Category How Apollo Flew to the Moon

In the Christmas season of 1968, with a little over two hours to go before they entered lunar orbit, the CMP of Apollo 8, Jim Lovell, pointed out that despite their pioneering journey, something was missing.

“As a matter of interest, we have as yet to see the Moon.”

“Roger,” came the reply from Capcom Gerry Carr. He pondered this point for a few moments then sought a degree of elaboration.

“Apollo 8. Houston. What else are you seeing?"’

The acerbic reply from LMP Bill Anders shed new light on the truth of the astronauts’ great adventure.

“Nothing. It’s like being on the inside of a submarine.”

“Roger."’ was all Carr could say, for Anders’s comment was so true.

Despite being nearer to the Moon than any human in history, and with the exception of some sightings that Lovell had made through the spacecraft’s optics, this crew had yet to view their quarry. This was partly because their three large windows had fogged up owing to a design problem with the sealant around them. Additionally, they had spent most of their time during the coast broadside to the Sun. twirling slowly in the barbecue mode. In this attitude their two good windows, which looked along the direction the craft was pointed, viewed only deep space.

As with many of the Moon-shots, Apollo 8 arrived over the western side of the lunar disk at the same time as the Sun was rising over the eastern side. The nearer they got to the Moon, the closer it came into line with the Sun until, in the final few hours before arrival, the spacecraft entered the Moon’s shadow’ and plunged the crew into darkness. Apollo 10 arrived at the Moon under similar lighting conditions and its commander Tom Stafford still gained no view’ of the approaching planet.

“fust tried looking out as far as 1 can. out the top hatch window-, and still can’t see the Moon; but we’ll take your word that it’s there.’’ By ‘top hatch’. Stafford meant the round window in the side hatch, though as they lay in their couches, it was above their heads.

“Roger. 10. That’s guaranteed; it’s there,” said Charlie Duke in mission control. Stafford’s LMP Lugcnc Ccrnan couldn’t catch a glimpse of it cither, but the next time he journeyed to the Moon as the commander of Apollo 17, he got an eyeful.

“Boy. is it big! We’re coming right down on lop of it!” he shouted. “I’ll tell you, when you get out here, it’s a big mamou.”

Cernan was one of the few’ Apollo astronauts who had a natural ability to convey to the lay person some of the emotion and depth of the Moon-flight experience. Although he had been there before, the approach of this serene orb stunned him. and in his memoirs he expanded on this first glimpse.

“Looking at the Moon from our vantage point w’as quite unlike seeing it from Earth, w’hen it is so distant. Now’ it was gigantic, a world of its ow’n. and it forced me to question what I was really seeing. Such scenes existed only in science fiction, for not even the simulators could impart the reality of such a moment. We plummeted tow’ards it, faster and faster, and the closer w’e got. the bigger it grew-.’’

Cernan w’as also struck at how this extraordinary initial scene even managed to mute his LMP. geologist/astronaut and incessant talker. Jack Schmitt.

“Dr Rock was also stunned by the sheer size of the planetoid lhai he had spent a lifetime studying. Never in his wildest dreams had Jack imagined such a sight, and he momentarily lost his ability to even speak. The Sun illuminated the high peaks and mountains, and the rims of giant craters and surface details emerged, bathed in gold or hidden in deep shadow-.”

Apollo 11 was the first flight to afford an approaching crew a view of the Moon.

They did not require a final mid-course correction, and this gave them the time and opportunity to turn the spacecraft around and for the first time see the Sun-blasted world they were about to explore loom in front of them. Mike Collins wrote about the shock he felt at what he saw.

“The change in its appearance is dramatic, spectacular and electrifying. The Moon I have known all my life, that two-dimensional, small yellow disk in the sky, has gone away somewhere, to be replaced by the most awesome sphere I have ever seen.”

This astronaut, one of the more poetic among the Apollo crewmen, had managed to glimpse the Moon at an opportune moment, just as they were about to enter its shadow. They then flew over an eerie landscape: the near side was dimly illuminated by Earthshine and a part of the far side which, owing to their vantage point was visible to them as a crescent, was as black as could be.

“To add to the dramatic effect, we find we can see the stars again. We are in the shadow of the Moon now. in darkness for the first time in three days, and the elusive stars have reappeared as if called especially for this occasion. The 360-degree disk of the Moon, brilliantly illuminated around its rim by the hidden rays of the Sun, divides itself into two distinct central regions. One is nearly black, while the other basks in tt whitish light reflected from the surface of the Earth/’

Collins was struck by the interplay of the three lighting effects on the Moon; the Sun’s corona, the dim light from Earth and the deep black of the star-peppered sky, combining to gently illuminate the lunar surface below’ with a bluish light.

“This cool, magnificent sphere hangs there ominously/’ he wrote in his autobiography, “a formidable presence without sound or motion, issuing us no invitation to invade its domain. Neil sums it up: ‘It’s a view’ worth the price of the trip.’ And somewhat scary too. although no one says that.’’

The next stage of the LM checks required the crew to think about the concept of the dead band, which is another of those curious terms in spaceflight where a simple concept lay behind opaque jargon.

