Category How Apollo Flew to the Moon

In the years after World War II. in the bowels of America’s aeronautical research facilities, a few remarkably gifted engineers were having ideas above their station. Thinking outside the box. as we now call it, they wondered how a manned spacecraft (women were never considered) that had been blasted outside Earth’s atmosphere, could possibly return without killing its crew. Two in particular, Max Fagct and Owen Maynard, were formulating a plan to bring together diverse technologies that were then maturing which might allow the dream of space travel to be realised. Many of these technologies were also concerned with the delivery of nuclear weaponry – the most prominent examples being the liquid-fuelled rocket, the ablative heatshield and the blunt-body re-entry vehicle.

Both the USA and the Soviet Union had been familiarising themselves with rocket technology gleaned from the defeated Germans of World War II. Having learned from rocket engineers who had worked for the Nazis, both superpowers had launched vehicles that had been either looted from central Europe or built locally on the basis of German experience. It soon became apparent that these rockets would be useful carriers for the newly developed nuclear warhead, able to dispatch these weapons across large distances in a short time. Both sides in the Cold War had nuclear armaments, and both realised that in the event of an exchange, early delivery of their warheads would be crucial to national survival.

Fast delivery of nuclear weapons required development of the intercontinental ballistic missile (ICBM) whose long, coasting flight could cross continents in half an hour. Though this class of missile was not required to go fast enough for orbital flight, much of its trajectory was spent in the vacuum of space and one of the chief problems encountered in this arrangement was dealing with the punishing heat the payload had to endure as it re-entered the atmosphere at hypersonic speeds.

After dispensing with solutions that tried to absorb the energy in a heat sink, engineers turned to the ablative heatshield. This was a layer of material on the outside of the warhead fabricated from materials that would ablate – that is, they would slowly char and burn away, carrying the heat and thereby protecting the bomb as it came hurtling back into the atmosphere. At the same time, the work of H. Julian Allen had shown that by forming the shape of a re-entering body into a blunt shield, the searing hot shockwave that always accompanied high-speed aerody­namics could be made to stand away from the fabric of the hull, and thus keep the hottest and most erosive gases clear of the vehicle.

Faget and Maynard investigated whether this technology could be arranged so that a person could sit inside the rocket’s payload instead of a warhead, enter space and return to Earth without being roasted, chilled, asphyxiated, crushed or drowned. An early implementation of their work was the one-man Mercury spacecraft, a relatively unsophisticated capsule that enabled America to log its first minutes and hours of manned space flight. Even before the first such flight was attempted, engineers had begun to consider the design of a successor that could sustain a three-

man crew for an extended flight in space and make a controlled descent through the atmosphere to land on the ocean. Although there was no specific mission for such a spacecraft, the engineers were intrigued by the possibility that it might be able to fly to the Moon.

The name for this spacecraft, Apollo, was coined in mid-1960 by the Director of NASA’s Office of Space Flight Programs, Abe Silverstein, who delved into Greek mythology for inspiration. Apollo was the son of Zeus and had associations with Helios the Sun god. In Silverstein’s mind, the idea of Apollo riding across the face of the Sun seemed an appropriate metaphor for the grand sweep of the programme that such a spacecraft might be able to undertake.

In May 1961, America had hardly dipped its toe in space with the 15-minute flight of Alan Shepard, when President Kennedy proclaimed a mission for this nascent spacecraft by challenging his nation to send a man to the Moon and return him safely to his home planet, and to do so within the eight and a half years still remaining of the 1960s. Kennedy’s early months as President had been troubled by a succession of ‘space firsts’ achieved by the Soviet Union, particularly on 12 April 1961 when Yuri Gagarin became the first person to fly in space by making a single orbit of Earth. Further trouble with an aborted invasion of Soviet-backed Cuba made Kennedy seek something that would raise America’s profile around the world. Landing men on the Moon, a goal that people within NASA were already thinking about, and carrying it out to a deadline, seemed like an enterprise at which his country could excel. The Apollo system would be pressed into this role.

