Category How Apollo Flew to the Moon

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.

When the spacecraft was sitting on top of the Saturn V, it included one extra element of the Apollo system that everyone hoped would never be used. If it had, it would have been a particularly bad day for all involved. Attached to the tip of the CM was a truss structure upon which was mounted a thin, pencil-like tower which included a powerful solid-fuelled rocket motor. This was the launch escape system (LES). Its lower section was a fibreglass and cork shroud that covered the shiny surface of the command module, called the boost protective cover (BPC). It shielded the CM from the heat of friction with the air during the first three minutes after launch, and from the blast of exhaust from the rocket mounted just above, should this be used.

If the Saturn V were to suffer a mishap, this motor would have burned for just eight seconds, but it would have produced a force equivalent to 66 tonnes weight and an acceleration in excess of 7 g that would whip the CM and its crew to safety. The motor’s exhaust exited through four nozzles that were canted to direct its blast away from the spacecraft. If the launch was normal, then after the first 3 lA minutes another smaller rocket motor near the top of the tower would pull the launch escape system, including the boost protective cover, away to fall into the Atlantic Ocean.

By now NASA had confidence in the Apollo CSM, but no one had yet flown inside the flimsy lander that was to take crews to the Moon’s surface. NASA ticked the D – mission box by flying the entire Apollo system, consisting of the main spacecraft and a fully configured lander, LM-3, in Earth orbit as Apollo 9. It was to rehearse all the manoeuvres that a Moon flight would require. It also marked the first time astronauts would entrust themselves to a spacecraft that had no heatshield and therefore would not be able to bring them home in an emergency, but it was a trust that would have to be gained if the LM was to take their colleagues to the Moon’s surface.

After a successful launch on 3 March 1969, the crew followed a ten-day timeline roughly similar to a lunar mission but without leaving low Earth orbit. This began with retrieval of the LM from its station on top of the S-IVB stage, one of many firsts achieved in this crammed mission. Controllers on the ground then commanded the now discarded booster to reignite its engine and leave Earth’s vicinity, as if dispatching an Apollo mission to the Moon. The spent stage eventually escaped Earth’s gravity to enter its own independent orbit around the Sun. After a number of firings of their SPS engine to set up the correct orbit, Jim McDivitt and Rusty Schweickart entered the LM, call sign Spider, and powered it up. David Scott remained behind in Gumdrop, the CSM. The names selected by the crews simply reflected the shapes of their spacecraft.

Schweickart was to venture out of the LM’s front hatch in order to test the type of

spacesuit and back pack that crews were to use on the Moon and demonstrate that, in the event of an unsuccessful doeking or a blocked tunnel, a crewman could make his way from one spacecraft to another by using external handrails, but this spacewalk was reduced in scope when Schweickart suffered a bout of space adaptation sickness on the day prior to his task. Managers still had little experience of this condition and allowed hint only to move out onto the LM ‘porch’ to prove the space-worthiness of the suit and back pack, while Seotl stood in Gumdrop’s hatch to retrieve experiments from the skin of his own spacecraft.

Four days into the flight, McDivitt and Schweickart sealed the tunnel between the two vehicles and undocked Spider. After a visual inspection by Scott, they fired the LM’s descent engine to move 185 kilometres away from Gumdrop and set up the conditions for a lunar-type rendezvous. After the descent stage had been jettisoned, they flew the ascent stage back to the command module, as would happen on a lunar mission, and eventually docked and transferred back to the company of Scott without difficulty.

For the remainder of the flight, the crew practised navigation techniques, made multiple adjustments to their orbit with their dependable SPS engine, and carried out experiments including multispectral photography of Earth in support of future Earth resources satellite programmes and the manned Skylab orbital workshop. Although less glamorous than the missions to come, Apollo 9 was a highly successful overture to Apollo’s climax: flying to the Moon.

