Category How Apollo Flew to the Moon

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.

The measurement of radial velocity relied on the Doppler effect, whereby the movement of a transmitter altered the received pitch or frequency of whatever wave was being transmitted. This is familiar to most people as the change in pitch of a constant note, perhaps from an engine or a horn, as it passes the listener. We hear its pitch drop as it passes by and departs. The same effect applies to radio transmissions where the frequency of the received signal changes slightly according to whether the transmitter is approaching or receding from the receiver. This frequency can be accurately measured and, by knowing the precise frequency with which it was transmitted, the velocity of the transmitter can be calculated. The system is similar in some respects to radar speed traps used to catch speeding car drivers.

To use the Doppler effect well, the frequency of the transmitted signal from the spacecraft had to be known with great precision. Onboard equipment to generate a signal of sufficient accuracy – one whose frequency was precise and stable despite the thermal extremes of space, w’ould have been excessively heavy and power-hungry. However, an elegant solution existed that kept the heavy equipment on the ground, yet could yield a measurement that was inherently more accurate.

Given that a powerful, aeeurate radio signal would in any case be sent to the spacecraft to carry voice and data from mission control, engineers simply arranged that it be modified on board in a known way. and retransmitted back to the ground, this lime carrying voice and data from the spacecraft. If the frequency of the signal from Earth was precisely known, then so was that from the spacecraft if it were not moving.

For Apollo, the ground station transmitted data and voice signals from mission control to the spacecraft on a carrier signal called the uplink. This carrier was synthesised from a very accurate frequency standard installed at the station. Ground stations supporting the Apollo programme had some of the most accurate frequency standards available at the Lime. For the CSM, the carrier had a frequency of 2.106.4 MIIz while that for the LM was 2,101.8 MHz. On reception by the spacecraft antenna, an onboard transponder Look this signal, multiplied its frequency by the ratio of 240/221 (about 1.086) and sent it back to Earth, using this new signal as the carrier for the downlink.

When received by the ground station, the precise frequency of the downlink was measured and compared to the uplink. If the precise 240/221 relationship was maintained, the spacecraft was neither approaching nor moving away from the ground station, such as when moving across the face of the Moon perpendicular to the line of sight to the ground station. A higher received frequency meant that the spacecraft was approaching; a lower frequency indicated receding motion. This was a very powerful system because it measured Doppler shift over both the up and down legs of the signal’s journey, doubling the sensitivity of the system to the point where it could even detect the velocity change caused by the minuscule thrust that was generated when the crew’ dumped their urine overboard.

Buried within the pie-shaped struc­ture of the service module were two (later three) tanks each of oxygen and hydrogen. Although these two sub­stances are excellent propellants for rocket engines, in this case propulsion was not their purpose.

A common notion is that space­craft usually derive their electrical power from the Sun via large arrays of photovoltaic cells. While this is

generally true for automatic spacecraft in the inner solar system and for the International Space Station, the high power demands of a typical Apollo flight would have required such large panels as to make them cumbersome. This size would not have been a problem during a coasting flight, but when the spacecraft’s large engine was fired, the mechanical stress from the aeeeleration would have required the panels to be folded away at the very time that their power was most needed. Л second alternative is to use storage batteries to bring eleetrical power from Harth. Although they could have supported a short flight, as they did for the early manned Gemini flights, they could not support the Apollo spacecraft for two weeks without being prohibitively heavy.

It was up to the Gemini programme to prove the concept of a third alternative, the fuel cell, as a source of clcctrieiiy for long-duration flights, f irst developed before the Second World War in Britain, the operation of the alkali fuel cell is remarkably simple. It acts like a battery by using the chemical reaction of two substances, in this case oxygen and hydrogen, and it makes the energy of the reaction available in the form of prodigious quantities of electricity. However, unlike a conventional battery, the reactants can be replenished constantly. As long as fresh reactants are fed past the electrodes the fuel cell does not run down. Even more remarkable is the fact that the waste product of this reaction is water that is sufficiently pure to drink.

The adoption of the fuel cell in Apollo therefore killed two design quarries with one stone. Not only did it produce lashings of electricity (a single fuel cell could generate well over one kilowatt of electricity at peak demand), the water it produced became a sort of lifeblood of the spacecraft. It quenched the thirst of the crew and rchydraicd their food in metered amounts through a pistol-style ‘squirt gun’ on the end of a hose. It also supplemented the cooling of the spacecraft’s electronic equipment by being evaporated into space, taking heat with it. Any excess was periodically discarded through an orifice in the spacecraft’s hull.

