Category How Apollo Flew to the Moon

Over the final few hours before they entered lunar orbit, the Apollo crews worked through an exhaustive series of checks and adjustments, interrogating the spacecraft’s systems about their ability to sustain life while in the Moon’s clutches, and on the engine’s readiness to do its job properly.

Another important task in the build-up to LOI was to change the spacecraft’s knowledge of which way was ‘up’. During the five or six minutes of the burn, the crew’ would want to avoid any appreciable errors in the direction of the engine’s thrust. Additionally, they needed to ensure that the guidance system could measure the effect of the burn on their velocity. As was usual before a burn, the CMP performed a P52 to check the alignment of the guidance platform, but this time special procedures were applied. Up to this point, the platform had been aligned with an orientation that suited the coast to the Moon and made the barbecue rotation easier to set up and maintain. Now the platform would be realigned to match a new

REFSMMAT[3] that suited the LOI burn, and so obviously it was known as the LOI RHFSMM AT.

First, the CMP carried out a realignment to refine the platform’s orientation in terms of the REFSMMAT they had been using. This yielded a measure of its inherent drift, a parameter that was always carefully monitored and no opportunity was missed to gain another data point. Once the amount of drift had been measured, the platform’s orientation was torqued around align with the new RFFSMMAT. This one had been chosen to match the attitude in which the spacecraft would make the upcoming burn. By lining up the coordinate systems of the platform and the spacecraft, the crew’s job of monitoring attitude during the burn became a lot easier. Their 8-ball attitude indicators would now read zero on all three axes w’hich made them much simpler to interpret, к is wise to be certain that your ship is pointing in the correct direction when you make major engine burns near planets (especially ones without atmospheres) as a mistake can lead to a crash.

Next, they put the entry monitor system (FMS) through a test to demonstrate that it could still accurately measure the change in speed brought about by the burn. This feature of the FMS, its ‘Dclla-v‘ display, w-as one of the redundant methods by which the engine could be commanded to shut down once it had achieved its task.

The spacecraft’s cooling circuits w’ere next to be checked. At first glance this may appear to be one of the less exotic systems, but if any flaw were to be found in cither the main or the backup circuit – especially if any leaks had formed in the radiator pipes due to micromcLcoroid damage – the crew would return directly to Earth.

More checks followed w’hich covered the caution and warning system, the tanks and valves associated w’ith the manoeuvring thrusters on both the service module and the command module, and the spacecraft’s supplies of oxygen, water and power. Once these essential tasks had been completed, the crew could begin to implement the burn itself.

Any journey in space is heavily influenced by the propellant available to achieve it. At the same time, the amount of propellant required is largely determined by the mass of the object that is to make the journey and how quickly the journey has to be undertaken. In simple terms, mass is everything. The alternative scheme, known as lunar orbit rendezvous (LOR) sought to limit the amount of mass that had to be propelled at each key point in the journey. A reduction in the quantity of propellant required for the Apollo spacecraft would also minimise the initial mass that would begin the journey, and thus bring the entire mission within the capability of a single Saturn C-5.

The advantages are best understood by working backwards through a mission. The only part of the spacecraft that could return to Earth was the heatshield – protected command module. To propel it out of lunar orbit required the propulsion capability of the service module and their combined mass defined the amount of propellant required for the task. Next, instead of taking a lot of redundant mass down to the Moon’s surface just to bring it up again, a dedicated lander would be designed specifically for the task, leaving the Apollo mothership, the CSM. in lunar orbit with the consumables and propellant to get home. This lander would only Lake two of the crew down to the surface, leaving the third to take care of the CSM. Moreover, there was no need for the engine, landing gear and the empty tanks that had enabled them to land on the surface, to come back up to lunar orbit. The crew with its gathered lunar treasures could return to the mothership in only the Lop part of the lander using a smaller engine and the propellant required for the task. As there would be no need to bring this remaining part of the lander back to Earth, it, too, could be discarded at the Moon. Therefore, the final propellant load for the CSM was made up by the fraction required to get the entire assemblage into lunar orbit, plus the fraction required to get itself to Earth. At each key point in the journey, the engines would work against only the mass that was absolutely necessary, and everything else would be discarded when its function had been fulfilled.

