Category How Apollo Flew to the Moon

I. OS and AOS: out of sight

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,


An example of the data storage equipment. (Courtesy Scott Schneeweis Collection/

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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.


To begin the process of splitting the two spacecraft, the electrical umbilical between them had to be disconnected within the tunnel and the docking mechanism put back in place. Two other umbilicals were reeonneeted to the docking equipment to pass telemetry and commands to and from the probe and to supply pow? er to operate its retraction mechanism. Then while the LM crew closed the hatch at the Lop of their spacecraft, the CMP put on his helmet and gloves, a safety measure for the next task of preloading the probe.

Up to that point, the two spacecraft had been held together by the 12 docking latches that gripped across the two docking rings and their seals. These latches, however, had to be manually released prior Lo undocking, thereby removing the primary means by w’hich the tw’o spacecraft w’ere joined. Therefore, to prevent the spacecraft from being pushed apart by the cabin air pressure, the CMP extended the probe to engage the three capture latches at its tip, each the size of a thumbnail, with the rim of the hole at the centre of the LM’s drogue. The probe was then tensioned to firmly engage these latches. When the main latches were released, the capture latches would have to hold against the air pressure that would try to push 34 tonnes of spacecraft apart hence the need for the CMP to be wearing his spacesuit. Before any of this, however, he had to disable some thrusters.

The strength of the probe was more than adequate to hold the spacecraft, except in one axis – roll. If the thrusters of the CSM were to impart a rolling motion to the stack, the force would be transmitted to the LM primarily through the probe arms and the little capture latches, subjecting them to dangerous shear. At this point, therefore, the CSM was inhibited from firing its roll thrusters. Once the probe was tensioned, it was safe to release all 12 docking latches an operation that also re­cocked them, ready to engage again when the LM returned to dock after its journey to the surface. The CMP then reinstalled the hatch at the apex of the command module. Only when he had checked that the air pressure in his cabin was secure, could he remove his helmet and gloves.

As with many operations on board Apollo, the procedures surrounding undocking and separation were carefully choreographed. Undocking was always carried out at a specific attitude and at a specific time, with the stack’s long axis towards the Moon’s centre. An attitude was given in the flight plan for the event and the stack was manoeuvred to this attitude some minutes prior to the undocking. Being in an inertial attitude, the stack would reach the correct orientation with respect to the Moon at a specific time, and this would be the moment of undocking.

Undocking w’as only ever carried out once during a normal mission. The second time the LM departed, it was actually cut free, along with the tunnel and all the docking equipment – a very final event that disposed of the ascent stage at the end of its mission. Coordinating the undocking with the event timer helped the crew to accurately run through a time-dependent sequence, as so often was the case for major mission events. With 30 seconds to go. the CMP set the EMS to monitor changes in velocity and started the movie camera. At /его, a switch that controlled the extension and retraction of the probe was momentarily pushed up to finally execute the undocking.

There were two procedures available to undock the spacecraft and it depended on the precise operation of the switch that extended the probe to decide which one was used. The switch had a momentary action which had two effects: it sent a command to the probe to fully extend, w hich it did regardless of how long the switch was held for; it also caused the probe to pull in the capture latches thus disengaging them from the drogue, but only for the duration of the swatch action. Therefore, to achieve a simple undock merely required that the switch to be held closed for the length of time it took the articulated probe to extend, so that when it reached its full 25-centimetre extension, the latches would still be disengaged and the LM would sail away.

The preferred method, however, was the ‘soft undock’ for w hich the extend switch was held for only a short period. Although this fully extended the probe, it allowed the capture hitches to re-engage with the drogue so that the LM would be held at the end of the fully extended probe. This method minimised unintended LM velocity


Airfares, the Apollo 14 LM recedes from Stu Roosa in the CSM Kitty Hawk. (NASA)

with respect to the CSM. Once the motions between the two vehicles had stabilised, the latches were released by cycling the extend switch once more. The CSM would then complete the separation by controlled firings of its RCS thrusters.

If the electrical command to release the capture latches were to fail, the probe included arrangements to allow a suited crewman to manually release them from either side of the tunnel: either the CMP could pull a handle from the CM side or a LM crewmember could access a button in the centre of the probe tip which poked through the hole in the centre of the drogue. In either case, the respective cabin would have had to have been depressurised and the corresponding hatch opened to allow access.

Undocking generally occurred shortly after the spacecraft came back into view of the Earth. When Apollo 15 reappeared after its planned undocking and separation,

Ed Mitchell in mission control enquired how it had gone. David Scott didn’t have good news.

