Category How Apollo Flew to the Moon

After the horizon check, and a check of the VHF radio, the crew entered a code to tell P62 that they were about to shed the service module. The spacecraft was then yawed 45 degrees to the left. Since the CM’s thrusters could not impart translation motions, only rotations, it was the SM’s task to manoeuvre clear in order to minimise the risk of a collision.

Jettisoning the service module was an intricate task left almost entirely to automatic systems that not only severed the interface cables and pipes safely, they also ensured that the SM moved clear. However, the SM wns the primary source of power, oxygen and cooling, and inadvertently separating the two modules any earlier in the flight would have had disastrous consequences for the crew. Therefore, as with most other events that occurred only once yet demanded the highest reliability, the SHCS took care of disconnecting the umbilical lines between the two modules, cutting the ties that held them together and controlling the SM’s evasive manoeuvres, all of which occurred in just a few seconds.

The process began when the CMP applied power to the logic circuits of the SECS and armed the pyrotechnic system, connecting dedicated batteries to their control circuits. After the arming sequence was complete, the flip of a switch began the separation sequence. As w’ould be expected, this switch was guarded by a metal cover to avoid accidental operation. The SECS then assumed control. A command was sent to a controller box in the service module. This started a timer to trigger the RCS jets on the SM. ready to move the now unwanted hardware away from the CM after separation. Many systems on board the SM remained active to support the separation process. In particular. tw’O fuel cells continued to supply power for the jettison controller and the thrusters, and to fire the pyrotechnics that severed the steel lies holding the CM and SM together. First the eleetrieal connections across the umbilical w ere carefully disconnected by an arrangement of cams and levers in the CM, powered by a small explosive charge that literally unplugged the cables between the two modules and then another pyrotechnic charge drove a guillotine through all the cables and plumbing that ran between them. On command from the SM’s event controller, each of the three strong tension ties that ran through the heatshicld to hold the twro modules together w’as cut by two separate explosive charges in a manner that allowed springs beneath each of six support pads to push the modules apart.

As the service module came free, its controller fired its jets both to pull it away from the command module and to impart a spin that attempted to stabilise its

motion. On early missions those thrusters that were working to pull it away continued to fire until depletion or failure, but despite these efforts to take the SM well away from the CM the complex dynamics of the remaining propellant in the tanks caused it to turn around and approach the CM again.

During their debriefing, the Apollo 11 crew were asked if they saw their service module.

“Yes. It flew by us," said Mike Collins.

“It flew by to the right and a little above us, straight ahead," added Buzz Aldrin. "It was spinning up. It was first visible in window number four, then later in window number two, really spinning."

This problem w’as cured by shorter separation burns.

After the SM had gone, the crew quickly checked the pressures in their RCS tanks, safed the system that had fired the explosive devices, and checked that their batteries still had enough pow’er for the final leg of the mission. The CM w as yaw ed back 45 degrees in order to face backwards along their flight path and then pitched down to the correct attitude for atmospheric entry. At this point. Collins noted how the weak thrust from water vapour leaving the steam duct interfered with his attempts to yaw. The vapour came from the evaporator, now’ their only means of losing heat. “When I got a yaw’ rate started, the water boiler would fight me, the rate would reduce to near zero, and 1 would then have to make another input."

At this point, in a heads-down attitude with the lift vector up and with everything verified, the CMP pressed ‘Proceed’ on the DSKY to give the autopilot control of their attitude. Its display showed their impact coordinates and a check of their heads – up, down status at wliich point ‘Proceed’ was again pressed to pass control of the re­entry profile fully into the computer’s hands.

Down the side of the PAD form was an area for comments that related to the burn, and in particular the set stars. As ever, the Apollo planners had looked for procedures and methodologies that would give the crews options to continue in the face of equipment failure. The set stars were to provide a backup attitude reference in ease the guidance platform failed to remain aligned to the desired RBBSMMAT.

Although it w’as something of an alphabet soup, the basic idea went like this. As well as the I M lJ with its gyro-stabilised platform, the spacecraft had two other sets of gyros: the body-mounted attitude gyros (BMAGs) and their associated electronics, the gyro display couplers (GDCs) that made sense of their output. Unlike the platform, which measured absolute attitude, the BMAGs really only measured changes in attitude. Therefore to determine absolute attitude, the GDCs had to work from a starting point and count the changes in attitude as measured by the BMAGs. It wasn’t as accurate as the IMU and was prone to drift but it would work for short periods. Normally, the GDCs got their starting point from the IMU merely by a press of the GDC Align button. But if the platform was lost, another way w’ould be needed to give the GDCs a valid starting point. Then, by processing the signals from the BMAGs. they could determine absolute attitude relative to the desired REFSMMAT. The aim of the set stars was to give the GDCs that starting point.