The cumulative weight savings made the LOR scheme highly attractive in engineering and cost terms, but it caused NASA to face certain operational realities which, in the early days of space flight, seemed daunting. As with EOR. having separate spacecraft meant learning how to rendezvous in orbit when both were travelling at what wrere then perceived to be incredible speeds. The ships would have to join together, or dock, to allow crewmen and cargo to transfer from one craft to the other. Neither of these techniques had yet been demonstrated in Earth orbit, but the LOR concept was calling for them to occur nearly half a million kilometres away in the lonely vicinity of the Moon. A failure of the rendezvous would doom the occupants of

the lander to certain death in lunar orbit, while a failure of the docking would require crewmen to don spaccsuits and move from one craft to another by going outside. At a time when no one knew what challenges the weightless environment would present to a crewman in a bulky pressure suit, this seemed to be a very risky thing to do.

Many in the burgeoning space community were aghast at the audacity of LOR. It seemed foolhardy and dangerous. However, convinced of the benefits, and with an almost religious /.cal, its leading advocate, John Houbolt. drove through layers of NASA bureaucracy and the entrenched positions of its various centres, in an effort to convince the organisation that there was little chance of getting to the Moon within the decade unless LOR was adopted.

NASA debated the mode issue for more than a year after Kennedy had laid dow n his challenge, during which Lime, direct ascent and its incredible Nova launch vehicle was largely discarded, leaving EOR, championed by von Braun, and LOR, which, because it included a specialied lander, had become Gilruth’s preferred option, as the competing schemes. As work on the spacecraft could not begin in earnest until the matter was settled, Joseph Shea from NASA headquarters asked each side to report on the other’s scheme – a management strategy that enabled von Braun to recognise the benefits of LOR. In June 1962, at a large meeting at Marshall, NASA acceded to Iloubolf s campaigning and chose LOR as the means by which they would get to the Moon.

With the mission mode settled, the definition, design and construction of the spacecraft could begin. The command and service modules would be built by North American Aviation. These craft were already well into their initial development, but their role could now’ be precisely defined; there being no need for a landing stage on the SM, for example. Major components for the SM had already been designed. It was decided to leave the thrust of its propulsion system at its original design value and Lake this capability into account in mission planning. Two versions of the CSM were to be built. The Block I spacecraft would be incapable of supporting a mission to the Moon, but w’ould allow experience to be gained in Earth orbit until the Block II became operational. The Block II would be the Moonship proper. Complete with fuel cells for power, hardware for docking, deep-space communications and a fully capable guidance and navigation system, the Block II CSM would be the linchpin in the Apollo story, ferrying a spidery landing craft to another w’orld. In a sense, the CSM was a mini-planet, providing everything three men w’ould need for Lw’o w’eeks in space during which they would undertake a journey that had been a dream of humans over the ages. In the event, the design of the Block II w’ould be forged in the lessons learned from the fatal flaw’s that would prevent the Block I from flying a manned mission.

By the spring of 1968, with two flights completed, the Apollo programme seemed to be hitting its stride. It had demonstrated all three stages of the Saturn V worked, the command module had survived its high-speed re-entry, and an early version of the lunar module had performed satisfactorily. Before the Saturn V could be declared fit to carry astronauts, a second А-mission was required. This flight was named Apollo 6 and. once again, events unfolded that threatened to stop the programme in its tracks.

After a successful lift-off on 4 April 1968. the first problem appeared towards the end of the SIC’s flight. Rockets have always been prone to vibrations along their length, but for about ten seconds immediately before the first stage was to shut down, the longitudinal shaking of the entire vehicle (known as pogo) became alarming. Meanwhile, at the front end of the rocket, a conical aerodynamic shroud that would normally protect the lunar module (not carried on this flight) was losing chunks of its outer surface. Since this section had to support the mass of the CSM multiplied by the g-forces of acceleration, its structural integrity was of some concern.

Halfway through the flight of the S-II stage, one of its five J-2 engines began to falter, prompting the instrument unit to shut it down. As it did so. another engine that had been showing no distress also shut down, causing the thrust from the other three to be applied asymmetrically. Considering that the Saturn’s control system had been programmed only to deal with a single-engine failure, it did a remarkably good job of compensating for the off-axis thrust and burned the remaining engines for longer on the residual propellant. The first burn of the S-IVB third stage successfully pul the vehicle into orbit, but a subsequent command to restart the engine failed. Some of the flight’s objectives were met, but if the problems could not be fixed, NASA would not dare to put men on top of the next Saturn V. as was being considered instead of a third Л-mission test.

In the event, engineers managed to find solutions for all these problems. The first stage vibrations were suppressed by the addition of helium gas to cavities in the LOX feed lines, which damped out pressure oscillations. Elaborate tests on the J-2 engine discovered a design fault in a liquid hydrogen fuel line that had not only caused one of the engines on the S-II to shut down but also prevented the S-IVB from restarting. Compounding the S-II problem, a wiring error had sent the shutdown command from the Saturn’s instrument unit to the wrong engine, shutting it down unnecessarily. The aerodynamic shroud had failed because frictional atmospheric heating as the rocket went supersonic caused trapped moisture and air within its aluminium honeycomb sandwich skin to expand, in turn causing the skin to peel off in sheets. This problem was remedied by making small ventilation holes in the shroud’s skin and adding cork insulation.