The high-energy reaction that occurs when hydrogen and oxygen are burned in the combustion chamber of a rocket makes it greatly favoured by rocket engineers. In the Apollo fuel cell, most of this energy was expressed as electricity, but although it could reach efficiencies of 70 per cent, the reaction still yielded significant amounts of heat. Some of this was used to warm the extremely cold reactants before they entered the cell; the rest was rejected through eight radiator panels around the upper circumference of the service module. An early version of the fuel cell flew on seven of the Gemini flights which gave engineers a chance to iron out the teething troubles with this promising technology. By the time the Apollo programme finished Apollo 13’s oxygen tank explosion notwithstanding – no Apollo flight suffered from a failure of their fuel cells. It was one of the many technologies and techniques for which the Apollo programme depended on Gemini to pioneer.

Among the limitations of the Apollo fuel cell was that it was very sensitive to the presence of impurities in the reactants. Even with hydrogen and oxygen of the highest purity that NASA could procure, the build-up of contaminants required that the cells be purged from time to time to avoid the resultant loss of electrical power. Oxygen purges were carried out daily, while hydrogen purges happened every second day. Three switches on the LMP’s side of the main display console allowed the gases to he routed to any of the three cells for this function, hlcclric heaters were included to ensure that the purging gas was warm enough to avoid it freezing the water in the cells.

Apollo’s implementation of black-and-white television w as, by far, the simpler of the tw;o systems, and the less greedy of radio bandwidth. The camera, as used on Apollos 8, 9 and 11 had just one imaging tube and operated at scan rates that would normally be called slow-scan television. The frame rate used was only 10 frames per second with 320 lines per frame. There was no interlace. The bandwidth of the signal (which dictated how well the image handled fine detail) was restricted to a very low value of 0.4 MHz (as compared to about 5 MHz for contemporary broadcast TV). The camera used a vidicon type of imaging tube that was notorious at the time for its excessive image lag, and this caused a ghostly smear to trail behind the moving image.

When the pictures reached Earth at this non-standard frame rate, they were electronically incompatible with just about every TV system on the planet. Converters were installed at chosen ground stations to generate standard US television signals from the lunar TV. The converter worked in two stages. The first simply consisted of another vidicon TV camera aimed at a small television screen. The screen displayed the images from the Moon at 10 frames per second while the camera, which ran at 60 fields per second, was allowed to capture the screen only when a full image had been completed, which was every tenth of a second. In other words, only one field in six from the camera contained a picture.

The second stage was to recreate the missing five fields. The single good frame from the TV camera was recorded onto a magnetic disk which then replayed it five times to reconstruct the full 60-fields-per-second TV signal, ready for distribution to Houston. The repetition of the fields and additional lag from the second camera added to the ghostly impression left by Apollo ll’s moonwalk coverage.

A question that is often asked is, if Neil Armstrong was the first man on the Moon, who operated the TV camera? ft is a spurious question because it assumes that all cameras must have a cameraman behind them. In fact, Eagle’s camera was mounted inside a fold-down panel next to the ladder. At the top of the ladder, Armstrong pulled a lanyard to open the panel and thereby reveal the camera. The most ergonomic and lightweight way to mount the camera on this panel was upside – down, so on Earth, the conversion equipment had a switch which the operator flicked to right the upside-down picture. Once both crewmen were on the surface, Armstrong lifted the camera from its mount and placed it on a stand from where the TV audience could watch proceedings. The operator threw his switch back to restore the image’s orientation.

One day in 2003,1 said to prolific author David Harland that there was one book in me waiting to get out. "Right," he said, "and it’s my job to get it out of you.’’ Before I knew what was happening. I had started putting down what I had learned about the Apollo flights and, slowly, this book began to take form. I hope David likes what I produced, and I greatly acknowledge his wisdom, advice and support throughout the process.

I am particularly grateful to Clive Horwood and all at Praxis Publishing, and Maury Solomon at Springer for not only taking this project on when I had never previously delved into the world of book writing, but also for considering it worthy of a second edition. I also thank Alex Whyte for his copy editing work on the first edition and David Harland for his work on this new and expanded edition.