What mission control did was to give the crew two stars – in this Apollo 15 ease, Vega and Deneb – and a set of three attitude angles. To make the backup realignment work, the CMP needed to manoeuvre the spacecraft so that the stars were arranged in the scanning telescope in a predefined manner. This placed the spacecraft in a known attitude. In that attitude, the three given angles represented the spacecraft’s attitude with respect to the desired RBFSMMAT. enabling the

BMAGs and GDCs to work from that attitude as their starting point.

Fortunately, no IMU ever failed in flight and this procedure was never used.

The next comment in the PAD – No ullage – reminded the crew that because the SPS propellant tanks were full there was no need to settle their contents prior to the burn.

When there was a need, an ullage burn by the RCS thrusters forced propellant to the outlet end of the tanks to minimise the possibility of gas from the empty part of the tank being ingested into the engine.

After stating the mass of the lunar module, the comments went on to deal with the problems pertaining to Apollo 15’s faulty circuitry in the primary control bank of the SPS engine. If they could only use the good В bank, the slightly reduced thrust would increase the burn duration by 11 seconds to 6 minutes 52 seconds. It was decided, however, that the crew should let the burn begin under automatic control with the В bank, and then after several seconds manually engage the A bank. Shortly before the expected cut­off, they would disengage the A bank and let the automatic systems terminate the burn with the good bank, lest the short circuit in the A bank override the shutdown command.

The final point in this PAD was that should the В bank itself fail, the crew were to use the A bank manually to achieve the required delta-v and enter lunar orbit. In this case, they would have had to ensure shutdown by the simple expedient of removing power from the SPS which they would have achieved by pulling a circuit breaker.

This lengthy PAD demonstrates how, even on what was considered a ‘nominal’ flight, great lengths were taken by mission control and the crew to provide options to successfully pursue the mission in the face of a wide range of possible failures.

While the commander was Lending to the LM’s guidance needs, the LMP continued his checks with the activation of the communications system. Communications to the CSM were handled by two VHF antennae mounted fore and aft. Communica­tions to Earth, as well as the same ranging and tracking functions as found on the CSM, used either of two low-gain S-band antennae, also mounted fore and aft. For higher data rates, a steerable, high-gain dish was mounted high on the right-hand side of the ascent stage and it included systems to keep itself aimed towards Earth.

For the descent, the electrical power for the LM came from batteries mounted in the descent stage. Originally, the lunar module’s manufacturer, Grumman, had intended to power it with fuel cells, in a manner similar to the CSM and most of the Gemini spacecraft. Managerial and technical difficulties, mostly concerned with the interdependence of the power system with other systems in the already exotic LM, conspired with the race to get the spacecraft ready on time to force a switch to using batteries as the power source. Though heavy, batteries had the enormous advantage of simplicity, and since the LM was intended to be powered for only a few days, their weight penalty was no worse than the fuel cells. As part of the checkout of the LM, their health was closely studied, as were the extra set required for the ascent stage. The ascent stage carried a separate small set of batteries because it had to operate on its own for only a few hours for the trip from the surface of the Moon up to the CSM.

Another major item in the LMP’s checklist was the cooling system. Whereas the CSM used radiators and evaporators to lose heat from a water/glycol coolant, the LM relied on a sublimator to achieve the same task. These devices cooled by having ice directly sublimate to waiter vapour in a vacuum, in the process taking heat away from the coolant. The lunar spacesuits used the same cooling technique. Because the LM had no source of water available as a by-product of fuel cell operation, a large water tank was included in the descent stage, with a smaller supply in the ascent stage. The water/glycol coolant was pumped between the LM’s electronics and the sublimator by redundant sets of pumps. The pressures delivered by these pumps were checked before committing the LM to the surface.

Manoeuvring the LM was effected by a set of thrusters similar to the RCS jets mounted on the service module. Where each cluster of jets on the service module had independent propellant supplies, those on the lunar module had a common propellant system that, if the need arose, could be topped up from the propellant used by the ascent stage’s main rocket engine. As a further difference, the service module’s RCS quads w’ere mounted on the spacecraft axes, while those for the LM were set at the corners of a square around the spacecraft to keep the windows and hatchway clear. Before it could be used, the propellant system had to be pressurised. An explosively operated valve was opened to allow helium gas into the propellant tanks, at which point the crew and mission control could verify that the pressures within the system were as expected.

The spacecraft’s design had assumed that the windows would face forward during the final approach to give the crew a view of their landing site, and that it would pitch into this attitude from a windows-up attitude. If the window’s were facing up, and the landing radar had to face downwards, then its antenna had to be mounted on the base of the descent stage on the side opposite the windows.

“Intrepid, Houston,” called Carr to Conrad and Bean. "You’re looking good at three [minutes].’’

“Okay, Houston," replied Conrad. He w’as waiting for two indicator lights on the DSKY to go out, which would mean that the landing radar was producing valid measurements of their altitude and horizontal velocity. “I have an altitude light out; and I have a velocity light out.”

“Roger.’’

Conrad then looked at the DSKY’s display for a number. "I’m showing minus 918. Minus 1.000. Looks good. How’’s it look to you, Houston?"