After a hearty but low-resi­due breakfast in the crew quarters located some kilo­metres south of the VAB, the prime crew walked into the suiting-up room where a cov­ey of technicians helped them to don the spacesuits which would protect them if the cabin unexpectedly depres­surised early in the mission. Careful checks were made to ensure that the suits were airtight (checking the pressure integrity in NASA parlance) before the crew were finally sealed in, drawing on a portable supply of oxygen which they would carry with them to the launch pad.

The reason for sealing the crew inside their suits so early was linked to the fire in the Apollo 1 spacecraft. In its aftermath, it was decided that the cabin would have a mixed atmosphere of nitrogen and oxygen prior to launch, to make the interior much less flammable. After launch, as the vehicle ascended and the outside air pressure diminished, the cabin atmosphere was allowed to vent overboard and be replaced by pure oxygen from the spacecraft’s tanks. As it did so, automatic systems ensured that adequate cabin pressure was maintained, never going below about one-third of the atmospheric pressure at sea level. In the space of several minutes, the pressure in the suits also dropped by two-thirds, and without preparation this could cause nitrogen in a man’s bloodstream to come out of solution and give him the ‘bends’ – a problem also faced by divers who rise too rapidly through a column of water. To avoid this condition, nitrogen was flushed out of the crew’s bodies by having them breathe pure oxygen for several hours prior to lift-off while sealed in their suits.

Three hours before launch, the crew arrived at the 320-foot level of the launch umbilical tower and walked along the highest of the nine access arms, nearly 100 metres above the launch platform. This arm led into the so-called ‘white room’, a controlled environment high above the Florida sands that gave access to the command module’s hatch. One by one they entered the cramped confines of the CM aided by the pad crew who strapped them tightly into their couches and changed their oxygen supplies from the portable kit to the spacecraft’s circuit.

The commander entered first and settled into the left couch from where he would be able to scan the instruments and watch for any issues affecting their trajectory. If trouble arose that threatened the crew, he would abort the mission by twisting the

translation control with his left hand. From Apollo 11 onwards, he had the option of flying the Saturn to orbit manually if the rocket’s guidance system failed, guided by the instruments in front of him. Normally the lunar module pilot (LMP) entered next, taking the right couch. In this position he could watch over the spacecraft’s systems. The command module pilot (CMP) entered last to occupy the centre couch. During ascent, his major role was to assist the commander in monitoring the progress of their climb and to operate the computer.

There was only one exception to this arrangement when Mike Collins, CMP on Apollo 11, took the right seat and entered before Buzz Aldrin, who was LMP. Collins felt that the start of the elevator ride at the bottom of the launch umbilical tower was really the start of his journey to the Moon. Walking across the ninth arm, he was impressed at the contrasts in his field of view. "On my left is an unimpeded view of the beach below, unmarred by human totems; on my right, the most colossal pile of machinery ever assembled.’1 By the time Collins was named with Neil Armstrong and Buzz Aldrin as the prime crew of Apollo 11, his crewmates had already served as backup crew on Apollo 8 and had practised launch procedures

with Buzz in the centre couch. It was decided not to change this, and so Collins trained for the right seat at launch.

In the confined space of the white room, Collins continued a space age tradition by giving the leader of the pad team, Guenter Wendt, a going-away gift; in this case, a tiny trout nailed to a plaque in recognition of Wendt’s tall fishing tales. There were often little gifts or pranks that helped to lift the tension in the edgy moments before a crew were shut inside the spacecraft. Armstrong gave Wendt a ticket for a ‘space taxi ride’ and Aldrin presented him with a Bible. In return, the German gave Armstrong a mock ‘Key to the Moon’. As the Apollo 14 crew were boarding, Alan Shepard, then the oldest of the active astronauts and already a grandfather, presented Wendt with a Second World War German Army helmet, and took receipt of a mock walking stick dubbed the ‘lunar explorer support equipment’.

