Category How Apollo Flew to the Moon

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.

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

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.

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.”

"What?”

"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.

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.’’

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

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

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

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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.

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.

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.