Category How Apollo Flew to the Moon

Even before Kennedy’s challenge, the lunar Tree-return trajectory had been recognised as a safe and efficient means by which a spacecraft could make the journey. The idea is attributed to Yuri Kondratyuk of the Soviet Union, who realised its possibilities for lunar flight in the early twentieth century. A major crater on the far side of the Moon is named after him. It was a wonderful solution to the problem, and one whose propellant needs were within the capabilities of the Saturn V. It could get a crew’ to the Moon within three days and allow the entire mission to be carried out within 14 days, well within the duration for which the Apollo spacecraft was being designed. Furthermore, if a fault arose in the SPS on the way to the Moon to prevent major manoeuvres, the free-return trajectory would bring the crew back towards Earth, and any fine tuning on the homeward leg would be within the capability of their RCS thrusters. This was an inherently attractive option for an agency that had a presidential directive to preserve the life of its human crews.

The free-return trajectory relied on using the Moon’s gravity as a steering device for the spacecraft. It is one of a range of techniques that have been used by interplanetary probes for decades to move around the solar system much more quickly than w’ould be possible with rockets alone. If a spacecraft coasts towards a planet from beyond that planet’s sphere of gravitational influence, it must have the same speed on both the incoming and outgoing legs when measured with respect to that body. However, the planet is moving with respect to the Sun so there is an opportunity for some exchange of momentum between the planet and spacecraft. If it passes by the planet’s trailing hemisphere, its heliocentric (or Sun-centred) velocity is increased as it gets a little gravitational tug like a skater holding onto a car. The result is that its orbit is made larger; speed has been gained without so much as a squirt of rocket propellant. This is often characterised as a slingshot effect.

Conversely, if the spacecraft passes the /^Mo°n

leading hemisphere of the body, the ex­change is away from the spacecraft. It gets a tug from the planet against its orbital motion so heliocentric velocity is reduced and its orbit gets smaller.

With the lunar free-return trajectory, the leading-edge case is taken to an extreme in which the spacecraft is made to swing all the way around the far side of the Moon and onto a path back to Earth, in the process tracing out an immense figure-of-eight.

Such a trajectory also affords a slower approach velocity with respect to the Moon, thereby minimising the amount of propel­lant required to achieve lunar orbit. A win – win scenario.

Once the free-return trajectory was factored into the TLI calculation, there were very few solutions remaining to the equations that calculated the burn for the S-IVB. Such equations took into account the motions of Earth, the Moon and the spacecraft as well as the other major bodies in the solar system whose gravity would to some degree influence the spacecraft’s path. They also accounted for the trajec­tory that the lunar module would take during its descent to the surface, particu­larly when Apollos 15 and 17 had to approach through mountain ranges. One

particular flight controller in mission control was responsible for procuring the details of a TLI burn that would achieve as many of the desired conditions as possible. The flight dynamics officer (FIDO) worked with a backroom team and a room full of mainframe computers to calculate a range of possible solutions whose starting point was the orbit that had been achieved around Earth. These could be optimised for fuel efficiency, duration of flight, suitability for entering lunar orbit, and flight safety in terms of their return-to-Earth characteristics. From these, he picked one which, by his judgement, was the best compromise; one that required the Saturn’s third stage to fire along its flight path in order to change the speed of the spacecraft by a certain amount at a certain time. It was then up to the J-2 engine of the S-IVB to supply that change in velocity.

Over the course of the Apollo lunar flights, the manner in which planners used free-return altered as NASA’s operational confidence increased. A pure version of the trajectory, one that would set the spacecraft on a path directly to Earth without

intervention, would fly around the far side of the Moon at an altitude of roughly 500 kilometres, depending on the precise Hanh/Moon/Sun geometry. Up to and including Apollo 11, the spacecraft was sent on a trajectory that was a near approximation to this and which, if lunar orbit insertion were to be impracticable, would require only minor burns of the RCS thrusters to steer to the desired splashdown site. Since the free-return trick only worked if the flight was kept within the plane of the Moon’s orbit around Harth, it limited the potential landing sites to those that lay along the track of the resulting lunar orbit. Since the Moon’s equator is within a few degrees of the plane of its orbit around Earth, the ground tracks for missions which flew a free-return trajectory were all near-equatorial.