The launch vehicle issues apart, the CSM-020 spacecraft successfully performed a number of remote-controlled manoeuvres and was recovered from the Pacific Ocean. Preparations for Apollo 7 continued because it would use a Saturn IB launch vehicle. It was decided that if this mission went well, the third Saturn V would indeed carry a crew.

Like most launches, the lift-off of an Apollo mission could only occur within well – defined spans of time known as launch windows. Launch could not be attempted

outside the launch window because some operational constraint would be exceeded. The major constraints on an Apollo launch window were propellant, communica­tions and the lighting conditions at the landing site on the Moon.

It had been determined that the best time to land on the Moon was in the lunar morning. Surface temperatures were moderate and the low-angle lighting made the landscape’s shape stand out. accentuating the topography and aiding the commander as he looked for a smooth place to set down. Therefore, a landing could be made at a particular site only once per month, and this restricted the launch to a single day each month. In case a launch had to be postponed for a day. NASA sometimes certified landing sites further west, where the lighting would be suitable two or three Earth-days later. Given the propellant available on board, planners then worked backwards from each landing opportunity to calculate when the launch had to occur.

Now the complexities of flight planning really became apparent. To get to the Moon, the engine of the third stage had to be reignited in Earth orbit and continue to burn propellant for a few minutes. By knowing where the Moon would be when the spacecraft arrived, orbital mechanics (of which more later) said that this ‘burn’ would have to occur on the opposite side of Earth. But NASA wanted this important burn to occur while there were good communications with mission control, including the hours immediately afterwards in case a quick return to Earth became necessary. This meant that the burn had to occur near Hawaii so that the spacecraft’s rise from Earth would be covered by a string of ground stations and communications aircraft across the eastern Pacific Ocean and the United States. As Earth turned, the Hawaii region moved into the correct position for the burn once per day, which further constrained the launch. However, since the S-IVB stage could not store its cryogenic propellants for more than a few hours in space, the number of Earth orbits prior to making this burn were restricted. It was decided to allow the crew a single full 90-minute revolution to check their spacecraft, and then head for the Moon during the second revolution. If the launch were to occur a little late, there was sufficient flexibility to delay the burn for the Moon by another orbit.

The launch window for Apollo 11 to allow Armstrong and Aldrin to reach their assigned target in Mare Tranquillitatis began at 09:32 Eastern Daylight Time on 16 July 1969 and lasted nearly 4‘Л hours. There were further opportunities to launch for sites further west two days and five days later. If those were missed, the same three sites became accessible a month later and indeed for each subsequent month.

From an altitude of about 30 kilometres until after the second stage had taken over, the abort rules changed slightly once again. By this time, the vehicle was so high and
the air was so thin that the canards at the lop of the LET would not have been able to ensure that the CM was in the correct attitude for jettisoning the tower. Instead. abort mode one-charlie required the crew to use an array of little rockets around the CM achieve the correct orientation.

This system of rockets, the RCS. was one of the erueial systems on the spacecraft. Both the command module and the service module had their own systems and they were the only way the spacecraft could control its attitude when the larger propulsion systems were not operating, which was most of the time. Later spacecraft would use the inertial properties of fasl-spinning wheels to provide something against which the spacecraft could push when adjusting its attitude – a scheme that offered the benefit of saving propellant. The command module’s RCS thrusters were only ever intended for use in an abort or in the final stages of an Apollo flight after the service module had been cut adrift.

Once in orbit, the crew could remove their helmets and gloves to give themselves a little more freedom, but for now would remain in their suits. As they busied themselves with their tasks, the cabin became cluttered as cameras and lenses were unstowed, ancillary equipment was fished out and installed, and the necessary system checks and alignments made. In addition to their spaeesuiis. the crew of Apollo 8 were still wearing life vests in case the CM had to ditch in the Atlantic after launch. As Jim Lovell was moving around, his life vest caught something and began to inflate from its internal gas supply.

"Oh, shoot!"

"What was that?’- asked his commander.

"My life jacket," he replied.

"No kidding?" laughed Borman.

Bill Anders was aware that, at this stage of the flight, their words were being recorded for later transmission to Earth and so he began a running commentary.

"Lovell just caught his life vest on frank’s strut."