“Okay, Houston; this is the Falcon. We didn’t get a Sep. and Al’s been checking the umbilicals down on the probe." When Л1 Worden had pushed the extend switch, neither the latches nor the probe extension had operated. The suspicion that the probe umbilicals were not properly connected was confirmed by Mitchell’s next message.

”Falcon, Houston. We have no probe temperature data], which indicates the umbilical is probably not well connected."

“Okay. Well, that’s just what he’s checking,” Scott informed. Worden had removed the forward hatch in order to gain access to the plugs and sockets of the probe umbilicals within the tunnel. Scott realised the danger in the situation and checked that Worden was aware of it also. “Hey, Al, I hope you made sure the extend, release switch was off when you went in there.” Scott’s fear was that if the switch to extend the probe had been placed in the ‘on’ position, and with the docking latches released, then when Worden reconnected the umbilical the probe would immediately extend, separate the craft and evacuate the cabin.

As soon as Worden had reseated the plugs in their sockets, mission control saw their telemetry change. “Apollo 15. Houston. We’re seeing the telemetry on the probe now. 1 presume that may have been our problem.” A new separation attitude was sent to the crew to reschedule the event for 26 minutes later.

Program alarms: part I

Apollo ll’s descent to the surface was, by far, the most challenging of all the missions because it was the first; and being the first, it tested procedures and systems that could not otherwise be exercised. Some were found to be wanting, because soon after Eagle had yawed around and the landing radar had begun to feed data to the computer, Armstrong made an urgent call.

"Program alarm.”

“It’s looking good to us,” said Duke in the Capcom seat, relaying a judgement on the data coming from the landing radar.

"It’s a 1202,” said Armstrong to inform Houston of the code that had come up on their DSKY. ”What is it?” he asked Aldrin. It was an error code from deep in the executive software, but neither of them had the foggiest notion what it meant. “Let’s incorporate.’’ he added, having heard Duke’s advice that the landing radar data was good. "Give us a reading on the 1202 program alarm,” Armstrong called to Houston some 15 seconds after the alarm had occurred.

The Guido flight controller. Steve Bales, was responsible for the LM’s guidance. He and his back room team knew the LM’s programming well, and did know what the alarm meant. The computer was reporting that it was overloaded, but Bales could tell from his telemetry that it was managing its primary tasks. So long as the error did not become continuous, it w’ould be able to cope. Armstrong was told that he should continue the powered descent.


There was little opportunity on Apollo 11 for Armstrong or Aldrin to wander far from the LM. Their time outside was so brief and they had been given so much to do. Lven though their PLSSs were capable of supporting a 4-hour moonwalk, managers had kepi this initial single excursion down to only 2Vi hours, and then packed that Lime with an enormous range of tasks. One of Aldrin’s tasks was to investigate their mobility in this new environment. This he would do in front of the TV camera.

“Td like to evaluate the various paces that a person can [adopt when] travelling on the lunar surface." He readied himself to walk towards the camera and across its field of view and then he began to narrate his various strides. "You do have to be rather careful to keep track of where your centre of mass is." Since lunar gravity did not bear down nearly as much as on Earth, the brain was less aware of where the centre of mass was. "Sometimes, it takes about two or three paces to make sure you’ve got your feet underneath you."

Aldrin’s first display w as a loping stride on alternating feet as he headed towards the camera, but his lightness meant that for much of the time, both feet were off the ground as each leg launched him forward on what was really the first example of running on the Moon. His inertia was much more of an issue because while his weight was reduced, his mass and that of his suit were unaltered and once in motion, they took conscious effort to bring to a halt. "About two to three or maybe four easy paces can bring you to a fairly smooth stop." He continued his stride away from the camera, deliberately changing direction a few7 Limes so everyone could see. "[To] change directions, like a football player, you just have to put a foot out to the side and cut a little bit."

So much for a walk/run. Next he tried bouncing, with two feet pushing forward together as he returned towards the camera. “The so-called kangaroo hop docs work, but it seems as though your forward mobility is not quite as good as it is in the more conventional one foot after another."

"I felt it was quite natural,” said Armstrong after the flight as he described his mobility on the lunar surface. "The one-sixth gravity was. in general, a pleasant environment in which to work, and the adaptation to movement was not difficult.’’ Planners had worried about how’ well humans would cope with a hugely reduced gravity, and this had been one of the reasons for the very conservative extent of the Apollo 11 EVA. Prior to the flight, many schemes were pursued to simulate sixlh-g and give astronauts a flavour of what to expect, but in the event, they adapted with ease. "In general, we can say it was not difficult to work and accomplish tasks." commented Armstrong. "I think certain exposure to one-sixth g in training is worthwhile, but I don’t think it needs to be pursued exhaustively in light of the ease of adaptation."

