Category How Apollo Flew to the Moon

Timing their tasks up to and beyond re-entry was important to help the crew’ to coordinate their progress through an increasingly busy checklist. To this end, they set their digital event timer to count up to, and beyond the moment of entry interface.

Their next task was to prepare the EMS by setting it to the starting conditions for re-entry, as read up in the PAD. The CMP moved the scroll to the start of the relevant monitor pattern, aligning the scribe to the velocity that would be expected when 0.05-g was sensed. The total distance for re-entry was entered into the digital display at the bottom of the panel and the dial of the roll indicator was aligned to ensure that its reference matched the GDC and that it would accurately show whether they were orientated feet-up or feet-down.

Then attitude control was transferred to the CM and the thrusters on both rings of the RCS were operated for a briefly to verify their operation. Once complete, attitude control was returned to the SM thrusters.

With just 25 minutes remaining to entry interface, the crew began to prepare the service module for jettison. A valve was closed to cut off the supply of oxygen from the SM tanks which left the CM reliant on the contents of a small tank mounted in its periphery. One of the three fuel cells was shut down to force the batteries to take more of the load and so help to warm them up. ‘I’he service module’s radio systems were switched off. Circuit breakers were pulled to remove power from a number of heaters that kept the radiators, the dump nozzles and the potable water tank warm. Once all these small tasks had been completed, a check was made to ensure that the spacecraft was still in the correct attitude.

Use of the computer was quite intensive during re-entry, especially since, if all was working wrell. it would be the computer that would fly the spacecraft all the way to deployment of the parachutes. This began with Program 61 (P61) which started re­entry navigation by measuring the acceleration acting on the spacecraft. Ii also accepted relevant information from the PAD to allow subsequent programs to control the re-entry. This included their planned impact latitude and longitude and whether they would be entering heads up or down, information that went into Noun 61; their maximum deceleration, their velocity and flight path angle at entry interface went into Noun 60. They then checked the contents of Noun 63, which held their range from the 0.05-g point to the landing site, their velocity at the 0.05-у event and the total duration of re-entry.

When they were happy with the numbers to this point, they pressed ‘Proceed’ and the computer moved to P62 which handled the jettisoning of the service module and placed the command module in the correct attitude for re-eniry. First the CMP had to carry out a horizon check at 17 minutes to go. All he had to do was look out to see where Earth’s horizon appeared with respect to a series of angle markings on the edge of the rendezvous window. It was expected that it would be coincident with a line draw’ll at the 31.7-degree mark. However, because the line of sight was dependent on exactly where the CMP placed his head, a tolerance of 5 degrees was allowed. If the horizon was outside these limits, the rules said that they should assume the G&N to be faulty and steer the ship manually.

Apollo 12 CMP Dick Gordon couldn’t even see the horizon, as he related after the mission: "It was dark and I never was certain that there was a horizon out there.” he told his debriefers. He was not concerned as they had already made so many attitude checks. "It really didn’t make any difference. We had already checked the alignment. We were satisfied with the IMU. We had a boresight star. We had a sextant star check. We knew where we were, and the DAP [digital autopilot] was working properly. We were confident the w:hole time, and I didn’t care whether I made that check or not.”

His commander Pete Conrad pointed out the inconsistency in this approach. ‘We ought to change the rule, because we actually violated the rule."

"Well, we actually picked up the horizon check later on during the entry.” pointed out Gordon. In fact, there was no real necessity for the check to be done 17 minutes prior to entry as the checklist included a graph. "You’ve got the chart of the Harth horizon angles versus time from entry interface and you can check that any time prior to entry interface. It’s a nothing cheek, and you can either do it or not do it. I couldn’t care less."

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.

Most of the preparation of the CSM and LM was completed while the mobile service structure was still around the space vehicle. All the food and equipment in the two craft had been packed into their designated storage spaces, and their propulsion tanks filled with storable propellant long before loading of the Saturn V began, ensuring that, on the day of launch, only a few final tasks remained to be completed by hand. Throughout the countdown, and for much of the journey to the Moon, the LM was without power and inert, saving its precious batteries for its foray to the lunar surface. The CSM, on the other hand, was a buzzing, vibrant machine wrhose health was monitored closely by flight controllers and contractors throughout the countdown in case a problem occurred.