Later missions evolved the trajectory to hybrid versions. The H-missions of Apollos 12, 13 and 14 started out from Harth on a free-return trajectory, but once safely on their way they performed a small burn to improve their approach characteristics, knowing that a corrective burn from either the SPS engine on the CSM or the large engines on the LM would be sufficient to re-establish a safe coast home. The latter contingency had to be used on Apollo 13 to restore a free-return after the SPS engine was disabled. The J-missions were injected directly into a non – free-return trajectory, one that would not bring them home directly. Again, they relied on either the SPS engine to effect a safe return or. if that was out of action, one or both of the large LM engines.

Keeping track of a spacecraft’s motion far away from Earth is a wondrous application of human ingenuity and knowledge. It requires a blend of heavy

engineering and subtle, precise electronics; the first to build and control huge dish antennae, allied to the reception and measurement of vanishingly weak radio signals.

Two techniques were used for Apollo tracking, and both of these were cleverly interwoven into the same radio signal that carried all the communication between the spacecraft and Earth: voice, television and telemetry from the spacecraft’s systems and data uploads from the ground station on Earth to the ship’s computer. Because so many functions were brought together into one S-band radio signal, the system was known as Unified S-band. Its tracking capability could determine range and radial velocity – that is, the distance to the spacecraft and how fast it was approaching or receding along the line of sight of the antenna system on Earth.

The Apollo command module (and to a lesser extent the lunar module) could be thought of as a mini-planet. All the basic requirements for human life over a two – week period were brought together inside a sealed conical machine less than four metres across and little over three metres high that could transport its occupants between worlds. Along with its ser­vice module, the spacecraft provided air, water, power and propulsion. It included a means of navigating across space and a way to negotiate Earth’s atmosphere upon return. It contained adequate supplies of food and warmth as well as the equipment that its crew would need for a programme of science during their journey. The life-support infrastructure of the Apollo spacecraft was under the watchful eye of the flight controller who sat at the EECOM console.

EECOM stood for electrical, environ­mental and communications, although evolving roles within mission control had removed the communications responsibility without changing the name.

The video imagery of Neil Armstrong descending Eagle’s ladder and stepping onto the Moon is among the most famous of all news clips. However, commentators from more recent and media-savvy times are often astonished to learn that television, which would be of tremendous public relations importance, was almost pushed aside as a distraction to the exacting job of actually achieving the prime goal of the Apollo programme. Wally Schirra resisted having a TV camera on Apollo 7, but the success of black-and-white television from Apollo 8, which brought pictures of a distant Earth and of the Moon’s dramatically illuminated terminator to the public, sold the idea to NASA. These broadcasts drew massive audiences from around the world and NASA soon realised that TV could play an important role in shaping how history would remember Apollo.

Several months prior to the historic moonwalk by Armstrong and Aldrin, NASA had intended to send them to the surface with only a 16-mm movie camera and a spool of film that was insufficient to record the entire moonwalk. Max Faget, one of the spacecraft designers, described it as “almost unbelievable [that the mission] is to

be recorded in such a stingy manner". The pendulum swung towards acceptance of the technology in the spacecraft and on the lunar surface. The commander of Apollo 10. Tom Stafford, embraced the idea of TV from a spacecraft and helped to push the development of a colour TV camera that allowed him and his crew to make regular transmissions from their orbit around the Moon. This further raised the importance of television in the minds of crews and managers.

Despite this acceptance, the frantic pace of flight development and the difficulty of developing a lightweight colour camera for the harsh conditions on the lunar surface meant that the Apollo 11 lunar module Eagle took only a simple black-and – white TV camera to Tranquillity Base, while the colour unit remained in the comfy warmth of the command module.

In later years. Jack Schmitt was withering in his assessment of NASA’s overall reluctance to adopt TV. "The first Apollo TV camera was just ludicrous. NASA just totally screwed up the specs in buying the thing and there was no excuse for it. Finally, we got a good, high resolution camera for 15, 16, and 17. Actually, Tom Stafford flewr it on 10. But the so-called Apollo Television Camera that flew on Apollo 11 w-as terrible – low resolution, black-and-white. Just not any good at all. It couldn’t take any kind of bright scene at all. On Apollo 8. we had to put every filter in the spacecraft in front of it just to Lake a picture of the Earth.’’

