while driving. Seats, armrests and footrests were unfolded to their final positions, as was the console with its T-handle. At this point, the commander could get on, power it up and take it for the short drive to where their gear had been stored in the other stowage quads on the LM. The rubber tyres of the MET were discarded in favour of a mesh design made from piano wire that could withstand a large amount of deformation. An inner tyre of metal bands provided additional protection against hard impacts with rocks. The rover could clear a rock that protruded up to 30 centimetres. In planning a mission, there was usually some worry about whether the chosen site would be navigable to a rover. Pre-mission photography tended to lack the resolution to show small boulders that would reduce what NASA called ‘trafficability’; the rover’s ability to get about the site without being impeded by excessive numbers of rocks too large to drive over.

Gamma-ray spectrometer

Complementing the x-ray spectrometer in an effort to characterise the surface composition was the gamma-ray spectrometer. This instrument was designed to detect two expected sources of gamma rays. One was from the nuclei of some
elements in the lunar surface, particularly iron, which will react to cosmic rays by cmiliing gamma rays of a precise energy. Another source came from the radioactive decay of other elements, especially the radioactive constituents of KREEPy material; potassium, thorium and uranium, whose gamma-ray emissions are of a w7ell-know7n energy. Mounted on the end of a 7.6-meirc boom that removed it from contaminating sources around the spacecraft, the gamma-ray spectrometer helped to paint a picture of the composition of the Moon along the spacecraft’s ground track.

Pressure integrity

With the docking successfully completed, Worden pressurised the tunnel between Endeavour and Falcon, then removed the forward hatch and docking equipment to inspect the 12 docking latches. Meanwhile, Irwin copied down a P30 PAD from mission control for a burn that would eventually Lake the jettisoned LM out of lunar orbit to crash on the Moon.

Once the LM’s overhead hatch had been opened, Worden sent the vacuum cleaner through the tunnel to help the LM crew to deal with the dust on their spacesuits. Scott and Irwin then began to transfer all required items to the CSM, with a list in the flight plan indicating where each item should be stored. The list included film magazines, rock and soil samples, food, used urine and faecal bags and one of the oxygen purge system (OPS) packages from the surface. The OPS. w’hich had been mounted on top of one of the PLSSs during the moonw’alks. w’ould be needed by Worden during the coast home to Earth, for his spacewalk to retrieve film magazines from the cameras in the SIM bay. It contained high-pressure oxygen bottles that would provide emergency air to a suited crewman in case of a problem with their primary umbilical air supply.

Items not required by Endeavour for the remainder of its mission, such as used lithium hydroxide canisters, a second OPS and the now-useless docking probe and drogue, were left in the LM to be jettisoned with it. In the light of the Soyuz 11 incident, this jettison was to occur with the crew fully suited up. As Irwin w:as the last to leave Falcon’s cabin, he closed its overhead hatch behind him. Once everyone was inside the command module, the forward hatch was installed and the cabin checked for leaks. At this point. Scott had to deal with a slight pressure leak in his suit. “Okay, we are going to be a few minutes here. We’ve got to pul some LCG plugs in our suits and it’s going to take probably about 10 or 15 minutes to gel all that done.”

This communication was the start of a confused episode which involved checks of the suit and hatch for pressure integrity. Scott’s boss, Deke Slayton, came on to the communications loop, betraying management’s concern at the crew’s deviation from the flight plan. Scott’s use of the plugs in his liquid cooled garment (LCG) w:as a minor remedy for a leak that was probably brought on by the wear and tear from the tenacious and abrasive lunar dust.

“Hey, one quick question. How come you guys need plugs for those suits?’’ asked Slayton.

“Well, because, apparently, the LCG connection on the inside won’t hold an air seal,’’ replied Scott. “So we’re getting them taken care of with these extra little blue plugs we got that are airtight on the inside.”

“Roger. We thought those plugs only were required w’hen the LCG was not on. We’re trying to crack that one for you down here, Dave. There’s something screwy here.’’

“Okay. Well, we’ll put these plugs in and run another pressure integrity cheek and see how it works.’’


