Category How Apollo Flew to the Moon

Six minutes into Apollo ll’s descent. Duke came up with a time for the crew. "Six plus 25. throttle dowm.”

“Roger. Copy,” said Aldrin.

‘’Six plus 25.’’ reiterated Armstrong.

Houston had calculated that they could expect the engine to start to throttle 6 minutes 25 seconds into the descent. It was part of a clever strategy that engineers had come up with to w’ork around the engine’s forbidden throttle settings in its high thrust range. They arranged that P63 should compute a course to a spot 4.5 kilometres short of the landing site. In some cases, strangely enough, this point could be below’ the surface but this didn’t matter since P63 never took the LM anywhere near it. This profile had been chosen to achieve two goals. First, it protected the engine from the erosion that would result from the forbidden throttle settings. Second, it yielded nearly optimal efficiency. The profile called for an initial thrust level that w as higher than the engine could achieve, to which the engine responded with its constant high thrust setting that is, 92.5 per cent of maximum, referred to as the fixed throttle position. For about 6.5 minutes of the burn, the spacecraft continued to lose speed and gently arc towards the surface while the engine continued in its high thrust setting. Hvenlually, the thrust required to achieve the profile fell below 57 per cent of maximum whereupon the program’s logic permitted the engine to move into its allowable throttle range which lay between 65 and 10 per cent of maximum. For the remaining 2.5 minutes of P63’s work, the computer could control the throttle and adjust it as necessary to compensate for any errors and drive the spacecraft’s trajectory towards an optimal flight path.

"Wow! Throttle down," called Aldrin. joyfully.

“Throttle down on time,” said Armstrong.

“Roger." said Duke. “We copy throttle down."

“You can feel it in here when it throttles down," noted Aldrin. "Better than the simulator." The crews had intensively simulated the descent, but the one thing the simulators could not provide was the g-force provided by the engine.

Navigation on the lunar surface was of no concern to Armstrong and Aldrin. They never ventured more than 60 metres from the LM. But as the crews of Apollos 12 and 14 made their H-mission traverses it became increasingly difficult to tell where they were as they roamed further on foot to locate features of scientific interest that geologists had identified from aerial photographs. At close quarters, the lunar surface is remarkably homogenous, especially on the plains. Huge areas consist of little more than large, time-worn craters pockmarked with smaller craters, all covered with a ubiquitous layer of grey dust and shattered rock.

The issue came to a head as A1 Shepard and Ed Mitchell set off to reach the rim of Cone Crater, their primary scientific target. Seen from above, Cone was a majestic 1,000-metre-wide hole in the ground but it was surrounded by a landscape that, at a human scale, w7as unrelentingly undulating. Worse, they had to climb a rough slope to reach it and since from their perspective atop the ridge the far rim of the crater was lower than its near rim, it would remain invisible until they were almost at its edge. It had been thought that the crater would have a raised rim that would make it easy to locate.

“Okay. We’re really going up a pretty steep slope here,” puffed Mitchell as his heart rate peaked at 128 beats per minute.

“Yeah. We kind of figured that from listening to you,” said Fred Haise in Houston. The heart rate of Shepard, a much older man at 47, had just risen to 150 as the effort of their climb and the frustration of their inability to locate the rim of Cone began to tell.

As the two laboured up the ridge, they pulled a little hand cart, the modular equipment transporter (MET) which was the engineers’ answer to the shortcomings of the hand tool carrier. It not only provided a place for the tool carrier, it allowed a much larger range of tools to be taken on a walking traverse as well as cameras, film magazines and sample bags; and it permitted a greater weight of rock to be returned to the LM. The MET rolled along on two rubber tyres pressurised with nitrogen. It was of some surprise to the crew that the wheels did not throw up rooster-tails of dust, but rather formed smooth ruts in the soil that reflected the Sun when viewed into the light. Included with the MET was a magnetometer to measure the local magnetic field at points along the way to Cone. Its sensors were mounted on a tripod which was deployed on the end of a 15-metre cable. After allowing time for the unit to settle, readings were gathered from a unit on the MET itself.

"I thought the MET worked very well." said Shepard alter the flight. "It enabled us to operate more efficiently than we would have otherwise.”

Mitchell agreed. "’We would have been in real trouble trying to move all that stuff out with just a hand tool carrier, and still get the same amount of work done.”