Apollo was one of the first applications of a digital Пу-by-wirc system whereby control of a vehicle was placed in the hands of a computer. In the Apollo guidance computer, programmers included a scries of algorithms that would fire the RCS jets as necessary to bring the spacecraft to a desired attitude with respect to the IMU platform, and hold it there. These algorithms were called the digital autopilot (DAP). However, the gimbals around the platform were able to measure angular errors in the spacecraft’s attitude to an accuracy of hundredths of a degree, and to have constantly corrected the slightest drift to such a Light tolerance would have made the spacecraft seesaw backwards and forwards as the jets incessantly fought to maintain the ideal attitude, wasting propellant in the process. Instead, a range of attitude error around the ideal was deemed acceptable and the thrusters did not fire within this band; they were said to be ’dead’. This error band, the dead band, could be set to be cither a half or five degrees from ideal, depending on how accurately the spacecraft had to be pointed. A narrow er dead band used more RCS fuel, because the thrusters tended to fire more often when the spacecraft drifted beyond the permissible deviation.

While still docked, the commander gave the LM’s RCS system a checkout, first by using the computer to Lest that the hand controls were producing the commands expected of them, the so-called ‘cold-fire’ checks; and then by firing all 16 thrusters for short periods in a ‘hot-fire’ test. Prior to carrying out these tests, he had to ensure that his crcwmatc in the command module had the CSM’s digital autopilot set for a wide, five-degree dead band. That is, although overall the thruster firings – fore aft, left/right. etc. should be neutral, they would briefly rotate the entire CSM/LM stack by a few degrees, and it would have been a waste of propellant if the thrusters on the service module had to battle to restore attitude.

The commander also ensured that telemetry from the LM was being sent to mission control at a high bit rate. This maximised the number of engineering parameters that could be received while the health of the RCS was checked.

Almost like a third eye on the forehead of the LM’s face, another dish sprouted from the ungainly LM cabin, preferring a direction that faced forward. This was the antenna for the rendezvous radar, one of the subsystems that allowed the ascent stage to seek, find and follow the CSM as it chased the mothership around the Moon during the rendezvous. The radar operated in conjunction with a transponder on board the CSM so that the LM’s computer could tell how far apart they w ere, and in which direction. As the commander put it through a self-test routine, the final steps towards undocking were completed.

Breaking the link

Perhaps il was a bitter sweet moment for the command module pilot as he watched his crewmates leave for the Moon. There would probably be some relief that the mission had reached this point, and increasingly looked like it was going to be successful. At the same time, there might have been a deep longing for an opportunity to take a ride to the surface knowing that there were only a few kilometres between him and a moonwalk. But for all the CMPs, there was terror in the knowledge that it required only one or two of many possible failures to occur, and he might have to light his SPS engine and return to Earth alone, as a marked man, having left his crewmates on the Moon.

Alan Bean found Dick Gordon’s reaction to sending his crewmates away on Apollo 12 remarkably sanguine. Gordon and Conrad had flown together on Gemini 11, although their friendship went back to Patuxent River Naval Air Station where they were both test pilots and good buddies. Although Gordon’s friendship with the likeable and super-competent Conrad had helped to seal his place on Gemini, the experience subsequently gained prevented him from taking a ride to the Moon’s surface. Deke Slayton, who decided Apollo’s crewing arrangements, generally gave command to the most experienced in a crew, and that was Conrad. At the Lime, he preferred the astronaut in charge of the command module Lo have had experience of rendezvous in ease the CSM had to rescue an ailing LM. Thus Gordon got to fly the CSM solo. That left Bean, the rookie and the third member of this friendly crew-, with a ride to the moondust. Twenty-three years later. Bean completed a painting that imagined Gordon being down on the surface with his two buddies. In his notes on that work, Bean stated. "Dick was the more experienced astronaut, yet I got the prize assignment. In the three years of training preceding our mission, he never once said. Tt’s not fair, I wish I could w? alk on the Moon too.’ 1 do not have his unwavering discipline or strength of character.”

Having just dealt with a shorted abort switch. Apollo 14 ran into more technical problems when they reached a point where they expected the landing radar to begin feeding data to the computer. Typically, crews expected the all-important radar to be working by 10,000 metres altitude, but as Aniares passed this point, the computer was still receiving no radar data from the antenna.

“Come on radar,’’ implored Ld Mitchell, the LMP, but the two lights on the DSKY stayed stubbornly illuminated. “Come on radar!”

A minute passed and Fred Haise in Houston informed them of when they could expect P63 to begin throttling the engine. "Okay, 6 plus 40 is throttle down, Aniares.”

“Roger. Houston,’’ said Mitchell. “We still have altitude and velocity lights.’’

By 7.000 metres, there was still no valid data coming from the landing radar and the two crewmen frantically tried to make it work, knowing that if they still had no success by 3.000 metres, they were bound to abort the mission, separate the ascent stage, and return to the CSM.

“Antares, Houston,” said Haise. “We’d like you to cycle the landing radar breaker.”

Mitchell pulled one of the little aviation-type circuit breakers to remove power from the radar, then pushed it in again. Quite often in electronics, a power-down, power-up cycle is all that is required to clear an abnormal operating condition and the earlier manual patch to the computer to deal with the abort switch short had created such a state. “Okay.” said Shepard. “Been cycled.”

