Category How Apollo Flew to the Moon

"Ignition.” announced Armstrong to Aldrin as Apollo 1 l’s LM Eagle began the human species’ first descent to another world.

“Ignition,” repeated Aldrin. “Ten per cent.”

On all the missions, at PDI the DPS engine was initially run at only 10 per cent of its maximum thrust for 26 seconds to give the computer enough time to sense whether the engine’s thrust was acting through the LM’s centre of mass, and if it was not. to move its supporting gimbals until it was. This ability to vector the thrust was not intended to steer the craft. It was too slow for that. Steering was provided by the RCS thrusters which altered the attitude of the entire craft to aim the thrust, leaving the engine’s gimbals to deal with longer-term centre-of-mass shifts.

Aldrin counted up to the end of the low-thrust phase. “24, 25. 26. Throttle up. Looks good!” Propellants poured into the engine as it went to its high-thrust setting.

Starting with Apollo 12, engineers added a modification to the computer’s programming to achieve pinpoint landings. As soon as Intrepid came around the Moon’s limb prior to landing, its velocity was compared with what would be ideal to achieve a pinpoint landing. From this, engineers could calculate the difference between where they wanted to land and where the computer, which believed it was on the right course and unaware of external factors which had perturbed its path, was actually taking them.

“Intrepid, Houston,” called Capcom Gerry Carr only 80 seconds into Apollo 12’s powered descent. “Noun 69, plus 04200. Over.”

“Roger. Copy. Plus 04200," confirmed Bean.

This was the important call that ensured that the LM would land where it was
supposed to, and it was extremely dangerous. Noun 69 held three values that represented an update to the position of the landing site in three dimensions. Changing one of those values by +4,200 feet (1,280 metres) shifted the computer’s idea of where they should land to a point further downrange, thereby fooling it into taking them where they wanted to go. When the crew had punched the number into the DSKY’s register, mission control took a look at the telemetry to verify they had done so correctly before confirming that they could ‘enter’ it into memory. Had the crew inadvertently entered the wrong data, they could easily have sent the LM out of control and been obliged to abort.

“Intrepid, Houston. Go for Enter,” said Carr once he had received word from other flight controllers that the crew had typed the update into the correct field.

“It’s in, babe,” said Bean.

“Intrepid, Houston. Looking good at two,” replied Carr as they passed the 2-minute mark into the burn.

Throughout P63’s regime, the DPS engine had to fire more or less into the direction of travel, especially during the initial minutes. As long as it did so, the LM could make rotational manoeuvres around the engine’s axis. On Apollo 11, the first few minutes of powered descent were flown with the windows, and there­fore the crew, facing towards the surface. Armstrong had a method of using the angle markings on his window to time the passing of land­marks below. Before ignition, it had given him a check of what their perilune altitude was going to be. This used the fact that the closer you orbit a body, the faster the landscape below appears to pass by. Then after the commencement of powered des­cent, he could compare the absolute time a landmark passed with a predicted time. Since they were travelling at about 1.5 kilometres a second, only a few seconds early or late signalled the extent of any miss. It was a simple but powerful techni­que.

“Looking good to us,” Capcom

Charlie Duke informed Apollo 11. "You’re still looking good at three. Coming up. three minutes."

"Okay, we went by the three-minute point early." said Armstrong. “We’re long." He w’as right, because they landed six kilometres further down-range from where they had planned.

Conrad dispensed with the idea of having the windows looking down at the start of PD1 on Apollo 12 since they had other techniques in the wings to determine their approach errors. As soon as they entered the descent orbit over the far side, he placed the LM into the correct windows-up attitude for PDI. As this was an inertial altitude, set with respect to the stars, it was not concerned with the position of the Moon, so a windows-up, engine-first attitude over the near side of the Moon was a windows-down, engine-trailing attitude over the far side, as Pete Conrad explained: “From that inertial attitude, we watched ourselves pass from face down, through local horizontal [i. e. feet down, facing forward], to pitch up at PDI. It gave us an excellent look at the Moon going around.’’ It also gave their steerable antenna a clear view1 of Earth from AOS right through to landing.