Scott’s subsequent successful suit integrity cheek pul the crew slightly behind their timeline, but Slayton’s intervention displayed the start of jitteriness in mission control about the crew and their tiredness when a slightly abnormal situation arose. Then, with only a few minutes to go before LM jettison, another pressure integrity problem became evident when Worden reported the pressure difference between the cabin and the tunnel. “LM/CM dcha-P is 2.5… 2.0, excuse me.”

“Copy, 2.0,” confirmed Bob Parker at the Capcom console.

The crew had used the tunnel vent valve to bleed air out into space from between the two spacecraft. Had it been completely evacuated, this pressure reading, given in pounds per square inch (psi), would show between five and six psi because it indicated the pressure difference across the forward hatch. With a good vacuum in the tunnel, the reading would be essentially the absolute cabin pressure. Their procedures called for the reading to be at least three psi prior to jettison. The fact that it was only two psi, having earlier read three psi, strongly suggested that air was entering the tunnel through the hatch of one or other spacecraft. Compounding the jitters in the MOCR was the knowledge that, on the way to the Moon, there had been confusion between Scott and the MOCR about the settings of this valve, which could either vent the tunnel or allow the crew- to monitor the pressure but not do both.

“Okay, the LM/CM delta-P doesn’t look exactly right to us. What do you think?” asked Scott.

“We’d like to get another pound [per square inch of pressure] out of there.” replied Parker. “We’re showing about 3.5 in there.” But mission control were not reading this directly. They had deduced this figure by subtracting the reading they had been given from the measured cabin pressure (5.5-2.0 = 3.5).

“Okay.” said Scott, as he and his crew looked for answers. “We had a suspicion that possibly the LM overhead dump valve was open, and it might be.” That is, it was possible Irwin had inadvertently left it open a little when he left the LM. Scott tried venting the tunnel further. "It’s up to about 2.3 now,” he reported.

The flight controllers in the MOCR discussed the readings with Scott a bit longer, before reaching a conclusion that was an extreme rarity in the history of flight control a mistaken conclusion. Parker radioed up. “Dave, we think that the increase in the cabin pressure during the suit integrity check could have raised it from your side.” However, adding more air to the cabin by inflating the suits for Scott’s pressure test would have had the opposite effect, increasing the pressure difference across the hatch.

Then Parker let slip about how the ground and the spacecraft had got out of sync with each other. "Stand by. Dave; confusion reigns down here.” In the light of this, mission control decided to hold off on the jettison, back out of the situation they were in, and have the crew disarm the pyrotechnic devices that were about to cut loose the LM. If the crew’ were to remove the hatch to inspect its seal, an accidental detonation of the armed LM jettison explosives would be catastrophic.

Scott and his crew brought the tunnel back up to the same pressure as the cabin, then removed the hatch but found nothing untoward. In any case, it was perfectly possible that contamination to the seal, perhaps from lunar dust, could have been blown off as the hatch was removed. Now that they had an extra two hours before the next jettison attempt, because it had to occur at a specific point in the orbit, mission control decided to use the Lime to test the hatch seal thoroughly. The crew reduced the pressure in the tunnel low enough to give a reading of 3.5 psi and Parker asked them to hold it there throughout their next far-side pass to see whether it had changed when they reappeared 45 minutes later. Scott and his crew were thinking about food and wanted to take their helmets and gloves off to eat: "I guess in that case, we’ll probably break the suits down and then run another suit check before we see you around the corner.”

“Okay, we’ll buy that," replied Parker.

“It’s about time for dinner.’’ said Scott.

“I knew there was a reason.”

By this time, it was 18 hours since Scott and Irwin had suited up for their gruelling final day on the lunar surface. They had not eaten for eight hours, and had been fully suited for much of the time since before launch from the Moon 6 lA hours earlier. The problems with their suit and hatch integrity were compounding their tiredness and they were going to be a further two hours behind. They were keen to get settled down to a much-needed meal break.

“Okay, we’re about 3.2 [psi] now on the delta-P,” reported Scott. “We’ll leave LM [meaning tunnel] in Vent.”

“Roger.” replied Parker. “I understand; 3.2 and still venting.”

The confusion was being compounded. Mission control had asked for the tunnel pressure to be held around the far side, but SeoiL had understood that he was to leave it venting. Then the managers worried whether the crew’ should remove their helmets and gloves in order to eat. Breaking open their suits w’ould necessitate another check of their pressure integrity before LM jettison. Despite having earlier concurred with the request from the crew to do precisely this, the suit integrity check would pump air into the cabin and affect the reading on their pressure gauge, so Parker notified them of a compromise: "You are permitted to break the suits down, but do not do the suit integrity check until you come back around the other side; wfe can Lake another look at that tunnel.”