The lesson had been learned, and from Apollo 12 onwards, colour TV cameras were taken to the Moon’s surface. To get TV back to Earth, engineers used a part of the bandwidth in Apollo’s radio system set aside for auxiliary signals, be it scientific data, recorded onboard data or television. Although both the black-and-white and the colour systems used this auxiliary communications channel, the implementation was very different.

Before describing how TV images reached our sets from the Moon, a cjuick lesson in terminology is in order. In the United States, conventional TV images were built up within a frame of 525 lines. A complete frame was sent 30 times per second but the lines within it w’ere not sent sequentially. Instead, all the odd-numbered lines were sent first, followed by all the even-numbered lines which meant that two interlaced scans of the image combined to make a frame. Each of these scans was called a field. To an approximation, the US television system of the late twentieth century had 60 fields per second of 262.5 lines per field, or 30 frames per second of 525 lines per frame although not all of those lines carried imagery.

All video imaging of the time was carried out by tube technology that generated electron beams within vacuum vessels. Ilot wires provided an electron source and coils of wire deflected the beam to scan the image. They were therefore hot, heavy and power-hungry.

It is hardly surprising that the Apollo programme, which was lauded as one of humanity’s greatest achievements, should have spawned a vibrant niche in publishing. In the wake of the missions, innumerable books commemorated the flights of the Apollo crews as publishers took advantage of the public’s interest. But then, within ten years, the story was held to be less fascinating and new books on Apollo became increasingly rare.

Things began to change, however, beginning with the twentieth anniversary of the first manned landing on the Moon in 1969. A generation who had watched Apollo on their parents’ television screens with wide-eyed wonder had grown up and taken the reins of society. To them and those w’ho were born after the landings, the programme became the product of the previous generation and. at this point, retrospectives began to appear. Apollo is now written as history rather than as current events. However, much that has become available concentrates on the programme’s conception and on those who transformed it from engineers’ dreams into a superpower’s goals. A particularly popular sub-niche is the astronaut biography, a somewhat variable collection of tomes that do much to relate the story of humanity’s only foray away from the grip of planet Earth. Other volumes relate, in varying levels of detail, what the intrepid explorers actually did during their far too brief spells on the surface of another world.

Remarkably few’ books discuss the practical aspects of how the voyage from the Earth to the Moon was achieved. The genre seldom describes the equipment that was used; nor does it relate the procedures and techniques that allowed the Apollo crews to accomplish their audacious task: in general, historians are not concerned with how a feat was achieved technically. Instead. Apollo’s written history comes in three dominant types: (I) the experiences and interrelationships of the people involved, (2) the political and social milieu in which they operated and (3) the polemic and ranting of those who arc doing the commentating. This is all well and good – to a point. The same applies to the modern media. The details of how something was achieved are considered to be the realm of the ‘geek’ or ‘nerd’, and the general public are thought, at best, to be disinterested in science and engineering, or at worst, incapable of understanding such rigorous topics.

One particularly thoughtful television programme of the late twentieth century looked at the conflict between reporting as the dissemination of facts, and reporting as the telling of human interest stories. Produced by actor Tom Ilanks, the series From the Earth to the Moon bravely dramatised the entire sweep of the Apollo programme and included an episode about the flight of Apollo 13, a mission that has become a byword for human doggedness and ingenuity in the face of overwhelming challenges. Rather than remake a story that had been well told in an earlier cinema release, the writers concocted a battle of wills between two characters – an older journalist who took the trouble read up on the technicalities and complexities of the mission and who cleverly and clearly did his level best to explain them to the public; and a young, upstart reporter whose mantra was human interest and who would not be seen reading the spacecraft’s checklist or NASA’s official press kit. A line from the upstart makes the case for the modern view. ‘‘You think America wants to know about PC burns and passive thermal rolls? That’s not news. man. That is ’Sominex’.” Somincx is a sleeping pill. He perceived an America that neither understood nor cared about science and had little interest in engineering niceties. What they wanted to read about was the emotional state of the families of the stricken astronauts.

But in the age of the internet, this uninformed public is swimming in an ocean of information, much of which is of dubious accuracy. Among this deluge of ideas is one that tests their understanding of historical truth. In recent years, whether for financial gain or just as a pseudo-intellectual prank, people have taken to questioning the veracity of Apollo’s greatest achievement. Websites abound that mock the very idea of America having achieved moonlandings in the 1960s and 1970s. They pick spurious holes in the historical record and rely on the ignorance of the public at large, feeding on a distrust of big government in order to sell books and TV to a section of society that savours and favours mammoth conspiracy theories.