Despite the rules that bound them to the TV camera’s field of view7, Armstrong pushed the envelope a little. Townrds the end of their excursion, he decided to go for a short run and headed 60 metres behind the LM to a small crater he had overflown on the way down. For the brief moments he could be seen, it was clear he had settled into the same loping gait that Aldrin had just demonstrated and which most of the moonwalkers wnuld adopt.

Ed Mitchell, Charlie Duke and Gene Cernan often used a gait that was in between the foot-by-foot lope and the kangaroo hop. In this, they pushed off on both feet but always kept a given foot in front of the other while landing w ith the rear foot slightly earlier than the front foot.

After the conservatism of the first moonwalk, Apollo’s managers let the program move up a gear to extend the reach of subsequent crews. Conrad and Bean used their first 4-hour EVA to set up science instruments. Then with a little time left over, they went 180 m beyond their new science station to the rim of a big crater to take some pictures. Their second EVA, also for four hours, was devoted to a walk of over 1.3 kilometres that made a great loop around a series of geological targets. The furthest of these was a small, fresh (meaning a few million years old) crater called Sharp sited 400 metres from the LM.

The two astronauts hustled around their circuit, loping easily from site to site in their bulky suits at about four kilometres per hour to give themselves as much time as possible at their stops. The early model of the suit was very stiff at the waist and this restriction made walking hard work when compared to the more flexible suits worn by the J-mission crews. During part of their journey, as they headed from Sharp to another crater, this one called Halo, Bean felt a change in the apparent air pressure in his suit that was later attributed to his vigorous movement causing the flow of air out of the suit to be momentarily interrupted, producing an overpressure that he felt in his ears.

The stiffness of their suits also made it difficult to kneel down and pick up rock. Though they had tools to help them, Bean came up with another idea when Conrad was about to go for another sample on the southern rim of the Surveyor Crater. "Wait, Pete. Eve got an idea.”


Подпись:"Pete, let me reach back here and grab this strap.” The strap was part of a bag attached to the rear of Conrad’s PLSS that was to carry parts from the Surveyor 3 probe they were about to visit. Bean realised that in the weak gravity, he could use this to lower his commander to the surface without Conrad having to bend his knees.

Since they had found the scoop to be a little tricky to use in the light gravity, Bean’s trick avoided it and allowed Pete to use both hands to reach out for the rock directly.

"That a boy,” said Conrad once he had the rock firmly in his hand. "[Pull me] back up!” The two astronauts were adapting and improvising in their new domain. Bean thought it would be useful technique for the next crew to visit Luna’s surface.

"Now, if they had a strap like that, they could just hold the other guy while he leaned over and picked up a
rock.” In the event, such clever techniques were rendered obsolete by the greater flexibility of later suits.

An additional problem that became apparent on their walk was that they had a lot of equipment. To help them, they had the hand tool carrier (HTC), a small, three­legged truss structure that not only held their tools, like a hammer, corer and scoop, it also had a bag in the centre to give them somewhere to place rock samples gained during their long traverse. A handle extended upwards to give the crewman something to grasp. "Boy, this hand tool carrier is light and nice compared to carrying it around on Earth,” said Bean as he began their long walk. Though it helped a lot, it could cause problems of its own, as Bean discovered when he tried to make a faster pace across the surface. “I’m carrying that thing and that interferes with your running,” he later explained. "You can’t run good, because it bumps into your legs and it’s just a big hassle. And it gradually got heavier because we kept putting rocks in it.”

The fact that it had to be grasped by a handle also raised a continuing difficulty that anyone in a pressurised spacesuit faces when they have to hold objects for a long period – they have to overcome the stiffness of their gloves. Under pressure, the gloves, like the rest of the suit, tried to adopt a particular posture which, for the Apollo suit, had the hands slightly outstretched with the fingers and thumbs curled inward a little. To grasp an object for a long period, the crewman had to constantly work against this pressure to maintain his grip around an object and soon, his forearm muscles would tire.