As soon as the world learned of Sputnik’s launch, it was clear that the United States lagged behind the Soviet Union in the lifting capacity of their launch vehicles. But this was no failing of their designers. Rather, most rocket research to this point had been in support of both nations’ nuclear weapons programmes and because US designers were better at building smaller, lighter weapons their rockets were smaller. The Atlas missile used for the Mercury orbital flights and the Titan for the Gemini programme were really delivery systems for nuclear weapons, and struggled to lift their manned payloads. It became habitual for designers to minimise payload weight as they strove to maximise the capability of their spacecraft within the constraints of the available rockets. One decision to save weight would have tragic consequences for what was to have been the first manned Apollo mission.

On Earth, the atmosphere consists of four-fifths nitrogen and one fifth oxygen, the latter being the gas that sustains life. To save the substantial mass of the
equipment required to supply two gases in a manned spacecraft, NASA decided that the cabins of its space­craft would be filled with 100 per cent oxygen, but at a low pressure to ensure that the crew received only the concentration of oxygen mole­cules to which their lungs were accustomed. This single-gas arrange­ment worked well throughout the Mercury and Gemini programmes, and was a sound engineering deci­sion, but as the first Apollo crew were preparing their spacecraft for flight, this nearly ended the pro­gramme.

On 27 January 1967, the AS-204 mission was three weeks away from its planned launch. It was so desig­nated because it was to use the fourth vehicle in the Saturn IB series.

Informally, it was dubbed Apollo 1.

The Apollo spacecraft, CSM number 012, was a Block I type and was sitting on top of an unfuelled launch vehicle. Its crew of three were strapped in for a ‘plugs out’ countdown simulation in which the ability of the entire space vehicle to function on its own power would be tested. The cabin had been deliberately overpressurised with pure oxygen in order to test for leaks, as had been done in ground tests for the Mercury and Gemini programmes. Five and a half hours into a simulated countdown that had made only halting progress, a fire began near the commander’s feet. In the super-oxygenated environment, this quickly grew into an intense conflagration that ruptured the hull of the spacecraft and asphyxiated the three crewmen – Gus Grissom, Ed White and Roger Chaffee.

NASA sustained heavy criticism from the press and the political classes for this tragedy. Some was directed at the manufacturer, North American Aviation, with accusations of sloppy workmanship. North American rebutted, pointing out that as it tried to build the spacecraft, NASA had insisted on interfering with the process by ordering a multitude of changes. In congressional hearings on the fire, astronaut Frank Borman appealed for support from the lawmakers. “We are confident in our management, our engineering and ourselves. I think the question is: are you confident in us?”

NASA learned many lessons from this accident and applied them to the rest of the Apollo programme. Some commentators have argued, convincingly, that there was a very real possibility that, had the fire not occurred, NASA would never have realised

its lunar dream. They point out that the shock of the deaths spurred all those involved in the programme, especially at NASA and North American, to make the Block II spacecraft into the great spacefaring ship it became. Without the changes imposed by the tragedy, casualties may have occurred later in the programme, possibly in space. At the very least, the development problems of the Block I Apollo spacecraft would probably have crippled the programme at a later stage had they not been brought into sharp focus so early on.

Although NASA wanted to keep this unflown mission’s name as AS-204, it acceded to the widows’ requests that the name Apollo 1 be reserved for their dead husbands’ flight. Crews had been in training for Apollos 2 and 3, scheduled for later in 1967, but they were cancelled.

Meanwhile, a few months after the Apollo fire the Soviet Union grieved at its first loss of a cosmonaut during a test of the new Soyuz spacecraft. Both nations therefore had to cope with setbacks in their race to the Moon.

PREPARATIONS FOR LAUNCH Leaving the VAB

Final preparations for the launch of an Apollo mission began weeks in advance in one of the cavernous aisles of the 160-metre-tall vehicle assembly building (VAB). On a large steel platform, 49 by 40 metres, a complete but unfuelled Apollo/Saturn space vehicle, itself 110 metres tall, was stacked. ‘Space vehicle’ was a term for a combined launch vehicle and spacecraft. It started its journey when a 2,700-tonne diesel- powered crawler/transporter that employed tracked tractor units derived from heavy open-cast mining equipment jacked itself up underneath the platform and carried the combined load of 5,700 tonnes out through one of the massive doors and along a 5 V2 – kilometre crawlerway to one of two launch complexes, 39A or 39B, from which the rocket would make its fiery departure.