The fact that one of the best documented events of history could be considered to be a hoax thrives partly because so few people actually know how the feat was achieved, or how the most basic laws of physics express themselves beyond the surface of our planet. I once spoke with a head teacher – an educated man in charge of over a thousand teenage pupils – who could quote Shakespeare as knowledgeably as he could discuss football. I asked him why the crews on board the Space Shuttle were seen to float about the cabin. “Because there’s no gravity in space, of course.’’ was the reply. At the time. I didn’t have the heart to enquire of him what kept the Moon in its orbit around Barth. I wasn’t trying to mock him but I wanted to understand the extent to which concepts derived from basic science were understood by the public. I soon learned that ignorance in science and engineering appears to be the norm.

The provocative suggestion that the Moon landings were faked is what evolutionary biologist Richard Dawkins would call a successful тете. Like the gene, it is self-replicating; an idea that has the requisite characteristics that allow it to sustain and be passed from one credulous mind to the next carried forward because it can easily replicate through a population who are largely scientifically illiterate. Distorting facts to support a false theory is a straightforward exercise, especially with a sprinkling of pseudo-scientific jargon, when the audience lacks the tools, and often the inclination, to examine them critically. To refute these false tales requires intellectual rigour and a well-grounded knowledge of the physical world, the possession of which would likely inoculate a person from taking such claims seriously in the first place. One of my motives for writing this book was to provide a little of the knowledge that might help to refute the absurd assertion that Apollo was faked.

Another reason behind the book was a desire to share something of my own personal journey in reaching an understanding of how this wonderfully audacious adventure was achieved. Like so many of my age. Apollo happened at an impressionable lime in my life. I was only just old enough to realise that a flight to the Moon would be an incredible, fantastic thing to attempt; at which point. I watched America promptly realise that dream before my eyes. The enchanting, almost primeval sense of wonder that this adventure left with me transcends its grubby, political roots, and has never really departed with childhood. Then the arrival of the internet in our household, soon after the 25th anniversary of the Apollo 11 landing, blew pure oxygen over the embers of this fascination. It lit a vigorous fire because, for the first time, I could find material that explained in great detail just how this difficult endeavour was executed. Equally important w;as being able to connect with others w’ho had been similarly touched by Apollo, eventually having the honour to link up with some of those who were lucky enough to have taken part.

This book and my personal journey through Apollo, discovering how it all functioned and what happened on another world, owe an endless debt of gratitude more to one man than to any others I have encountered along the wny. Geographical distance has so far prohibited me from having the opportunity to shake his hand and thank him personally, as he lives on the other side of the w’orld. Yet the internet allows me to count him as one of my closest friends. Eric Jones Look on the monumental task of compiling a journal of the first era of lunar exploration after he became frustrated at how’ his country had shelved the lessons learned after they had spent billions of dollars going to the Moon. Inspired by J. C. Beaglehole’s journals of Captain James Cook’s explorations 200 years earlier, he recounted and explained every moment the Apollo crew’s spent on the lunar surface. By making his efforts freely available on the internet, I and people from around the world came on board, adding our time and talents to make the Apollo Lunar Surface Journal w’ebsite one of the most remarkable historic documents from the twentieth century.

My chosen role was to extend the journal to include the flying portions of the missions the journey to the Moon, from the Moon and the time spent in orbit around it. Taking Eric’s w’ork as my model, I set out to explain w’hat w;as occurring moment by moment, and w’hile doing so. I learned more than I could have imagined about how. at a broad level, the Apollo jigsaw fitted together. This book is my attempt to pass on this knowledge to a wider audience.

Most of the book will Lake the reader through the various stages of the Apollo flights, from before their spectacular launches at Kennedy Space Center in Florida to the decks of the aircraft carriers that recovered the crews from the Pacific Ocean a week or two later. For newcomers to the subject, I have devoted the first two chapters to an outline of Apollo’s genesis and achievements; and have included a bibliography at the end of the book that will provide years of excellent reading for those who wish to delve further.

This is the tale of how Apollo flew’ to the Moon and how the United States of America brought together the finest of its people and skills to achieve a dream as old as the human race, to turn the ancient light in the sky into a world to be explored. It describes the efforts involved not only in successfully flying to the Moon, but also in returning safely, to provide knowledge and a new perspective on the human position within the cosmos.