“Come on in!” Mitchell urged, then. ’’Okay!” as the lights went out and the radar began to function normally at only 5,500 metres. “How’s it look, Houston?” called Shepard.

Shepard, at 47, was the oldest of the Moon-bound crews and the only Mercury astronaut to go to the Moon. Many have wondered whether he would have attempted a landing without the radar. Most believe that if he had tried, the narrow’ margins of propellant w ould have obliged him to abort further down.

In contrast to Apollo 14‘s late acquisition of radar data, Young and Duke got a pleasant surprise when Orton s landing radar began to deliver data much earlier than expected on Apollo 16. Compared to the other landing flights, Orion’s descent began at a much higher altitude over 20.000 metres, probably due to some over­compensation made for the influence of the mascons on Apollo 1 5. They were then surprised when their landing radar started to work while they were still 15,000 metres or 50,000 feet up. w’hich was 50 per cent higher than expected.

“Look at that!” exclaimed Young. “Altitude and velocity lights are out at 50k!“

“Isn’t that amazing," agreed Duke.

“Look at that data, Houston,” said Young. "When do you want to accept it?”

“Okay, you have a Go to accept.” said Jim Irwin once the flight controllers had passed on their agreement.

”Okay,” replied Duke. "It’s in.”

"Tm at the foot of the ladder.” Neil Armstrong brought a quiet coolness to the moments before he took humankind’s first step on the Moon. "The LM footpads are only depressed in the surface about one or two inches, although the surface appears to be very, very fine grained, as you get close to it. It’s almost like a powder. [The] ground mass is very fine.”

Armstrong was not telling science anything it did not already know. Previous unmanned probes and objective theorising by lunar geologists had established that the lunar surface would be finely powdered, beat up from an incessant rain of objects over extremely long time periods. But, first and foremost, Armstrong was an engineer and test pilot and one of the best in the business. What test pilots do is observe and describe in physical terms, and that was exactly what he was going to bring to this endeavour.

“I’m going to step off the LM now.”

With his right hand holding onto the ladder. Armstrong placed his left foot onto the dust of Marc Tranquillitatis. "That’s one small step for [a] man; one giant leap for mankind.’’

With the moment appropriately marked, Armstrong continued onto the surface and tentatively began to adapt to moving around in the weak gravity field. He also returned to his descriptive roots. "Yes. the surface is fine and powdery. I can kick it up loosely with my toe. It does adhere in fine layers, like powdered charcoal, to the sole and sides of my boots.”

In the years leading up to this moment, one scientist had attracted the attention of the press, ever hungry for a story, by suggesting that the LM or an astronaut would be swallowed up by a great depth of dust which, he theorised, would have taken on a Tairy-castlc’ structure. Thomas Gold’s theory was based on interpretations of radar observations which showed that the surface consisted of very loose material, which is indeed an accurate description of the Lop few millimetres. However. Gold took this observation and wove it into a tale of great seas filled with electrostatically supported dust. In fact, the large size of the LM footpads is attributed to his influence. Gold continued to provide reporters with a yarn of possible catastrophe even after unmanned Surveyor landing craft had successfully touched down using footpads designed to impart the same pressure as the LM pads. These spacecraft also returned images of boulders resting on the surface and orbiting spacecraft had imaged great swathes of ejected blocks from large craters that had clearly not sunk into the dust. But of this period, geologist Don Wilhelms wrote, "One would think that the presence of all this dust-free blocky material would have weakened the Gold-dust theory, but no amount of data can shake a theoretician deeply committed to his ideas.”

Armstrong was demolishing such worries once and for all. "I only go in a small fraction of an inch, maybe an eighth of an inch, but I can see the footprints of my boots and the treads in the fine, sandy particles."

Always aware that a problem could cause the EVA to be terminated at any Lime, Armstrong’s initial moments on the surface were carefully planned. He had a short moment to ensure he would have no difficulty moving around, then he and Aldrin used a looped strap to send a Hasselblad camera down from the cabin. The hmar equipment conveyor (LEC) was NASA’s reply to a fear that it might be difficult and time-consuming to carry items up and down the ladder, particularly boxes of rock samples. The LEC w-as discarded after Apollo 12 as crews came to better understand the ease with which heavy loads could be handled on the Moon. With the camera attached to a bracket on his chest mounted RCU. Armstrong proceeded to take a series of overlapping pictures that could later be merged into a panoramic view of the landing gear. Then, after a reminder from Bruce McCandless in Houston, he used a scoop to gather a contingency sample of the soil near Eagle.

"Looks like it’s a little difficult to dig through the initial crust,” noted Aldrin from Eagle’s cabin.

"This is very interesting,” said Armstrong. "It’s a very soft surface, but here and there where I plug with the contingency sample collector, I run into a very hard surface. But it appears to be a very cohesive material of the same sort.” Armstrong

had discovered what is, to people used to Earth, an odd property of the soil. This part of Mare Tranquillitatis had seen essentially no volcanic activity for well in excess of three billion years. The only large scale processing of the rock had been by countless impacts, and the vast majority of these were small. Each had helped to pulverise the surface into a layer of dust and boulders about five metres thick called the regolith and each had served to shake the subsoil until it became extremely well compacted yet unconsolidated material. So while the top few centimetres were loose, the subsurface seemed hard and unyielding.