Once the crew’ reappeared from behind the Moon, Parker quizzed them. “How’ did the hatch integrity check go?’’

‘"Well, we’ve just had it in Tunnel Vent all the way around the back side as I think you suggested," replied Scott.

“Did you have a look at holding it in delta-P to see how it was holding on that?’’ queried Parker.

“No, we just left it in Tunnel Vent all the way around the back side,” reported Scott. “That’s what we’d thought you’d said to do. We can check it now.”

By now’, Glynn Lunney. the flight director on this shift, w’as becoming frustrated at the difficulty his team were having in getting this crew put to bed. Parker called up, “15, why don’t you bring it up to 3.5. and let us watch it for a w’hile. I think w’e garbled something there.’’ Lvcryone was keen that they jettison the LM only when the seal on the forward hatch w’as good.

In-flight exercise

Although Apollo occurred in the first decade of manned space flight, doctors had already begun to test the body’s reaction to weightlessness during the Gemini programme and had noticed how muscle tone and bone mass were lost after only a few days. Bxcrcisc was believed to be the key to mitigating these effects but this was next to impossible in the cramped confines of a Gemini spacecraft – the exerciser was an elasticated strap, and the astronaut would loop one end around a foot and then pull against the tension. The greater volume afforded by an Apollo cabin permitted some limited exercise, especially when the couches were folded away. Every Apollo flight therefore carried an ‘Exer-genic’ or ‘Exergym’ exerciser, a commercial gadget consisting of a rope with handles that passed through a cylinder. The resistance to pulling the rope could be adjusted.

’’We all did a little bit of exercise almost every day." said Armstrong after his flight. "We used either isometrics or callisthenics in place, or the Exer-genie. It got a little hot and stored a lot of heat, but it was acceptable.’’

Collins elaborated on how their little gadget was dealing with the heat from friction. ’If you got a good workout on the Exer-gcnie, it got so hot that you couldn’t really touch it.”

As military test pilots for the most part, these men tended to take their exercise seriously in life outside NASA. Collins did daily runs, and Scott and Irwin often played handball. Armstrong was the exception when it came to exercising for its owm sake. As strong, fit individuals, they felt the need to exercise hard, even using the spacecraft’s structure, as the Apollo 12 crew related. "The thing we had for exercise,” said Gordon, "other than just moving around using the struts and the flat areas in the LEB for doing pushups and armpulls or whatever you wanted to do. is the Exergym. We all used it on the way out a couple of times a day for maybe a half hour each time. I didn’t use it at all coming back. Л1 didn’t use it coming back because the Exergym rope was frayed. Pete was using it on the wray back when he noticed that fibres were coming loose. So we elected not to use the exerciser at all on the way back.”

Surface crews found that the demands of working on the lunar surface was a hard exercise in itself. The CMPs. on the other hand, needed as much extra workout time as they could get, as Worden did on Apollo 15. "The Exergym is good for keeping some muscle tone,” he said after the flight, "but I found that there was just no way I could get a heart rate established and keep it going. I finally decided on a combination of two exercises. I used the Exergym a little bit. just to keep my shoulders and arms loned, and I ran in place. I took the centre couch out and flailed away with my legs, just like running in place as a matter of fact.’’

Crews regularly wore biomedical sensors on their skin that allowed the Surgeon in mission control to monitor their normal heart rate and breathing, and also while they were exercising. "I didn’t say anything to the ground.” continued Worden, ‘but the doctors watching the biomeds called up and said, ’Hey. you must be exercising. We can see your heart rate going up.’ And they kept me advised of what my heart rate was. It worked out very nicely. I thought, because they could tell you that you’re up to 130. going up to 140 (beats per minute). Then 1 would exercise a little bit harder, and true, even though I wasn’t exerting any pressure on anything, just moving the mass of your legs around really gets your heart going. As a matter of fact, I thought I’d strained some muscles that I had never used before because I was just free wheeling my legs and wasn’t exerting any pressure on anything. I found out that with the centre couch out, there’s just almost the right amount of room. In fact, the same thing could be done up in the tunnel area. You don’t need a whole lot of space.”