к may be appropriate to mention here a few points on general terminology, as American engineers, scientists and technologists have a habit of constructing long descriptive names for their ideas and systems, which they promptly shorten to an acronym. Their use of the resulting words and phrases then settles into a form that is often chaotic and contradictory. In this book. I have been unable to avoid filling the text with some of the same arcane acronyms that clog up so much discussion of technical matters. However, many that I have used are so ubiquitous in the Apollo story that they will soon be seen as old friends, and readers will be well served by making them a familiar part of their vocabulary. Notes have been included in the glossary in order to point out the various and inconsistent wnys in which acronyms were pronounced.

Through my own science and engineering education, I have a bias towards the use of SI units and. as a result, those units have been used throughout the book. This is a controversial path to tread as it is often pointed out to me that those who carried out the achievement used ‘English’ or ‘Imperial’ units, and that it would only be proper for books on the topic to do likewise. This argument holds no more weight for me than the suggestion that books on Egyptology should exclusively be written in cubits. SI is the dominant system of scientific and engineering expression in the world. NASA began to use it for its science publications in 1970 and even Apollo’s onboard computer did its internal sums in metric. It therefore seems appropriate to explain Apollo in units that will have the widest possible understanding. Where English units are used in dialogue, a suitable SI equivalent will be near at hand.

So here’s my book, and I hope you like it.

IN David Woods Bearsden Scotland January 2011

Despite the weight advantages that were gained from the adoption of the lunar orbit rendezvous concept, an Apollo spacecraft and lander were still a huge mass to lift off Earth and send to the Moon, and a very special rocket would be needed for the task.

In every way, the Saturn V epitomised the sheer audacity of the Moon programme. It was big – in size, thrust and weight; it required huge facilities to build, test, transport and launch; and its engines consumed massive quantities of propellant at prodigious rates. It also demanded fine, subtle control of the
enormous forces it produced, and it stretched the will of NASA even to have conceived it. The fact that they did was perhaps because, at the outset, its designers had considered an even larger vehicle. In comparison, the Saturn V may have seemed relatively straightforward but when the time came to turn ideas to reality, its procurement strained the IJS aerospace industry every bit as much as the spacecraft it carried.

The lineage of the Saturn V led back to a pre-war German amateur rocketry club, the VfR (Verein fur Raiicnsehffahrt or Society for Space Travel) where a young Wernher von Braun first shone as a gifted rocket engineer and motivator of men. As Adolf Hitler rearmed Germany, its military, denied conventional long-range artillery by the Versailles treaty, took an interest in the successes of the VfR and how its new rocket technology could be applied to sending warheads towards an enemy.

Von Braun headed a group of engineers based at Peenemunde, a peninsula on the large island of Usedotn on Germany’s Baltic coast, where his Л-4 rocket, fuelled by alcohol and liquid oxygen, was developed. Towards the end of the war, conventional warheads were installed on the A-4 and the propaganda ministry renamed the rocket V-2 or Vergeltungswaffe 2 ( Vengence 2). An explosion in Chiswick, London, on 8 September 1944, signalled the first use of the V-2 as a weapon. Thousands of these rockets, built largely by slave labour under the control of the notorious Schui/.siaffel (the SS) were launched in an attempt to ruin the morale of the British population.

As the Allied forces marched across Huropc in the war’s final days, teams of intelligence specialists searched for useful military technology. Von Braun knew that the knowledge and experience of his engineers would be a great prize for whichever Allied power reached them first. The Soviets w ere closer but he preferred the western option, and arranged for his team to surrender themselves to the American forces. Additionally, he helped his captors to retrieve hardware and documents that would prove useful to them. Though he shamelessly used the military as a means to develop his rocket, von Braun had something else on his mind space travel.