The number, called ‘dcita-H’ was telling him that their height, as measured by the landing radar was 1.000 feet or 300 metres lower than the computer’s estimation based on its knowledge of their orbit.

“Roger; it looks good. Recommend you incorporate it,” said Carr as the flight controllers passed on their wisdom.

“No sooner said than done. Let me know witen it converges. I’m going back to my normal displays.” With this declaration that the radar’s height measurement was reasonable, Conrad commanded the computer to accept the radar data, compare it to its current estimation of their height and rate of descent, then attempt to lly a compromise between the two. Having done so. il then revised their trajectory to high gate and repeated the cycle until its estimation of their height converged with the data coming from the radar. This gradually folded the new’ data into the flight path without causing a sudden transient. The delta-H figure on Apollo 12 was small – radar and computer were almost in agreement. Had the radar shown them to be 10.000 feet or 3.000 metres higher than the computer believed them to be. an abort would have been called for because they would have run out of fuel before reaching high gate.

When Armstrong and Aldrin arrived at Tranquillity Base, their flight plan gave them a rest period before they were to embark on their historic moonwalk. NASA’s operational conservatism had not wanted to load them up with too much in one day. But many believed that the crew would be too keyed up to rest at that point and there were tentative plans to bring their EVA forward. One hour after landing. Armstrong announced his decision. “Our recommendation at this point is planning an EVA, with your concurrence, starting about eight o’clock this evening. Houston time. That is about three hours from now. We will give you some lime to think about that."

Charlie Duke promptly gave Houston’s reply. “We thought about it; wre will support it. We’re Go at that time.’’

Aflcr the mission, Armstrong explained their thinking. “There were two factors that we thought might influence that decision. One was the spacecraft systems and any abnormalities that we might have that we’d want to work on; and the second was our adaptation to one-sixth g and whether we thought more time in one-sixth g before starting the EVA would be advantageous or disadvantageous at that point. Basically, my personal feeling was that the adaptation to one-sixth g was very rapid and [it] was very pleasant, easy to work in. and I thought at the time that we were ready to go right ahead into the surface work and [so I] recommended that."

For the most part, crews had their first surface EVA on the day of landing. Apollo 16 was one exception. The 6-hour delay in its landing had changed mission control’s view. “You probably gathered that we want y’all to sleep first," said Jim Irwin in Houston.

“That suits us," said Duke, not entirely honestly. “Jim. I feel exactly like I thought I was [going to feel], I really want to get out, hut I think that discretion is the better part of the valour here.’’

Decades later. Duke regretted the decision. “As a hindsight observation, it wasn’t a good idea. I think we should have gotten out first. I had a tough time getting to sleep. My mind wras just racing and I wanted to get out. I was thinking about all that was coming up; and the excitement had just passed; and. you know, my mind was just whirling. 1 think wn had so much adrenaline pumping we could have gone, probably two days – forty-eight hours – without any problem. So, looking back, I think we made a mistake. But it’s done."

The other exception to EVA on landing day was Apollo 15. They had a SEVA!

SEVA

David Scott’s flight was the first J-mission, the first to have a rover and the first where science defined the mission. Back in 1964, when geology training began for the prospective lunar explorers, Scott’s teachers spotted that, more than the other jet – jockeys that passed through their hands, he possessed a mind that was receptive to scientific enquiry. When his Moon mission came up. his enthusiasm and excitement for Apollo’s scientific quest easily transferred to the other members of his crew, and to those subsequent members of the J-mission crews who, like him, came from a test – pilot background. Don Wilhelms, who played a major part in transforming Apollo into a hunt for geological answers, described Scott, Jim Irwin and Л1 Worden as "the geologic crew”.

Scott had been impressed by the teachings of geologists like Caltech professor Lee Silver and USGS staffer Gordon Swann who had been brought in to give the crews training in field geology, as distinct from the previous classroom instruction. When approaching a new’ geological site, one of Silver’s techniques was to find a high spot, even the vehicle that had brought them there if needs be, to gain an overview of the site before digging into the detail of stones and bedrock. This site survey gave the bigger picture; what the major rock exposures were, what type of rocks they held and at w’hat angle the layers of rock dipped. Scott decided that since there were no plans for an EVA on the day of their landing, he w’ould make a stand-up extravehicular activity (SEVA) instead.

The arguments to convince managers that a SEVA was worthwhile included a need to maintain the crew’s circadian rhythm. At the time, it was felt that a full 7- hour EVA after landing would have meant an excessively long day. Yet. to sleep so soon after arrival would lead to a shorter than normal day. The SEVA would fill the gap and help to release the excitement they would naturally feel at having landed on the Moon. Also, Apollo 15 s northerly site had not been well mapped by the Lunar Orbiter probes whose high-resolution imagery had been expended near the equator. In particular, no one really knew w’hether Iladley provided a suitable surface for the rover, and if significant problems became visible from up top then his descriptions would be useful in revising the planned drives.