Irwin added, "We strained against the struts, against the bulkhead, and against the straps. This was kind of an isometric form of exercise. I think it’s almost as good as the Exergym.”

Ken Mattingly was not a huge fan of the exercise they had from a practical standpoint. “I just can’t believe that the amount of exercise 1 had justified 30 minutes [of my time],” he said after Apollo 16. ‘T really think I’d have been just as well off to just forget the whole thing.”

When it came to the CSM, Mattingly was regarded as an expert and he felt that the spacecraft’s environmental control system (ECS) might not handle the extra heat of someone who was really working out: “If you go out there and work up a sweat, really do exercise like you ought to. the ECS will not handle that kind of a load. The ECS is marginal. It’s designed for three marshmallows laying there. It isn’t designed for you to go out and do any exercise. The other thing I worried about was lying there and banging into things, because you can’t do any reasonable exercise and maintain your body position."

Sometimes crewmen exercised so vigorously that the entire spacecraft felt it. "Bob, this is Jack," called Schmitt to Bob Overmyer at the Capcom position. "I’m going to try to get a little exercise. I’d be interested to know how high I can get my heart rate just fooling around up here."

"Okay, we’ll keep you posted. Jack."

As Schmitt started exercising. Cernan noticed that the CSM’s barbecue roll was deviating. "I just figured out what happened on my PTC. here.” he told Overmyer. "With his exercises. Jack is shaking all of America in all three axes."

"Roger. He finally got to 115 on the heart rate," said Overmyer.

"Yes, the rate needles are bouncing back and forth a half a degree," laughed Schmitt as he watched his movements show on the FDAI needles that indicated rate of rotation.

Even EECOM. watching the spacecraft’s tanks, could see the effects.

17, we’ve got a serious one here.” joked Overmyer. "You might be interested. ЛИ that exercise banging around in there has destratified tank three 02. so it stirred it all up good.” The movement of the spacecraft had achieved the same effect that an internal fan had prior to Apollo 13’s explosion. It had disturbed the unwanted separation of density layers in the tank.

”Yes, glad we brought him along then." returned Schmitt’s colleagues. "We found some use for him.”


As the mission entered its final hour, one chore for the astronaut in the right couch was to install a 16-millimetre movie camera in a bracket next to his rendezvous window. This would record the view from the window looking backwards along their glowing wake. The camera did not have a direct line of sight, but rather was mounted off to one side and used a small 45-degree mirror to see out.

In the meantime, a final few items were stowed away for the re-entry. These included the ORDbAL box, the COAS optical sight, the chlorine injector and gas separator of the water squirt gun. An important aspect of stowage for re-entry was to ensure that objects were not only well secured but that their weight, soon to rise more than six times their earthly weight, and their edges and points, did not strike a crew member or cause damage, especially to the aft hull. Additionally, items above the crew had to withstand the sudden shock of the spacecraft’s impact with the ocean’s surface.

With 50 minutes to go, tasks leading up to their meeting with entry interface were coming thick and fast. The heaters that were preheating the CM thrusters were switched off, and a check was made of the two batteries that would fire the pyrotechnic devices. These were separate from the spacecraft’s three main batteries.

If either battery indicated less than 35 volts, extra energy was taken from the main supply to ensure operation. The third of the CM’s other three batteries was connected across both of the main power busses. The batteries would become the spacecraft’s primary electricity supply after the fuel cells were cut adrift with the service module.

Л check was made to see that their backup attitude reference, the GDC. was not drifting excessively. If it was, then two instruments that were relying on its output had to be treated as suspect: the right-hand FDAI and the RSI, the latter being the instrument that showed the direction of their lift vector and therefore a key item on a manual re-entry.

Get the hell out of there: the AGS

As ever, there was a backup system for the PCNS, although in this case the philosophy was a little unusual because, in the event of failure, it was not meant to replace the PCNS in order to allow the mission to continue. As its name indicated, the abort guidance system (AGS pronounced biggs’) was intended to be used only in ease of an abort.