In 1956. the US Department of Defense made the IJS Air Force responsible for procuring the country’s long-range missiles. Over the succeeding years, the USAF and the companies that worked for it developed the Atlas. Thor and Titan missiles. In the meantime, von Braun’s group, still part of the US Army – initially based at the White Sands Proving Ground in New Mexico but now’ at the Army Ballistic Missile Agency in Huntsville, Alabama – had already devised the Redstone and Jupiter missiles. The former was America’s first rocket capable of sending a payload into orbit. After the Soviet Union started the space race by launching Sputnik on 4 October 1957. von Braun’s Juno I rocket (a Redstone with solid-fuelled upper stages) countered for America by placing the more scientifically useful Explorer I into high orbit. However, rockets were inextricably lied up with the nuclear weapons they were designed to carry, and because by now’ America had lightweight nuclear devices, its rockets tended to be lower pow’ered and less useful as lifting vehicles for spacecraft. Soviet nuclear weapons were large, heavy affairs, and therefore their rockets, being more powerful, made better space vehicles. Realising this shortcoming, von Braun’s team first added solid-fuelled rockets to the top of their Jupiter missile, which was essentially a scaled-up Redstone, to make the Juno II space launcher, and then progressed to the development of a heavy-lift booster specifically for space use. Initially designated the Super Jupiter, this booster would cluster first-stage engines and tanks to achieve the desired thrust. This project evolved into the Saturn. As early Saturn development continued into I960, von Braun’s team found themselves transferred to NASA with, at last, a civilian role for their rockets. Their facilities in Huntsville became the Marshall Space Flight Center, with von Braun as its director. Once President Kennedy’s lunar goal had been set, the development of the Saturn rockets became a key part of the civilian space effort, with their final design being firmly linked to the needs of the Apollo spacecraft they would carry.

Though an entire family of launchers were envisaged, only three Saturn rockets came out of the programme. The Saturn 1 was primarily a development scries that proved the concepts of engine clustering in order to achieve high thrust levels as well as testing early Apollo hardware. The Saturn IB used an improved form of the first stage and made use of a new, highly efficient rocket stage, the S-IVB. which was manufactured by the McDonnell Douglas Company. This stage would be crucial to the Apollo programme. As well as being the second stage of the Saturn IB. it formed the third stage of the Saturn V, in which role it would provide the final impetus to take all NASA’s manned spacecraft to the Moon. The Saturn IB was a man-rated vehicle that could take either a CSM or a LM, but not both, to Earth orbit.

The big member of the Saturn family of launchers was the Saturn V, so-called because, in von Braun’s mind, there were three paper configurations after the Saturn I that were never built. In truth, its NASA’s early plans shifted, there were many configurations that were put on paper to match machines to missions. Many of these used varying quantities of two engines that were under development, the F-l and the.1-2, but it was the iconic Saturn V, which could lift both the CSM and the LM, that utilised them to fulfil the lunar goal.

Most of the major components and procedures required for a Moon landing had been tested, though not always in the context in which they would be needed during a lunar flight. In order to minimise the surprises that might face the landing mission, NASA wanted to practise a complete lunar mission as far as they dare, short of beginning the final descent to the Moon. This dress rehearsal flight, the F-mission, was accomplished by Apollo 10, flown by Tom Stafford, Eugene Cernan and John Young. Their spacecraft were named after characters in Charles Schulz’s popular cartoon strip Peanuts, who had also featured in NASA campaigns to promote quality control in the programme. The CSM was therefore named Charlie Brown while the LM took the name Snoopy.

Launch took place on 18 May 1969 and. for the first time, all the functions of the CSM to take a lunar module to an orbit 110 kilometres above the Moon had to work. Once the two docked spacecraft had successfully entered a lunar parking orbit, the crew settled down for their first night in the Moon’s vicinity as their ship hurtled above its surface at 5.800 kilometres per hour. Next day. Stafford and Cernan entered Snoopy’і cabin, separated from Young in Charlie Brown, and took the LM into the same low orbit from which a landing mission would make its final descent. This orbit brought Snoopy down to an altitude of less than 14,500 metres above the lunar terrain from where Stafford photographed and described the approach to the landing site that had been selected. The most important task was to prove that lunar

orbit rendezvous would work as planned. After they jettisoned the descent stage, the ascent engine was used to set up an orbital situation similar to that which would be presented after lift-off from the Moon. NASA’s management had once been wary of the idea of two speeding craft being brought into close proximity while in orbit around another world, and they wished to prove that their techniques worked prior to committing a lander to the surface. This successful rendezvous and docking finally cleared the way to a landing attempt.