From Scott’s point of view, the best reason for the SEVA was the science, as he later recalled. "To be able to stand there and just look at all that stuff. I mean, that was just a mindblower to be able to just stand up there and gaze around and report what you saw% knowing full w’ell that Lee Silver and Gordon Sw’ann and the guys in the Backroom are listening to every word.”

As well as the SEVA, Scott and the geologists wanted to take a telephoto lens. "We spent a lot of time and energy justifying the 500-mm lens,” remembered Scott. "And the final trade-off was in abort propellant. They reduced the amount of abort propellant on the landing by witatever the mass of the telephoto lens w’as. There was a lot of scepticism on whether it would be useful at all. But, gosh, you go out in the field with a bunch of geologists and you can’t get to the mountain and it becomes obvious that a telephoto picture is a lot better than nothing.”

Once he had got his head out of the overhead hatch. Scott began to photograph the virgin site using a 60-mrn lens to take an all-round panorama and then his telephoto lens to capture features of special interest. With that out of the way, he began what, according to Wilhelms, ranked as "the best geological description by an astronaut on the Moon.”

"All of the features around here are very smooth,” reported Scott. "The tops of the mountains are rounded off. There are no sharp jagged peaks or no large boulders apparent anywhere.”

That was good news. They would have little difficulty driving the rover across the landscape as planned. Scott worked his way around the view, describing everything he saw while giving clock-face references with respect to the LM’s west-facing aspect. "As I look on down to my seven o’clock, I guess I see Index Crater here [in] the near field. But, back up on Hadley Delta, to the east [at Silver Spur], why, again I can see a smooth surface. However, I can see lineaments. I’ll take a picture for you.”

The Moon’s immense antiquity drapes almost all its bedrock with a thick blanket of beat-up, pulverised rock. Since bedrock is what geologists like the most, by virtue of it being in situ, any sign of it on the lunar surface gets their attention, especially when they see layering. Scott’s lineaments, on the feature they had happily named Silver Spur for their geological mentor, appeared to be such layering.

"There appear to be lineaments or lineations dipping to the northeast, parallel [to each other]. And they appear to be, maybe, three per cent to four per cent of the total elevation of the mountain, almost uniform [spacing]. I can’t tell whether it’s structure or internal stratigraphy or what. But there are definite linear features there, dipping to the northeast, at about, oh, I’d say 30 degrees.”

Silver Spur was too far away to visit, but with his telephoto lens Scott was able to gain clear images of the structures for the geologists.

For the two later missions, the justifications for a SEVA lessened and it was dropped. Scott would be the only commander to look out of the top of his LM in what Jim Irwin likened to Desert Fox in his Panzer.

SUITING UP

If a crew intended to leave the lunar module soon after landing, their preparations were reduced by virtue of being already suited. In fact, for the first three landings, suits were never removed while on the Moon. Their task primarily involved changing over from the LM’s oxygen and coolant water to that supplied from their back pack, the portable life support system (PLSS, pronounced ‘plies’). Once they had put their helmets and gloves back on, there were a large number of checks to be made.

The LM provided very little room for two crewmen wearing suits with PLSSs attached, so one of the first tasks was to lay out all the items they would need at positions where they could get to them without undue difficulty. They checked their zippers, the double locks that would hold their gloves and helmets in place, then positioned themselves to put their back packs on. The sequence of steps in the process was very carefully choreographed. Among all the unstowing of cameras, boots and other paraphernalia was one important task; to deploy a VHF antenna on the outside of the LM by turning a crank. One of the crew had to manoeuvre to the back of the cabin to get at it. When crews had eight hours or more hard work outside while sealed inside a suit, some food was welcome while an occasional drink was essential, so for the later missions, bags of water and a food bar would be installed within the neckring. Then coatings of anti-fog treatment would be applied to the inside of their helmets.

Before they added their oxygen purge system (OPS) to the top of the back packs, a check was made of the pressure in the oxygen tanks within. Each OPS had two spherical tanks with the gas inside stored at an extremely high pressure. A small gauge was provided to check its value. Depending on how a crewman’s PLSS or suit had failed, the OPS would provide at least 30 minutes of emergency oxygen and cooling by the regulated discharge of the contents of its tanks. In fact, the OPS carried more than three times as much oxygen as did the PLSS itself but the latter was merely recycling the gas in a loop and topping it up as necessary as it was consumed.

As the Apollo 14 crew worked through their suiting up procedures, they reached the point where they had to don the PLSS. Mission control had them call out where in the checklist they were. “Okay, Houston,” called Ed Mitchell. “We’re at that point where we hand the real PLSS out and get the lightweight one.”

Once again, the crews could not help but compare their amazing experience on the surface of the Moon with the endless rehearsals they had put in before the flight. During their training exercises or when they had checked out Ant ares itself, they had used a real PLSS up to the point of donning it, but the back packs were uncomfortably heavy in Earth’s gravity and unwieldy in the tight space of the LM cabin. To avoid damaging flight hardware, the real PLSS would be exchanged for a lightweight mock-up. Ron Blevins was an instructor who would typically make the exchange.