Over the past decade, as I probed deeper and deeper into how Apollo worked, I came across many other people who are all part of a loose group in society who. like me, watched the dream of Apollo being fulfilled as impressionable children, trie Jones was pivotal in bringing the depth of Apollo to a much wider audience through his unmatched Apollo Lunar Surface Journal. This journal is simply the best Apollo resource available, largely due to trie’s boundless generosity. We all owe him a huge debt of gratitude.

When 1 took my idea of an ‘Apollo Flight Journal’ to trie, he wisely pul me in touch with Apollo computer expert and self-proclaimed ‘geek’, Frank O’Brien. Frank was a little sceptical of the value of the AFJ at first, but soon changed his mind as we discovered the many layers of complexity that went into an Apollo flight. Frank’s knowledge is vast and deep, and I could never have penetrated Apollo’s subilcty without his guidance.

While writing this book, I leaned on others for advice, feedback and resources. In particular, I’d like to thank fellow Apollo students Tim Brandt, Ken MacTaggart, Lcnnie Waugh and Scott Schneeweis for comments and suggestions. Scott was also kind enough to supply photographs of items from his comprehensive collection of Apollo hardware.

In my professional life in TV broadcasting, I work among people who combine technical ability with creativity. Some have had to put up with me for years rabbiting

on about the Moon and Apollo. Л long time ago. Hedda MacLeod made it clear to me that she believed that I really ought to write a book and it is Hedda I thank for planting the seed that really made me think it in the realms of possibility. Some of my colleagues agreed to become guinea pigs, reading drafts and guiding me where I was going. I am therefore grateful to Martin MaeK. eny. ie and Ken Stirling for their feedback. With my cover, I wanted to hark back to the nuts-and-bolts science fiction I grew up with, where a flight to the Moon was a fantastic, futuristic promise. 1 also wanted to indicate to a prospective reader that the book’s contents would not be overly technical and I am grateful to Stew-art Ramsay for realising my vision for this edition.

I w’ould also like to thank my sister. Hilda Harvey, for her advice, feedback and encouragement. Hilda is no space geek but, while reading my drafts, she made me believe that the book could be made amenable to the wider public. In a similar vein, I am proud to mention my lovely wife, Anne, and my two wonderful sons. Stephen and Kevin, who were always there as sounding boards when I struggled for words and phrases and who read the text for me throughout our proofreading. My parents, Allan and Violet, always fostered my interest in space and spaceflight, buying their young son a telescope and sending me on astronomy courses. To them and all my sisters. I send love and thanks. I would also like to thank John Lightbody for his drawings that accompany my explanations of orbits and the state vector.

For me, Apollo 15 commander David Scott is the epitome of the Apollo lunar explorer. He has also been incredibly generous to those who have been compiling the Apollo journals. I am hugely grateful to him for his support throughout and for contributing the Foreword to this book. Other Apollo participants who were generous with their talcs over the years include astronauts A1 Worden and Jim Lovell; and members of mission control Gerry Griffin, Sy Liebergot and Chuck Dcilerich. Thanks also go to Stephen Garber and the staff at NASA’s History Division. For over a decade. Stephen has always been greatly supportive of the Apollo Flight Journal, upon which this book is based.

Among the justifications for producing a second edition was feedback from readers who pointed out occasional errors in the first edition or who provided me with a deeper understanding of various issues. In this respect. I am grateful to LM computer expert Don Eyles who shared his astonishing tales of the cutting edge of computer technology in spaceflight. Also to Jim Scotti for his input. Additionally I felt that I should expand the book with details of the surface exploration of the Moon and for my research, there was no better material than Eric Jones’ ALSJ and, once again, I am in Eric’s debt for this wonderful resource.

The majority of pictures that I have used in this book are courtesy of NASA and all effort has been made to determine the copyright owners for other images. In those few cases where this was difficult. 1 have used the images anyway owing to their historic importance, and relevant copyright owners should contact me and I will be pleased to give due credit in later editions.

Any errors in the book are my own but if any are spotted, please pass them to me via my publisher or the book’s website, w-ww-.hafttm. com. They will be considered in the event of a reprint or new edition.

The F-l rocket engine is still one of the most powerful liquid-fuelled engines ever built and one with a distinguished service record. An uprated version of the engine, the F-1A, underwent trials and reached 17 per cent greater thrust but it was never brought into service. The Russian-designed RD-170. which saw service in the late 1980s, gave 12 per cent more thrust with greater efficiency.

