Category How Apollo Flew to the Moon

The Block II Apollo spacecraft

The command module was a stubby, conical craft almost entirely covered with a hcaishield that was thickest across its base, or aft. to sustain most of the punishment

Preparing the Apollo spacecraft. Left, Apollo 13’s CSM Odyssey. Right, Eagle, the Apollo 11 LM.

of re-entry. The outer rim of the cone was packed with small tanks, thrusters, various antennae and two small ports for the ejection of waste water and urine. The apex of the cone had a removable probe mechanism to enable it to dock with the Apollo lander and a tunnel through which the crew could transfer between the two
spacecraft. Parachutes were carefully packed around the outside of this tunnel, along with other paraphernalia of the Harth landing system.

The main bulk of the CM’s volume was taken up by the pressure hull which accommodated three crew members and much of their electronic equipment. For lightness, the hull was constructed from two layers of aluminium sheet with an aluminium honeycomb in between. During launch and re-entry, the crew lay on couches with their backs to the aft of the spacecraft. Seating positions were defined by the roles of the crew. At the time of launch the commander (CDR) occupied the left couch for access to most of the flight instruments; the lunar module pilot (LMP) usually took the right couch and had responsibility for the spacecraft’s systems, as this was where many of the relevant switches and displays were located; and finally the command module pilot (CMP) generally had the middle scat, with his head next to the spacecraft hatch. For major manoeuvres in space, the CMP occupied the left seat. Directly in front of the crew and ranged around the entrance to the tunnel was the main display console – a vast panel of some 400 knobs, switches, meters and displays with which most of the flying of the spacecraft was achieved, in association with various hand controllers that sprouted from the armrests. Above the console, in the eye-lines of the commander and LMP. were two small forward­facing windows. Other panels, windows and compartments were arranged around the crew. In particular, at the CM P’s feet was the lower equipment bay which included all the gear he would need to navigate the spacecraft, a task for which he was responsible.

Attached to the rear of the CM was the cylindrical SM that supplied most of the consumables; electrical power, water, air and cooling. It also carried an array of parabolic antennae for deep-space communication, thrusters for attitude control (iattitude being a wrord in spaceflight that means a spacecraft’s orientation; i. e. which way up it is) and the spacecraft’s main means of propulsion, an ultra-reliable, service propulsion system (SPS) engine that protruded from the rear. This engine took the crew’ into orbit around the Moon and. when their exploration had been completed, sent them back towards Harth. Over 16 tonnes of propellant for this engine was stored in four large tanks that occupied most of the service module’s bulk. Smaller tanks of oxygen and hydrogen provided the reactants for three fuel cells where a little chemical magic provided the crew with not only electrical power but also water good enough to drink. The tanks also supplied breathing oxygen for the CM’s cabin until separation from the SM.

The Apollo programme became a juggernaut towards the end of 1968 as flights were launched every two or three months in the race to achieve Kennedy’s deadline. The C-mission of Apollo 7 gave the Block II spacecraft, without a lander, its first manned test. It began on 11 October 1968 with a launch by the smaller Saturn IB vehicle. Its crew of Wally Schirra, Donn Eisele and Walt Cunningham spent 11 days orbiting Earth, which was more than enough time for a mission to get to the Moon and back, and it proved the CSM to be a worthy, spacefaring ship.

As soon as the spacecraft achieved orbit, the crew separated it from the S-IVB second stage and attempted to practise the type of turnaround manoeuvre that would be required of future flights, when the lander would have to be plucked from inside its protective shroud. As soon as he saw the S-IVB, Schirra noticed that one of the four hinged petals of the shroud had not fully deployed. He cancelled a simulated approach manoeuvre for fear of it hitting the spacecraft, and later recommended that the panels be jettisoned instead.

Throughout the early part of the mission, the crew concentrated on achieving their most important goals: firing the main engine repeatedly to make rendezvous passes with the S-IVB (whereupon it was noted that the balky shroud petal had properly deployed) and proving the operation of their navigation system. With these tests satisfactorily performed, the crew spent their remaining time carrying out secondary tests of the Apollo system and completing a programme of Earth photography.

Despite the operational success of the mission, history tends to remember it for the breakdown in relations between the crew and flight controllers in mission control. A dose of the common cold made its normally wise-cracking commander increasingly grumpy. In weightlessness, the symptoms of cold are exacerbated by the inability of the congested head to drain itself. The other two crewmen, both rookies, were drawn into the soured atmosphere with the result that, having irritated management, neither man flew in space again. Schirra had already announced his retirement from space flight.