“Roger,” laughed Capcom Bruce McCandless. “I’ll have Ron come on up the ladder.”

Crews did find the LM cabin to be a much more pleasant environment when the one-sixth-g of the Moon ruled the physics of their work. “The only time I found the lunar module really confining was when we started moving the PLSSs around,” remembered Charlie Duke. “We move one of the PLSSs on the floor between us, and we pick it up and put it somewhere else. We’d done that in training and John just struggled to get that thing up, because it was heavy! You know, 155 pounds [70 kg]. They had a lightweight mock-up which 1 think was a little bit lighter. But up there, in that one-sixth g, boy, he just picked it up with one hand. It was just tremendous. It made you feel like Superman.”

With the OPS attached to the top of the PLSS, connections were made between the two, including a feed to the antenna on top through which the crew would keep in touch with each other and Earth. Then they would don the PLSS. “Getting the PLSSs on was very difficult,” recalled Gene Cernan. “It was a two-man operation. The PLSSs were strapped to the side walls, at working height, and you had to back up against yours and the other guy would then unhook it from the wall and help you get it strapped to your back. The LM was so small that it was a very difficult operation, even in soft (unpressurised) suits.”

To control and monitor most of the functions of the suit, a remote control unit (RCU) was attached to the chest. It had a quantity gauge for oxygen; switches that controlled their water pump and fan assembly, and their communications; and there were flags that indicated the status of the back pack. These gave warnings for oxygen, water and carbon dioxide as well as for the suit’s pressure. On the front was a bracket upon which a crewman could attach his Hasselblad camera. Once the suit’s systems had been connected up, it was time for checks of the communications

between each other and with mission control. That’s when Conrad and Bean came unstuck.

The checklists the crews used were very thorough. Every connection and every switch position was detailed for the men to check, and the procedures were practised repeatedly before the mission. The problem for the Apollo 12 crew was that they had done this so many times that they thought they knew it. More importantly, they were used to their training hardware. A1 Bean explained: ‘’The gear used down at the Cape [for training] have the comm switches on them but you don’t have to use them at all for comm.” Hence they did not have to actually operate the switch during training, as Conrad pointed out, “fn training, we always just read it in the checklist and kept on going.”

“Here’s an example of the gear we’re using [on the Moon] not being configured precisely like the gear we use in practice; and that cost us,” recalled Bean. In fact it cost them nearly 20 minutes in a machine whose operational lifetime was measured in hours. Differences between training and flight hardware would bite them again when their TV camera was damaged by being inadvertently aimed at the Sun. The crew had only ever practised with an inert mock-up and had no appreciation of the flight unit’s susceptibility to direct sunlight.

With the final oxygen connections made from the OPS to the PLSS, each crewman took a last long drink from the LM’s water supply before they sealed themselves off by donning their helmets and gloves. Care was taken to ensure their microphones were properly positioned as there would be no opportunity to reposition them once outside. They then transferred the suit’s oxygen and cooling water hoses from the LM to the PLSS and put on their hard-wearing outer gloves. Because they would get no cooling from the PLSS until they were in a vacuum, crews from Apollo 12 onwards made sure to give themselves an extra shot of cooling water from the LM through their liquid-cooled garment before this changeover.

The next step was to make sure the leakage from their suits was acceptable. Rather than build suits that were completely airtight, engineers accepted a slight leak rate in the knowledge that the PLSS supply would be adequate to make up for the loss across the duration of the EVA. This ‘pressure integrity’ check required the suits to be inflated above cabin pressure until the gauges on their cuffs indicated 3.7
to four psi. The oxygen supply was then turned off and the gauge watched for a minute. Their check­list stipulated that they should not observe a drop greater than 0.3 psi, some of which was merely due to them breathing the oxygen or the gas getting into all the nooks and cran­nies within the suit.

Lost air

Once everyone was happy, it came time to let the air out of the cabin, or depressurise, to use the parlance. There were two valves the crew could use to achieve this; one on the forward hatch that led out to the ladder, and the other on the overhead hatch that would lead to the tunnel and the CSM once they redocked. The handles for these dump valves had three positions; open, closed and an intermediate position called ‘automatic’ which was its normal setting in which the valve acted to protect the spacecraft against excessive internal pressure. It remained closed until the cabin’s pressure reached a threshold of about one third of nonnal Earth atmospheric pressure with respect to the outside. It then opened to vent air until the pressure dropped below the threshold.

The procedure for depressurisation was to set the dump valve in its open position and drop the cabin pressure from its normal reading of about five psi until it indicated 3.5 psi and then stop. They then monitored their suits to ensure that they were also dropping to maintain the correct relative pressure. If all was well, the dump valve was returned to the open position until the cabin was evacuated enough that the forward hatch could be opened. When sensors indicated less than 3.5 psi, the EVA was deemed to have begun and Houston would begin to time the crew’s progress.