Given the limited capabilities of the lunar module, it was considered to be almost impossible to accomplish a landing without a working guidance system. The highly optimised nature of the approach trajectory and the complexities of keeping the spacecraft balanced on top of the descent engine’s thrust meant that there was very little room for error. A fully functioning backup system would have been excessively heavy. Yet there were reasonable concerns about the possibility of the PGNS failing while two men were descending to the rocky surface of a hostile world. Its systems were complex, exotic and very new. It was decided, therefore, that if the PGNS did fail, the descent to the surface should simply be aborted, the descent stage should be jettisoned, and the ascent stage should fire its engine to return to orbit. To achieve this, designers added a separate pared-down guidance system, the abort electronics assembly, which included a computer at its heart, the AGS, which was even simpler than that in the PGNS. Instead of having its own heavy IMU, the AGS received its attitude reference from a set of body-mounted gyros and accelerometers. ‘These were intrinsically less accurate and more prone to


Photographed during the flight of Apollo 11, this is Buzz Aldrin’s station in Eagle’s cabin. In the centre is the DEDA, Aldrin’s interface with the AGS. (NASA)

drift than a full IMU, but they would only be required for a short period while the abort took place.

Throughout a normal descent, the lunar module pilot closely monitored the AGS to ensure that its knowledge of velocity and position kept track of the PGNS. At regular intervals, he fed it updates from the more accurate system and then watched how the two compared. Then, if the crew lost the PGNS, the AGS was ready to take over and automatically guide them to a safe orbit, from which the CSM could rescue them.

The LMP worked with the AGS using an interface that was basic even compared to the DSKY, the data entry and display assembly (DEDA). It had one single 5-digit display and a simplified keyboard. Since it had little in the way of a user-friendly interface, he had to get down to its machine level to use it. For example, to access its memory he had to supply the address where a value was stored in a manner that will be familiar to people who used the very early microprocessor machines of the 1970s. In order to achieve high functionality, he had to understand it well and be slick at interrogating it. It could keep an LMP very busy. By Apollo 17, Jack Schmitt and the engineers he worked with on simulations had done so much with the AGS that they believed they could have used it to continue to a landing had the PGNS failed.

A blob of solder

While Alan Shepard and Ed Mitchell were activating Antares on Apollo 14, flight controllers noticed that a single digital bit they were monitoring was being intermittently set. This bit reflected the state of the Abort pushbutton and appeared to indicate the button had been pushed, which it had not. Fred Haise, the Capcom,

passed on the news to the crew: "Antares. we’re showing the abort bit set, and we’re working on a procedure to reset it.”

‘’Okay. That’ll be great, thank you," said Mitchell. "We’re pressing on with the DPS pressurisation.”

A Tew minutes later as mission control watched telemetry, Haisc asked Mitchell to help with a little troubleshooting. "We’d like to do a Verb 11. Noun 10, Enter; 30 Enter; and look at that bit again."

This dialogue with the computer asked it to display a digital word which included the suspect bit on the DSKY. The crew now had a visual indication of wiiich way the bit in question was set.

"While we’ve got that display up. Ed.’! said Haise. "could you tap on the panel around the Abort pushbutton and see if we can shake something loose?’’

Mitchell tapped around the pushbutton and quickly saw how the bit’s state responded. "Yes, Houston, it just changed while I was tapping there.”

"You sure tap nicely.’’ said Haise.

"I’m pretty good at that,” replied Mitchell.

Without a LM to disassemble after the mission, engineers managed to work out that the problem was a short circuit caused by a metallic object that had been inadvertently sealed within the Abort pushbutton itself on its manufacture. This, and the similar problem that beset Apollo 15’s SPS, meant that NASA and its contractors began to x-ray all their switches for internal contamination.

Apollo 14‘s problem was that if the bit were to be set at the moment of PDI, then instead of commencing the descent, the computer w’ould use the DPS to abort the mission and return to a safe orbit. As Antares passed around the Moon’s far side, a team from Mi l’ led by Don Eyles figured out a workaround. It required Mitchell to feed instructions to the computer just before ignition so that PDI could occur automatically and the abort bit would be ignored. ‘I he procedure also required them to manually raise the engine’s thrust to maximum at 26 seconds, and then punch in more instructions to allow P63 to continue with the bit being ignored. Mitchell managed to enter all the verbs, nouns and values as required, saving a S500 million mission from w’hat was probably no more than a rogue blob of solder.