It was then Aldrin’s opportunity to climb down the ladder as Armstrong photographed him. “Now I want to back up and partially close the hatch, making sure not to lock it on my way out.”

“A particularly good thought,” laughed Armstrong.

“That’s our home for the next couple of hours and we want to take good care of it.” The checklist had called for the hatch to be partially closed, probably to prevent the shaded interior radiating its heat into space.

Ever the test pilot, Aldrin continued to narrate his descent of the ladder. “It’s a very simple matter to hop down from one step to the next.”

“Yes. I found I could be very comfortable, and walking is also very comfortable. You’ve got three more steps and then a long one.”

Aldrin practised leaping between the ladder and the footpad, then, before he stepped onto the surface, he turned to take in the landscape.

“Beautiful view!”

‘‘Isn’t that something!” said Armstrong. “Magnificent sight out here.”

“Magnificent desolation.”

Snow on the Moon

As A1 Bean waited to follow’ Pete Conrad out of Intrepid’s cabin, he moved to his window to adjust a movie camera that would record their work on the surface. Just after Conrad had taken a contingency sample of lunar soil, both men heard a warning in their headsets.

“Uh-oh, did I hear a tone?” said Conrad as he tested his mobility on the surface.

“Yeah; I’ve got an H20 A,” said Bean.

“You do?”

“Yeah. I wonder why?”

The ‘A’ flag and tone was telling Bean that his cooling system wras failing. For a few’ minutes he tried to troubleshoot the balky PLSS as Conrad got started on tasks around the LM.

“Okay. I think I know what happened. Houston.” said Bean as he spotted the cause. After the flight, he explained the circumstances: "1 happened to glance down and noticed the door was closed. I realised what had happened. The outgassing of my sublimator had closed the door, with the result that I didn’t have a good vacuum inside the cabin anymore. I quickly dove to the floor and threw back the hatch. The minute I did. a lot of ice and snow went out the hatch.”

“What did you just do, Al?” asked Conrad when he saw’ resultant display.

“Man, I just figured it out.”

“You sure did. You just blew water out the front of the cabin.” Then correcting himself, “Ice crystals.”

“That’s what had happened to the PLSS. The door had swung shut, […] and probably bothered the sublimator. ‘cause it wasn’t in a good vacuum anymore. So 1 opened the door and it’s probably going to start working in a minute.”

“I should hope so. laughed Conrad. "When you opened the door, that thing shot iceballs straight out the hatch.”

The first time David Scott opened Falcon s forward hatch, he treated Jitn Irwin to a similar display of ice particles visible out of his window. “It’s blowing ice crystals out the front hatch,” laughed Irwin. “It’s really beautiful. You should see the trajectory on them.”

“I bet they’re Паї, aren’t they, Jim?” asked Capeom Joe Allen. “The trajectories?” Allen wras a scientist by training and as soon as he heard about the flying crystals, he immediately began to think about how they would move.

“Very flat. Joe,” answered Irwin to Allen’s query.

For the first traverse with a rover on the Moon, Scott and Irwin headed southwest towards a spot where Hadley Rille took a sharp turn below the flank of Mount Hadley Delta. "Well, I can see I’m going to have to keep my eye on the road,” commented Scott as he negotiated the undulating terrain of the plain. "Boy, it’s really rolling hills, Joe. Just like [Apollo] 14. Up and down we go.”

The chaotic nature of the landscape kept him busy as he worked the rear-wheel steering to avoid fresh, steep-walled craters and occasional rocks large enough to be hazards.

"We’re going to have to do some fancy manoeuvring here,” remarked Scott. "Okay, Joe, the rover handles quite well. We’re moving at an average of about eight kilometres an hour. It’s got very low damping compared to the one-g rover, but the stability is about the same. It negotiates small craters quite well, although there’s a lot of roll. It feels like we need the seat belts, doesn’t it, Jim?”

“Yeah, really do,” agreed his LMP.

“The steering is quite responsive even with only the rear steering,” continued Scott. “I can manoeuvre pretty well with the thing. If I need to make a turn sharply, why, it responds quite well. There’s no accumulation of dirt in the wire wheels.”

“Just like in the owner’s manual, Dave,” said Allen.

The speed that Scott could maintain was typical for the rover and though it may not appear to be very fast, the combination of the heavily cratered surface and the light lunar gravity made the vehicle pitch and roll enough to take the wheels off the ground. “Man, this is really a rocking-rolling ride, isn’t it?” laughed Scott.

Irwin concurred. “Never been on a ride like this before.”

“Boy, oh, boy! I’m glad they’ve got this great suspension system on this thing.”

Young and Duke’s first traverse took them west for a short distance away from the LM. “Man, this is the only way to go, riding this rover,” said Duke.

But a disadvantage of driving west was that the Sun was behind them and so there were few shadows in front of Young to help him judge the terrain ahead. Essentially, all objects directly ahead were hiding their own shadows. Worse, at the point directly opposite the Sun, the so-called zero-phase point, the backscattered reflection from the tiny crystals in the regolith became particularly bright to the point of being dazzling. It limited their speed as Young fought to discern the obstacles ahead in the glare.