As mission control worked towards a final decision to stay, and as the crew got on with their tasks, there was usually some discussion about how close to their target point they had managed to land. Armstrong had known long before touchdowm that he and Aldrin w’ere going to land w’ell past their intended destination. Shortly after arrival at Tranquillity Base, he raised the question with Houston. "The guys that said that we wouldn’t be able to tell precisely where we are. are the winners today.” It was no great surprise, given the hair-raising nature of their descent. "We were a little busy worrying about program alarms and things like that in the part of the descent w’here w’e w’ould normally be picking out our landing spot.” he continued, "and aside from a good look at several of the craters w? e came over in the final descent, I haven’t been able to pick out the things on the horizon as a reference as yet.”

‘;No sw’eat,” reassured Charlie Duke in mission control. "We’ll figure it out.”

It took a long Lime for anyone to figure it out. Throughout the time he spent in orbit alone, Mike Collins never once managed to view’ Eagle through the sextant, which was hardly surprising, given that it had landed six kilometres from its intended site and the plain of Mare Tranquillitatis is a featureless wasteland of crater imposed

upon crater. The best estimate was made by the geological team at mission control who noted how Armstrong had described flying over a large blocky crater to a "fairly level plain with a large number of craters”.

The factors that led to the inaccuracy of the Apollo 11 landing were overcome to guide Pete Conrad to a pinpoint landing on Apollo 12. He knew where he was, but had trouble telling mission control because he was reading the map incorrectly. Four hours later, as Dick Gordon coasted overhead in Yankee Clipper, he had his eye firmly affixed to the sextant eyepiece.

"Houston, I have Snowman.” The familiar pattern of craters stood out when the computer was asked to point the optics at the intended landing site. At first, he confused Intrepid with the Surveyor 3 spacecraft that had arrived 31 months earlier. Then it clicked.

"I have Intrepid. I have Intrepid,” said Gordon excitedly. "He’s on the Surveyor Crater; he’s about a quarter of a Surveyor Crater diameter to the northwest.”

"Roger, Clipper. Well done,” replied Capcom Ed Gibson. Surveyor Crater formed the torso of the Snowman and was where the eponymous spacecraft had been located.

"I’ll tell you, he’s the only thing that casts a shadow down there.”

If the planned walking traverses to geologically interesting sites were to be fulfilled, an accurate landing was mandatory for Apollos 12 and 14. Just 600 metres off could make a destination unreachable. Alan Shepard brought Antares down within a mere 50 meters of his target, the best of the programme, but for the J – missions, only the commander’s bragging rights were in jeopardy by such a miss. Having a rover rendered such inaccuracies moot.

As soon as Lunar Rover-1 had been released from Falcon, David Scott hopped on and began to check its instrument panel: "Okay. Amp-hours, 105 and 105. Amps, of course, are at zero.” The battery power seemed a little low as the batteries should have had 121 amp-hours each. His reading of current made sense because he had not yet begun to drive. He continued, "Okay, volts: on number 1 I’ve got about 82, and
number 2 is reading zero. Hmm." Fortunately battery 2 was fine and the zero reading was an instrumentation fault. Nonetheless, they had not even begun to drive yet were seeing a slew of niggling problems.

Scott then began to check the rover’s major functions. “Hey. Jim. you can probably tell me if I’ve got any rear steering.’’

"Yeah, you have rear steering.” replied Irwin.

"Okay. But I don’t have any front steering.”

The rover had been designed with independent steering on both front and rear wheels. This not only made it highly manoeuvrable, it also gave redundancy. Loss of one steering system would not inhibit it much. Scott and Irwin set off regardless, and completed their first traverse satisfactorily.

Next day, before they set out on their second traverse, Joe Allen in Houston asked Scott to take another look at the front steering. On powering up the system Scott got a pleasant surprise. "You know what I bet you did last night. Joe’.’ You let some of those Marshall guys come up here and fix it, didn’t you?” NASA’s Marshall Space Flight Center had been responsible for the development of the rover, w hich was built by Boeing.

"They’ve been working. That’s for sure,” said Allen.

"It works, Dave?” laughed Irwin.

"Yes, sir. It’s working, my friend.”

"Beautiful.” said Irwin.

"Lot of smiles on that one. Dave,” said Allen. "We might as well use it today."