The F-l began as an Air Force programme in 1955, which NASA then nurtured for its bigger missions. It was ideal as an engine for a first-stage cluster in a huge booster owing to its prodigious power, but its gestation was as difficult as any in the Apollo;Saturn story. In operation, a single engine consumed a total of nearly three tonnes of kerosene and liquid oxygen every second and produced a force that could balance 690 tonnes of mass. It became obvious in its development that simply scaling up the design of contemporary engines was not going to work. Injecting so much propellant into a huge 90-centimetre chamber often led to brutal combustion instability that destroyed engine after engine. It took nearly five years of trial and error for engineers at the Rockeidyne company to tame the F-l to the point where a

small bomb could be ignited within its combustion chamber and the resultant instability would dampen itself out within half a second.

Apollo 11 departed the Kennedy Space Center in the early morning of 16 July 1969 on a mission that would culminate in an attempt to land on the lunar surface. It is widely quoted that over a million people gathered in the vicinity of Cape Canaveral to witness the start of what promised to be a defining event in human history. For the first four days, its crew of Neil Armstrong as commander, lunar module pilot Edwin ‘Buzz’ Aldrin and the command module pilot Michael Collins followed a path

blazed by their predecessors. They even took time out to give viewers to the TV networks an extended tour of their lunar module, Eagle, with an improved colour camera.

On the fourth day, Armstrong and Aldrin left the command module, Columbia, in the charge of Collins, undocked, and fired Eagle’s descent engine to enter the descent orbit around the Moon. As communications proved to be somewhat troublesome, Armstrong reorientated the LM slightly in order to improve reception. On approaching the point where they were to reignite the engine for the landing phase, Armstrong timed the passage of landmarks to determine whether their trajectory was as it should be, and saw that they seemed to be a little ahead. The engine was ignited on time, and after several minutes of continuing to monitor the passage of the landscape below they rotated the LM to allow its radar to take altitude and velocity measurements.

At this point, things became hair-raising, especially for the flight controllers in Houston who lacked the crew’s situational awareness. Thanks to a subtle flaw in the spacecraft’s electronic systems, the computer began to complain of being overloaded, ft displayed debugging codes that were never meant to be seen during a flight and which most people at mission control, as well as the two men in the spacecraft, had little knowledge of. However, just two weeks before the mission, the LM computer experts had studied a large number of such codes, including those that the crew were seeing. Given that the vehicle was otherwise operating normally, they recommended that the descent continue.

Armstrong was able to monitor where on the lunar landscape the computer was guiding them as Aldrin read out relevant numbers. When he saw that their destination appeared to be a boulder field near a large crater, he put himself in the control loop earlier than planned, and manoeuvred to smoother ground 300 metres further along the flight path. Meanwhile, mission control began to worry about a shortage of propellant. When only 15 seconds remained before mission control would have advised the crew to abort the landing attempt, Eagle successfully realised John F. Kennedy’s goal on 20 July 1969 by touching down in the southwest comer of Mare Tranquillitatis.

In the minds of the crew the difficult part of Apollo’s goal had been achieved, yet the public was more eager to witness an event whose scale was much more human and personal. This was the moment when a man made a boot impression in the lunar dust. Armstrong later pointed out that the moonwalk carried far fewer dangers than manoeuvring seven tonnes of flimsy spacecraft loaded with explosive propellants down onto an unknown rocky surface on the end of a rocket flame, while surrounded by a hard vacuum. Nonetheless, it was inconceivable that a crew would land on the Moon and not walk on the surface!

Over the subsequent hours, in one of the most memorable television events in human history, Armstrong and then Aldrin descended the ladder onto the lunar surface. Observed by a black-and-white television camera whose mode of operation gave them a ghostly appearance, they took photographs, collected samples and set up three simple scientific experiments: a small seismometer, a laser reflector and a solar wind collector. The social significance was not forgotten when the flag of the United States was raised on behalf of the nation that had paid for the venture. Additionally, a plaque was unveiled to inform any future visitors to Tranquillity Base that its first visitors "came in peace for all mankind” and the two explorers took a telephone call from President Richard Nixon. After 2 V2 hours, the moonwalk ended. Armstrong and Aldrin took their exposed film and a box of lunar samples up to the ascent stage, repressurised the cabin and tried to get some fitful sleep before

performing lift-off for the second time in less than a week. Their return to Collins in Columbia and the trip back to Earth were uneventful, concluding with a landing in the Pacific Ocean on 24 July.