Thick soil and an increasing incline were adding to their difficulties. "The grade is getting pretty steep,” warned Mitchell, clearly breathing heavily. "And the soil here is a bit firmer, 1 think, than we’ve been on before. We’re not sinking in as deep."

"‘That should help you with the climb there.” said Haise encouragingly.

"’Yeah. It helps a little bit.”

Shepard had another way of making the climb easier.

"’Al’s got the back of the MET now, and we’re carrying it up. I think it seems easier.”

"Left, right, left, right,” prompted Shepard as he tried to quicken their progress up to Cone.

The backup crew of Gene Cernan and Joe Engle were listening in next to Haise. "There’s two guys sitting next to me here that kind of figured you’d end up carrying it up."

“Well, it’ll roll along here,’’ explained Mitchell, "except we just move faster carrying it.”

Always mindful of the need to return before their consumables ran out, and having already awarded a 30-minute extension, mission control enquired of their progress.

"Л1 and Ed. do you have the rim in sight at this Lime?”

"’Oh, yeah,” confirmed Mitchell.

"’It’s affirmative.” said Shepard. "It’s down in the valley.”

But they had misheard ’rim’ as ‘LM‘.

"’Em sorry,” said liaise. ""You misunderstood the question. I meant the rim of Cone Crater.”

"’Oh, the rim. That is negative,” said Shepard. “Wc haven’t found that yet.”

The problem they faced was that they were being fooled by the terrain that surrounded the crater and. by heading for the highest ground, they were being taken south, to one side of their goal. They were not lost in the strict sense of the word. The LM was easily visible from their elevated route and there would be no difficulty in finding their way back. On behalf of mission control, Haise called a halt to the quest. "Ed and Al. we’ve already eaten in our 30-minute extension and we’re past that now. I think we’d better proceed with the sampling and continue with the EVA.”

It was a bitter moment for the two explorers. They had come 400.000 kilometres to the Moon and had laboured uphill for over a kilometre to sample the deepest ejecta from the rim of Cone Crater. The superficial metric of their success would have been to take possibly spectacular pictures that looked across the crater to its far rim.

"It was terribly, terribly frustrating,” remembered Mitchell twenty years later, on how it felt to have seemed to have failed. "Coming up over that ridge that wc were

going up, and thinking, finally, that was it; and it wasn’t – suddenly recognizing that, really, you just don’t know where the hell you are. You know you’re close. You can’t be very far away. You know you got to quit and go back. It was probably one of the most frustrating periods I’ve ever experienced. There’s no feeling of being lost. I mean, the LM is there; we can get back to the LM. It’s not reaching and looking down into that bloody crater. It’s terribly frustrating.”

Like most of the Apollo astronauts, Mitchell came from a high-achieving military background and was used to reaching goals that had been set. This particular goal was also the pinnacle of a crewman’s career and the apparent failure was a blow to a pilot’s pride. But in truth, Apollo 14 admirably fulfilled the scientific task it had been set. Their objective was to gain deep samples from the rim of Cone and since their closest sampling point was later established to be only about 30 metres from the true edge of the 1,000-metre crater, they had, without realising it, been successful.

Scientists had always been vocal that Apollo crews must place high-quality scientific instruments on the lunar surface. After all, many in the scientific community saw manned spaceflight as a sink for funds that ought to go to unmanned craft. If man on the Moon was being shoved down their throats, then at least something useful ought to come from it. Arrangements were made to develop a system of instruments that would work off a common pow’er source and radio system. It was known as the Apollo lunar surface experiments package (ALSHP) and would be deployed on the surface with sufficient radioactively-sourced pow’er to last years. Across 1965 and 1966. principal investigators were recruited by NASA to design the instruments which Bendix would develop and supply. Early plans assumed that the first landing would to have tw’o moomvalks; one to deploy the ALSEP and the other for a geological traverse. However, as the planning for the initial landings continued, it became clear that an ALSEP would not be ready in time for the first crew. There would be a single short moonwalk which w’ould combine the history and ceremony of the moment with a small amount of sample gathering and photography. ALSEP would have to wait for subsequent missions.