"It’s hard to get off, too," commented Lovell. The three crewmen soon realised that the vest had been inflated with carbon dioxide, and if Loo much of that gas were to be dumped into the cabin it w ould overw helm the lithium hydroxide canisters that were intended to absorb the toxic gas in their own exhaled breath. Anders came up with the solution: " fell you what we’ll do: we‘11 dump ii oul with the vacuum cleaner

over the side there.1′ The CM’s vacuum cleaner worked simply by dumping cabin air overboard, taking dirt with it. By feeding the eonients of the life vest down the vacuum cleaner, the problem was solved.

Although they only had about 2 Vi hours in Earth orbit, the Apollo crews usually considered that to be enough Lime to eompleie a rigorous scries of systems checks and still have an opportunity to look out of the window at the w’ondrous sights passing below. For some crewmen, this would be their first experience of spaceflight, but this was not so for the Apollo 11 crew, all of wiiom were Gemini veterans.

“ frees and a forest down there.” said Mike Collins, as they flew somewhere over the western United States. "It looks like trees and a forest or something. Looks like snow and trees. Fantastic. I have no conception of where we’re pointed or which way we’re going or a crapping thing, but it’s a beautiful low-pressure cell out here.”

This crew, and many of the other Apollo crewmen, had flown in the cramped confines of the earlier Gemini spacecraft – a couple had even been squeezed into the tiny one-man Mercury capsule. Apollo gave them a bit more space to move around. “I’m having a hell of a time maintaining my body position dowm here,” noted Collins after he had manoeuvred down into the lower equipment bay where the eyepieces for the optical instruments were stored. “I keep floating up.”

“How’ does zero-g feel?” asked Neil Armstrong of his crew. “Your head feel funny, anybody, or anything like that?”

“No, I don’t know, it just feels like w’e’re going around upside down.” replied Collins who was still transfixed by the experience.

The Lime in Earth orbit was something that all the crews wished could have lasted longer. “Jesus Christ, look at that horizon!” yelled Collins on seeing howr quickly the Sun rose in orbit, even though he had already witnessed the spectacle during his Gemini mission in 1966.

“Isn’t that something?" echoed Armstrong.

“God damn, that’s pretty; it’s unreal.”

“Get a picture of that.” suggested Armstrong.

“Oh, sure. I will,” replied Collins who then had to contend with the compact and complex space that was an Apollo cabin. “I’ve lost a Hasselblad. Has anybody seen a Hasselblad floating by? It couldn’t have gone very far. big son of a gun like that.” "

Eugene Cernan. the commander of Apollo 17, noted how their night-time launch affected their experience in orbit. "Launching at night, we just had a somewhat different view of the Earth than most other flights have had. The first real view we got of being in orbit was pretty spectacular because it happened to be Earth sunrise and that’s a very intriguing and interesting way to get your first indoctrination to Earth orbit.”

Certainly Cernan’s LMP, Jack Schmitt, flying for the first Lime, did not hold back in describing what he saw as he saw it, a characteristic this scientist astronaut would exercise both on the Moon and in orbit around it. For example, while flying over the dark United States, he described the lights of the American towns and cities to Capcom Bob Parker. "Man’s field of stars on the Earth is competing with the heavens. Bob. 1 think we got the Gulf Coast showing up now. by the band of lights."

Half an hour later, over the daylii hemisphere, he applied some terminology with which he was familiar to the delicate patterning he saw in the great cloud systems that lay below: "Bob. we’re over what might be intermediate to low strata that have a very strong crcnulalion pattern – pulling out some geological terms here. 1 don’t think I’ve ever seen anything like it flying [an aircraft].”

The exposed desert landscapes of the Sahara brought him back to thinking about rocks. "Bob, wc had almost a completely weather-free pass over Africa and Madagascar. And the scenery, both aesthetically and geologically, was something like I’ve never seen before, for sure. There were patterns like I haven’t even seen in textbooks. Maybe I haven’t been looking enough, but some of the desert and grassland patterns had the appearance of ice crystals almost.’’

The crew of Apollo 12 had been entranced when they saw countless tiny pinpricks of light across the night-time expanse of the Sahara Desert as nomads sat by their campfires. The Apollo 16 crew also spotted this reminder of the human race’s relationship with flame, one that had lifted them off the planet.

"Look, look, John.” said Duke.

"What?’’ asked Young, ever unflappable.

"The fires. Out the right side. Looka there!-’ said Duke in some wonder. He had heard the stories from the Apollo 12 crew about them. "1 hey were right. They were really right. Beautiful!”

"What’s that?” asked Young.