The crawler/transporter was one element of a mobile launch system that had been devised by members of Wernher von Braun’s rocket team. They had suggested that for a vehicle as large as an Apollo/Saturn V, the task of stacking its stages and installing the Apollo spacecraft on top would be best carried out inside a large hangar. The resulting VAB was a huge box-shaped building 52 storeys tall at the focus of Kennedy Space Center (KSC). In a sense, it was from here that a journey to the Moon began.

If the space vehicle had to be stacked indoors, a method had to be devised to get it out to the launch pad. Barges within dedicated canals were considered, as were great layouts of railway tracks, before the mobile launcher concept was decided. This called for a platform upon which the space vehicle would be fixed until the moment of launch. A tower even taller than the rocket sprouted from one end of the platform, which took the height of the whole affair to 136 metres. This launch umbilical tower (LUT) supplied the space vehicle with its essential services by way of nine huge arms that reached across from the tower. These tended the vehicle until they were swung away either prior to launch, or in some cases disconnecting and

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

DOI 10.1007/978-1-4419-7179-1 3. © Springer Science+Business Media. LLC 2011

pulling away only as the rocket began its flight. Though they weighed an average of 22 tonnes, these arms could quickly accelerate away from a rising rocket, and just as quickly brake to a halt close against the tower.

With the VAB doors open, the crawler/transporter lifted the platform clear of six supporting pillars and carted the entire assemblage away at about 1.5 kilometres per hour. To accommodate the pressure of the crawler’s 456 treads, each weighing nearly a tonne, a specialised roadway had to be constructed wherever it needed to go. A metre depth of support ballast fonned a bed for a layer of aggregate that bore the immense weight of the crawler and its load as they were steered along their journey to the launch pads.

After 42 seconds of flight, the rules of what would happen in case of an abort changed slightly to take account of the fact that the space vehicle had tilted over substantially and had also gained plenty of horizontal speed. The mission had moved into abort mode one-bravo which lasted until it reached an altitude of 30.5 kilometres. If an abort were to be called during this period, the escaping command module would no longer be in danger of falling into the Saturn’s debris, and the small motor intended to steer it out to sea would no longer be required. What was needed, however, was a way to ensure that the CM would turn to face the correct direction after being pulled from an aborting Saturn. This was because tests had shown that at hypersonic speeds, it was possible for the CM/escape tower combination to be stable in a nose-first attitude. The tower could not be safely jettisoned in this mode because the airflow would ram the boost protective cover onto the CM hull, which would not only prevent the tower pulling away but would also prevent the deployment of the parachutes.

Once the LES had pulled an aborting spacecraft free, two skin sections near the top of the tower, known as canards, would deploy after the escape motor had done its job. The drag they created would thereby force a turn-around manoeuvre, if one were needed, to face the heatshield in the direction of travel, and enable the tower to be jettisoned cleanly and the parachutes to be safely deployed.

One of the original concepts put forward for getting to the Moon was the direct ascent mode, whereby a rocket from Earth left the planet on a direct trajectory for the Moon. A major objection to this plan was that it included no opportunity to check whether the spacecraft had come through the stresses of launch unscathed before committing it and its human crew to a lengthy journey in deep space. The competing arrangements. Earth orbit rendezvous and the ultimately successful lunar orbit rendezvous concept, did include a period in parking orbit around Earth as part of their mission plan, giving time to make a comprehensive check of the spacecraft’s systems.

Earth orbit was an important staging point on Apollo’s lunar. journey. It was 2’A hours or so during which the crew and the flight controllers on the ground checked every system they could and prepared for the 5-minute translunar injection burn. For the first time in the flight, the spacecraft was being exposed to a true space environment after having endured the vibration and shock of launch and ascent. It was generally the task of the lunar module pilot (LMP) to work through a series of checklists to verify the status of every major system on board the spacecraft. In this, he was normally aided by the commander.