When the cabin was sealed, the pad team retired to a safe distance and the swing arm was rotated 12 degrees away, to place the white room close alongside, ready to be returned should an emergency arise that required the crew to exit the spacecraft. At this early stage, the crew kept themselves occupied with checks of their ship and worked with the spacecraft test conductor at the local control centre to examine as many vital systems as possible. Communications were tested – in particular a special circuit set aside for calling an abort to the mission. Tanks for the service module’s four sets of little manoeuvring thrusters, the reaction control system (RCS), were pressurised to force propellant towards the thrusters and enable them to work. The guidance system was initialised; the spacecraft would not direct the rocket but it had to know where it was and it needed to keep track of where the Saturn V was taking it in case the commander had to assume control. Then with five minutes remaining in the countdown, the arm carrying the white room was swung away from its interim position around to the opposite side of the launch tower, to position it as far away as possible from the plume of flame that the rocket would leave in its wake as it lifted off.

On Apollo, it was usual to cut, eject or actuate parts of the vehicle using carefully arranged explosives that ranged from small squibs that operated helium valves on the lunar module, to the two great rings of explosive bridge wire that ran around the interstage. These were powerful enough to cut the vehicle in two. Notwithstanding
the fact that the Saturn contained enough propellant to simulate the bang of a small nuclear device, its structure was festooned with explosives, including the propellant tanks themselves. Enormous strips of shaped charges had been placed along the sides of the tanks to rip them open in case of an abort while in the atmosphere. This would enable the propellants to disperse before the vehicle impacted the ground. The only time the crews were able to see one of these charges in operation was when the LET and the boost protective cover (BPC) that surrounded the command module were jettisoned. Four explosive bolts that held the base of the tower to the top of the CM were detonated to free both it and the BPC that was attached to it. A small solid – rocket motor near the top of the LET was enough to pull them away from the already accelerating spacecraft. For the first time, the crew could see through all five windows, rather than one or two portholes built into the BPC. Ed Mitchell described what he saw out of his windows at the Apollo 14 post-flight debrief. "When the escape tower and the BPC jettisoned, there was quite a bit of noise and flash associated with it, and quite a bit of debris that came off.” Mitchell was probably witnessing the detonation of the bolts that held the tower to the CM. "It was louder and more dramatic than I expected,” he added. His commander Alan Shepard reinforced the observation: "There wasn’t any question about the fact that it went.” "It’s like all the pyro functions,” added Stu Roosa. "You know they happen.” John Young took two rides on the Saturn V to appreciate the spectacle; first in the centre seat on Apollo 10, then in the left seat on Apollo 16. "You can see the whole works go off. I didn’t see it on Apollo 10, but I sure saw it this time.”

When Apollo 12 had launched after a rain shower, Pete Conrad, who was still trying to take in the repercussions of their lightning strike, noted how the boost protective cover had failed to be as watertight as expected. "The tower and BPC went as advertised; but when they did they unloaded a whole pile of water on the spacecraft again, and this water streaked down the windows and froze immediately. At the same time, the water picked up particles from the LET jettison motor and deposited a white ash in the form of oil droplets and streaks all over the windows. The ice sublimated later, en route to the Moon, but it left white deposits in the form of spiderweb-like things in the corner crevices and as a white deposit on the windows.”

Even before Kennedy’s challenge, the lunar Tree-return trajectory had been recognised as a safe and efficient means by which a spacecraft could make the journey. The idea is attributed to Yuri Kondratyuk of the Soviet Union, who realised its possibilities for lunar flight in the early twentieth century. A major crater on the far side of the Moon is named after him. It was a wonderful solution to the problem, and one whose propellant needs were within the capabilities of the Saturn V. It could get a crew’ to the Moon within three days and allow the entire mission to be carried out within 14 days, well within the duration for which the Apollo spacecraft was being designed. Furthermore, if a fault arose in the SPS on the way to the Moon to prevent major manoeuvres, the free-return trajectory would bring the crew back towards Earth, and any fine tuning on the homeward leg would be within the capability of their RCS thrusters. This was an inherently attractive option for an agency that had a presidential directive to preserve the life of its human crews.