While on the pad. the CSM was pow’ered by electricity supplied from the ground. For flight, power came from fuel cells that made eleeiricity by reacting cryogenic

hydrogen and oxygen from tanks in the service module. Two days prior to launch, these tanks were filled with reactants and their contents checked for contamination before they were permitted to enter the fuel cells. Rechargeable chemical batteries augmented the spacecraft’s power requirements in space, and these were fully charged as part of the launch preparations.

One member of the backup crew, usually the backup command module pilot, entered the command module prior to the crew’s arrival and ran through an extensive checklist to ensure that every switch, knob and indicator was correctly set for launch. There were hundreds of these, each taking a line of the checklist. In the later Apollo flights, there were over 450 lines to be checked before the prime crew arrived. With that done, the backup crewman usually waited for the crew to arrive
before he retired to the launch control centre to con­tinue working with them by voice up to the point of launch.

The reaction between hydrogen and oxygen is one of the most powerful sources of rocket thrust there is, with the exception of some highly exotic combinations that include fluorine as an oxidiser. Engineers are always keen to use it where possible owing to its high efficiency, its relatively benign properties and the clean exhaust it leaves behind – superheated steam. On the Saturn V, both the second and third stages used these two propellants in J-2 engines to pick up where the S-IC left off, achieve most of the speed required for Earth orbit and boost the Apollo spacecraft towards the Moon. Five engines were mounted on the S-II and a single example powered the S-IVB.

Outwardly, the J-2 might seem like a smaller version of the F-l engine but beyond having a chamber and nozzle fabricated from fuel-warming pipes, there were few similarities between these motors. In particular, because the temperature of the fuel was barely above absolute zero, the J-2 had to cope with two cryogenic propellants. Prior to start-up, and to prevent the propellants turning to gas as soon as they entered the engine block, the J-2 was pre-chilled by feeding a small amount of propellant through its components.

Two entirely separate Lurbopumps. driven from a single supply of hot gas, forced liquid hydrogen fuel (henceforth called LIF) and LOX into the combustion chamber. The high-pressure llows coming from both pumps were fed via control valves to the injector, where they entered the combustion chamber. The injector was very different from the drilled metal plate in the F-l engine. It consisted of a stainless steel mesh arrangement through which over 600 Lubcs-within-tubcs passed. These

carried LOX through their central passage and LH2 through the outer. Some fuel was diverted to cool the mesh.

An important component of the engine was a valve that could reduce the flow of LOX to the injector. This operation altered the thrust a little, but the point of having it was to allow the consumption of LOX and LH2 to be balanced over the length of the bum in order to ensure that the propellants were used equitably. Another important component was the spherical start tank – which was actually a tank within a tank. The inner tank held helium, whose pressure operated the engine’s valves. The outer tank held LH2 which would be released at engine start to spin the turbopumps before hot gas from a gas generator took up the task. In the restartable version of the J-2 for the third stage, this outer tank could be refilled with LH2 in preparation for its second burn – the burn that would leave Earth orbit and head for the Moon.

The starting sequence for the J-2 engine was equally as complex as that for the F-l and only a summary is within the scope of this book. The basic ignition source was the augmented spark igniter, NASA terminology for what was basically a spark plug which lit a flame source in the combustion chamber and which operated continuously throughout the bum. The main fuel valve was opened and LH2, pushed only by the fuel tank’s pressure, began to flow around the pipes that formed the engine’s walls, conditioning them to the fuel’s extreme chill. After a short delay,

Diagram of the J-2 engine. (NASA)

the hydrogen in the start tank was discharged through the turbines of the two turbopumps to make them spin. The gas generator then began to burn LH2 and LOX to produce hot gas that would continue to power the pumps. The LOX valve was then opened which allowed LOX to begin to burn in the combustion chamber with the LH2 that had been circulating through the chamber walls. As the turbopumps spun up to full speed, this valve was slowly opened to bring the engine gently up to its rated thrust.