On launch day, final preparations began with the countdown clock at 13 hours before lift-off, or T minus 13 hours in the parlance of the rocket men at KSC. At this time the lower of the S-IC’s two massive tanks was filled with refined kerosene (RP – 1) propellant. Whereas the rest of the Saturn’s propellant tanks would carry volatile and super-cold hydrogen and oxygen that had to be loaded during the final hours prior to lift-off, the RP-1 could remain in its Lank for extended periods. The use of highly refined kerosene gave more predictable combustion properties.

Next, the Saturn’s cryogenic tanks were prepared to accept their loads. Both hydrogen and oxygen are gaseous at normal pressures and temperatures. To use them in a large rocket, their quantities had to be concentrated within their tanks and there are only two ways to achieve that: one way is to store them under very high pressure, but to achieve the required strength, their tanks would have been hopelessly heavy; and the other way is to reduce their temperature sufficiently to liquefy them. To be used in the Saturn V, oxygen needed to be at minus 18.1’C. becoming liquid oxygen (hereinafter referred to as LOX), while the very light hydrogen fuel had to be chilled to minus 253 C, only 20 degrees above absolute /его.

Use of cryogenic propellants in any rocket demands elaborate insulation but the effort is worth the benefit as liquid hydrogen is a very high-energy fuel. In particular, the tanks had to be carefully conditioned before their contents could be loaded. This required removal of every trace of water vapour from the tanks and. in the case of the hydrogen tanks, even air had to be purged to prevent the nitrogen in it from freezing and contaminating the fuel. To achieve this, dry nitrogen cleared out the S­IC’s LOX lank and increasingly cold helium gas was pumped through the tanks of the upper two stages.

Once the tanks had been purged of contaminants, the propellants could be loaded, but even that had to be carefully handled because, compared to the temperatures of the liquids that they w’ould receive, the walls of the cryogenic tanks were hot and the propellants boiled furiously when initially introduced. It was similar to pouring water into an empty saucepan that has been sitting on a flame for too long. To begin with, propellant was pumped in slowiy and allowed to boil, removing heat as it did so and further chilling the tank. Lvcnlually a pool of liquid settled at the bottom of the tank, at which point the ground crew began to pump in propellant at the maximum rate. When the tanks were nearly full, a slow fill rate was resumed to fill them completely and compensate for the ongoing boil-off, caused by the ambient Florida heat that was leaking into the tank. This state was maintained until several minutes before launch, when the venting valves were shut and the pressures inside the tanks were allowed to build to their operating values.

The first staging event: cutting up the ship

Theory had demonstrated early on that a rocket constructed using a series of modular stages would allow engineers to loft payloads into space with much greater efficiency than trying to use a single stage, at least with current technology. The Saturn V was therefore built as a three-stage vehicle. When each stage was exhausted, it was cut adrift to coast on alone to its fate while a fresh stage took up the task of getting the spacecraft into orbil and thereafter to the Moon. The moment when one stage Look over from another was called staging, and was always a complex and carefully choreographed event controlled by the instrument unit’s computer and sequencers. In the space of a few seconds, a nearly exhausted stage had to be shut down in a controlled manner, physically separated then retarded while the propulsion system of the fresh stage was ignited and brought up evenly to full thrust. The manner in which this was achieved depended on the specific staging event.

Before the first staging event could Lake place that is. w’hen the S-IC gave way to the S-II – the final manoeuvre of the tilt sequence had to be made. Throughout most of the S-IC’s burn, the Lilt sequence had been gently rotating the vehicle towards the horizontal. However, at staging, the Saturn vehicle would to all intents and purposes be cut in half. As it was undesirable to have two very large pieces of hardware in a state of rotation next to each other, the final manoeuvre of the tilt sequence was till arrest. This minimised the rotation of the stack so that the S-IC and the interstage ring between the two stages could depart without significant risk of cither of these disposed items coming into contact with the engine bells of the S-

II.

Once the rocket’s attitude had stabilised, sensors in the propellant tanks signalled to the Saturn V’s computer that depletion was near, at which point the four outboard engines were shut down and Timebase 3 began. The logic programmed into the sequencers ensured that this new timebase could only start

if Timebase 2 (activated at centre-engine shutdown) had already occurred to avoid the possibility of it being accidentally started on the ground, for it coordinated all the events relating to staging and the control of the S-II stage for the entire duration of its burn.

The separation of the S-IC and the S-II stages was technically known as a dual plane separation since the vehicle was to be cut around its girth in two places on either side of a ring called the interstage. This ring or skirt was a 5.5-metre section of the Saturn V’s skin which acted as a spacer between the first and second stages to accommodate the latter’s engines. It had been decided that the ring should not separate with the S-IC, lest any rotation between the stages caused it to damage the engines. On the other hand, its long-term presence was likely to cause overheating problems around the S-II engines. The solution was to separate the S­IC first, wait 30 seconds until the second stage was firing smoothly and rapidly accelerating, and then cut the skirt free. Failure of the skirt to separate was considered sufficiently serious to abort the mission, since to carry so much extra weight would have seriously degraded the stage’s performance. The skirt did fail to separate on the last Saturn V to fly – the unmanned launch of the Skylab orbital workshop to orbit in May 1973. Fortunately, the S-II tolerated the heat and the lower payload weight allowed it to reach orbit with a mere 2.5 per cent of its propellant remaining.