Careful checks were made of the environmental control system to ensure that the supply of oxygen to the crew was under proper control, that the system was capable of providing necessary cooling for the many electronic and mechanical systems, and that it would maintain the cabin at a comfortable temperature for the crew. Special attention was given to the radiator panels on the side of the service module, and the evaporators in the command module which supplemented the radiators when cooling needs increased. Pipes had been fabricated into the radiators to take the hot fluid from the spacecraft and cool it by radiation into space. If a leak had sprung in one of these panels, there would be no mission to the Moon.

Neither would a journey to the Moon be possible if a problem arose in one of the sets of thrusters arranged around the spacecraft. These small, low-thrust rockets were the business end of the reaction control system (RCS). There were two sets: one each in the command and the service modules. While the thrusters built into the command module would only be required upon re-entry, those on the service module were crucial to just about everything that happened; from simply aiming the spacecraft for the collection of science data to controlling its attitude for critical operations such as guidance sightings, engine burns and thermal control.

In some romantic sense, the S-IVB stage had the most bittersweet, almost tragic fate of all the Saturn components. These large, perhaps elegant stages had been faithful servants to their Apollo masters, who they dutifully sent onwards to the Moon. They were spared the ignominious crash into the sea that befell their larger brethren, the S­IC and the S-II. Instead, they were sent away from Earth to meet a celestial end. Of the ten manned Saturn V third stages – nine of which were Moon bound – half were sent to impact the Moon at high speed in the name of science and lunar seismometry, while the others coasted away from the Earth-Moon system to follow lonely orbits around the Sun, perhaps for all time.

After the departure of their Apollo payload, they became spacecraft in their own right, controlled from Karih or by the systems in the Saturn’s instrument unit until either their batteries ran out or the ground stations ceased to track the receding hulks. The people who controlled the S-IVB from Earth used what little residual propulsion the stage had remaining to achieve these final ends.

After translunar injection, both the S-IVB and the spacecraft were on very similar trajectories which were initially highly elliptical Earth orbits; but the intervening gravitational influence of the Moon would determine the final fate of both craft. While the Apollo spacecraft continued on a path to lunar orbit, the S-IVB was given one of two fates.

For the early lunar Apollo missions, a decision was made to ensure that the S-IVB would be taken well clear of the spacecraft and, in effect, dumped in solar orbit. To achieve this, its remaining propulsion was used to slow it down, so that while the spacecraft would pass the Moon’s leading hemisphere, the stage would be targeted to pass the Moon’s trailing hemisphere and receive a gravitational slingshot that would eject it from the Earth-Moon system. This was the fate of four of the Apollo S-IVB stages and they arc out there, drifting still. Although the Apollo 9 mission never went to the Moon, its S-IVB was nevertheless sent out of Earth orbit as a rehearsal and it, too. orbits the Sun. Like the others, it is slightly inside Earth’s orbit and periodically catches up with Earth.

As Apollo 17 headed out from the Moon, the crew saw something in the distance flashing at them regularly. Jack Schmitt had seen it earlier and Cernan had caught a glimpse of it. "Hey, Bob, I’m looking at what Jack was talking about,” said Cernan to Robert Parker, their Capcom. "It is a bright object, and it’s obviously rotating because it’s flashing. It’s way out in the distance. It’s apparently rotating in a very rhythmic fashion because the flashes come around almost on time.”

They discussed the idea of turning the spacecraft around to enable them to look at the object with their optics, which were mounted on the opposite side. What could be seen out of the windows could not be viewed through the optics any more than windows at the front of a house could be used to look around the back. Anyway, Schmitt was in the habit of using a 10-power monocular to view – Earth’s weather patterns and when he trained it on the object, he reckoned it to be their S-IVB, some way off.

"One unique thing about it. Bob. is that it’s got two flashes,” said Cernan. "As it comes around in rhythmic fashion, you get a very bright flash, and then you get a dull Hash. And then it’ll come around with a bright Hash, and then a dull flash.”

‘"That’s the side of the S-IVB.” said Schmitt, “and then the engine bell. Gene.”

Cernan didn’t believe him. "The commander doesn’t think that I can see the engine bell on that thing,” commented Schmitt.