Flying to the Moon when you don’t have a lot of propellant to hose around is like a stone throw – a ballistic lob across 400,000 kilometres of space between two worlds. The impulse for this ‘throw’ came from the S-IVB stage of the Saturn V which added an additional three kilometres per second to their speed with a burn that was nearly six minutes long. As this translunar injection burn progressed, it modified the spacecraft’s circular orbit into an increasingly long, stretched elliptical orbit whose apogee reached further and further into space. By the time the S-I VB had shut down, it had set the Apollo spacecraft on an elongated orbit around Earth that had a perigee of only 170 kilometres, but whose apogee would have taken it to an altitude of over half a million kilometres except for the intervention of the Moon!

The precise details of Apollo’s throw to the Moon, its duration, direction and timing, depended on a collection of constraints. These were often contradictory, but they narrowed the possible options for the S-IVB‘s burn to a unique but very useful trajectory. One constraint was propellant, which was a very expensive commodity by virtue of the fact that it had to be lifted off Earth’s surface. Consequently, planners tended to prefer trajectories that did not demand long burns of rocket engines. As a result, flying to the Moon was not going to be a quick affair. In modern Limes, means have been found of reaching the Moon that require very little propellant, but these result in complex trajectories that can take weeks or months to complete. The consumables carried by the Apollo spacecraft would never last long enough, and the physical endurance of the crew in the confined space would be sorely tested. For a manned mission, there comes a point where the advantages of reduced propellant requirements become more than matched by the increases in food, power and radiation shielding required by the crew.

Another constraint was the landing site. When Apollo reached the Moon and inserted itself into lunar orbit, that orbit had to pass over the landing site. Therefore, to save propellant, their Moonward trajectory had to be shaped to reach the best position near the Moon from which to achieve this orbit, and do so travelling in the same direction as the orbit. Planners also had to consider the lighting conditions at the time of landing and the thermal conditions on the Moon’s surface. By landing in the lunar morning, crew’s could benefit both from the low-angle sunlight which made the surface topography clear, and from the benign thermal environment that exists between the chill of the night and the heat of the day.

The choice of burn was further constrained by crew’ safety considerations. This was paramount in the minds of planners in view of Kennedy’s stipulation that a crew be returned safely to Earth. Any option that maximised NASA’s ability to bring an endangered crew home was eagerly adopted. The engineering mantra was: if there is a problem with which you cannot deal directly, then do as little as possible lest you make things w’orse. What the planners really wanted w’as an option that would still allow the crew’s safe return even if the spacecraft’s main propulsion system had completely failed. Fortuitously, a suitable option existed – the frec-return trajectory.

In order to describe a flight path in space, the trajectory experts simply need to know two things: where the spacecraft is and how fast it is going at any particular time in the flight. But before they can express these two concepts they must have some frame of reference against which to measure them.

If I were throwing a stone across my back garden and wished to define its path – assuming I had access to the necessary measuring equipment – I might be able to state that one second into its flight, the stone was four metres from my neighbour’s fence behind me, three metres above the lawn and two metres from my house wall. For the same moment in time, I could also analyse the stone’s speed, stating how fast it was moving away from the fence, its speed away from or towards the lawn, and how fast it was moving with respect to my house. In total, for that moment in time, I would have six numbers that would not only define the stone’s position and speed in three dimensions, but could also be plugged into Newton’s laws of motion to predict the stone’s continuing journey.

Describing a spacecraft’s trajectory is exactly analogous. Its position and speed at a given time arc expressed in three dimensions with respect to some reference or sense of which way is up. Position is expressed as three coordinates; each plotted along the X, у and jr axes of the current reference. Likewise, speed is resolved to three velocities, a definition of speed where the direction of motion is taken into account; and again, they are plotted along the. v, у and г axes. This set of six numbers is collectively known as the state vector. Computers can use the state vector as a starting point in their calculations to extrapolate the flight path forward and, by allowing for the gravitational fields of any bodies in the solar system that would significantly influence the spacecraft’s motion in space, predict where it will be at any time in the future. Since the lives of the Apollo crews depended upon the accuracy of a spacecraft’s state vector, a lot of effort was expended in refining it.