When the entire Apollo stack of LM and CSM arrived at the Moon, it was placed in an orbit that would pass over the landing site at the time of landing. After the LM had set off for the surface, the CSM returned to a 110-kilometre orbit if it wasn’t already there. While the surface crew carried out their exploration, the Moon continued to rotate on its axis and. in most cases. Look the landing site away from the orbital plane of the CSM. The exception was Apollo 11. for which the landing site and the CSM’s orbit were more or less aligned with the equator, with the result that the landing site did not significantly stray from the CSM’s ground track. On all the other landings there was sufficient tilt in the CSM’s orbit to require a plane change manoeuvre, and given the LM’s minimal fuel reserves, it was most efficient for the CSM to make it. Therefore, at some point while his colleagues were on the surface, the CMP in orbit executed a plane change manoeuvre.

Changing the plane of the orbit required a burn of the SPS engine of between 10 and 20 seconds. Unlike height adjusting burns that added or subtracted energy from the orbit by firing along the spacecraft’s direction of motion, a plane change burn was usually made at righi-angles to the orbital plane, often near the point where it crossed the Moon’s equator. Preparations for this burn w ere just the same as for any other SPS burn, except that it was usually made using only one of the two engine control systems in view of its short duration and, similar to the circularisation burn, everything had to be done by the CMP alone. In mission control, FIDO calculated ihc details of a burn that would achieve the objectives with minimum use of propellant. This information was written on a PAD and read up to the lone crewman, ready to be entered into the computer under Program 30.

Richard Cordon was the first CMP to fire the spacecraft’s big engine alone in lunar orbit. Rather than make the burn just before the LM returned, he carried it out the previous evening, at the end of the day they landed. "I realised at this time that it had been a real long day and I was tired and more prone to make mistakes. I certainly didn’t w-ant to be making mistakes during an SPS burn.’’ Normally for any SPS burn, two of the crewmen worked together through a checklist using the ‘challenge and response’ technique designed to ensure that no step w:as missed a luxury the solo CMP did not have. Gordon’s solution was to have mission control listen to him as he went through each step. Fortunately, unlike most SPS burns, the plane change was made while in communication with Harlh and he had 14 minutes between acquisition of signal (AOS) and the actual burn.

“When I came around this lime and had AOS. I chose to go to VOX operation and read the checklist as I performed it. to the ground so they could monitor exactly where I was, exactly what I was doing, and would be abreast of the status of the spacecraft at all times.” VOX meant that his transmissions were controlled by a voice-operated switch. Each time the CMP spoke, his words were transmitted to Earth, and there was no need to operate a push-lo-lalk button. "It gave me the assurance that I was reading the checklist correctly, not leaving anything out. Now, 1 would think that the ground probably appreciated this. They knew exactly where I was in the checklist, what I was doing, and if I was behind and if I was ahead, so if any particular problem came up. they knew that I was with it or behind it.” Without the weight of his two crewmales and their lunar module, the SPS burn felt much more sporty, as Gordon noted post-flight: ”’l’he acceleration, of course, is much more noticeable than with the LM docked.”

CSM to the rescue

Given a normal mission, the role of the CMP might seem to be minimal in the upcoming orbital ballet of rendezvous, but NASA’s defence-in-depth policy ensured that he would have plenty to keep him occupied. It is true that the LM was always the active participant, as it was its responsibility to get off the Moon, into lunar orbit, then find, track and pull alongside the CSM. But the CMP had the role of rescuer in case the LM failed to execute the rendezvous. For this possibility, he had practised a wide range of scenarios where the CSM would become the active spacecraft and w’ould hunt down an ailing LM.

In an effort to get around the terribly short period of time that an Apollo CSM was permitted to orbit the Moon, barely a w? eek at most, scientists added a small, 35.6- kilogram subsatellite to the SIM bays of Apollos 15 and 16. This was ejected just before the crew? headed home. Its function was to investigate the various particles and fields in the lunar environment. ‘Particles and fields’ is an expression used within the planetary science community for the investigation of planets and their environments whereby, rather than taking pictures of a planetary body, measure-

ments are taken of the force fields, molecules and radiations that surround and interact with it. In the late 1990s, this work was continued by the Lunar Prospector probe.

The subsatellites added an extra complication to the missions’ flight plans because the scientists did not want them to be placed into the CSM’s normal orbit. Orbits around the Moon are inherently unstable. Given enough time, the influence of the mascons beneath the lunar surface and the tug of Earth’s gravity will cause an orbiting body to hit the surface. The subsatellite had no means of propulsion with which to compensate for these perturbations and if it were to be deployed from the CSM’s normal orbit, its lifetime would have been measured in weeks. But by manoeuvring the CSM prior to deployment it would be possible to extend its life towards a year.