Cutting the skirt was not the only event involved in staging. Immediately after the first stage had shut down, two opposing sets of solid-fuel rockets ignited to pull the two halves of the Saturn apart. Up to eight forward-firing retro rockets were mounted in the conical fairings at the base of the first stage and a similar number of rearward-firing ullage rockets were fitted to the outside of the skirt. Ullage is a

brewer’s term for the space in a bottle or cask that is left unfilled. When applied to rocketry, it refers to the portion of a propellant tank’s volume that is filled with gas. When starting a liquid-fuelled rocket engine, it is usually unwise to allow gas from the tank to enter the engine pumps. However, when the first stage engines shut down, the rocket was temporarily in freefall and the liquid in the S-II’s propellant tanks was free to slosh around. Designers eliminated the problem of gas-filled voids entering the engines by using ullage rockets to settle the contents of the tanks prior to second-stage ignition. Simultaneously with the ignition of the solid rockets, an explosive cord at the top of the S-IC was detonated to cut the skin across the first separation plane and cast loose the dead weight of the useless stage from the rest of the rocket. For the final three heavily loaded Apollo missions, the Saturn V’s payload capability was improved by deleting of some of the retro rockets on the S-IC and all of the ullage rockets on the skirt. By this point, the conservative engineers under von Braun had gained enough experience to know what was really working and what added little to their creation’s performance.

One second after the first stage was jettisoned, the five J-2 engines of the S-II stage were commanded to start. The momentum of the empty first stage caused it to continue upwards in a ballistic arc for some time, then fall into the Atlantic Ocean some 650 kilometres from the launch site in an area of sea cleared of shipping. 29 seconds after S-II engine start (30 seconds after S-IC separation) – a period deemed sufficient for the smooth running of the second stage to be established – explosive cord cut the skin around the top of the interstage ring to allow it to fall away from the accelerating second stage.

Frank Borman was an all-business astronaut who, during a post-flight debriefing, summed up the violence of Apollo 8’s first staging event on his terms. The S-IC/S-II
separation was nominal; the crew was thrown forward in their seats, as you would expect in a staging. Then the g – load was shifted from four to about one. Consequently, you noticed the change in thrust quite distinctly. There was some indication of light flash at staging through the hatch window.”

Other crews were less reserved in their reaction, especially in the moments after the experience. ‘‘Man, that staging was quite a sequence!” exclaimed Tom Stafford, Apollo 10’s commander. Shortly afterwards, his LMP Eugene Cernan asked of mission control, “Charlie, are you sure we didn’t lose Snoopy [their lunar module] on that staging?” Charlie Duke replied “No, I think Snoopy is still there with you. You’re looking good.”

On Apollo 13, rookie Fred Haise was completely unprepared for the shock. “Man, I’ll tell you, that first stage. When that shut down, man, I thought I was going through the instrument panel. I was so surprised.” His commander Jim Lovell was already a veteran of the Saturn V. “I should have warned you,” he told his crewmate.

Ken Mattingly, on the other hand, had been warned. “I was well braced for it. I’m sure glad I was. That really gets your attention!” Sitting next to him was John Young, commander of Apollo 16, who had flown the Saturn V previously and knew from experience what to expect. He was supposed to be gripping a T-shaped abort handle which, by a simple twist counter-clockwise, would have brought the mission to an end. He was worried by the possibility of the event’s violence causing him to unintentionally operate the handle. “I was holding on to the bottom of the T-handle, at that point, because I sure didn’t want to do the wrong thing.”

Part of what made the first-stage separation so exciting was the launch vehicle’s entire structure being unloaded. Up to the point when the remaining four F-l engines were shut down, the 110-metre length of the Saturn had been slightly compressed by the 4-g acceleration they imparted. When their force was suddenly removed, the structure tried to bounce back to an unloaded state like a spring. Later Apollo crews were prepared for the shock, as Ed Mitchell on Apollo 14 noted: “When the outboards cut off, as we had previously been briefed and as previous crews had discussed, there was a sharp unloading. I
expected to be thrown against the instrument panel, and I had my hands out to brace against it. But it was not as much as I expected.’’