The Apollo programme had been designed to be aggressive from the outset, with launch facilities at KSC constructed for multiple or closely spaced launches. Now, with the moonlanding successfully accomplished, and America’s spending on the Vietnam War draining the nation’s purse, the scale of lunar exploration was cut back by Congress. Nevertheless, the sheer momentum of the programme brought another landing attempt only four months later.

At T-20 seconds, swing arm 2 was retracted from its position connected to the top of the S-IC. As it arced back to the tower, the guidance system on the Saturn was finally set for the flight. In the instrument unit above the S-IVB stage, there was a conventional gimbal-mounted guidance platform – the type that can hold its orientation while the vehicle around it rotates. As the Earth turned with the Saturn V on the pad, the platform kept its alignment with respect to the stars. If an onlooker could have watched it over a few hours, it would have appeared to rotate, making one full turn each day. Both CM and LM contained similar devices, and these will be discussed more fully later in the book.

The Saturn’s guidance platform provided two important pieces of information needed to guide the space vehicle to the required orbit. One was knowledge of the direction in which the rocket was pointed. This was derived from the platform’s property of maintaining its orientation and thereby provide a reference against which the vehicle’s orientation (normally referred to as its attitude) could be measured. The second came from a set of accelerometers mounted on the platform
with which the instrument unit’s computer could sense the movement of the rocket, and hence, its three­dimensional path from Earth into orbit and later the manoeuvre to head for the Moon.

The orientation of the Saturn as it sat on the pad, and indeed the orientation of the pad itself, was not haphazard. It had been carefully thought out prior to being built. Each pad was aligned to the cardinal points of direction, with the flame trench aligned exactly north-south. The launch vehicle was brought to it with its umbilical tower to the north. From here, the most efficient head­ing was to fly directly to the east, so the rocket was presented with the spacecraft’s hatch also facing east. This way, when the rocket ascended, tilted over and entered orbit, it did so with the spacecraft windows essentially facing Earth and its navigation optics facing out into space.

At this point, it is worth outlining the vehicle’s coordinate system. The Saturn V’s plus-.v axis ran along the length of the rocket and out through the top of the escape tower. It plus-у axis ran through the vehicle towards the umbilical tower and therefore was pointed north. The plus-2 axis ran through the vehicle to point west. As the crew lay on their couches, their heads aimed east and therefore towards the minus-2 axis.

Most Apollos did not fly directly east but on a heading a little north or south of east to ensure that they reached the spot over the Pacific Ocean where the burn for the Moon would be made. The heading taken by the launch vehicle was known as its flight azimuth, a figure quoted as degrees from true north where a heading due east was said to have an azimuth of 90 degrees. The azimuth was directly related to the groundtrack of the vehicle and the inclination of the subsequent orbit. As KSC was at a latitude of 28°, an easterly azimuth would lead to an orbit whose inclination would also be 28°. A smaller azimuth of say 72°, would produce a steeper inclination.

For the flight, the orientation of the platform had to be aligned to match the flight azimuth, but this could only be done a few seconds prior to lift-off. Had the platform been aligned early on and left uncorrected, Earth’s rotation would have rendered the alignment invalid by the time lift-off occurred.

What was done on the Saturn V was to align its orientation with respect to a theodolite that was mounted some way from the pad between the crawlerway tracks. A small window in the side of the instrument unit was provided for this purpose. The

Diagram to explain the effect of flight azimuth on resultant orbital inclination.

platform’s alignment was then held rigid until T-17 seconds, the time of guidance reference release when it was set free. From then on, it held its orientation with respect to the stars. This moment has been immortalised in the recordings from that era when the NASA public affairs officer announced to the world, "Guidance is internal”, ff the countdown were stopped after T-17 seconds, a new flight azimuth would have to be calculated and the platform realigned to it.