Ken Mattingly, CMP on this mission, reminded his commander: "The fires of Africa. They’re there. Like he said. Isn’t that spectacular?”

"Thai is really beautiful!” said Duke.

"Can you see them. John?” asked Mattingly.

"Yeah, I see them. Yeah, yeah. Good gosh!-’

"There must be a hundred or so,” added Duke. "What are they from?-’

"Nomads,” said Mattingly. "All the nomads and stuff that are out there.”

Sharp-end forward

While the crew busied themselves to ensure that their ship was healthy, the S-IVB had not been idle as it prepared for its main burn. Throughout the one-and-a-half orbits made before TLI. a set of small rocket thrusters attached around its base kept the stack pointed forward into the direction of travel. The vehicle was still in the upper fringes of the atmosphere and this sharp-end-forward attitude presented the smallest area to the hypersonic air flow’, thereby minimising frictional heating. They also kept the cabin windows facing Earth and the spacecraft optics on the opposite side facing out to the stars for the CMP’s navigational duties.

This sharp-end-forward attitude was also required for TLI. so it made sense to maintain it throughout the Earth-orbit phase and avoid having to make large attitude adjustments that would have stirred up the propellant in the part-used tanks just prior to the burn. An early unmanned Lest (light had shown that it ought to be
possible to rotate an S-IVB, but excessive motions of the stage had to be avoided in case large slosh waves were generated within the tanks. Un­fortunately, Apollo 15’s S-IVB mana­ged to lose a quarter of a tonne of LOX when it readjusted its attitude too quickly. The stack had entered orbit in an excessive nose-down atti­tude and the slosh wave that resulted from the readjustment managed to reach a vent. Fortunately, the loss did not impact the mission.

Attitude control of the S-IVB stage was somewhat different from the technique used on the first and second stages of the Saturn V. While these stages could use their main engines to turn the ship in all three axes, the S- IVB’s single engine could only gimbal in two axes to provide control of pitch and yaw. It had no means to control roll. Additionally, unlike the two lower stages, the S-IVB was required to maintain its attitude during coast­ing flight when no power was avail­able from its main engine. The engineers’ solution was the auxiliary propulsion system (APS) which used two modules affixed to the base of the stage’s cylindrical section, each of which held four small rocket engines that burned hypergolic propellant from their own tanks. During powered flight, only the APS roll engines had to operate because pitch and yaw were effected by gimbaling the main engine. After the main engine had shut down and the stage had begun to coast, the APS modules assumed control of all three axes: roll, pitch and yaw.

While in Earth orbit, the crew avoided using their RCS thrusters as any motion imparted by them would be immediately counteracted by the APS thrusters whose commands came from the Saturn’s instrument unit. One exception was a short firing made to check their operation. Pete Conrad on Apollo 12 made a particular point of testing his spacecraft’s RCS thrusters with a few short pulses. His vehicle had sat in heavy rain prior to launch and he was convinced that this would have affected the upward-facing thrusters. “I was still worried about the water in those thrusters. I wasn’t convinced, in my mind, that we had not frozen some thrusters full of ice as there was water on the windows. Everybody thought [the water on the windows] would disappear and it hadn’t. I was concerned about those service module RCS thrusters, but the ground assured me they were working okay and it was alright with us.

CROSSING CISLUNAR SPACE

There is a poetic beauty to the Apollo flights which lies in the fact that the crews navigated between worlds by sighting on the very same stars their ancestors would have employed to guide boats and ships across the oceans of Earth. The maritime connection even extended to the instrument used for the task, because the Apollo spacecraft had its own sophisticated version of the sextant, an optical device used for centuries by sailors to measure angles between Earth’s horizon and the Sun and stars. Yet sighting on celestial objects was only one of a range of techniques that NASA brought to bear on the problem of guidance and navigation, skills that had to be mastered to ensure that 400,000 kilometres of space between Earth and the Moon were crossed in both directions accurately and safely. These skills required consummate finesse in the measurement of extremely subtle parameters, and high mathematical competence to interpret the results correctly, as excessive errors were utterly and lethally unforgiving. This region of space, encompassed by the Moon’s orbit around Earth, is termed cishmar space, pronounced with a soft ‘c’. Finding a way across it is therefore called cishmar navigation.

On one occasion, while Apollo 8 was returning home. Jim Lovell was busy with his programme of navigation exercises and as he punched away at the DSKY, its attitude light went out. ‘ For some reason.’’ he called to Mike Collins in mission control, "we suddenly got a Program 01 and no attitude light on our computer.”

Program 01 was only meant to be used at the start of a mission to initialise the IMIJ platform. In effect, the computer had lost its knowledge of which way was up.