Apollo 17’s launch was distinctive by being the only night-time launch of a Saturn V. and its commander Eugene Cernan. who was also having his second ride on the big rocket, commented in his debrief on some effects at staging that he never saw on his first ride: “I don’t think it’s ever been recorded on a daylight launch before, but as soon as the S-IC shut down, the trailing flame of the S-IC overtook the spacecraft when we immediately went into that zero-g condition. And. for just a second, as the S-II lit off, we went through the flame. It was very obvious. We could see it out of both windows. 1 particularly could see it out of the left-hand window. It was not a smoke; it was not an orange fireball; it was just a bright yellow fire of the trailing flame of the S-IC; and it happened for just a split second.” In fact, the Apollo 15 commander David Scott did notice something similar when he commented to his crewmates soon after his mission’s daylight staging: "Okay. I got the big fireball going by at staging. I don’t know whether you saw it or not. That beauty really goes.”

Jack Schmitt’s recollection of the Apollo 17 staging echoed just about everyone else’s. "Just pushing 4 g on the thing and it quits just like that. I was prepared for it because Gene had said. ‘Hey. brace yourselves because it is going to happen.’ and it happened all right. It just fiat quit when we went from 4 g to zero.” Cernan then added a full stop to the crews’ experiences of the S-IC. “The great train wreck.”

The attitude adopted by a spacecraft when it orbits a planet is completely dependent on its mission. Observatories like the Hubble Space Telescope usually hold a fixed attitude with respect to the stars to allow optical systems to gather light from distant objects. Their attitude control is said to be inertial and, as a result, they continuously change the face they present to the world below as they go around in orbit. But the vast majority of satellites in orbit around Earth are required to aim cameras, antennae or other paraphernalia at the ground, and this is usually achieved by having the entire spacecraft slowly rotate so as to ensure the appropriate equipment can be brought to bear on their subject. This method of flying around a planet or moon with one side constantly facing the surface is known as orbital rate because it requires an orbital rate rotation to be set up. If it takes 90 minutes to orbit Earth, as is typical for a low Earth orbit, then by pitching down at a rotational speed that also takes 90 minutes per revolution, one side of the spacecraft can be made to face the ground at all times. Apollo often used orbital rate motion in Earth orbit and in lunar orbit because so many operations required a ground-based frame of reference. As well as pointing cameras and instruments at the surface, the attitude required for many manoeuvres depended on where the ground was with respect to the spacecraft and the crews felt it would be easier to monitor and control the bums if they did so with reference to the ground.

As it happened, the guidance system for Apollo was designed by a team from MIT who thought entirely in inertial tenns. As trying to monitor an orbital rate rotation was a little problematic for a spacecraft that had been designed to show inertial attitudes, a means had to be found to make the 8-ball rotate at orbital rate in sympathy with the spacecraft’s rotation so that it, too, would display attitude with respect to the ground. The solution was one that reflected the hurried nature of the programme. There was
insufficient time within Kennedy’s end-of-decade deadline to redesign the guidance system to implement such a feature. Instead, engineers added a workaround; a little box. given the acronym ORDEAL, that the crews had to install after they attained orbit. The acronym stood for orbital rate display, Earth and lunar, and its operation was simple. On the assumption that an orbital rate attitude was simply a progressive pitch – down motion (which it w’ould be if the orientation of the S-IVB was as planned), the ORDEAL supplied a calibrated drive signal that caused the 8-ball to pitch at the same speed. With this box properly set up. the crews could read off their attitude with respect to the ground. For later flights, they learned how to use the ORDEAL to monitor their attitude during the 1 LI burn, which was also carried out in an orbital rate attitude. If the burn could be accurately monitored, the commander could take control of the stack in the event of a failure of the S-IVB guidance system, thereby further increasing the redundancy of the entire Apollo system.

Guidance and navigation underpinned much of the challenge of Apollo. Indeed, it is crucial to any form of spaceflight and, consequently, major systems on board the command module and lunar module were devoted to it, as were a large number of consoles in the mission operations control room (MOCR, pronounced to rhyme with ’poker’). The front row of the MOCR, known to its occupants as The Trench’, was where the flight dynamics guys sat, and on a single shift two flight controllers, GNC (guidance, navigation and control) for the command module and Control for the lunar module, kept a close watch on the hardware with which guidance and navigation was performed. A further three – the guidance officer (Guido), the flight dynamics officer (FIDO) and the retrofire officer (Retro) – thought about nothing other than where the spacecraft was, where it was going and how it could return to

W. D. Woods, How Apollo Flew to the Moon, Springer Praxis Books,

DOI 10.1007/978-1-4419-7179-1 6. © Springer Science+Business Media. LLC 2011

Earth if something went wrong, respectively. As this is a necessarily complex topic that is not always amenable to verbal description, and one that requires a certain amount of three-dimensional visualisation, it is worth taking some time out to discuss the problem and consider how it might be solved.