As has been said herein repeatedly, the Saturn V was a big vehicle and its length helped to contribute to one of the most uncomfortable sensations most crews had to tolerate during their ascent to space – pogo. This was named after a stick-like toy of the 1960s on which children could bounce along, aided by a large spring that stored the energy of each jump. Engineers found that many rockets, not least the Saturn V, were prone to severe longitudinal vibrations which they called pogo for the obvious reason. Such vibrations could, and sometimes did, cause serious damage to vehicles. Like all structures, the Saturn V could resonate at particular frequencies if excited by an appropriate mechanical stimulus. As it happened, it was not short of such stimuli for it operated in a highly dynamic environment where as many as five huge engines pushed forward and liquids by the tonne flowed to the rear; all of which made the vehicle especially susceptible to vibrations along its length. Ugly mechanisms took hold whereby small, perfectly normal variations in thrust affected the pressure of the propellant lines. This resulted in further thrust variations that interfered with the flow of large volumes of propellant coming down the pipes, inducing ever larger surges. The natural resonance of the rocket’s structure sometimes reinforced these vibrations. Worse, the resonances constantly changed as the tanks emptied, sweeping through a substantial range of frequencies. To further complicate matters, payloads and mass distributions were altered from mission to mission, w’hich changed the nature of the pogo vibrations and made it difficult to design the problem out of the structure. Contrary to popular misconception, pogo was not related to the tendency of liquid propellant to slosh about in the tanks, although this phenomenon also had to be suppressed by the installation of anti-slosh baffles within the tanks.

The second unmanned test flight of the Saturn V (Apollo 6) suffered a spell of pogo in its first stage that would have nearly shaken a crew senseless had there been

anyone on board. Engineers suppressed the S-IC pogo problem by pumping helium into cavities in the propellant lines to make them act like shock absorbers, but it was never completely eliminated and affected most flights to some degree.

As Apollo 8 rode its S-II, Frank Borman relayed his impressions. “Okay. The first stage was very smooth. And this one is smoother.’’ Perhaps he was trying to keep the flight controllers from worrying, because his crew and others on the early Saturn V launches had noticed that pogo was especially strong towards the end of the second stage. According to his post-flight debrief, it bothered Borman. ‘’Quite frankly." he said, “it concerned me for a while, and I was glad to see S-II staging.”

By Apollo 10, engineers had decided that the easiest way to avoid pogo in the S – II would be to shut down its centre engine early, but when Apollo 13 ascended on its second stage, the pogo vibrations became so severe that switches designed to detect improper thrust in its central J-2 engine were inadvertently activated and the engine shut itself down earlier still. Jim Lovell was in command: "Houston, what’s the story on engine 5?” Capcom Joe Kcrwin in mission control didn’t know why the centre, or inboard, engine had quit but he was being told that the other engines were doing a good job. “Jim, Houston. We don’t have a story on why the inboard – out was early, but the other engines are Go and you’re Go.’’ Fred Haise was monitoring the rate at which their height was changing, a quantity known as h-dot. and immediately saw that they weren’t gaining height as rapidly as expected. “Okay. We’re a little bit low on h-dot now, but that’s to be expected." The loss of the centre engine was not as problematic as might be expected, because the other four engines continued to give a balanced thrust and the instrument unit compensated to some extent by burning them for longer, using up the remaining propellant. The crew continued monitoring.

“Didn’t like that inboard [shutting dow n early].1’ said Lovell as the S-II drove on with four engines, but his CMP Jack Swigert gave him comforting news. “Okay, we’re 1,400 feet a second low? on V|. That’s not too bad.’’ The quantity > q was their inertial velocity. Swigert realised that although they were 430 metres per second slower than they should have been, the Saturn had enough in reserve to make up the shortfall. “Watch the trajectory closely, Jack.’’ urged Lovell.

“You’re S-IVB-to-orbit capability now.” announced Swigert eight minutes into the mission. If the S-II gave up completely, the S-IVB third stage would have enough power to take them into Earth orbit. Mission control reassured them that the Saturn wasn’t defeated yet.

“Thirteen, Houston. Looking good at eight minutes."

“Roger.” replied Lovell before asking Haise. "How’s those systems. Fred? Are there any…” His lunar module pilot was quick to reassure, “They’re looking good." Swigert continued his analysis. "Okay, now. h-dot is low?. Jim, [but the] S – 1VB ought to pick you up.’’ The S-II was not meant to Lake Apollo to orbit anyway and the final burst of speed was provided by a short burn by the S-IVB. Lovell was concerned that if the S-IVB had to make up more speed to compensate for his ailing S-II, it might not have enough propellant left over to send Apollo 13 to the Moon, but it reached orbit with sufficient propellant for its lunar mission. After diagnosing the premature shutdown on this flight. NASA altered the pipeline feeding LOX to the centre engine to make sure that this particular source of pogo was suppressed in future flights.

Once in Earth orbit. Lovell summed up the experience: ‘‘There’s nothing like an interesting launch.” he said, not knowing how ‘interesting’ Apollo 13 was to become.