‘’Stand by one, Jim.” said Collins. "We’re working on a procedure for getting you cranked back up again.”

"Okay.”

Lovell had meant to enter Program 23. the navigation program, and then use Star 01. A slip of the fingers and a couple of missing keystrokes meant that he had entered Program 01 in error. Unfortunately, there was no Undo button. In view of the huge amount of work Lovell had to perform on this pioneering flight, and his severely disrupted sleep patterns, an occasional slip was to be expected. At least it had occurred when it had no impact on the mission. To recover from the error, he had to realign the platform from the beginning using the stars. Mission control had to uplink a refresh of their REFSMMAT and check other data in memory because of what the computer had forgotten during the reset; all of which took about an hour of their З-day coast home.

Amused, both Borman and Anders constantly ribbed Lovell about his error for the rest of the journey.

“13, we’ve got one more item for you, when you get a chance,” said Lousma. Liebergot had been getting poor data from the quantity sensors and had been calling for more frequent stirs. “We’d like you to stir up your cryo tanks.”

“Okay. Stand by,” replied Swigert.

A minute or so passed as Swigert began to stir all four tanks sequentially. Suddenly, the data stream to Earth began to drop out, interrupting the flow of information about the spacecraft to the controllers’ displays. Something had disturbed the spacecraft’s attitude and caused its dish antenna to lose lock. Then a call came from Swigert. “I believe we’ve had a problem here.”

“This is Houston,” said Lousma, his voice suddenly taking a more authoritative tone. “Say again, please?”

Lovell immediately took over. “Houston, we’ve had a problem.”

He then launched into a technical discussion of what was happening on board the space­craft. “We’ve had a main bus В undervolt.” The CSM was losing power.

So began a 4-day drama that gripped the world and seriously threatened the lives of the crew. The story was traced back 18 months, to when an oxygen tank originally intended for Apollo 10 was dropped several centimetres. Although the tank appeared to be unda­maged, a tube to allow it to be filled and emptied may have worked loose.

It was then installed as the number two oxygen tank in Apollo 13’s service module. Three weeks before launch, the tank was filled during a routine test, and technicians found that it was slow to empty afterwards. Their solu­tion was to switch the tank’s heaters on and boil the gas out. The second major thread in the story then kicked in.

The heater circuits included thermo­static switches designed to prevent the

tank from overheating. When originally designed in the early 1960s, NASA’s engineers had specified that spacecraft systems should run on 28 volts, but they later instructed their contractors to rate all electrical items for 65 volts instead, as this voltage was to be used at the launch site.

Unfortunately, the message was not passed to the sub-subcontractor who supplied the switches. When the tank became too wann during the attempt to empty it, the thermostat tried to open the circuit, became welded shut by an arc of electricity that it could not handle, and continued to feed power to the heaters until the temperature within the tank exceeded 500°C. As a result, the insulation on the wiring was baked and became brittle.

At 328,300 kilometres from Earth, as Apollo 13 coasted towards the Moon, the agitation caused by tank 2 being stirred brought exposed wires into contact, and the short circuit ignited their insulation. A vigorous fire ensued within the tank, fed by the extremely dense oxygen and the combustible materials that constituted the tank’s innards. The pressure rose rapidly until the tank wall ruptured with such a force that the entire panel from that side of the service module was blown off. The consequential disruption to the plumbing allowed the oxygen in the undamaged tank 1 to leak out into space as well, thereby depriving the command module of its source of power and air. Since power was necessary for the operation of the SPS engine, the catastrophe also deprived the CSM of its propulsion.

ft might have ended there had the blast occurred on Apollo 8 – four days away from home, heading away from Earth with the crew slowly dying of asphyxiation in a dead ship – except for Apollo 13’s lunar module Aquarius. Luckily, it was still attached with its supplies unused and its engines fresh. NASA had even studied the possibility that one day the LM might be used as a lifeboat, and had tested a burn of a LM main engine while docked to a CSM during Apollo 9. Although it was far from
ideal and could not re-enter Earth’s atmosphere, the lunar module had plentiful oxygen, a working RCS and two reasonably powerful engines. The CM was the only part of the spacecraft that could bring the crew safely through the atmosphere. If they could use the LM to bring them to Earth, the CM’s remaining consumables would be preserved so that it could take them to the ocean.