Guidance is how to make a spacecraft go where intended. Given its constraints in propellant and payload, what path does a spacecraft need to take to get from one place to another?

Navigation is how to determine where a spacecraft is. How can its position and velocity be determined at any given time as it passes between worlds?

Control is the operation of the hardware to ensure that a spacecraft reaches its intended destination. Contrary to a misconception raised by the entertainment industry, spacecraft do not fly around the cosmos with their engines ablaze; with the exception of recent craft whose ion engines have thrust levels measured in millinewtons. Instead, everything in space, be it spacecraft, our Earth or the entire galaxy, generally coasts along following ballistic paths, moving in freefall with their motion determined by gravity. A major step towards guiding a spacecraft is to understand how those paths work, and how they might be measured, predicted and then controlled.

At its simplest, Apollo’s path to the Moon can be likened to throwing a stone. If you throw a stone almost vertically, it follows a sharply curved path which travels

most slowly when it is near the top of that path. Apollo’s trajectory was directly comparable. After the spacecraft had entered Earth orbit, it was ‘thrown’ towards the Moon by the S-IVB stage on an extended, curved trajectory away from Earth, governed by the same natural laws that took your stone along its ballistic arc; one that, in the absence of intervention, would reach a slow peak before falling back to Earth.

The trick for lunar flight was to work out the time and duration of the S-IVB’s throw that would cause the Moon’s gravity to intervene on the spacecraft’s path in a beneficial manner. By careful calculation, NASA’s trajectory experts picked the correct place and time, and just the right amount of shove from the S-IVB’s engine, to set up a rendezvous between the spacecraft and the Moon. For the three days during which Apollo coasted on its extended ellipse out from Earth, the Moon moved a quarter of a million kilometres along its own orbit (about one-tenth of a complete revolution).

Had the technology of spaceflight produced some fabulous ship with unlimited propellant and great power, the trip could have been made much more quickly by having the crew fire their engines all the way there and back, using brute force to expedite the journey. Unfortunately, no space faring nation has yet had that luxury as even ion engines must operate for weeks or months for their low thrust to have an appreciable effect. Lifting propellant off Earth’s surface and into space is an extremely expensive proposition. Having paid dearly to get it there, it must be used in the most efficient manner possible in order to avoid carrying any more propellant than is needed for the task. It was this type of thinking that allowed the Apollo missions to be accomplished with a single Saturn V in the first place. Until the advent of low-thrust ion engines that operate for months on end, most space flight was

conducted with chemical rockets that used short engine burns to modify long coasting nights.

It is helpful to think of the problem in terms of our stone-throwing analogy. Imagine that someone kicks a football high across a field and that you throw your stone from underneath, at the right time with the right force to hit the ball. This would be analogous to a spacecraft being sent to the Moon and impacting its surface – not something Apollo would want to do. but a fate that deliberately befell the Ranger probes in the early-to-mid-1960s. But if the stone had been travelling a little more slowly, the football would fly across its path early, leaving the stone to coast past the ball’s trailing side. If the stone had been moving a little faster, it would reach the football’s flight path before the football arrived, and would therefore pass in front of it. finally, if there was a way to control the timing and force of the throw accurately, and to monitor the stone’s flight path very carefully and change it if required, then the stone could be made to pass the leading side of the football by any number of millimetres we wished.

Getting an Apollo spacecraft to the Moon was a very similar exercise except that a football has no appreciable gravity and the Moon docs, and this had to be taken into account. If the trajectory produced by the ТІЛ burn was correct, the spacecraft would be pulled around the lunar far side, miss the Moon by about 100 kilometres, and be slung in the direction of Earth in an approximation of the intrinsically safe free-return trajectory. Midway around the far side was when the spacecraft had to intervene by firing its SPS engine. This would be done near the point of closest approach, against the direction of travel in order to slow’ the spacecraft and enter lunar orbit. But what would happen if the Moon-bound trajectory was not precisely right? If the spacecraft were moving towards the Moon too quickly, it would reach the Moon’s distance too early and pass over the surface at a higher altitude. A failure of the SPS engine would leave it to sail on into deep space, never to return, at least not with its crew still alive. At the other end of the error scale, an ever-so-slightly slow coast would take them closer to the Moon, creating a very real risk of the spacecraft impacting the lunar surface as it swung around the far side.