On Apollo 11, the depressurisation proved to be a lengthy procedure. Unlike subsequent missions, Eagle’s forward dump valve had been fitted with a bacterial filter which halved the rate at which air could depart the cabin. As the amount of air within the LM decreased, so did the pressure that was pushing it through the depress valve and thus the rate of evacuation tailed off. It took about three minutes for the cabin pressure to indicate only 0.2 psi but even this was too high for a crewman to directly open the hatch. It had to get nearer to 0.1 psi. Yet at such low pressures, the cabin began to be replenished by internal sources that further slowed the depressurisation, as Armstrong explained after the flight. “It took a very long time to depressurise the LM through the bacteria filter with the PLSS adding gases to the cockpit environment or something adding some cabin pressure. The second was that

we weren’t familiar with how long it would take to start a sublimator in this condition. It seemed to take a very long time to get through this sequence of getting the cabin pressure down to the point where we could open the hatch, getting the water turned on in the PLSS, getting the ice cake to form in the sublimator, and getting the water alarm flag to clear so that we could continue. It seemed like it took us about a half hour to get through this depressurisation sequence.” To speed up the process on subsequent flights, not only was the filter dropped, crews took to grabbing the corner of the hatch and peeling it open.

The sublimator that Armstrong was referring to was the central element of the PLSS’s cooling system. Water was fed directly to the vacuum where its evaporation and eventual sublimation from ice to a gas cooled a separate water circuit that cooled the crewman. This emission of vapour was the reason that later crews pre­cooled themselves before disconnecting their water from the LM. It was better not to start the sublimators until the hatch was open and a vacuum had been established in the LM.

Once the hatch was fully open, the commander manoeuvred himself onto the floor, kneeling with his feet towards the open hatch. Guided by his LMP, he crawled backwards through the hatch onto the porch, a small platform between the hatchway and the top of the ladder. When Neil Armstrong got onto the porch, he reached to his left and pulled a nearby D-ring which allowed one of the side panels of the descent stage to hinge open. A small TV camera was mounted upside down on the panel and strategically aimed to document his descent into the history books. On

Earth, TV converters at the ground stations had circuitry to flip the image the right way up.

In the expectation that a descending LM was likely to deform the crushable interior of the landing legs and compress them, the lowest step of the ladder was set the best part of a metre above the footpad. However, on all missions, there was little appreciable compression of the gear, necessitating a gentle leap from the bottom step to the footpad in the low lunar gravity. Only then could a boot print be made in the lunar surface.

One particular problem with the rover became a serious nuisance for the Apollo 16 and 17 crews. The commanders on those two missions had a habit of storing their geological hammers in pockets sewn onto the shins of their suit legs. Though they could reach down and grab the hammers when needed, they had difficulty seeing them, given that the chest-mounted RCU blocked their view. Unfortunately, as they worked around the rover, both John Young and Gene Cernan caught the right-rear fender extension with the hammer and broke it off.

Young broke his fender partway through their second day and after that, anytime they drove, they and everything on the rover were showered with dust. Unlike the rubber wheels on the MET which tamped down the soil into smooth ruts, the open framework tyres of the rover lilted dust and threw it into rooster tails. Engineers began to worry when the blackened covers began to warm up the batteries and the dust made the radiators less efficient.

Then Cernan did the same thing on his first EVA. He initially tried to use sticky tape to reattach the fender but dust affected the adhesive and it proved to be less than successful. That night, mission control came up with a repair which Young tested in a suit before he passed the details on to Cernan in the morning. In the LM, Cernan took four maps from a book and taped the stiff cards into one large sheet. He took these outside along with two clamps that were normally used to mount little

utility lights onto the protective frame of their alignment telescope. At the rover, the clamps held the card onto the existing fender’s support structure to fashion what was a very successful repair.

The Apollo programme left behind an archive of data and over a third of a tonne of samples, which, to repeat the publicist’s mantra, really did keep scientists "busy for years” and they have formed the bedrock on which theories of planetary formation and evolution have been built. Prior to the space age, planetary science was in the doldrums, with only blurred photographic evidence to feed scientific curiosity. With Apollo’s scientific harvest, planetary science entered an age in which ground truth – actual rocks gathered in situ could inform new theories and help to sort the wheat from the chaff.

Our current understanding of how the Moon was formed first gained acceptance at a eonferenee in Hawaii in 1984. This idea, chiefly proposed by William Hartmann and Alistair Cameron, has yet to be toppled. It is a story of birth rising out of incomprehensible violence.

Our solar system formed about 4,600 million years ago out of a coalescing cloud of dust and gas known as the solar nebula. Most material ended up in the Sun, but some formed a disk out of which the planets gradually grew, or accreted – a process whereby gravity causes loose material in space to gradually gather into ever larger bodies. The light pressure and solar wind from the new star tended to push lighter substances out to the further reaches of the system while heavier substances tended to remain in the Sun’s vicinity. This created predominantly rocky planets near the Sun. gaseous giants further out, and frozen worlds beyond the point at which even gases become liquid or solid.