Thanks to the usual predictability of the gathered media, Apollo 10 is more often remembered for the ‘son-of-a-bitch’ language Cernan used when a pilot error caused the LM to gyrate unexpectedly as the descent stage was being jettisoned. The journey home was uneventful except for the unprecedented colour television coverage of a receding Moon that was beamed to Earth soon after Charlie Brown’s SPS engine was fired. Stafford had promoted the importance of TV on Apollo, not only to the public, but also to engineers and lunar scientists. The 8-day flight of Apollo 10 put NASA on the home straight, leaving the G-mission with no unknowns except the landing itself.

At the very tip of the Saturn V stack, the rounded point of the launch escape lower (LET) included eight small holes which led to a device called the Q-ball. Shortly before launch, a cover was removed that had protected these holes from debris and insects. Their function was not dissimilar to the pitot tube seen on conventional aircraft for measuring airspeed, because the Q-ball similarly measured air pressure as the vehicle rammed through the atmosphere during the ascent. However, the point of eight holes on the Saturn was not to measure airspeed, but to measure whether the air was hitting them equally and thereby determine whether the rocket was flying straight and true through the atmosphere. The angle-of-attack that they sensed was displayed on a dial for the crew’s benefit. If the flight had to be aborted, and the spacecraft pulled clear by the escape tower, the Q-ball would then help to determine which way round the command module and tower were flying.

With two minutes to go, a crewman pulled a knob in the cabin which stopped coolant flowing to radiators on the side of the spacecraft. Normally, in space, these panels received hot liquid from the cooling system and shed that heat by radiation. However, as the rocket ascended, the frictional heat generated by passing through

the atmosphere warmed the radiators, temporarily making them counterproductive, and so for the few minutes of ascent they were bypassed.

With much of the atmosphere behind it and the second stage working smoothly, the

Saturn changed the way it guided itself to orbit. So far. it had not compensated for any distance that the wind and other forces had pushed it from its ideal flight path. Nor had it tried to correct for any under – or over-performance of the engines. Instead, during the tilt sequence, the instrument unit had merely kept track of where it was at any moment. The new guidance regime was given a typically NASA-cse name of the iterative guidance mode. While in force, equations in the Saturn’s computer plotted the most efficient flight path from ‘wherever the vehicle was’ to The point in space where it wanted to go’ – in this case, insertion into a parking orbit around Earth. To anthropomorphise the situation, what the computer was thinking was, “OK, 1 know where the wind and such has pushed me to. What must 1 do to reach the position and speed that I have to reach?” As the S-II powered ahead, the computer monitored the vehicle’s progress and sent steering commands to the four gimballed outer engines as necessary to achieve the desired result. This steering was maintained for the rest of the S-II burn. It was then suspended and the stack’s attitude held steady until the S-II had dropped away and the third stage had ignited to pow’er the vehicle to orbit. With the S-IVB thrust established, the steering recommenced.

To help visualise the flight path to the Moon, imagine a vantage point looking dowm at Earth and the Moon from the north, with an Apollo spacecraft about to make a TLI burn for a free-return path. The spacecraft’s orbit around Harth is towards the east (anticlockwise in our visualisation), a direction it shares with the rotation of Harth. and indeed almost everything else in the solar system. The TLI burn is Limed to occur at a point in this orbit that is essentially on the opposite side of Earth from where the Moon and spacecraft will come together in three days. The burn places the spacecraft on a path that ordinarily would be a long, slow elliptical orbit with an apogee half a million kilometres out from Earth, somewhat beyond lunar distance. Meanwhile, at TLI the Moon is about 40 degrees further back in its path around Earth, moving at one kilometre per second towards the point where its orbit intersects the spacecraft’s path. The timing of TLI is such that the spacecraft reaches lunar distance shortly before the Moon comes along.

As it heads out, the influence of Earth’s gravity on the spacecraft gradually weakens while that of the Moon, approaching from the side, grows stronger; it attracts the spacecraft and deflects it from its original anticlockwise elliptic path. By virtue of careful calculation of the required trajectory, the spacecraft is aimed to pass near the Moon’s leading hemisphere and be pulled around its far side travelling in a clockwise direction. If nothing else is done, and if the timing and direction are correct, such a trajectory will, under the Moon’s gravitational influence, swing right around the lunar far side and fall back to Earth. Such a path would make its closest approach about midway around the far side and in order to stay in the Moon’s vicinity, the crew; must fire their main engine against their direction of travel at this

point. This bum will take energy out of the spacecraft’s trajectory, slow it down and make it enter a clockwise, or retrograde lunar orbit.