It was obviously critical to the lives of the crew1 that the spacecraft be placed onto the ideal Moon-bound trajectory. It was equally important to determine whether or not that trajectory was being followed and if not, to do something about it. One of the inherent problems in ballistic flight is that tiny perturbations early in a trajectory have large effects when propagated forward to a destination. Even an apparently perfect trajectory from the ТІЛ burn contained errors, initially too small to measure, but whose effects became apparent through time. Additionally, ground controllers had to understand the many factors that could interfere with Apollo’s trajectory, the most significant of which was the size and direction of the push given to the spacecraft by the S-IVB at translunar injection. Despite being well-engineered and controlled, this rocket stage, like any rocket in existence, was unlikely to deliver a perfect burn. There was always some small deviation from the ideal that would later have to be compensated for. Additionally, as the spacecraft coasted to its destination, housekeeping manoeuvres carried out by the crew using the RCS thrusters tended to affect the trajectory. Also, the expulsion of any liquids or gases

by the crew as part of their daily operation generated tiny thrust forces. Water vapour from the spacecraft’s cooling system, waste water from the fuel cells as they generated electricity, and urine from the crew were all necessary emissions that generated sufficient thrust to deflect the trajectory. To compensate for all these compounded perturbations, the crew had to make small correction burns. However, they had to know how much correction to make, and to do this it was necessary to measure their trajectory with extreme accuracy.

THREE MEN IN A SUBMARINE

A large part of the Apollo journey was spent in coasting flight; a period of time, usually somewhere between the Moon and Earth or in orbit around the Moon, when the three crewmen waited to reach a destination or when the command module pilot was waiting for his two crewmates to return from their exploration of the lunar surface. Although this part of the flight held little interest for the news media, NASA made sure there was plenty to keep its crews occupied. Exotic conditions like the command module in deep space had cost the taxpayer dearly and did not occur often, unlike the continuous time in weightlessness offered by later space stations which stayed in low Earth orbit. Being in deep space exposed crews to an environment beyond the shielding effects of Earth’s magnetic field. As a result, mission planners, managers and controllers very rarely allowed them to relax during a flight. This was particularly true of later flights, when the business of just keeping the spacecraft running had become somewhat routine.

The openness with which NASA conducted its primary objective, whereby it allowed unprecedented access by the media to most of what it did, demanded that its costly missions should at least appear to extract as much as possible from every minute of the flight even beyond the goal of reaching the Moon. If the crew were not busy dealing with the upkeep of their mini-planet or of their own bodies, they would find themselves involved in a series of scientific experiments, out-the-window observations, television broadcasts, changes to the flight plan or the execution of carefully calculated adjustments to their trajectory.

Apollo used two radio-frequency ranges for communications: VIIF and S-band. Originally. NASA had intended to implement the radio systems that they already had available to fulfil the disparate requirements of voice, data, television, as well as the need to track the spacecraft out to the Moon. But it soon became clear that this would involve the installation of multiple items of hardware, with severe weight penalties, and so, as far as possible, the engineers strove to integrate these requirements into a single system. The result was the Unified S-band or USB system.

USB used frequencies above 2 GIIz which were well suited to long-distance operation, but their highly directional nature made the USB system less suitable during the final stages of re-entry, or when the crew were talking between vehicles at the Moon. For this, line-of-sight VHF frequencies were used.

The antennae to support these radio systems were arranged all around both spacecraft. For the most part, the CSM hid its antennae within the smooth lines of its hull in order to preserve its streamlined shape for the ascent through the atmosphere or the later re-entry. In comparison, the LM appeared to bristle with various dishes, helixes and rods as function overcame form on a ship that needed no streamlining.

The most prominent component on the CSM was an array of four dishes known as the high-gain antenna (HGA). To preserve the spacecraft’s aerodynamics, it was

mounted at the service module’s base and folded next to the engine bell until the SLA panels were jettisoned. It was then brought out to the side on the end of an arm. Its mounting was articulated, and could swivel under automatic or manual control to aim at Earth. Like an adjustable torch, the array itself could be electronically configured in three ways: wide beam, medium beam and narrow beam. Each focused the antenna’s pattern into tighter beams to concentrate its ability to send and receive the USB radio signal over the large distances to which the spacecraft travelled.

The HGA had four receiver horns at its centre. To keep lock on Earth in narrow – beam mode, the receiver electronics compared the signal strength coming from each horn and sent steering signals to the antenna’s motors as appropriate to equalise the strength across all four. While Apollo 12 orbited the Moon, this system failed and as they departed for the Moon in Intrepid, Conrad and Bean were able to watch what the antenna was doing.

“Houston. This is Intrepidcalled Conrad. “If it would be any help to you, Yankee Clipper’s S-band antenna is just wandering – it’s just oscillating back and forth in two directions, like it can’t hold lock.”

“It looks like it’s in some sort of continual search mode,” added Bean.

Mission control suspected that the fault was affected by heat and to localise the cause, they arranged a test during the coast back to Earth. The CSM was held in an attitude that pointed the SPS engine at the Sun, thereby applying solar warmth to the HGA for an extended period of time. Various modes of operation were used and the problem was narrowed down to a set of microwave electronics in the antenna.