Scott agreed. "Boeing has a secret booster somewhere to take care of their rover!”

Previous experiments on and around the Moon had shown the lunar atmosphere to be incredibly tenuous. Scientists hoped to characterise what little there was by looking at the emissions it gave off in the ultraviolet region of the spectrum as atmospheric atoms fluoresced in the presence of solar ultraviolet radiation. They were surprised that this spectrometer could find no trace of an atmosphere around the Moon except for a transient atmosphere generated by the exhaust from the LM engines.

Radar to the Moon

One of Apollo’s major contributions to planetary science was to help to push the development of radar on an orbiting vehicle as a tool to investigate the surface and subsurface of a planet or a moon. From the simple experiments conducted on Apollo, such technology has gained profound capabilities that allow’ the shape or topography of a surface to be accurately profiled. Moreover, depending on the nature of the soil it can ‘see’ buried structures to a depth of one kilometre, and since it does not require light, it allows unlit and cloud-covered terrain to be viewed.

Starting w ith Apollo 14, radar tests became a normal part of the CMP’s solo tasks when the spacecraft’s S-band (around 2,200 MHz) and VHF transmissions (around 260 MIIz) w’ere aimed at the Moon to be received by large dish antennae on Earth. These bistatic tests w’ere so called because, unlike most radar setups in w’hich the transmitting and receiving antenna is one and the same, here two antennae were used, separated by a distance similar to the target distance. As a result, these tests required no additional equipment on the CSM and w’ere simply a bonus from using what wras already available. Researchers could determine the electrical properties of the surface by seeing how’ the strength of the reflected radio wave varied with its incident angle. And the interplay between the spacecraft’s orbital motion and the resultant Doppler effect on the signal’s frequency allowed discrete lunar features to be ‘seen’ in the signal’s received spectrum.

For Apollo 17’s lunar sounder, researchers took the technology to the next level. Specialised antennae were mounted on the service module to send pulses of radio energy towards the Moon and to receive the reflection, including any modification due to its interaction with the surface. Results from the radar w’ere recorded optically on film for later analysis on Earth.

The lunar sounder w’as the prototype for radar systems that successfully imaged the cloud-covered landscapes of Venus and Titan, searched for underground geology and ice deposits on Mars, and mapped the surface topography of most of Earth.

The Moon after Apollo 393

The Apollo lunar module has a special place in the hearts of those who study the Apollo programme. In every way, it was an extraordinary flying machine. Its systems were often at the very edge of what humans could wield at the time, from its advanced lightweight computer to the supercritical helium technology that pressurised its tanks. Its pared-down, minimalist form was often derided by the press as ungainly and spidery, like a bug. But its beauty derived not from any need to slip through a planet’s atmosphere like an aircraft does. The beauty of the LM was in its function: it took men to another world for the first time in the history of the human race, and did so in the spirit of exploration, as a weapon of peace in a battle for the minds of people.

Even in its last act, it continued to add to our knowledge of the Moon, because in most cases it was commanded to impact the lunar surface in the name of science. Most of the Apollo crews left seismometers as well as other experiments scattered across the surface. When the LM ascent stage hit the surface at nearly 6,000 kilometres per hour, it sent shock waves through the interior of the Moon that were picked up by the emplaced instruments and radioed back to Earth. Geologists used these signals to decode the internal structure of our natural satellite.

Each of the six descent stages that safely lowered their human cargo to the lunar surface are still to be found there. Each ended its useful life as a launch pad for the

ascent stages, and then began its lonely, perhaps futile wait for humans to return. If we never go back to the Moon, these relics will all sit there silently, forever immobile except for the changes that come about every lunar day and night as they experience the fierce heat of the unfiltered Sun or the deep chill of space.

Every so often, a miniscule dust particle will fall at an extreme velocity and punch a tiny crater in one of them. Less The Apollo 17 descent stage left behind at Taurus – often, a meteorite will impact Littrow. (NASA) somewhere in the distance,
launch a sheet of ejecta across the landscape and coat a descent stage in a thin layer of finely pulverised rock. With a slowness that our minds can barely conceive, over terms measured in millions of years, the Apollo descent stages and all the other human artefacts we set across the Moon’s surface in that golden age of exploration, will erode, sandblasted by the incessant rain of dust that still collects on all the worlds of the solar system. Simultaneously, they will be gradually covered with dust, scarred skeletons buried within the regolith until, perhaps 500 million years into the future, the only sign of our visit will be a few dusty mounds – like sandcastles on a beach that have been washed away by the tide. That is. if we never return.