In the early days of Apollo, around the Lime it became a mission to the Moon as opposed to being a generic, advanced spacecraft whose role had not been defined, managers expected that the state vector would be determined solely by the crew. Apollo had become a part of the Cold War, a grab for prestige by the United States at a time when they and the Soviet Union stared at each other, each with nuclear weapons in hand, waiting for the other to blink. There were serious concerns that the Soviets might try to interfere with the Apollo flights, perhaps by jamming radio transmissions, therefore it was decided that the guidance and navigation system should be completely autonomous. Once dispatched to the Moon, the crew should be able to navigate, conduct their mission, and return home entirely without assistance from the ground. This philosophy drove the design of the spacecraft’s guidance system by the Instrumentation Laboratory of the Massachusetts Institute of Technology. However, by the time Apollo w? as ready to fly. a lot had changed.

In the mid-1960s, NASA had begun to send probes to the Moon that were either deliberately crash-landed (the Ranger probes), soft-landed (the Surveyors) or sent into orbit (the rather unimaginatively named Lunar Orbiters). These were mainly for reconnaissance purposes in support of Apollo, with some scientific gain coming as a bonus. With time, the people running these missions became increasingly adept at controlling their spacecraft from the ground. Moreover, techniques to accurately determine the state vector of the remote probes from Earth using radio tracking were refined to levels of exquisite accuracy. Also, although Cold War rivalry was still real, it had become less belligerent. NASA therefore decided that ground-based techniques would be the prime means of determining the state vector for the Apollo missions. The crew would still make their own separate determination, but only as a backup to be used in case of emergencies.

NASA’s manned space experience began with Alan Shepard’s 5-minute sample of weightlessness in May 1961 on board the first manned Mercury spacecraft. In less than eight years, through the Mercury and Gemini programmes, NASA became increasingly sophisticated in Earth-orbit operations, which culminated in October 1968 with the 12-day ‘shakeout’ flight of Apollo 7 when the first manned Block II

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

DOI 10.1007/978-1-4419-7179-1 7. © Springer Science+Business Media. LLC 2011

spacecraft was put through its paces. Then, with the first Moon-bound flight of the pioneering Apollo 8 on 21 December 1968, managers found themselves vexed by a very basic problem – the disturbed sleeping patterns of the three-man crew in the limited volume of the command module. Something had changed in the nature of spaceflight.

Throughout the early years of planning for the Apollo lunar flights, it had always been assumed that the best arrangement for sleep would be a rotation system in which at least one crew member would always be awake to monitor the systems. Apollo 7 followed this regime. Donn Eisele, the command module pilot, took his rest period alone. The commander Wally Schirra and lunar module pilot Walt Cunningham took theirs simultaneously. The crew never reported problems with this arrangement during their debriefing; although any tiredness could have been masked by the commander’s irritating head cold and perhaps the irritability all three men displayed could have been exacerbated by lack of sleep. In any case, Apollo 7 was confined to Earth orbit and its crew were not in the position of being the first humans to orbit another world, never mind not being the first to leave Earth’s vicinity.

filings started well on board Apollo 8 with only a bout of space motion sickness from the commander Frank Borman to cause any medical concern. The coast to the Moon was relatively uneventful and the crew kept the ship running smoothly. The major activity was by Jim Lovell as the CMP, who practised the cislunar navigational techniques that subsequent crews would use. However, as the spacecraft’s flight progressed, the crew found that their planned sleep patterns became increasingly disrupted.