More than at any other time, the toughness and competence of mission control and the huge array of supporting staff behind them came to the fore to overcome the almost intractable problems that Lovell, Swigert and liaise had to deal with. The range and depth of hazards they faced cannot be overstated, and each was handled with a creativity and tenacity beyond expectations. The LM seemed to lack sufficient battery power for the return. Its RCS thrusters were never intended to steer a ship that had a 30-Lonnc dead weight hanging off the end of it. There w-crc problems of guidance, of communication and tracking, of excess carbon dioxide, oflack of food, of sleep deprivation, of cold and discomfort. In addition, in the command module there was the problem of condensation over a mass of electronics that had to work on re-entry.

Thanks to a successful Hollywood movie in the 1990s. the story of Apollo IS and its successful return to Earth has become a by-word for the never-say-die. failure-is – not-an-option doggedness that turned this flight into the successful failure of the Apollo programme.

While the crew7 conducted their checks, engineers on Earth carefully measured the effect of both the final mid-course correction and, if appropriate, the jettisoning of the SIM bay door. From this, FIDO calculated the definitive LOI-1 burn. All the information associated with the burn was written onto a pad of no-carbon-requircd paper that allowed six copies to be made at once. The top layer w;as written using a red ballpoint pen in order to help to distinguish what was to be read to the crew7.

This list, referred to as a PAD (pre-advisory data), w’as read carefully by Capcom over the air,7ground communication circuit and copied by one of the crew7, usually the LMP. onto an identical form. Immediately afterwards, it was all read back to Earth w’here several flight controllers checked it to confirm that its contents had been correctly copied dowm. Much of the information in the PAD would later be entered
manually into the computer as the first stage of prepara­tion for the automatic con­trol of the burn.

It seems like a woefully low-tech way to relay data to the spacecraft by using voice, and although there had been some consideration of add­ing a teleprinter to an al­ready crammed cabin, the idea was shelved in view of the limited space, the weight of the apparatus and the tight schedule imposed on the programme by Kenne­dy’s challenge. Anyway, the use of voice, paper and pen is a wonderfully lightweight method of data transfer. The solution didn’t have to be elegant. It just had to work.

The PAD for the Apollo 15 lunar orbit insertion man­oeuvre was read up to Jim Irwin in the command mod­ule Endeavour by scientist/ astronaut Karl Henize.

"Okay. LOI, SPS/G&N; 66244; plus 121, minus 012; 078314591.”

The PAD was little more than a list of numbers that were almost indecipherable to the uninformed ear.

“Minus 28975, minus 07764, minus 00441; all zips for roll, all zips for pitch, all zips for yaw.”

As well as the impenetrable numbers, the jargon-rich language owed much to the military aviation background that most of its participants, both crew and flight controllers knew well.

“01696, plus 00584, 30001, 641, 29939; 25, 2671, 228; the rest is NA.” Henize read out these numbers in strict order from the top copy of the form, with each digit occupying its own box. On board Endeavour, Irwin wrote the digits onto his form, one in each box. Months of training and simulation meant that in many cases, the crews knew what kind of numbers they should expect, which helped to trap errors. The PAD finished with a series of comments related to the burn.

“Set stars are Vega and Deneb; 264, 090, 349. No ullage; LM weight, 36258. Single-bank burn time is 6 plus 52; and just a reminder that, if bank В doesn’t burn,

we are expecting you to go into lunar orbit on bank A.” This latter comment was directly related to the electrical short in Endeavours SPS circuitry.

Apollo thrived because, when dealing with the hostile, unforgiving space environment, particularly in the vicinity of the Moon, its people worked through the technical and operational aspects of the task with great care. Hvery item on this PAD was well defined and the procedures for passing such lile-or-death information to its recipients were strictly adhered to. The following puts meaning to the numbers and phrases.

LOI, SPS/G&N The first two items stated the purpose of the PAD and the systems that would be used to achieve the manoeuvre that it described. In this ease, the PAD is for the lunar orbit insertion burn that would place Apollo 15 into its initial orbit around the Moon. The burn was carried out by the SPS engine and associated equipment under the control of Endeavour’s primary guidance and navigation system.

66244 – In its calculations, the computer needed to know what mass the engine had to push against. This was in two parts, the CSM and the LM. Since the PAD form only had space for the CSM mass, the LM mass was given later. It is an interesting aside that the people at NASA were still using the term ‘weight’ when, strictly speaking, they should have been using the term ‘mass’. Mass is a measure of the amount of matter an object has. and in modern times the standard unit of mass is the kilogram. Weight is a measure of the force exerted by the mass on whatever is supporting it, which varies according to the gravity field it is in. At this point, the Apollo CSM was weightless though its mass was just over 30,000 kilograms (66,244 pounds as given in the PAD). This figure was determined by pre-flight measurements, and by carefully accounting for consumables on board the spacecraft, including the jettisoned panel.