“Driving down-Sun in zero phase is murder,” moaned Young. “It’s really bad.” He elaborated further during his post-flight debrief. “Man, I’ll tell you, that is really

grim. I was scared to go more than four or five kilometres an hour. Going out there, looking dead ahead. I couldn’t see the craters. I could see the blocks alright and avoid them. But I couldn’t see craters. I couldn’t see benches. I was scared to go more than four or five clicks. Maybe some times I got up to six or seven, but I ran through a couple of craters because I flat missed [seeing] them until I was on top of them.”

The speed record for driving a rover probably goes to Gene Cernan on his second lunar EVA when he and Schmitt were driving down a scarp that crossed their valley floor. On the Moon, slopes tend to be smoother than level ground because the gradient tends to make particles move preferentially downhill with every micrometeoroid hit and the constant downslope movement smooths out the features. "What was it. 17’A or 18 clicks we hit coming down the Scarp. Jack?" claimed Cernan nonchalantly. Schmitt merely laughed at the suggestion.

Л record that Apollo 17 s rover can claim is to have taken its crew furthest from the LM. On EVA 2’s outbound leg. Cernan drove 9.1 kilometres to a site at the foot of the South Massif, fully 7.4 kilometres distant from Challenger, its safety and a ticket home to Earth.

John Young took his rover to its own record – the highest point up a hillside reached by any crew at 170 metres. The rover’s easy glide up Stone Mountain was in stark contrast to the labour-intensive struggle Shepard and Mitchell had climbing 85 metres up a much shallower slope in an effort to reach Cone Crater. At the Cincos craters. Young came to a stop and looked around to see the view. Duke was the first to speak up and tell Tony England about it. "Tony, you can’t believe it, this view looking back. We can see the old lunar module! Look at that, John.” But Young was too busy getting the rover parked in a position where it would not begin to slide back downhill. He had the experience of Apollo 15 to draw from.

On their second traverse. Scott and Irw’in had taken their rover onto the lower slopes of Mount Hadley Delta. Among their goals, they wanted to find anorthosite, a white crystalline rock that geologists believed would be a sample of the Moon’s primordial crust. The mountain was thought to be a block of that crust and the hope was that by driving a short distance up the hill, they would find a sample that had been dislodged from further up. Scott generally tried to park inside the lower rim of a crater as this tended to be a relatively level spot on the otherwise sloping ground. However, at one point, he was attracted to a boulder in which Irwin had spotted a hint of green in its dusty surface. On giving up on one parking spot above the boulder because the slope was too steep, Scott found that as he went to dismount at a spot below the boulder, he could feel the vehicle slide. In fact, the rover was sitting with one of its wheels off the ground. To keep it in place, he had to ask Irwin to stand below it and hold onto it while he investigated and sampled the green boulder.

Getting off the Moon and returning to the relative safety of the command module was a feat that literally defined the mission plan. NASA even named it lunar orbit rendezvous (LOR) in view of the important benefits the technique promised in overall weight savings, including that of the launch vehicle. Yet, to many in NASA in the early 1960s, it seemed suicidal for one tiny spacecraft to launch and attempt to pull up alongside another tiny spacecraft, each whizzing along at some 5,800 kilometres per hour, around another world nearly half a million kilometres away. At that time, no one had even attempted rendezvous in the relative safety of Earth orbit in spacecraft that could at least return to the ground if things went awry. Space navigation techniques were rudimentry at best. There were no GPS satellites around Earth and even spaceborne radar techniques were merely theoretical. It was a measure of the managers’ faith in their engineers and scientists that they felt confident to march ahead with an apparently hare-brained scheme which, if it were to go wrong, would doom two men to certain death in lunar orbit.

Once LOR had been chosen as the preferred mission mode, NASA needed to practise the techniques of rendezvous around Earth. Through 1965 and 1966, the ten missions of the Gemini programme turned rendezvous from a frightening unknown manoeuvre into a routine operation. Appropriate procedures were learned through successive flights beginning with simple tasks:

• Could the manoeuvrable Gemini spacecraft station-keep with its spent upper rocket stage?

• Could two independently launched spacecraft rendezvous and station-keep?

• Could a spacecraft rendezvous and then dock with an unmanned target?

• Could it achieve the same feat within a single orbit?

All these lessons built NASA’s confidence in its procedures, and were directly applicable to Apollo’s need to rendezvous and dock around the Moon. The Gemini programme is often overlooked by writers eager to tell the story of how NASA prepared to venture to the Moon. But without it, Apollo could never have succeeded within President Kennedy’s deadline. Years later, David Scott, Gemini 8 pilot and veteran of Apollos 9 and 15, reflected: "You go away back, it was a big mystery

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

DOI 10.1007/978-1-4419-7179-1 13. © Springer Science+Business Media. LLC 2011
doing a rendezvous. Magic mysterious stuff! Now it’s just straight off – choof, bang.”

The NASA-ese term for the manoeuvre that brought the spacecraft out of lunar orbit and homeward to Earth was trans-Earth injection (TEI). In simple terms, it was very similar to the TLI manoeuvre that sent the crew Moonward in the first place in that its task was to add more speed to the spacecraft in order to raise the high point of its orbit sufficiently to propel it from one world to another. To achieve this, their orbital velocity had to be raised by nearly one kilometre per second. With only meagre thrust available from the RCS thrusters, the big engine on the service module was the only means of gaining so much speed.