“I have the Shape SPS/G&N PAD, when you’re ready for that,” said Joe Allen. He was ready to read up the details of the bum that would shape Apollo 15’s orbit in preparation for the subsatellite launch.

Jim Irwin usually took on the task of copying down the pads for this mission: "Okay, Joe. I’m ready on the Shape PAD.” Occurring only two and half hours before TEI, the manoeuvre required only a З-second bum of the SPS engine to raise their orbit’s apolune and perilune from 121.1 by 96.7-kilometre values to 140.9 by 100.6 kilometres respectively.

The shaping burn was made successfully just before Endeavour went behind the Moon for its penultimate lime. Then, around the far side. Worden executed ‘Verb 49′ in the computer, w’hich instructed it to bring the spacecraft’s attitude around to one that would place the long axis of the subsatellite perpendicular to the ecliptic and therefore perpendicular to the Sun. The launching mechanism was designed to spin the subsatellite as it was ejected from its receptacle in the SIM bay. This stabilised the small craft as it drifted aw-ay from the CSM. allowing the solar panels around its body to receive the sunlight required to power it. When they came back around the near side, the crew’ armed the pyrotechnics of the ejection mechanism while mission control monitored the spacecraft’s telemetry. An hour and 20 minutes before 1 HI, Allen piped up: "Endeavour. W’e verify your SIM pvro bus arm, and your rates look good to us down here. Over.”

"Okay,” replied David Scott. "We’ll go Free.”

As it w’as desirable for the spacecraft to be as still as possible for the deployment, time had been allowed for its rate of rotation to settle down within the half-degree dead band around the ideal launch attitude. Then, rather than risk the thrusters firing just as the subsalelliie departed, the control mode for attitude was switched to Free essentially disengaging the autopilot and allowing the spacecraft to drift. This w’as the first time that such a satellite ejection had occurred on a NASA spacecraft. "And wn know one of you will be watching out the window,” reminded Allen. "We’re particularly interested if the spin of the satellite is sweeping out a cone or if it seems to be a fairly flat spin as it comes out.” What Allen meant was that the satellite should be spinning around its long axis. It w’as important to the long-term future of the little spacecraft that this rotation be as even as possible as it departed.

However, there was still enough rotation in the CSM to take it slightly out of the desired attitude. ”Endeavour. we’re requesting you go back to Auto and do another ’Verb 49’. please. We see you drifted off about a degree.”

"In work," obliged Worden.

A suggestion then came from someone in mission control that the CSM should constantly correct its attitude until just before the launch. "Okay. Endeavour called Allen. “We’re recommending that you go back to Free at launch minus one minute."

"Okay; Free at launch minus one minute." confirmed Irwin. Mission control was still considering this one. Allen came on the air/ground a minute later with a revised procedure: "Endeavour, we’ve got a new update for the last instructions. Go Free at launch, please.”

Scott took a turn to reply: "Roger; Free at launch.”

By minimising the time spent Free they would reduce the scope for drifting off attitude.

Scott counted down the moments to launch: "Three, tw’O, one. Launch. We have a barber pole."

The subsatellite and its deployment mechanism moved along a track, opening a door in the process. It then engaged a switch that Tired the pyrotechnics to release it. allowing a spring to push it away from the spacecraft. A pin engaged in a curving
groove in a cylinder and imparted a rotation to the subsatellite which was stabi­lised by the deployment of three antennae. Scott saw a talkback indicator go to its ‘barber pole’ state. Once launch was complete, the deployment mechanism was retracted and this placed a grey flag in the indicator.

“And a grey,” confirmed Scott. “Tally Ho!”

“Can you see much?” asked Allen.

“Oh, looks like it might be oscillating maybe 10 degrees at the most,” said Scott as the long, hexagonal satellite drifted away, its three long, thin antennae sweeping out arcs in the sunlight. “A very pretty satellite out there. We get about two flashes per rev off each boom, and it seems to be rotating quite well. Very stable.”