About 40 million years after the solar system’s birth, two nascent planets were orbiting the new Sun at similar distances and it was only a matter of time before they met. The larger body, our proto-Earth, received an off-centre impact by a body half its diameter in a tremendous cataclysm. The iron cores of the two worlds merged and a large amount of mantle material was ejected to form a giant cloud of debris around what was now Earth.

In a relatively short Lime, some accounts suggest within only a year, this ejected material had itself coalesced to form a new, smaller world – the Moon. As it did so, the huge energy of its fast accretion melted its outer layer to form an ocean of molten rock, or magma, that lasted long enough to fractionate – like a salad dressing that has been left in a cupboard for too long. As the lighter components rose to the Lop. they cooled and crystallised to form a solid crust. They were typically light-coloured and rich in aluminium. Below the crust, in the mantle, the rocks were heavier and richer in iron. The regions that were last to solidify gathered up those elements that had difficulty fitting into the crystal lattice, leading to them being described as KREEPy.

The solar system was still a mass of debris for the first 800 million years of its existence and large impacts were commonplace on all the planets. The Moon retains the scars of this bombardment in the form of large craters, often overlapping one another, all over its lighter-toned surface. During this time, it sustained a particularly large collision when an object gouged out the South Pole-Aitken Basin, a 2.500- kilometre depression on the Moon’s far side. About four billion years ago, the

impact of large objects seems to have peaked before tailing off suddenly. The dark patches wc now see on the Moon’s near side were mostly formed within huge circular basins that were formed by the largest of these impact events. Of particular interest to the lunar science community was the Imbrium Basin, which was dated to 3.91 billion years ago from Apollo samples. Scars from its formation can be traced across much of the near side and therefore its age provides an important benchmark for the relative ages of other superimposed features. As noted, the formation of this basin excavated rock from deep within the Moon that had KREEPy characteristics.

About half a billion years later, prodigious quantities of lava, rich in iron and magnesium, were erupted through the fractured crust. It filled the basins and other low-lying areas to form enormous smooth basalt plains to which we applied romantic names like Mare 1 ranquilliiatis, Mare Serenitatis and Oeeanus Proccllar – um. The last gasps of this activity probably died out ‘only’ about a billion years ago but its peak was around 3.3 billion years ago. Since then, little has changed on the Moon. The material from the bottom of the Apollo 15 deep core had lain undisturbed for 500 million years. Every few tens of millions of years, there is a very large impact that produces a spectacular fresh crater and sprays the landscape with a new layer of rubble and dust. Apart from that, the occasional large object and a slow but incessant barrage of hypervelocity grains of dust sandblasts the top layer of the surface. Across the eons, the topography becomes rounded off and the landscape is draped with a thickening blanket of ground-up rock, the lunar regolith.

This is the kind of profound knowledge produced by careful, focused exploration. As later generations of probes extended our reach into the depths of the solar system, their new data has elaborated on the story of planetary genesis gleaned by men who explored a new w orld in person and applied the power of human intelligence.

MISSION ACCOMPLISHED… NEARLY

With their exploration to the lunar surface finished, their rock samples stowed and the orbital science programme completed, it was time to return to the home planet. At this point, the Apollo spacecraft consisted of just the CSM, the LM ascent stage having been jettisoned and, in some cases, made to crash on the Moon for the benefit of the seismometers emplaced by the crews.

Return to Earth was achieved by the last major firing of the SPS engine. This burn had terrified managers for years, and amply fed the hunger of newspaper and television journalists for riveting speculation about doomed astronauts marooned in their cocoon of failed technology around a forbidding, desolate planet while waiting for a time when their own exhalations would begin to asphyxiate them even as they heroically struggled to repair their flawed ship. The terror and hyperbole was driven by the knowledge that, while a failure to enter lunar orbit would have resulted in a return to Earth, failure of the burn to leave lunar orbit would, by all analyses, have led to the deaths of the crew. As no fail-safe system existed, the SPS had to be totally reliable.

A FIERY RETURN

Arguably the most audacious feature of an Apollo flight was to have the crew re­enter Earth’s atmosphere in the manner that they did. In the final minutes of a mission, a lump of metal and plastic, three crewmen and a few dozen kilograms of moonrock, altogether weighing nearly six tonnes, came barrelling in from outer space at speeds approaching 11 kilometres per second as Earth’s gravity hauled them in. As it entered, the air in front of the blunt end of the command module was brutally compressed in a shock wave that generated temperatures approaching 3,000°C. All that stood in the way of the crew being incinerated by this extraordinary heat was a coating of resin and fibreglass that NASA’s engineers reckoned could withstand the punishment.

