Category How Apollo Flew to the Moon

When Apollo was blazing its pioneer­ing trail to the Moon, the nascent space industry had yet to set up a comprehensive, worldwide communi­cations network using geostationary satellites and ground stations. It would take the efforts of another generation to arrange an infrastruc­ture that would allow crews to at least talk to mission control at any point in their orbit. Apollo crewmen could talk to mission control only for intervals of up to seven minutes at each ground station as they passed over a scattering of them along their orbital path. As with many aspects of Apollo, the exact configuration of these stations changed from mission to mission as operational experience was gained and priorities changed.

Early missions supplemented their coverage with extra ground sites. A scattering of specially equipped ships filled the gaps between the main sites.

Stations were sited on islands or on board ships strung across the Atlantic Ocean leading from Cape Canaveral to provide coverage for the ascent to orbit. A station on one of the Canary Islands off the coast of Africa permitted communications on the opposite side of the Atlantic, and another on Madagascar was used during the early missions for coverage heading out over the Indian Ocean. An outpost near Canberra in eastern Australia gave coverage on the opposite side of the world. An important station was set up on Hawaii, in the middle of the Pacific Ocean, which covered at least part of the spacecraft’s departure for the Moon. This was supplemented with ships and Apollo range instrumentation aircraft (ARIA) which filled in the gaps before a siring of stations across the continental United States gave constant coverage to the Atlantic. The ARIA were EC-135 jets – similar in structure to the Boeing 707 jetliner that were specifically equipped to support Apollo communications by relaying voice and recording telemetry.

During each short period of communication, data about the state of the crew and spacecraft were exchanged with updates from mission control. Another vital job for some of the ground stations at this time was to use large radar antennae to track the speed and position of the spacecraft as accurately as possible by reflection off its skin. This refined mission control’s knowledge of iis trajectory; information that was necessary to ensure an accurate burn towards the Moon. In particular, the station on the Canaries could provide an initial orbital determination and Carnarvon in Australia refined the determination antipodal to insertion.

When Neil Armstrong and Buzz Aldrin went outside the lunar module Eagle for their historic moonwalk, one of their tasks was to place a seismometer on the surface that would study moonquakes after they departed. However, the project to produce this instrument was conceived in a hurry. Its power came from two small panels of

The apparent brightness of astronomical objects is slated in magnitudes. A bright star is about magnitude 0. one at the limit of human eyesight is magnitude 6 while the faintest star visible with an Harth-based telescope is about magnitude 25

solar cells and, unfortunately, although it had small radioisotopic heaters, it was seriously damaged by the chill of its first lunar night. It was turned off during the next lunar day.

It fell to the next crew, from Apollo 12, to place on the Moon the first full science station, known as ALSEP, which included a seismometer that drew its power from a self-contained power unit. Subsequently, all missions that reached the Moon’s surface, with the exception of Apollo 17, emplaced seismometers to create a network of stations spread across the near side. From Apollo 13 onwards, all S-IVB stages were steered onto trajectories that led to a violent end, each forming a new crater on the Moon’s surface.

Flight controllers had two major sources of propulsion with which to control the trajectory of the spent S-IVB. The two APS modules had some leftover propellant, and there was still a small quantity of LOX that could be jettisoned through the J-2 engine nozzle under pressure from whatever heat was leaking into its tank. Minor additional thrust could be achieved by dumping the remaining hydrogen from the fuel tank and the helium gas from the pressurising system through two propulsive vents.

Control of the nearly dead stage was seldom very accurate and controllers never brought their rocket stage down on the Moon closer than 150 kilometres from the planned target. Nevertheless, they were able to track them accurately to their end and the impacts pro­vided lunar geologists with seismic events of known energies occurring more or less in known locations.

With each successful S-IVB impact sending lunar shockwaves to in­creasing numbers of seismometers, the quality of information that could be derived from the travel time of the sound waves improved.

The final network could provide triangulated readings from any im­pact, natural as well as those due to the S-IVBs and the discarded ascent stages of the lunar modules, yield­ing detailed information about the The lunar crater formed by the impact of Apollo Moon’s interior. 13’s S-IVB north of Mare Cognitum. (NASA)

The designers of the Apollo spacecraft were always careful to build redundancy into their systems to ensure that a single point failure could not put the crew in jeopardy a philosophy that extended to the guidance and navigation system. The designers were very aware that any of its exotic components could fail at any point in the mission. To this end. the command module had a second control system which, although it shared many components with the G&N system, could operate entirely independently. This stabilisation and control system (SCS) could maintain attitude and allow’ the crewr to make accurate manoeuvres and, if necessary, even manually control the SPS engine. Like the G&N system, it used gyroscopes, but these w:ere arranged in a different way to the gyroscopieally stabilised IMIJ.

The gyroscopes for the SCS were not attached to a stabilised platform. Instead, they were fixed to the spacecraft structure and therefore had to move with it. Being mounted in this way prompted the name body-mounted attitude gyros (BMAGs). Like all gyros, they had a tendency to want to remain in one attitude, and when the spacecraft rotated, they exerted a force on their mountings. As this force was a measure of the rate of rotation, the BMAGs were very suitable for measuring how fast the spacecraft was rotating rather than yielding absolute attitude. However, by processing the rate information within electronic boxes it was possible to derive the absolute attitude. The resultant values for attitude were highly prone to drift, much more so than those from the IMIJ, so it was important to regularly realign them to match. Prior to intensive use of the SCS. the crew would press a button to update the BMAGs’ electronics with the spacecraft’s attitude from the IMlJ’s platform. If the IMU ever became unusable, the crew had an emergency procedure whereby the attitude information from the BMAGs could be aligned by sighting on the stars.

One of the regular tasks for the CMP was the perfectly routine stirring of the service module’s tanks that contained the cryogenic oxygen and hydrogen reactants for the fuel cells. Each tank was essentially an efficient vacuum flask whose contents were best described as being a very dense fog rather than a liquid. As the gas was drawn off for the fuel cells or for the cabin air, the pressure in the tanks reduced slightly. If gas pressure falls but the volume stays the same, then according to the gas law that shows how pressure, volume and temperature are related, the temperature will also fall. Therefore, electrical heaters, w’hich could be switched on automatically or manually as required, were installed to help to maintain the tanks at their operating pressure.

Two long devices ran through the middle of each tank. One was a set of heating elements wrapped around a supporting tube. Two fans w’ere mounted, one at either end of the tube, to stir the tank’s contents. The other was a probe that determined the quantity of gas remaining in the tank. It consisted of a tube within a Lube and it measured the electrical characteristics across the gap between – a quantity known as capacitance. The capacitance of the probe depended on the density of the gas between the tubes, and this could be calibrated to infer how’ much gas was present in the tank. However, in the weightless environment of space, the gas tended to gather in layers of differing densities against the probe, which skewed the readings. This was where the fans came in. At regular intervals, they were switched on to stir the contents of the tanks in order to homogenise its density and allow’ an accurate reading. When EECOM Sy Liebergot asked Capcom Jack Lousma to ask CMP Jack Swigert on Apollo 13 to stir the tanks in Odyssey s service module almost 56 hours into the mission, the result became part of popular culture.

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.