The Apollo 15 subsatellite worked well for seven months before its telemetry failed. Apollo 16’s fared less well because mission control had decided to save the iffy SPS engine for the TEI manoeuvre and therefore cancelled the burn to shape their orbit. The subsatellite operated perfectly for 34 days before the changes in its orbit caused it to impact somewhere on the far side. The main result was a greater understanding of how the solar wind interacts with the Moon. In particular, the magnetometers on board each subsatellite also provided detailed information of the remanent magnetic field that some areas of the Moon exhibited in the form of miniature magnetospheres which warded off the solar wind.

Other tasks that had to be completed prior to the TEI bum on a J-mission included the retraction of instruments and paraphernalia that projected from the SIM bay. As the mapping camera was operated while extended out along a track to give the stellar camera a view to the side, the entire device was supposed to be retracted. However, this mechanism failed on Apollo 15. On Apollos 15 and 16, two instruments were operated on the end of 7-metre-long booms that could not withstand the load of an SPS engine bum. Although these booms were excluded from Apollo 17, it had two long antennae that projected out to either side of the service module, and these had also to be retracted. If any of these protuberances failed to retract, the crew had the option of jettisoning them, as was done when Apollo 16’s mass spectrometer boom failed.

The path of an Apollo spacecraft from the Moon to Earth began with the trans­Earth injection burn around the Moon’s far side. This set the spacecraft on an S – shaped trajectory which, had Earth not possessed an atmosphere, would have caused the spacecraft to loop around at an altitude of about 40 kilometres before returning to deep space on a very long elliptical orbit. Of course, Earth does have an atmosphere and any spacecraft on a trajectory with a 40-kilometre perigee is bound to plough into its gases where the immense kinetic energy will be dissipated as heat.

N Moon at ‘■ PTEI+2 days

Diagram of Apollo’s trajectory from TEI to re-entry.

If the spacecraft were to penetrate deeply enough into the atmosphere to lose the momentum needed to return to space, then providing that it has appropriate protection it will instead reach the surface. The return trajectory of the Apollo spacecraft was designed to achieve this in a highly controlled manner.

A modification of this technique is commonly used by unmanned spacecraft as a means of arrival at other planets. The conventional technique is to slow’ from approach velocity to orbital velocity by the consumption of a large quantity of propellant in a long burn. However, substantial weight savings in propellant can be made if the spacecraft carefully dips into the upper reaches of a planetary atmosphere, where it can lose small increments of velocity. The initial insertion burn can then be much shorter and the resulting mass reduction will also reduce overall mission costs by enabling the spacecraft to be launched by a smaller, cheaper rocket.

If Apollo’s targeted perigee w’ere too low. the entry angle would be steeper than ideal. ‘This would increase both the heat impulse that the heatshield had to deal with and the deceleration forces that the crew would have to endure. It would also tend to shorten the entry flight path – perhaps by more than the CM’s flight characteristics could compensate, forcing a landing short of the planned point. In the extreme case, it would be lethal, either by excessive g-load or by incineration. A higher than ideal perigee, and hence a shallower angle, would result in a longer entry path and lower g-forces. but at the risk of the spacecraft failing to shed enough energy to enable it to be captured before it rc-emerged from the atmosphere to coast out into space on a long elliptical orbit. Since the solo command module had no means of propulsion and very limited supplies of power and oxygen, failure to be captured by Earth’s atmosphere at the first attempt would be fatal for the crew’.

By the time P67 took over, the command module had slowed below the velocity required for orbit. The. job ОҐР67 was to continue to control the lift vector and the associated g-forces, rolling the spacecraft this way and that as necessary to guide it to its planned impact point while the speed decreased to only 300 metres per second. In addition to controlling how far the spacecraft would fly by directing the lift vector up or down. P67 also compensated for whether the trajectory was taking the spacecraft to either side of the target. By rolling up to 15 degrees to either side, a useful amount of lift was aimed to the left or right without impairing the lift in its up down axis. During P67, the DSKY displayed how much sideways angle they were steering and how far the computer reckoned their current impact point would be from the ideal, both left;right and long, short.

"Here comes the water again on my feet,” laughed Stafford on Apollo 10.

“What water’.’" asked Young.

“From the freaking tunnel! It’s cold, John baby,” said Stafford.

“Three g’s,” called Young.

“It’s going to pulse the lift vector up,” said Stafford.

“Fourg’s. Going to go lift vector up,” announced Young.

“We’ll let her shoot, lift vector up,” said Stafford, happy that the computer was doing its job.