This method of aiming an antenna is now considered obsolete in that to point at its target the beam must be slightly off-axis – i. e. be imperfectly aimed. The preferred method is to use a spacecraft’s computer and its guidance system to know where to aim the antenna. Since the computer always knows the spacecraft’s attitude and it can work out where its target, e. g. Earth, is, then it can directly drive the antenna to a very precise angle that maximises the antenna’s performance.

As the HGA was mounted to one side of the spacecraft, it could only be used when that side was facing towards Earth. For other occasions, four omnidirectional antennae were flush-mounted around the periphery of the command module. As the spacecraft rotated, these could be switched into use as necessary to ensure constant communications with the ground. Being omnidirectional, their pattern of reception and transmission was unfocused; their usefulness was limited because they could not carry large amounts of data. As a result, the use of the communications system for high-bandwidth data had to be carefully choreographed with the attitudes adopted

by the spacecraft for its various tasks. If the flight controllers wanted a detailed look at the spacecraft’s systems, then the telemetry had to carry much more digital data. From near-lunar distances, this could only be handled by the I IGA. Similarly, the high-bandwidth demands of television required this dish array when in the vicinity of the Moon. There were occasions, however, when tests were performed using only the omnidirectional antennae to send TV or high-bandwidth data to the large dishes of the Manned Space Flight Network. For VHF communication, the CSM sported a pair of scimitar antennae housed in semicircular mouldings on each side of the service module. These were primarily for communication with the LM.

The LM had a single dish antenna for high-bandwidth communications to Earth and a pair of omnidirectional antennae mounted fore and aft. It also sprouted three VIIF antennae: two mounted fore and aft for communication with the CSM and one that would be raised from the roof in order to link the two crewmen walking outside on the lunar surface, both with each other and with mission control. It had two other antennae, but these were for radar rather than communications.

The CSM was a streamlined and sturdy craft, designed to ascend through the atmosphere of Earth, and. in the case of the command module, withstand a punishing re-entry. In contrast, the lunar lander was a true spacecraft because it was entirely incapable of flight in an atmosphere.

Knowm as the lunar module (LM). its construction w:as entrusted to the Grumman Aircraft Engineering Corporation. This was a truly exotic ship in which every aspect of its major systems pushed the know-how7 of the engineers who designed it. When originally conceived, it was called the lunar excursion module and therefore received

the acronym LEM. However, in 1965 managers decided that the use of excursion was too flippant as it suggested that the crews were going on a vacation. The name was shortened to lunar module but the pronunciation as ‘lem’ stuck.

The LM needed to be sturdy enough to withstand the acceleration and vibration of a launch from Earth and the shock from a rough landing on the lunar surface. Yet it also had to be as light as could humanly be achieved in order not to outweigh the ability of both the Saturn V and the CSM to deliver it to lunar orbit. Its largest engine had to be throttleable to provide adequate control of the astronauts’ descent to the lunar surface without the aid of wings or runways. Its flight path was controlled by two small computers in an age when such machines tended to occupy entire floors of buildings. Its engines had to be utterly reliable, even though the propellant systems operated at extreme pressures.

Prior to Apollo no one had dealt with the realities of designing a lunar module, which meant that Grumman could start with a clean sheet. Even before they won the LM contract, their engineers had produced preliminary designs. They then worked through several iterations before settling on the final spacecraft. The need for the LM to be a two-part ship was a corollary of the LOR concept. It operated as one vehicle until the moment of departure from the lunar surface. Less fundamental aspects of the LM, like the number of legs and the seating arrangements, required some extra thought. Three legs would have been the lightest arrangement and most adaptable to an undulating terrain, but a failure of any leg would be bad news. Five legs provided excellent stability and safety but the layout conflicted with the arrangement of the tanks for the propellant, and would have necessitated more structure and more mass. Four legs proved to be a suitable compromise.

The lower or descent stage was a cross-frame that carried an engine in its centre

surrounded by four propellant tanks. At each end of the cross-frame, a landing leg was mounted, one of which included a ladder. The bays of the frame between the landing gear were used as stores for the equipment the crews would need when their roles changed from that of spacecraft pilots to lunar explorers, and, on later flights, would provide somewhere to carry a fold-up electric car.