A common problem occurred whenever a crewmember spoke with mission control, as his chatter u’ould disturb the slumber of his colleague. Why this should be a problem on this flight when it never previously arose may be that, unlike an Earth – orbit mission of the lime, there were no long periods during the coast when radio silence was enforced by the sparse distribution of the tracking sites. At the time, conversation with mission control during Earth orbit flights could only occur when a ground station was in view’ and that was often for less than ten minutes at a time. During the coast to the Moon, the flight controllers in mission control were in permanent communication with the spacecraft and Capcom would not only speak to the crew’ whenever an operational need arose but would also engage in idle chat, with the result that communication was often ongoing. Also, with three men occupying the very cramped confines of the cabin, any activity to carry out chores tended to disturb the crewman sleeping on or underneath his couch. In the command module there was no place to escape from colleagues and there were no sleeping bags. By the time the Apollo 8 crew reached their destination, they were all somewhat groggy from their attempts at napping, and it took the adrenaline produced by the excitement of making ten orbits around an alien planet to help them to perform their duties successfully and safely. All three were captivated by the forbidding, stark landscape that was passing beneath their window’s. They worked hard at their full schedule of photography, TV broadcasts and navigational sightings and all the time kept up a busy chatter with mission control until the seventh orbit, when Borman decided to discard the timeline for the remainder of their orbital sojourn. Although it was an incredibly sophisticated machine for the technology available in the 1960s, the spacecraft could only achieve its capability by being necessarily complex and intricate. There were countless ways in which a tired crew could kill themselves through inappropriate operation of its many controls. Borman knew this, and ordered Jim Lovell and Bill Anders to take a rest before the trans-Earth injection burn.

Mission planners took the Apollo 8 experience to heart when they reviewed the sleeping arrangements for subsequent flights. It w’as decided that since the controllers on the ground had a better view of the spacecraft’s systems through telemetry, the crews would sleep concurrently, following Houston time, with one crewman wearing a headset in case Houston felt the need to wake them up. As subsequent missions became increasingly complex and demanding, this change in
the sleep regime allowed crews to sleep well and helped them to cope more easily with the immense physical and mental strain they had to endure when their opportunity came to explore another world in the very limited time Apollo could give them.

One iconic symbol of the ‘space programme’ was the strange ‘beep’ that constantly seemed to punctuate the conversation between mission control and the spacecraft. Anyone mimicking or lampooning the spacemen felt the need to pepper their speech with the curious Lone that came on the audio feed to the press and broadcasters from NASA. Despite their association with something hi-tech, their purpose was rather prosaic. They were called Quindar tones and all they did was control a switch.

The USB radio system required that a radio carrier be sent to the spacecraft at all times for tracking purposes (sec Chapter 6). 4’his is in contrast to the situation with a walkie-talkie where the carrier is transmitted only w’hen the push-to-talk button is pressed. However, it was not desirable for Capcom’s microphone to be constantly live on the uplink to the spacecraft. He had to talk to others in the mission operations control room (MOCR) and indeed to any other site on the communications network, and his microphone could also pick up nearby conversation. Nor w as it desirable for the long-distance line from Houston to the ground station to reach the spacecraft as it was prone to interference. A decision was made to allow’ the line from Houston to be fed to the spacecraft only when Capcom wanted to speak, and so a method had to be found to tell the ground station when he had pressed his push-to-talk button and when he had released it. As it would have been expensive to have arranged a separate circuit just to carry a signal to tell the ground station to switch, engineers used a technique called in-band signalling w’hereby the signals to switch w ere not only sent on the same line as the Capcom’s words, they occupied the same audio band. These signals w’ere the Quindar tones.

When he w’anted Lo speak Lo the crew7. Capcom pressed his push-to-talk button. This produced a quarter-second burst of 2.525 kHz tone that w;as placed on the line to the ground station. There, it operated an electronic switch Lo connect the line from Houston to the uplink. When Capcom had finished speaking and released his push-to-talk button, another quarter-second burst of tone, this time of a slightly lower pitch of 2.475 kHz, signalled the switch to remove the Houston line from the uplink.

Retro’s second offering eoncerned the abort situation when the spacecraft had already entered lunar orbit. For the entire period that an Apollo spacecraft flew in orbit around the Moon, mission control ensured that the command module pilot always had the data he needed to return to Earth, even if an extreme situation meant that he had to come home on his own following the loss of the LM. These were trans-Earth injection (TEI) burns, one of which would eventually be used in the normal course of events to bring the entire crew home. Before they attempted LOI. Retro prepared details of these get-you-home burns suitable for use after one and two lunar orbits. Then, as their time around the Moon progressed, he continued to supply regular updates to ensure that they were never without a return ticket’.

Meanwhile, if all was going well, the fruits of FIDO’s efforts could be brought to play; first to refine the spacecraft’s approach to the Moon, and then to execute the LOI burn and place the spacecraft in lunar orbit.