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

DOI 10.1007/978-1-4419-7179-1 14. © Springer Science+Business Media. LLC 2011

As with the TLI burn. TEI was based on a Hohmann-type transfer. In the context of the Earth-Moon system, this meant that to reach Earth, the burn had to be carried out on the side of the Moon opposite Earth. In other words, the TEI manoeuvre had to be carried out over the Moon’s far side. The duration of the burn was calculated to raise their near-side apolune towards Earth until their trajectory became open ended, or hyperbolic. It was then no longer an elliptical orbit but had become an S – shaped path that would allow them to fall to Earth.

As usual, timing was everything. Mission planners needed to arrange a welcoming committee, which included an aircraft carrier, to recover the spacecraft and crew’. Although the command module was designed to land on water, it was not a boat. It wallotved sickeningly in even mild swells, and the nature of its precious cargo of crew’, rocks and film – and indeed the interest of the world – ensured that the US government made every effort to organise an appropriate reception, courtesy of the US Navy, for when the spacecraft returned. How’ever. as aircraft carriers and their escorts could not be moved around the Earth’s oceans very quickly, a prime landing area was designated in the middle of the Pacific Ocean where the largest recovery force would be stationed. Smaller forces were on standby at other designated sites on the other major oceans.

When deciding on a trajectory for the coast home, the Retro flight controller had to weigh a number of constraining factors. If re-entry was to be successfully negotiated, then whichever trajectory from the Moon to the Earth was used, the CM had to arrive at the top of the atmosphere at a shallow angle of 6.5 degrees, give or take a degree – a condition that occurred more or less on the opposite side of the Earth from the Moon’s position when the TEI burn occurred. The latitude of the splashdown site would be within Earth’s tropical region for the majority of possible trajectories – i. e. between the tropics of Cancer and Capricorn – because the Moon’s orbit hardly strayed from the ecliptic, to which Earth’s axis is inclined at 23.5 degrees. Other solutions w’ere possible, but would have required too much propellant to achieve. The Apollo system worked on a propellant shoestring and planners could not be profligate with the stuff, which constrained the possible trajectories further.

An even narrower set of trajectories was selected by the 24-hour rotation of the Earth. Retro knew’ that the command module would fly about 2,000 kilometres from its point of atmospheric entry to its point of splashdow’n. and there was only one moment in each day when the revolving Earth brought the recovery site 2,000 kilometres downrange of the start of entry. He therefore had to decide w’hether he wanted the crew’ to make a faster return or w’ait one full rotation and keep it leisurely – a decision made in view’ of the state of the consumables on board. The faster return used slightly more propellant but caught Earth one rotation early in case other consumables were low. Otherwise, a slower trajectory would allow extra lime for more science if all other considerations allowed.

Heatshield designs for Apollo originated in the re-entry vehicles that were developed in the 1950s for nuclear warheads. These weapons were launched on ballistic arcs out into space by large rockets to increase their speed and minimise delivery times. However, their elaborate mechanisms had to negotiate the great temperatures generated by re-entry and so they were protected inside a vehicle whose outer wall included some form of heatshield. Initially heat sinks were used, but development of this Cold War technology led to the use of ablative materials for w’arheads and this technique wns used for many spacecraft heatshield designs, including Apollo, until the introduction of advanced ceramics such as those used in the Space Shuttle and the planetary probes of the 1990s and beyond. NASA’s return to the Apollo style of spacecraft in the early 21 st century, known as Orion, also saw a return to the same ablative material for its heatshield.

The ablative heatshield works by allowing itself to succumb to the enormous heat of re-entry. As it does so, it slowly chars and peels away, or ablates from the shield, taking heat with it and revealing a fresh surface to continue the ablative process. As long as the thickness of the shield is greater than the depth of erosion caused by re­entry, the heatshield will function properly.

The command module heatshield wfas made from shaped sections of steel honeycomb sandwich, which provided a substructure onto which a fibreglass honeycomb was bonded. There were over a third of a million cells in a complete CM heatshield. and each was carefully filled by hand with a reinforced epoxy resin using a specially designed squirt-gun. After curing and inspection, any imperfectly filled cells were carefully drilled out and refilled. The heatshield came in three sections: the aft shield was up to seven centimetres thick and Look the brunt of the re-entry heat; the centre shield tapered between four and two centimetres and covered most of the conical surface; the forward shield or apex cover completed the taper and was wrapped around the upper cone of the hull where the parachutes were stored. Each w’as shaped on large lathes to the correct thickness and rcchccked before being affixed around the internal structure of the command module.

Once a deceleration of 0.05 g was detected, the hard work of entry began. The spacecraft was in the tenuous but thickening atmosphere and there was no longer any need to hold it in a particular attitude because its inherent aerodynamic stability was dominant. Therefore P64’s first task was to discontinue attitude-hold and begin to ensure that any unwanted motions in the pitch and yaw axes were damped out. The three displays on the DSKY showed their roll angle as commanded by the guidance computer, their velocity and the g-forccs associated with their deceleration. It was no coincidence that these values were duplicated on the EMS; both systems operated independently, and if one failed, the other could be used to complete the re-entry.