The free-return trajectory relied on using the Moon’s gravity as a steering device for the spacecraft. It is one of a range of techniques that have been used by interplanetary probes for decades to move around the solar system much more quickly than w’ould be possible with rockets alone. If a spacecraft coasts towards a planet from beyond that planet’s sphere of gravitational influence, it must have the same speed on both the incoming and outgoing legs when measured with respect to that body. However, the planet is moving with respect to the Sun so there is an opportunity for some exchange of momentum between the planet and spacecraft. If it passes by the planet’s trailing hemisphere, its heliocentric (or Sun-centred) velocity is increased as it gets a little gravitational tug like a skater holding onto a car. The result is that its orbit is made larger; speed has been gained without so much as a squirt of rocket propellant. This is often characterised as a slingshot effect.

Conversely, if the spacecraft passes the /^Mo°n

leading hemisphere of the body, the ex­change is away from the spacecraft. It gets a tug from the planet against its orbital motion so heliocentric velocity is reduced and its orbit gets smaller.

With the lunar free-return trajectory, the leading-edge case is taken to an extreme in which the spacecraft is made to swing all the way around the far side of the Moon and onto a path back to Earth, in the process tracing out an immense figure-of-eight.

Such a trajectory also affords a slower approach velocity with respect to the Moon, thereby minimising the amount of propel­lant required to achieve lunar orbit. A win – win scenario.

Once the free-return trajectory was factored into the TLI calculation, there were very few solutions remaining to the equations that calculated the burn for the S-IVB. Such equations took into account the motions of Earth, the Moon and the spacecraft as well as the other major bodies in the solar system whose gravity would to some degree influence the spacecraft’s path. They also accounted for the trajec­tory that the lunar module would take during its descent to the surface, particu­larly when Apollos 15 and 17 had to approach through mountain ranges. One

particular flight controller in mission control was responsible for procuring the details of a TLI burn that would achieve as many of the desired conditions as possible. The flight dynamics officer (FIDO) worked with a backroom team and a room full of mainframe computers to calculate a range of possible solutions whose starting point was the orbit that had been achieved around Earth. These could be optimised for fuel efficiency, duration of flight, suitability for entering lunar orbit, and flight safety in terms of their return-to-Earth characteristics. From these, he picked one which, by his judgement, was the best compromise; one that required the Saturn’s third stage to fire along its flight path in order to change the speed of the spacecraft by a certain amount at a certain time. It was then up to the J-2 engine of the S-IVB to supply that change in velocity.

Over the course of the Apollo lunar flights, the manner in which planners used free-return altered as NASA’s operational confidence increased. A pure version of the trajectory, one that would set the spacecraft on a path directly to Earth without

intervention, would fly around the far side of the Moon at an altitude of roughly 500 kilometres, depending on the precise Hanh/Moon/Sun geometry. Up to and including Apollo 11, the spacecraft was sent on a trajectory that was a near approximation to this and which, if lunar orbit insertion were to be impracticable, would require only minor burns of the RCS thrusters to steer to the desired splashdown site. Since the free-return trick only worked if the flight was kept within the plane of the Moon’s orbit around Harth, it limited the potential landing sites to those that lay along the track of the resulting lunar orbit. Since the Moon’s equator is within a few degrees of the plane of its orbit around Earth, the ground tracks for missions which flew a free-return trajectory were all near-equatorial.

Later missions evolved the trajectory to hybrid versions. The H-missions of Apollos 12, 13 and 14 started out from Harth on a free-return trajectory, but once safely on their way they performed a small burn to improve their approach characteristics, knowing that a corrective burn from either the SPS engine on the CSM or the large engines on the LM would be sufficient to re-establish a safe coast home. The latter contingency had to be used on Apollo 13 to restore a free-return after the SPS engine was disabled. The J-missions were injected directly into a non – free-return trajectory, one that would not bring them home directly. Again, they relied on either the SPS engine to effect a safe return or. if that was out of action, one or both of the large LM engines.