On the day after TEI. the J-mission command module pilots took centre stage. The SIM bay cameras used long lengths of photographic film stored in large cylindrical magazines. These were still in the service module and that would be discarded in a few days time to burn up and be destroyed in Earth s atmosphere. The film therefore had to be brought into the command module’s cabin, the only part of the spacecraft that would survive re-entry. To bring it in, the CMP had to perform a spacewalk.

Extravehicular activity (EVA) is the name NASA gave to what everyone else calls walking in space, or on the Moon. At the time of Apollo, it specifically referred to time spent in a spaccsuil when the air pressure outside the suit could no longer sustain life. It was another of those essential techniques for Apollo that NASA had learned during the Gemini programme.

Edward White, on Gemini 4, was the first NASA astronaut to leave his spacecraft. Part of his brief was political: America wanted their spacewalking astronaut to stay out longer than Alexei Leonov had done for the Soviets a few months earlier. White floated on the end of his umbilical and gingerly tried various techniques for moving around outside a spacecraft. Apart from some exertion while getting back into the cramped cockpit, White made EVA appear easy.

When Eugene Cernan attempted to don a jet-powered back pack on Gemini 9. he was the first to attempt to do substantial work during EVA. Cernan quickly found how difficult it w as to maintain control of body position in a completely Newtonian environment. The slightest twist, turn or push against the spacecraft would send his mass flailing away from where he wanted to go. Soon, the stress of his exertions began to take a toll on him, and because the suit could not cope with the heat he was generating, his visor began to fog up on the inside. When he was exhausted, he was called back by his commander, Tom Stafford. Almost unable to see through his visor, he drew7 himself hand over hand along his umbilical back to the hatch. Re­entering the cabin proved to be even more terrifying, as the two crewmen, their suits stiff with the air pressure within, battled to get Cernan far enough down into his seat to be able to shut the hatch over his head.

In the light of wrhat could have been a horrifying incident. NASA began to treat EVA very seriously indeed. During the final three flights of the Gemini programme. Mike Collins. Dick Gordon and Buzz Aldrin refined the techniques needed to work efficiently and safely outside a spacecraft. Handholds and foot restraints were added, a methodical approach was taken to all the movements needed for the EVA. and an underwater training facility was developed to simulate weightlessness.

Eor all the experience that was gained on Gemini, little actual EVA time was logged during the Apollo Moon programme in terms of a crewman floating outside the hatch of a weightless spacecraft. A lot of outside activity wxts logged on the Moon, but this was in a one-sixth g environment that the crews found very pleasant. During the Earth orbit operations on Apollo 9. David Scott and Rusty Schweickarl made tentative forays out of their CM and LM respectively: Scott just putting his head and shoulders out of the CM hatch while Schweickart egressed fully and placed his feet in so-called ‘golden slipper’ foot restraints on the LM’s porch for a test of the life-support back pack that the crews were to use on the Moon.

From Apollos 10 to 14 no crewman left the CM hatch for an EVA, although all crews trained for the possibility that, in the event of a docking problem, they might have to transfer from the LM to the CM via an EVA through the side hatches of both vehicles. When spacewalking finally returned to Apollo on the last three missions, it was something special. When Alfred Worden, Ken Mattingly and Ron Evans w-ent to retrieve film magazines from the SIM bay they became the only humans to have performed an interplanetary EVA.

Preparation for the EVA took a considerable amount of time because the spacecraft was packed with boxes of rock samples from the lunar surface. Also, three crewmen had to get suited up in the confined space of the CM, each methodically helping the other to check the integrity of his suit. Oxygen for all three men came from the spacecraft’s suit circuit with a particularly long umbilical being required to enable the CMP to get to the SIM bay. This made for a cluttered cabin with gloves and helmets floating among loops of hoses while three astronauts clambered into cumbersome spacesuits, two of which, after 20 hours of work in the dust and dirt of the lunar regolith. were extremely dirty.