‘"Roger, Jack. Is that w’ith the monocular you’re looking at it?” asked Parker.

“He couldn’t see the engine bell if he had ten monoculars.” said Cernan wryly.

‘’Bob, a couple of revolutions ago when I was looking at it, I had a much brighter view and I believe I was looking at it broadside.” said Schmitt. "It looks to me like it may be flashing more or less end-on now. But it’s not as bright now as it was a while ago. I just hadn’t put it together as maybe being the S-IVB. I thought it was just some other particle out there.”

”IIey. Bob,” said Cernan later. ‘’We got two of those flashers out there. They could be SLA panels. I don’t know. They’re alike in intensity and pretty regular in the bright and dim flashes they come out with, and they’re widely separated.”

We’ll never know whether Cernan and Schmitt were seeing the S-IVB stage or a couple of SLA panels. We do know that other crew saw flashes from discarded equipment. But as events transpired it was not the last meeting the human race would have with an Apollo cast-off.

After Neil Armstrong and Buzz Aldrin had landed their LM Eagle on the Moon and while they were preparing for their foray onto the surface, Mike Collins made strenuous efforts to locate the tiny LM among the monotonous wastes of craters on Mare Tranquillitatis by aiming his sextant where mission control reckoned they were. Getting the optics around to face the lunar surface and back again involved carrying out a number of manoeuvres. It was during the manoeuvres after one such viewing opportunity that Collins got a call from Capcom Owen Garriott.

“Columbia, Houston. Over.”

“Columbia. Go,” replied Collins.

“We noticed you are manoeuvring very close to gimbal lock. I suggest you move back away.”

“Yes. I am going around it,” said Collins. “Doing this СМС-auto manoeuvres to the PAD values of roll 270, pitch 101, yaw 45.”

“Roger, Columbia,” said Garriott.

“How about sending me a fourth gimbal for Christmas,” commented Collins, showing his annoyance at the restriction imposed by gimbal lock. Garriott could not make him out. “Columbia, Houston. You were unreadable. Say again please.” Collins let it lie in the spirit of their triumph. “Disregard.”

What was rankling him was a weight-saving decision that had the downside of

substantially complicating some of the operational aspects of a mission because it limited the range of attitudes that the spacecraft could adopt. Because of this, the crew and flight controllers had to avoid orientating the spacecraft in certain directions with respect to the guidance platform.

This characteristic was inherent in the system as designed by the MIT team who had settled on a three-gimbal mounting for the platform, similar to the system they had designed for the Polaris missile, and unlike the four gimbals employed in the Saturn V instrument unit and the Gemini spacecraft. Collins was a veteran of Gemini, and knew the advantage given by the fourth gimbal. But there were solid reasons for implementing only three gimbals. As well as saving the weight of a heavy outer gimbal, trade-offs included greater accuracy and a reduced tendency for the platform to drift in its orientation. However, the three-gimbal arrangement had this unfortunate side effect whereby, if the gimbals were moved in a particular fashion, the assembly lost its ability to maintain the platform’s alignment – a condition termed gimbal lock, and one that meant that the system had lost all knowledge of the direction in which the spacecraft was pointing. Having a fourth gimbal would have avoided this problem. As a result, care had to be taken during the flight-planning process, and throughout the flight itself, to avoid risking gimbal lock.

If gimbal lock did occur, it was a time-consuming procedure to fully realign the platform, possibly with the loss of important operational work. Even worse, if it occurred in the run up to a time-critical man­oeuvre where good platform align­ment was important, there would be no time available to take corrective action. To help crews to steer clear of it, two areas on the 8-ball’s surface were marked in red – it was these red spots that prompted the reference to the Pool ball. Manoeuvring the spacecraft in a manner that would bring either of these spots towards the centre of the display meant risking gimbal lock.