The upper stage of the LM was the crew quarters. Since it would lift the crew off the Moon, it was known as the ascent stage. A pair of propellant tanks protruded like cheeks on either side of a horizontally mounted cylindrical pressure hull, and a small rocket engine was set in the centre of the stage. Early designs for the cockpit included seats and large, high-visibility windows, as in a helicopter. In spacecraft design, there is a tendency for the mass of a spacecraft to rise as engineers go from initial concepts and estimates to final hardware. The Apollo LM could not afford such increases and the constant pressure to minimise the spacecraft’s mass continued even after its first successful mission. Engineers conceived the innovative idea of removing the seats because they realised that a crewman’s legs would make excellent shock absorbers for the low g-forces encountered during descent. Also, in the low gravity of the Moon, standing would be effortless. This change had a profound effect on the layout of the ascent stage. Had the crew been seated, their heads would have been placed well away from the windows and this would have

resulted in huge areas of heavy glass to give an adequate field of view. A better solution was to have the two crewmen stand close to the front wall of the spacecraft, where they could look out of two small downward-tilted triangular windows from which they could see an approaching landing site and steer towards it. This arrangement saved a large amount of mass. Major electronics systems were placed to the rear to balance the crew, four sets of thruster packages were placed at each comer for attitude control, and a collection of antennae were mounted on the roof, where function dictated. The result was a remarkable manned spacecraft that was perhaps aesthetically ugly, yet whose form was well matched to the function it had to perfonn.

Even before Apollo 7 was launched, managers were dreaming up something special for Apollo 8: an audacious six-day flight to the Moon in a hastily arranged mission which turned an otherwise unfavourable set of circumstances into a blessing.

Apollo 8 had originally been planned as the D-mission, a test of the entire Apollo system including a lunar module in low Earth orbit, on the assumption that Apollo 7 would successfully carry out the C-mission. However, the first man-capable LM was not ready for flight owing to a litany of problems: stress fractures had appeared in some of its structural components; the type of wiring used on the intended spacecraft

was prone to breakage; and the engine for the ascent stage was prone to combustion instability. Bereft of a LM, managers were unwilling simply to repeat Apollo 7, so they altered the mission sequence and brought the deep-space goals of the E-mission forward, but without a lander.

Furthermore, they took the gutsy decision to send the CSM all the way to the Moon and place it into lunar orbit. Although this would fulfil some of the goals of the E-mission (deep-space tracking, deep-space thermal control, lunar navigation), the fact that it would be a CSM-only flight prompted NASA to label it the C-prime – mission. Although it would provide operational experience needed to manage lunar missions, its unstated purpose was to reach the vicinity of the Moon before the Soviet Union. Intelligence reports suggested that the Soviets were preparing to send a crew on a flight that would loop around the back of the Moon and head straight
back to Earth, and the propaganda value of such a circumlunar mission would be immense. On the other hand, if the Americans could get there first, and enter orbit around the Moon, they could claim to have essentially won the space race as long as the Soviets did not achieve a landing.

On the morning of 21 December 1968 Frank Borman, Bill Anders and Jim Lovell rode a Saturn V away from Earth to become the first people to swap the Earth’s gravitational hold for that of another world. The three-day long coast out to the Moon gave Jim Lovell plenty of time to practise monitoring the ship’s trajectory by taking sightings of Earth, the Moon and the stars. On 24 December 1968, Apollo 8 took its crew around the lunar far side where they fired its SPS engine to enter lunar orbit to begin 10 revolutions, each lasting two hours. As they coasted 110 kilometres above the cratered surface, the crew closely examined two sites that were being considered for the first landing and, along with tracking stations on Earth, practised techniques for navigating while orbiting the Moon. Much of Earth’s population with access to television watched with amazement when the crew made an extraordinary Christmas-time black-and-white television broadcast made on the penultimate orbit, during which they read the first few verses from the Bible’s Book of Genesis while the stark early morning landscape of the Moon passed in front of the camera.

If their burn to enter lunar orbit had failed, the crew would have simply slingshot around the Moon and returned to Earth with little intervention – just as the Soviets intended to do – but the burn had been performed successfully and the spacecraft had entered orbit. It was Apollo 8’s next manoeuvre that really scared the managers. Although the SPS engine had been designed for reliability, everyone was aware that its failure would doom the crew to stay forever in the Moon’s grasp. Worse, because the engine bum would take place around the Moon’s far side, no one on Earth would be able to monitor its progress, and instead would have to wait until the spacecraft re-emerged, hopefully on a path for home. Shortly after midnight in Houston, Texas, on Christmas Day, Apollo 8 reappeared around the Moon’s eastern limb exactly on time, with Jim Lovell’s playful words to mission control, "Please be informed, there is a Santa Claus.”

Apollo 8’s voyage to the Moon raised the morale of the many thousands who were working brutally long hours to achieve the landing goal, and by allowing navigation, thermal control and communication procedures to be tested it gave NASA the operational experience it needed to make future lunar trips. On a philosophical level, the flight gave the human race its first glimpse of its home planet as seen from another world. In addition to television views of Earth from a vantage

point between the two worlds, while orbiting the Moon the awed astronauts photographed their home planet rising over a barren lunar horizon. These photographs would later become a catalyst for the rise of the environmental movement and were true icons of the age.