The CM P carried an additional emergency supply of oxygen from a package his crewmates had brought back from the Moon. The oxygen purge system (OPS) normally sat at the Lop of a crew-man’s PLSS back pack when used on the lunar surface. Its function there was to act as a standby in case of a failure of the suit or the PLSS. If a hole were to open up in the suit or a problem were to occur with his oxygen supply, the OPS could supply oxygen from very high pressure tanks that would give the crewman extra minutes to deal with the situation. Although the back packs w ere jettisoned on the surface, the OPS were returned with the surface crew in case they had to support an EVA from the LM to the CSM. One of the OPS was transferred to the CSM to give their colleague an emergency supply during his EVA.

A television camera and a 16-mm movie camera were mounted on a pole which, once the hatch door was fully open, was inserted into a receptacle in the door so that the EVA could be filmed and Houston could watch a live view of its progress. The spacecraft’s attitude was changed so that the Sun shone obliquely across the SIM bay, but not directly into the cabin. All of the RCS thruster quads were then disabled, except for the one opposite the SIM bay so that minor rotational manoeuvres could still be carried out while the CMP was outside.

When all the crewmembers were safely sealed into their suits, the air in the cabin was expelled to space by opening a valve in the main hatch. Then, once the internal pressure in the cabin had dropped to a very low level, the hatch was opened, venting the last wisps of gas.

“Okay, Houston. The hatch is open." Cernan was now in command of his own mission, Apollo 17. and an EVA expert himself. He kept a close eye on his two rookie crewmen. Jack Schmitt and Ron Evans. Schmitt’s role on the EVA was to stand in the hatch and keep an eye on Evans, look after the umbilical and take the film magazines from him, passing them in to Cernan. Evans was going to manoeuvre himself along the side of the service module.

“Iley, there’s the Earth, right up ahead,” said Evans as he positioned himself in the hatch. “The crescent Earth.”

“Okay, Ron. You’ve got a Go for egress.” informed Cernan.

“Beautiful," replied Evans.

“Okay, and just take it slow," said Cernan. speaking from experience.

The door shielded the hatchway from the Sun. “Man. that Sun is bright," said Evans as he cleared the door. He was wearing a lunar extravehicular visor assembly (LEVA) that had been worn by one of his colleagues on the Moon and brought with them in the ascent stage.

“Pull down that visor, Ron. You’re going to need it,” advised Cernan.

“Yes.” " "

“You’re a long way from home. We don’t want to lose you.’’

The LEVA went over the crewman’s clear helmet to provide additional protection from the light and heat of the Sun. and as an extra shield against micrometeorites. It included two visors that could be pulled down if needed. The inner visor was clear for additional ultraviolet blocking. The outer visor was coated with an extremely thin layer of gold to reject both light and heat. This gold visor has become part of the astronaut’s iconography, being seen in all the most famous images showing a man on the Moon.

On Apollo 16. it was Ken Mattingly who made the EVA to the SIM bay. "Okay. How about if I get rid of the jett bag first?” One of the first tasks was to throw out the trash. All the disposable items they could find were packed into one bag that was jettisoned by gently pushing it into deep space, probably to enter Earth’s atmosphere and burn up a few days later.

“Bye-bye, bag. Okay. Okay, I’ll go out and get the TV.”

As Mattingly squinted in the Sun, he reminded Charlie Duke to bring down his visor too. “Ooh! Charlie, you’ll need the outer visor as soon as you get into the hatch.”

Mattingly manhandled the coils of his umbilical out of the hatch then put the camera pole into its receptacle in the door. He then worked his way along the handles, hand over hand, to the SIM bay, inspecting the side of the service module as he went. Training had shown that the best way to reach the bay was to move along the service module until hovering above the instruments, then use the handholds around the bay to get into the correct position to engage the feet in a restraint. Once there. Mattingly glanced towards where the spacecraf t was pointed and caught sight of the Moon with most of its disk illuminated by the Sun.