The mechanism that causes gimbal lock is not easy to describe. As previously discussed, the platform was mounted within three nested gimbals. Each had a rotation axis which was arranged 90 degrees away from the axis of the adjacent gim­bals). In normal circumstances, this

arrangement allowed three degrees of freedom because the axes were pointing in three different directions. The problem arose when an attitude was adopted that allowed the axes of the outer and inner gimbals to line up. When this condition was approached, the gimbal system lost its ability to isolate the platform from the spacecraft’s rotations because there were now only two degrees of freedom. All the gimbal axes were now on a single plane, so any rotation of the spacecraft around an axis outside that plane could not be accommodated by any of the gimbal axes. Since the region that flirted with gimbal lock was defined by the current REFSMMAT. it followed that whenever the spacecraft needed to manoeuvre to an attitude that might approach gimbal lock mission control had to give the crew a new REFSMMAT. The platform could then be realigned so as to allow the otherwise awkward attitude to be adopted.

Of course, all this was worked out during mission planning. As many as eight REFSMMATs might be used during a mission, switching as the operational requirements changed. Each Lime the REFSMMAT changed, the platform would be duly realigned. But the avoidance of gimbal lock w as only one reason for changing the REFSMMAT. A more important reason was to aid the monitoring of critical events, such as launch, re-entry, and the major manoeuvres that required long engine burns or where the correct attitude of the spacecraft was paramount. By aligning the guidance platform to an appropriate REFSMMAT that matched the required attitude for the manoeuvre, the crew would find the 8-ball display easier to interpret when the spacecraft was pointing in the correct direction. Attitude errors, which must be avoided during an engine burn, could therefore be easily spotted and corrected.

When Apollo 8 made the first manned flight to the Moon, only three REFSMMATs were required. The purpose of the mission was simple – get to the Moon, orbit it ten Limes, and get back. There was little reason for the spacecraft to adopt widely varying attitudes. For the journey to the Moon, the platform was aligned to the launch pad REFSMMAT. This represented the launch pad’s attitude in space at the moment of launch, as determined with respect to the stars. This orientation made it easy for the crew? to monitor the progress of their ascent from Earth’s surface into orbit and during the translunar injection burn. Later, as they approached the Moon, they realigned their platform to a new REFSMMAT which coincided with their ideal attitude for the engine burns that Look them into lunar orbit. Choosing such an orientation for their platform made it much easier for them to monitor their attitude on the spacecraft’s displays. Once they had left the Moon, Jim Lovell realigned Apollo 8’s platform to a suitable REFSMMAT for re-entry into Earth’s atmosphere, again to make it easier to monitor this critical event.

As missions progressed, the number of REFSMMATs increased to facilitate a more sophisticated use of the hardware. Apollos 12 and 14 had six REFSMMATs while for the advanced J-missions that concluded the programme there were eight. The start of the journey began with the platform aligned with the launch pad. as with Apollo 8. Coasting to the Moon required the spacecraft to be turned slowly to evenly distribute heat around its surface. As this rotation had to avoid gimbal lock, a special REFSMMAT was invented. Each of the three major manoeuvres by the

CSM at the Moon had its own REFSMMAT in order to simplify crew monitoring. Two additional REESMMATs were defined to aid the lunar landing and lift-olT. each representing the orientation of the landing site at the times of these events. Finally, as with Apollo 8, a REFSMMAT was defined for re-entry.

After the loss of power on board Apollo 13. its crew found themselves in the uncomfortable situation of discovering what happens to the cabin temperature of a spacecraft after a power cut. With no electricity running through the systems of the CSM and very little in the LM. hardly any heat was being generated – heat that the crew’ depended on for warmth and comfort. Despite the unfiltered rays of the Sun falling on the ship, a chill permeated the cabin until an equilibrium temperature of only 6: C was reached.

In a properly functioning Apollo spacecraft, the substantial amount of electronic gear that it contained generated copious quantities of heat and. for the designer, the problem was to keep the spacecraft cool. Heaters w’ere only installed for items of peripheral equipment that felt either the chill of space or the chill of the cryogenic gases. To control internal temperatures, the CSM had a sophisticated cooling system that took heat from where it was not wanted, sent some of it to where it was wanted, and rejected the rest into space.

The electronic boxes in the command module were mounted on metal plates, known as cold plates. These w’ere cooled by pipes that contained a mixture of w’ater and glycol the same mixture used in the radiator of a car to cool the engine block. By the time it had passed through all the cold plates, the coolant was quite warm

and, if required, could be used to heat the cabin air, which again is something similar to the system in a typical automobile (at least one without air conditioning).