Pete Conrad near Surveyor Crater. Beside him is the hand tool carrier. (NASA)

Before the flight, Bean had tried to physically condition himself for the mission’s arduous workload hut after the flight he said that if he had properly understood the exertions he would have modified his approach. “The big pain with that tool carrier is that you have to hold it out from your body so that your legs don’t bump into it as you walk, which means you have to hold it by one hand. That’s not a big deal when it’s light and there are no rocks in it; but when you start filling it with rocks, it gets to be a pretty good stunt to hold it out there for long periods of time. I was running two and a half miles a day towards the end of the training period to get my legs in shape, and my legs never suffered a bit. If I had it to do over again, I would run about a mile a day and spend the rest of the time working on my arms and hands, because that’s the part that really gets tired in the lunar surface work.” Indeed, Bean passed this adviee onto his successor, Fred Haisc, while still on the Moon.

Though Conrad did not have to haul the tool carrier around, he found he had other problems with his hands. “I didn’t notice that my hands got tired as much as I noticed that they got sore. When you work for four hours and use your hands, you have a tendency to press the end of your fingertips into the end of the gloves. Although my hands never got stiff or tired, they were quite sore the next day when we started the second EVA.”

Bean pointed out another phenomenon in his gloves when working with the tool carrier. "Iley. one thing I’ve noticed, Houston, carrying the tools. You don’t feel any of the temperature here. Sun’s out nice and bright, but it’s nice and cool in [the suit]; except when you’re carrying something metal, like the hand tool carrier, or the shovel, or something. Then your hand starts to get warm." At first, he attributed this to the metal being heated in the direct sunlight, but he later reassessed the cause. "Maybe it isn’t that the tool is hot. When you grip your hand around there, then the air can’t flow in your hand area any more. So your hands don’t have the air circulation they normally do. Before, they were just kind of floating in the middle and the air’s being blown around. But once you grip, then the air can’t get down in there.’’

Last act

After their third and final traverse, each J-mission commander drove his rover to a spot roughly 100 metres east of the LM so that the TV camera could look at the sunlit rear of the LM and carry out the one remaining major task left to it – to treat


With one wheel off the ground, the rover is held stable on a slope by Jim Irwin while David Scott photographs a foreground rock, tongs added for scale. (NASA)

TV viewers on Earth to the spectacle of the launch of the LM ascent stage and hopefully to follow it as it powered towards the western horizon. As a nod towards the area at the Kennedy Space Center from which invitees and dignitaries would watch Saturn V launches, on his flight Young dubbed the rover’s final position the VIP site.

In the event, the spacecraft’s rapid rise proved difficult to track. When the INCO flight controller commanded a camera move, it took two seconds for the command to reach the pan/tilt head on the rover. It then took a further 1.5 seconds for the move to be seen on Earth if the viewer was watching an unprocessed, uncoloured feed from the Moon. Colour processing added more time. Taking these factors into account as well as the planned time of lift-off and details of the planned trajectory, a command was sent to the camera early enough that it moved in the right way at the right time.

On Apollo 15, Granvil Pennington could make no such attempt. The camera’s tilt mechanism had begun to slip as the temperature rose, and he didn’t dare command any kind of tilt for fear of never being able to return the camera to horizontal. Instead, viewers watched Falcon smartly disappear beyond the top of the picture. The timing wasn’t quite right for Apollo 16, but on Apollo 17 viewers could follow Challenger as Ed Fendell tracked it from lift-off to pitchover and beyond.

For as long as the rovers’ batteries held out, or until a circuit breaker on the Apollo 15 rover popped in the rising temperatures of a high lunar Sun, geologists continued to use the TV camera to view the landing site under the slowly varying illumination of the long lunar day. There was even an unsuccessful attempt to gain coverage of Challengers discarded ascent stage as it was targeted to impact the South Massif.

Around the time that the rovers were being driven across the lunar surface, the press divided the S38 million price tag by the number of rovers delivered and poked fun at how expensive these cars were. A different analysis would point out that these vehicles dramatically raised the efficiency of the surface crews and that, despite the extreme engineering demands place on them, the rovers performed extremely well. Given that the American taxpayer had already invested huge sums in getting to the Moon, the extra expenditure on the LRV more than paid for itself by allowing the system as a whole to give a much greater return on investment.


It will help the reader to understand the concepts behind rendezvous if a short diversion is taken into the field of orbital mechanics. At first glance, this topic seems arcane and, if studied rigorously, it is. Additionally, it can appear counter-intuitive but the basic concepts behind the subject are easy enough to grasp, and are really an extension of the orbital lessons discussed in Chapter 4.