“Oh, man. Man, the old Moon’s out there. Okay, going after the pan camera. Okay, here comes the hard cover – gone.’’ He threw the outer metallic cover of the camera cassette away, then removed the soft, inner cover that was held on by Velcro and discarded this too. "Soft cover has gone. Okay. I’m going after the hook.’’ He attached a tether to the magazine as the Sun beat down from his right making him glad he was wearing Young’s LEVA. “Boy. that old visor of yours that outer visor on the glare shield really comes in handy.’’

Mattingly continued to remove the giant cassette. "The pip-pin is out, and I’m throwing it away. Okay, get my feet out. There’s one. There’s two. Okay.” As soon as his feet came free, he involuntarily rotated as if he were doing a handstand on the spacecraft and had to pull himself back in with his hands. With the magazine tethered to his wrist, he manoeuvred across the module towards Duke in the hatch and passed the heavy object across for Duke to send through to Young inside. Mattingly had another job to do while outside. He was to expose an array of test chambers containing microbes directly at the Sun for 10 minutes. This was an ultimately inconclusive experiment to determine the effect of the space environment on microbial growth.

As Mattingly sat astride the service module with his back to the hatch, Duke caught sight of Mattingly’s wedding ring tumbling in the strong sunlight. It had been lost earlier in the flight and they had been unable to find it among the cabin’s many nooks and crannies. By the time he thought to retrieve the ring, it had floated out of his reach and on into the void of space. Duke thought it a shame the ring would be lost forever but elected to say nothing of it to his crewmates because they had other things to concentrate on in this hostile environment. Yet of all the directions the ring could have taken away from the command module, Duke was astounded to see that it was headed directly for the back of Mattingly’s head. He then watched in fascination as it bounced off the LEVA and began a perfect Newtonian path back towards the hatch. Duke was not to be foiled a second time. As soon as the ring came within range, he carefully grasped it.

"Guess what I caught floating out the hatch?” said Duke, gleefully, if somewhat misleadingly.

"What’s that?” asked Mattingly.

“A ring.”

“Oh, is that right?”

“Yeah. I think it’s yours,” laughed Duke. “In fact, it had already gone out and hit you and was coming back when I caught it.”

“Boy, how’s that for luck?” laughed Mattingly

When Ron Evans got his opportunity to go outside his CSM America and retrieve the film cassettes, no one could have been happier about it than he was. Throughout his EVA, he hummed and chatted with a boyish delight. Working away outside the spacecraft, he waved at the camera.

“We see you waving,” informed Houston.

“Hey, this is great!” said Evans, his voice up an octave in the joy of being outside of the cramped cabin where he had been for over 10 days. “Talk about being a spaceman? This is it!”

Evans took a look at the damage that the SM skin had sustained during the flight. "Okay. Beautiful! Hey. the paint on here – it’s a silver paint and it’s just got little blisters on it. You just kind of peel it off with your fingers." Then he described his position between the two worlds. "I can see the Moon back behind me! Beautiful! The Moon is down there to the right – full Moon. And off to the left, just outside the hatch down here, is a crescent Earth." He noticed how the atmosphere scattered the sunlight towards the night side, extending the crescent. “But the crescent Earth is not like a crescent Moon. It’s got kind of like horns, and the horns go all the way around, and it makes almost three-quarters of a circle.”

Having manoeuvred into the correct position, he tried to settle into the golden slippers. “Okay, I’m having a little trouble, right now. just torquing down to get my foot in the foot restraint, for some reason. Okay, the right one s in. And the left one’s in. Hey, pretty stable right here. Let go of both hands? See?"

Happy as he could be. he went after the radio sounder’s cassette. “Okay, let’s try the old cassette. We’ll push down on it until it goes past centre. Ah-ha! I think that was more than [the expected] two pounds of force to come out. but it came out. And I’ve got the film.”

He brought the cassette to Schmitt and then returned for the panoramic camera’s film, all the time revelling in the experience.

“Houston, this is – Let’s see. when you’re EVA, they use your name, don’t they?” He wasn’t simply T7′ now.

“Ok ay, Ron,” humoured Houston. “Yes, sir, we’ll use it, Ron.”

“Houston, this is Ron. okay?” announced a gleeful Evans. “You hear me okay, I guess, huh?”