In truth, the heatshield that surrounded the Apollo command module was very conservatively engineered for two main reasons. When the spacecraft’s design was frozen, engineers still had a poor knowledge of how the superheated air of re-entry would flow around the upper walls of the spacecraft. Although this surface did not bear the brunt of the heat, they decided to cover almost all of the hull with the heatshield material. Additionally, the original specifications had required that the shield should tolerate a much longer passage through the atmosphere, 6,500 kilometres, than ever proved necessary. The command modules that returned from the Moon typically flew for only about 2,200 kilometres through the atmosphere, which nearly halved the overall amount of heat the shield had to endure. In practice, although the heatshield took a lot of punishment across its curved aft section, much of its conical surface was barely singed by re-entry. Even the reflective Kapton tape that had been glued to the spacecraft’s exterior for thermal control in space was usually found to be still adhering to much of the hull. On recovery, pieces of Kapton were occasionally peeled off by those in attendance and kept as souvenirs.

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

DOI 10.1007/978-1-4419-7179-1 15. © Springer Science+Business Media. LLC 2011

"Well. men. wc’rc getting close." said Frank Borman as Apollo 8 neared the planet to make the programme’s first lunar return.

“There’s no turning back now’." added Bill Anders.

Jim Lovell continued their obsession of stating the obvious. “Old mother Earth has us," he said.

As they waited for the first stage of the re-entry, the computer moved onto P63. This program wns purely to initialise the upcoming sequence and start calculating the re-entry parameters. It maintained their attitude and w’aited for the accelerometers to sense 0.05 g. The crew had little to do but to look out of their windows, which wnre facing backwards along the flight path, affording a view’ of the Moon setting behind Earth’s bulk at a precisely known Lime. On Apollo 12. Pete Conrad became almost lyrical about the scene as Yankee Clipper coasted over the Western Pacific.

“Hey, there’s the horizon. Hot damn. Hello, w’orld! I ley, you’re going to get Moonset right on the schnocker!"

“Yes," agreed Gordon.

“It’s coming pretty fast,” enthused Conrad. "We is flat smoking the biscuit. God damn! We’re going! Whooce!”

"’We’re going 35,000 feet per second,” said Gordon, keeping an eye on the DSKY as they edged towards 11 kilometres per second.

“Were hauling ass is right,” said Bean. ”Goi some high clouds and some low clouds down there. Got a lot of ocean.”

‘"You’re going to have Moonsct pretty quick,” said Conrad. Right on lime, the Moon seemed to descend into the murk of Earth’s atmosphere.

“Hey, that’s something else. Look at that. I wish I had a picture of that.”

""Where is it?” asked Bean.

"‘Right out the centre hatch," said Conrad. Since he was occupying the centre couch, it was the hatch window that gave him his view outside.

"’Hey, Al. turn your camera on,” called Gordon, knowing that Bean had the movie camera set up in the window7. “Maybe you can get a picture of it for a couple of seconds.”

"’The camera’s going this way. and that’s up that way,” replied Bean. It was not in the field on view.

"’Too far away, huh?” said Gordon.

After the flight, Conrad spoke of the impression the view7 left on him. “Moonset really w-as spectacular. It’s too bad we didn’t have a camera to photograph that. It was a full Moon; and it was exactly aligned in the yaw plane behind us. Just wntching that thing settle behind a beautiful, lit daylight horizon, with clouds above the Pacific, was phenomenal.”

The Apollo 15 crew7 were so engaged with the view of the blue planet speeding by that they missed Moonset.

"’Oh, man. are w’e moving, too!” said Al Worden. "Son of a gun! Sheeoo!”

""Yes, indeedy,” said David Scott, who had made an Apollo re-entry before, albeit from a slower speed in Earth orbit. "You ought to be able to see it out the hatch window.”

""Oh my. I sure can,” said Worden. "‘Sure a lot of mountains down there. How about that!”

“Shit. I think that’s Alaska out there. That would be right, wouldn’t it?” said Irwin.

“Yes,” said Worden. "Keep an eye out for the Moon.”

""Yes, keep an eye out for the Moon.” agreed Scott.

"’We’ve done it. Oh. we’ve missed it." said Worden. ‘"We were too busy watching the Earth.”

"T’m not sure there’s much you could do about it to correct it anyway,” said Scott. Indeed there was nothing since the CM possessed no propulsion.

Being an arbitrary construct, entry interface passed with little more than a mention from the public affairs announcer. Of greater importance to the crew7 was when P63 sensed 0.05 g. about 30 seconds later, at w’hich point it triggered the EMS to begin monitoring their entry. Its scroll began moving to the left as their velocity decreased, and its range, velocity display showed either how7 far across the ground they had still to fly or how7 much velocity they still had to lose. Simultaneously, the computer was automatically advanced to P64 to fly the initial part of the re-entry flight path. There was no fixed altitude at which 0.05 g occurred, because it depended completely on their velocity, now 11 kilometres per second, the shape of the spacecraft and the local atmospheric conditions.