To lay down the groundwork for this we need to establish some basic ideas. Unless some kind of propulsion is being used, all movement in space is governed by the gravitational attraction of the bodies (stars, planets, moons, asteroids, etc.) among which things move. In general, the gravity of the nearest large body dominates, so for the purposes of this explanation we shall ignore the pull from other bodies. Any spacecraft in orbit moves around the central body in an ellipse. Even a perfectly circular orbit is treated as a special form of ellipse whose eccentricity value is zero.

There are three principles to bear in mind with orbital motion. First, a spacecraft in a higher orbit takes longer to go around than one in a lower orbit. At first glance, this appears obvious because there is a longer circumference to travel, but that is only part of the story. The more important point to grasp is that it really is a slower orbit. The spacecraft is moving at a slower linear speed because the pull of gravity from the central body becomes weaker with distance, and hence a lower speed can maintain the perpetual fall that is orbital motion. As an illustration, the Saturn V inserted the Apollo spacecraft into an orbit only 170 kilometres above

Подпись:Earth, taking only an hour and a half to go around at a linear speed of 7.8 kilometres per second. Geostationary satellites, which are the mainstay of global communications and televi­sion satellite broadcasting, orbit 35,800 kilometres above Earth’s equator, take 24 hours to get around once and travel at only 3.1 kilometres per second.

With this in mind, we can see a method by which one spacecraft can manoeuvre with respect to another, assuming that both are travelling in the same orbital plane. If the target ship is ahead, a pursuer can catch up with it by manoeuvring into a lower orbit, which is achieved by firing

Orbital mechanics 397

image234"against the direction of travel, as if trying to get away from the target. We said it was counter-intuitive. The burn will cause the pursuer to fall into a lower orbit, which will have a shorter period and a higher linear speed. This will allow it to catch up with the target. The difficulty lies in choosing the exact moment to start climbing back into the original orbit, which we shall deal with later.

image235"The reverse is also true. If the pursuer is ahead in the orbit, it can ‘slow down’ by accelerating forward, which causes it to rise to a higher and therefore slower orbit. It can then drop down again when the target has caught up.

image236"The concept of changing from one orbit to another is a common requirement in space­flight and is embodied by our second principle which we have already met in Chapter 4 as the Hohmann transfer orbit. It is the most efficient and simplest way to change an orbit whereby firing a spacecraft’s engine along the direction of motion at one point in the orbit will increase its speed and thereby raise the altitude that will be reached on the opposite side of the orbit. Firing against orbital motion will slow the space­craft and lower the altitude of the opposite side of the orbit. Control of the total impulse from the burn allows control of the altitude that will be reached at the opposite side. We have met this already in the way the CSM and LM made burns around the Moon’s far side to raise and lower their near-side altitude.

To move from a lower circular orbit to a higher one, a burn must be made in the direction of motion until it is calculated that Diagram of basic rendezvous the point in the orbit opposite the spacecraft, techniques, now the apogee, is at the height of the

intended circular orbit. Once the spacecraft has coasted around in its orbit to its apogee, another burn must be made along the direction of motion to raise the perigee until it equals the apogee’s altitude.

So far we have dealt with two spacecraft within the same orbital plane. The third principle behind orbital mechanics deals with the situation when the two objects are

in different orbital planes. This is a common requirement, since few launch sites are located cqualorially yet many satellites need to reach a geostationary orbit above the equator. For example, a spacecraft launched from Cape Canaveral will necessarily have an orbital inclination of at least 28 degrees: this being the latitude of that site. The most efficient way for a spacecraft to move from one orbital plane to another is for it to make a burn at the point in the orbit where the two planes intersect, known as a node. Unfortunately, the physics of the situation dictate that all but the smallest of plane changes will be expensive in propellant – indeed to move a communications satellite from a 28-degree low Earth orbit to its geostationary outpost requires almost as much energy as would be required to send it to the Moon! For Apollo, it was vital to minimise plane change manoeuvres, especially for the LM’s ascent stage where propellant margins were very Light.

When to go

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

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

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

Thing without wings

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

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

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

To skip or not to skip

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

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


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

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

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

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

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

"Do you?" asked Anders.

"Right below’ us."

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

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

"What’s wrong?" he demanded.

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

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


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

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

"Yes," confirmed Lovell.

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

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

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

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

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

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

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

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

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

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

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

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

"That’s incredible,’’ interrupted Cernan.

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

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

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

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

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

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

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

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

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

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

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

Collins agreed, "God damn. I guess."

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

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

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

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

"It looked grey." interjected Collins.

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

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