“Roger. Ron,” replied Houston. “Read you loud and clear.”

“Okay,” laughed Evans. “Oh. this is great. I’ll tell you!”

“Yes, we thought it was Mr. America.” With a pun on the name of the CSM, Houston was gently mocking him, but in his happiness he took it in good heart.

“Well, it is. Something like that. Oh, boy! Beautiful Moon! Full Moon down there. 1 can see the engine bell sitting back here. That’s a pretty good-si/ed thing, Loo. And, of course, the VHF antenna is still sticking out there.”

Evans held up the panoramic camera’s thermal cover for Houston to see. “Can you see that? The thing I’m holding up. It’s the cover that’s on the outside of the pan camera. It’s a thermal cover, see. that covers up the cassette.”

“Roger. Yes, we see it. Ron," confirmed Houston.

“Whooooee!" laughed Evans as he tossed the cover past the camera’s field of view.

Houston noticed. “We just saw’ that cover.”

Humming away to himself, Evans attached the magazine to his wnist and pulled it free. "Out she comes. Nice and easy. This is a heavy son of a gun. Not heavy up here; it just has a lot of momentum to it. Once she starts pulling in one direction, it just takes a lot of force to stop it.”

As he hand-walked his way along the handholds, he noted how the heavy object moved. “Hey. it’s just kind of coming along with me. I’ll just let her do that. Hey, she’s just floating there. Thai’s good. Nice and slow, because you don’t want that
thing banging around too much up there, I don’t think. That’s the way it ought to be done, isn’t it?”

Evans passed the cassette to his colleagues and returned to the SIM bay for the mapping camera cassette. As he did, he found that he was becoming used to the mechanics of getting around in space, and decided to go back down to the SIM bay by aiming his feet directly for the restraint. "You know, I’ll just go backwards down there.” He hummed away to himself as he shimmied down the spacecraft.

"That’s an unorthodox way to enter the SIM bay, but it works. Once you get your feet in there, you almost feel like maybe they might come out, you know,” he laughed. "So I’m not sure you really trust them. The right foot’s in good and tight. Hello, Mom!”

"We see you, Ron. Looking great,” called Houston. The MOCR were enjoying the show too.

Evans then called to his children. "Hello, Jan. Hi, Jon. How are you doing? Hi, Jaime. Let’s see, I’m supposed to rest, though, aren’t I? What would you like to know about the SIM bay? Looks great.”

When compared to the light-hearted and longer EVAs by Mattingly and Evans on the later flights, A1 Worden’s foray to the SIM bay on Apollo 15 was a fast, efficient

affair where, although he enjoyed the experience, he wasted little time on the view. However, as he started to bring the mapping camera’s cassette towards the command module, he paused to look at Irwin standing in the hatch: "Jim, you look absolutely fantastic against that Moon back there. That is really a most unbelie­vable, remarkable thing.” Worden had no camera with him to record this unique interplanetary view, but after they returned to Earth, artist Pierre Mion carefully reproduced the scene in a painting.

When Worden had finished off by making an examination of the SIM bay on Houston’s behalf, he and Irwin got back into the cabin, took the camera pole with them and closed the hatch. As they waited for Endea­vour’s cabin to repressurise, Scott remarked how quickly Worden had undertaken the EVA. "You should have stayed longer.” Perhaps Scott Pierre Mion’s recreation of A1 Worden’s view was aware of how much time he had towards Jim Irwin and the Moon. spent outside on the Moon’s surface,
and here was his colleague’s chance to feel the exhilaration of spacewalking, yet he spent less than half an hour outside. Bui Worden, like so many crewmembers, brought an entirely businesslike attitude to his work. On one level, it would have been nice to have got the job done then spend a little time just enjoying the view’ and the experience. On the other hand, he w-as in a situation that had many possibilities for danger, where a technical problem could quickly develop into a lile-threatening scenario.

The cabin was repressurised from a rapid repressurisation system consisting of three small oxygen tanks that had been topped up prior to the EVA. These w’ere now’ emptied into the spacecraft. Also, the oxygen from the OPS tank was bled out to add to the pressure. The next time the door was opened, the command module would be silling on the waters of the Pacific Ocean.

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.