The primary task for P64 wns to slow the spacecraft below’ orbital velocity, about 7.8 kilometres per second, thereby ensuring that it could not return to space on a long and lethal orbit of Earth. Within P64’s regime, the deceleration loads quickly built up to a peak above six g while the program repeatedly tested their flight path, evaluating whether a safe re-entry trajectory had been achieved.

By this time, a substantial shock wave had developed just ahead of the heatshield as the CM rammed into the tenuous gases of the upper atmosphere, instantaneously subjecting them to extreme compression and heating them to temperatures similar to the visible surface of the Sun, ionising them and surrounding the spacecraft in a sheath of plasma that effectively blocked radio communication. Eor about three minutes of the initial re-entry, this ‘blackout’ meant that mission control had no visibility into the craft’s systems and could do nothing except wait. idle. The crew’, meanwhile, unable to communicate, concentrated on monitoring their flight path, although on Apollo 14. CMP Stu Roosa reported that he could hear his Capcom "just fine”.

When Apollo 13 had to abort and return to Earth early, an unintended shallowing of their flight path caused blackout to last much longer than expected, in the process raising the tension of an already dramatic situation with the prospect that the craft had burned up. This radio-opaque plasma sheath is an inherent problem for all re­entering spacecraft but the Space Shuttle managed to circumvent it by establishing communications through the rear of the sheath via a geostationary communications satellite.

‘What is that?” said Anders on his first and only space re-entry. "Airglow?”

”That’s right, you’ve never seen the airglow,” said Lovell. Both he and Borman had re-entered on Gemini 7 and. like a couple of old-timers, thought they knew’ what was coming. ”Take a look at it.”

"You can’t get your pin without seeing the airglow7.” kidded Borman, referring to the gold astronaut pin that Anders would receive after Apollo 8 landed.

"That’s right,” joked Lovell.

Anders laughed. "I see it, I see it. Let’s see, is this where I’m supposed to ask how many g’s, Lovell?”

"That’s right,” answered the experienced spaceman, "you ask how many g’s.”

There were no rookies on Apollo 10, but that did not inhibit their surprise at the spectacle of being inside a re-entry from lunar distances where much more kinetic energy was expressed.

"Here comes the glow, John,” said Tom Stafford as they approached the 0.05-g event.

“Here it comes, babe.’- said Eugene Cernan.

“Shit, you’d belter believe it," said Stafford. "Okay. Stand by for rc-ent…” He interrupted himself. "Oh. look at that.”

"Look at that.” repeated Cernan. "God damn. God damn.”

"Just looks like daytime.” said Stafford, who then counted the g-forccs building up. "Point two, point three, point four. We’re trimming in good.”

"Here comes some g’s. babe,” said Young.

"Oh, you’d better believe here comes some g’s,” said Stafford, "Here comes the water, too. Just sit back anyway." Water that had condensed around the cold apex of the cabin began dripping over them.

"Okay, there’s one g,” said Young. But the sudden onset of a deceleration equivalent to Earth’s gravity seemed worse than it was to a crew that had been weightless for over a week.

"Shit!” cried Stafford. "You got to be kidding. Jose.”

"It seems like about 10 [g],” estimated Cernan.

On board Apollo 8. even the ’veterans’ Borman and Lovell were brought up short by a sight once described as like being inside a fluorescent tube. "God damn, this is going to be a real ride; hang on.” called Borman as the light outside and the g-forces built up. "Eve never seen it this bright before."

"Quite a ride, huh?’’ said Anders.

"Damndest thing I ever saw.” agreed Borman. "Gemini was never like that, was it, Jim?”

"No, it was a little faster than this one,” said Lovell, referring to the length of time they were staying in the high-speed region of flight.

"I assure you I’ve never seen anything like it,” said rookie Anders. "Cabin temperature’s holding real good. Up one degree.’’

After the flight, Borman was upbeat about the experience, "’flic ionisation on these high-speed entries is fantastic. The whole spacecraft was lit up in an eerie iridescent light very similar to what you’d see in a science fiction movie. I remember looking over at Jim and Bill once, and they were sheathed in a white glow. It was really fantastic.”

But this was no sci-fi movie. This was the real thing. The Apollo 11 crew were more descriptive of the sights that accompanied the onset of re-entry. "Along about 0.05 g, we started to get all these colours past the windows.” said Collins at their post-flight debrief. "Around the edge of the plasma sheath, there are all varieties of colours – lavenders, lightish bluish greens, little touches of violet, and great variations mostly of blues and greens. The central core has variations on an orange-yellow theme. It’s sort of a combination of all the colours of the rainbow really. The central part looks like you would imagine a burning material might look. Orangish. yellowish, whitish, and then completely surrounded by almost a rainbow of colours.”

"I thought there was a surprisingly small amount of material coming off.” added Aldrin.

"That’s right; there didn’t seem to be any chunks as there were on Gemini,’’ said Collins.

“There was a small number of sparks going by." added Aldrin. “You could definitely see the flow pattern. Looking out the side window, you could get a very good indication of the angle of attack by the direction of motion of the particles. That didn’t seem to change very much. When a thruster would fire, you could pick it up immediately, because it deflected the ion stream behind you.”