The warm liquid was then pumped to the service module where it was passed to one of two large radiators built into the spacecraft’s skin. Normally, one of these radiators would be basking in the full heat of the Sun while the other was being chilled by the cold of deep space. Automatic controls fed the coolant to whichever radiator was colder and the heat from the command module was released through radiation. For the sake of redundancy, the spacecraft had two independent parallel radiator circuits in case one developed a leak or became blocked. Automatic systems monitored the outlet temperature of these radiators to keep them from freezing. The cold water/glycol mixture was then returned to the command module where it could absorb more heat from the spacecraft’s electronics.

These radiator panels were not designed to lose all of the command module’s heat but they provided a simple, passive method of dealing with most of it. A second, much more active system was built into the command module to take care of peaks in the spacecraft’s heat output – for example, during preparation for a burn when most of the spacecraft’s systems were powered. This was the evaporator, often referred to by the crew as a ’boiler’, which worked because energy is required to convert a liquid to a gas.

When water evaporates, it takes heat from its surroundings, which is why we sweat when we are too warm. The evaporator used the same principle, but by introducing the water to the vacuum of space the evaporation was much more vigorous, making it a very efficient cooling system. In the spacecraft’s evaporator, spare water generated by the fuel cells was fed through metal plates that contained many tiny holes. Beyond the plates, the water encountered a mass of porous stainless steel called a wick, the other side of which was exposed to space. The evaporation of water from the wick kept it cold. Pipes from the coolant system were passed through this assembly and the water glycol within them gave up its heat to the vaporisation process.

The water vapour from the evaporator was led to space through a duct, called the steam duet, which exited from a port just below the crew’s left-most window. A valve in the duct controlled the loss of vapour to ensure that the wick remained wet and did not freeze. A frozen evaporator was considered a danger because there was a risk that the expanding ice could breach the spacecraft’s pressure hull. Redundancy dictated that there should be two evaporators, one each for the primary and secondary cooling systems.

If the mission had Lo be aborted before the LOI burn. Retro’s response would attempt to achieve two things. It would ensure that the spacecraft was set on an accurate path, not only to Earth but also to a landing site, usually in the mid-Pacific Ocean, where a recovery fleet would be on station. In addition his solution would strive to increase the speed of the spacecraft slightly, so that it would return home 24 hours (one Earth axial rotation) earlier than would occur without intervention. On Apollo 8, Retro had planned to use two separate burns to achieve these goals: & flyby manoeuvre that would have been carried out just before the spacecraft disappeared around the far side of the Moon; and a pericynthion plus 2 manoeuvre, or ‘PC — 2′ for short, to he made Lw’o hours after their closest approach to the Moon. On later missions, both of these functions were combined into a single planned abort contingency.

The PC —2 abort burn was actually used on one oeeasion as part of the effort to get the ailing Apollo 13 back to Earth. On this mission. Retro had to calculate the burn using the descent engine of the lunar module, the SPS engine being unusable owing to the loss of power in the CSM.

Apollo was conceived as a two-part spacecraft. The three-man crew occupied the conical re-entry section, from which they controlled the mission. This command module (CM) carried much of the equipment the crew needed for their flight, and everything they needed for re-entry. Most of their consumables (air, water, power) and their chief means of propulsion and cooling were carried in a cylindrical section attached behind the command module’s aft heatshield. This service module (SM) remained attached to the CM for most of the flight, the two sections acting as one spacecraft under the acronym CSM, for command and service modules. On the return journey the SM was discarded shortly prior to re-entry into Earth’s atmosphere. This distinctive cone-and-cylinder arrangement, with a nozzle sticking out of its aft end, became the archetypal spacecraft in the minds of many children who grew up at this time, fascinated by space flight.

Early plans envisaged taking some arrangement of the CSM all the way to the Moon’s surface as part of a larger vehicle that would sport a set of landing legs to enable the combination to touch down. Although this would have been a rather unwieldy craft to land, the requirement to lift the CSM off the Moon dictated the thrust of the spacecraft’s large main engine.