Next were two three-figure values with signs – phis 121 and minus 012. These were angles, measured in hundredths of a degree, and with their decimal points, could be written as — 1.2Г1. and 0.12′. Known as the pitch and yaw trim, they were the angles to which the crew had to swivel the main engine’s nozzle so that, at the start of the burn, its thrust would act through the spacecraft’s centre of mass. Throughout the progress of the flight, the position of the centre of mass changed as propellants were used up, the lunar module departed and redocked, and as the tnoonrocks were transferred to the command module. All this shifting of mass was carefully accounted for by mission control so that the spacecraft’s flight characteristics would be known w’hen planning a manoeuvre. Once the engine was running, the spacecraft’s centre of mass would change as propellant was consumed. The G&N system could sense these shifts and would steer the nozzle as required in order to steady the spacecraft throughout a long burn.

The large nine-digit number – 078314591 – represented the lime of ignition, normally referred to as ‘Tig’, down to hundredths of a second as measured against the ground elapsed time (GET) clock. In this case, ignition w:as to occur at 78 hours. 31 minutes. 45.91 seconds after launch. Although GET was notionally measured from the moment of lift-off from Earth, in cases where the launch was slightly delayed the GET could be adjusted during the flight so that subsequent event times
would match their place in the flight plan – this was done, for example, on Apollo 14.

Change in velocity: delta-v and frames of reference

The main purpose of the LOI burn was to slow the spacecraft down by a desired amount. This change in velocity is really a vector quantity because its direction is as important as its magnitude. Known within the industry as delta-v, it is normally resolved into three components (v, у and z) which were given by the next three numbers in Henize’s list – minus 28975, minus 07764 and minus 00441. Such was the primitive nature of the Apollo computer that there was no provision in it for entering or displaying a decimal point. Instead, the position of the decimal point was fixed in the programming and on the form. Everyone associated with the machine knew where the decimal point was in any particular context. Here, 28975 simply meant that the burn was for a change in velocity along that axis (but, being negative, in the opposite sense to the direction of the axis) of 2,897.5 feet per second.

So what about these three axes? By now it should be clear that coordinate systems were, and are, ubiquitous in spaceflight, and become especially important when dealing with engine burns. The firing of an engine in space results in a change of velocity and it is necessary to define the direction of that change in relation to a frame of reference; a known set of Cartesian coordinates against which it can be plotted. We can use any frame of reference we like but it is customary to use one that makes the calculations easier, and the one that is generally favoured is called local verticalj local horizontal (LVLH).

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This frame of reference is constructed relative to a line drawn from the spacecraft to the centre of the body it is orbiting, or whose gravitational sphere of influence it has entered. Imagine a point where this line intersects the planet’s surface. We can further imagine a flat plane at this point parallel to the horizontal. Obviously, as the spacecraft moves around the planet, the absolute orientation of this plane keeps changing but it provides a useful reference for orbital velocity computation. In this arrangement, the — z axis points towards the planetary centre, the —л – axis is in the direction of orbital motion parallel to the local horizontal and the + v axis is perpendicular to the orbital plane.

With this, we can make more sense of the velocity components given in the PAD. The large negative figure for the. x component. 2.897.5 feet per second, meant that the burn was largely retrograde, against the spacecraft’s motion, which is exactly what would be expected, given that they were trying to lose speed. The figure for v. 776.4 feet per second, meant that the spacecraft was being pushed sideways as part of the process of ensuring that it ended up in the correct orbital plane for the landing site. The figure for z, -44.1 feet per second, was small in comparison and was away from the Moon’s centre. Converted to metric units, these velocities were expressed as -883.2, -236.6 and -13.4 metres per second.

“All zips for roll, all zips for pitch, all zips for yaw" Again we have to deal with frames of reference for these three numbers, all of which are zero. However, whereas delta-v used local vertical/local horizontal as described above, these numbers were given with respect to the guidance platform, itself aligned to our old friend, the current REFSMMAT. For every burn, mission control gave the crew a set of three angles that represented the attitude of the spacecraft in terms of roll, pitch and yaw directions and these were always stated with respect to the current RHFSMMAT. But since the platform’s orientation had been aligned to match the spacecraft’s calculated orientation for the burn – the so-called LOI RHFSMMAT – then the attitude angles for this burn were necessarily all zeros, or ‘zips’ as Ilenize put it. This arrangement meant that the FDAI (flight director attitude indicator) or ‘8-ball" in front of the crew showed zero in all three axes. This provided an easy means of monitoring the direction in which the spacecraft was pointing, in case the crew had to take manual control.