Charlie Duke was surprised at how effective the thrusters were when he re-entered on Apollo 16. “When it decided to roll, boy, it just took off. You could see the horizon through the ionisation sheath, both out window five and the rendezvous window four."

He then spoke about detached Mylar strips he saw out his window. Considering the punishing temperatures being experienced on the opposite side of the spacecraft, it was remarkable that this plastic film could survive. “There was Mylar on window five that was flapping back and forth across the window that was there at touchdown. It had come up right at CM SM sep. I had seen that strip fly by. When we started getting the g’s it flopped up over the window, sort of stayed there and wiggled the whole time, which amazed me."

“Here’s tw o g’s." said Stafford. The deceleration was ramping up for CM Charlie Brown.

“Okay, baby; you keep flying it." Young urged the computer. “Three g’s."

“There’s one minute gone," said Stafford.

“Four g’s," said Young.

“Fiveg’s." they announced together.

On the g meter, they watched the deceleration peak at 6.2 g’s.

“Hang on. It’s getting better." said Cernan.

“It’s going dowm," said Stafford. “We’re starting to roll."

“Go, machine,’’ said Young. “It’s rolling good.’’

“Come on, baby; fly," urged Cernan.

“It’s good. It know s just what it’s doing." said Young as P64 manoeuvred to force the spacecraft towards Earth.

“It’s rolled lift vector down," observed Stafford.

“Go on. Keep that lift vector down," said Young. A spacecraft that was going down was one that would not fly back out into space.

“Ooh, only three g s." said Stafford. The pulse of g-force that went beyond six had been brutal.

“Oh, man." said Cernan. "That first [g-peak] was a bitch."

As high-performance jet pilots, astronauts had learned howr to breathe under such crushing conditions, tightening their chest muscles and taking short grunting breaths.

After the peak g-load had subsided. P64 maintained a four-g deceleration until it had determined that their velocity had dropped below the speed required for orbital flight. Then there was no way it could exit the atmosphere and enter orbit. It was going to land somewhere. When this condition was met, there were two possibilities for the rest of the re-entry.

Apollo missions w’ere intensively monitored from Earth. Indeed, because the flight controllers had deep technical visibility into the spacecraft’s systems through telemetry, and huge computing and personnel resources on hand in case of problems, they became accustomed to nursing its crews and machines over the days of the coast to the Moon. It was then a bit of a wrench when some of the most critical events in an Apollo flight, particularly the entry into and departure from lunar orbit, had to occur with a 3,500-kilomctre-diamcter lump of rock obscuring the view/

In future years, operations around the Moon might be supported by a telecoms satellite that will enable communications between Earth and crews that operate around the far side. In the time of Apollo, there w’as no such luxury, and contact depended on line of sight from the Moon to one of the three main ground stations distributed around Earth. But the engineers w’ere not to be denied. On board each spacecraft was a multitrack tape recorder, the data storage equipment (DSE). whose function was to digitally record a suite of measurements from around the spacecraft, particularly the SPS engine, and replay them to mission control on a separate radio channel w’hen communications were restored.

As the Moon pulled the spacecraft around its far side, communications were instantly and completely cut off at the moment an Apollo disappeared behind the limb. NASA referred to this event as loss of signal (LOS) and it occurred with alarming predictability by virtue of the deep understanding the trajectory experts had of an Apollo’s flight path. The first time it occurred w’as during the Apollo 8 mission, and Frank Borman found the accuracy of Houston’s predictions awe­inspiring. At the precise time that he had been told communications would disappear, they did.

“Ceeze!" he said to his crewmates, there being no one else to hear. “That was great, w’asn’t it?’’ Then he mused: “I wonder if they’ve turned it off.’’

Bill Anders laughingly replied: “Chris [Kraft, the boss in Houston] probably said, No matter w’hat happens, turn it off’ Bill’s humorous suggestion was that, in order not to worry the crew’ if the predictions had not been as accurate as they had hoped,

Kraft would have ordered the people at the transmitting station to turn off the radio signal at just the right moment. Borman wondered, however. When next they spoke to Capcom Gerry Carr, he reported: “Houston, for your information, we lost radio contact at the exact second you predicted.”

Carr confirmed that that was what had happened.

Borman probed further. “Are you sure you didn’t turn off the transmitters at that time?”

“Honest Injun, we didn’t,” was Carr’s joking reply.

The thing about LOS and its counterpart, acquisition of signal (AOS), was that they were both highly predictable events. AOS, in particular, had the useful property of being entirely dependent on what occurred around the far side by way of engine burns. Thus, on Apollo 14, for example, the precise time that the spacecraft would disappear behind the Moon’s leading limb had been calculated to the second, as usual. Additionally, mission control knew that if a problem had prevented the LOI burn from occurring, the spacecraft would not be slowed in its path and would reappear around the eastern limb only 25 minutes 17 seconds later, set on its hybrid free-return course towards Earth. On the other hand, if the LOI burn was executed as planned, the spacecraft, having been slowed, would stay out of radio contact for 32 minutes 29 seconds. Any deviation in the burn from that detailed on the PAD would show itself by the deviation of AOS from the predicted time.