“Go, baby. Just fly,” cried Cernan.

“Okay, Houston, we re showing six miles short right now," called Stafford, "and we’re coming on in; pulling about four g’s and this machine just flying like cra/y. Boy. it’s really going."

“Well, I’ll tell you, this thing is beautiful,” said Young in admiration for what the little module was capable of doing. The computer reckoned they were going to land 11 kilometres short, and had rolled the CM mound to a feet-up. lift-up attitude to gain them more distance.

“And we’re pulling about 3.5 g’s now. We’re rolling right 60 degrees, and we’re practically on top of the target. FMS is reading 21 miles to go. Okay, we’re coming down. I guess we’re about 150 К right now.”

They were still 47 kilometres up but most of their horizontal speed was gone. Stafford began to think about how to keep track of what was increasingly a vertical plummet.

“Steam pressure. Get the steam pressure,” he called.

Before Apollo 8 flew, Bill Anders had come up with an idea to give the crews a backup means of determining how high they were, rather than just depending on their altimeter. The CM already had a meter that indicated the pressure in the steam duct that led from the evaporator. Since it was reckoned that its reading would begin to rise at an altitude of 90,000 feet or 27.4 kilometres, they could start a watch as soon as it came off the zero peg and check the timing of subsequent events from there, particularly the deployment of the parachutes. This w:as the reason mission control had included a Liming for when they could expect 90.000 feet in the entry PAD. The technique worked well when Anders used it on his flight, but subsequent crews found it less reliable.

“The steam pressure peg was somewhere between Five and 10 seconds late." explained Ed Mitchell during Apollo 14’s debrief. "At 6:36 it still had not started to move up so I switched to secondary. It hadn’t moved either, and I remarked about it to Al. About that time, it [finally] started to move.” Mitchell was concerned that if relied upon, it could be misleading. "If we had to enter on that and go by times, I think we might have been in trouble.”

Duke found that it was even later than predicted on Apollo 16. “The only thing that was off nominal was the steam pressure was 32 seconds late. I started watch on the time that steam pressure pegged. If we had to call the times based on that, we‘d have been late.”

Once the spacecraft’s velocity had slowed to less than 300 metres per second, P67 changed the computer’s display to show – the range to splashdown and the current latitude and longitude of the spacecraft. By now they were essentially above the

recovery site, 20 kilometres up and falling more or less vertically. P67’s final act was to inhibit further thruster firing by transferring spacecraft control to the stabilisation and control system (SCS) before the program was terminated by a press of the ‘Proceed’ button.

OVERJOYED

“Hello, Houston, the Endeavour’s on station with cargo, and what a fantastic sight.” David Scott could not contain his delight at his close-up view of the Moon as soon as Apollo 15 came back into communications with Houston on emerging around the eastern limb.

Capcom Karl Henize in mission control empathised: “Beautiful news. Romantic, isn’t it?”

“Oh, this is really profound; I’ll tell you. Fantastic!” enthused Scott.

Not everyone thought Scott’s outpouring appropriate. According to lunar geologist Don Wilhelms, the no-nonsense Alan Shepard, commander of the previous mission, Apollo 14, was heard to grumble: “To hell with that shit, give us details of the burn.” But Scott’s ship was sound. Everything was going well, and they were merely greeting their friends on Earth. Data about the burn was indeed about to be relayed. As soon as the LOf burn had finished, the crew had interrogated their computer for its estimate of their orbit. They had also written down the residuals – velocity values that represented the difference between what they had asked of the engine, and what it had actually given them.

Engineers were keen to know how well the precious engine with its troubled A bank had worked, and the raw burn data from the crew was the first indication of its health. Another was the tone of the crew. However, the meat and drink for the engineers was stored on the DSE tape recorder, which, after each far-side pass was wound back and its data transmitted to Earth. This engineering data revealed every nuance of the engine’s performance, fn addition, the voice track supplied the crew’s conversation from before, during and after the burn. If there had been some glitch, the ground wanted to understand it before the engine was again called into action.

The next task for the flight controllers was to determine as precisely as possible the trajectory through space that was being followed by the spacecraft. With the crew’s burn report, they had preliminary confirmation that the engine had achieved the LOI burn as planned, a fact that was supported by the precise moment that the

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

DOI 10.1007/978-1-4419-7179-1 9. © Springer Science+Business Media. LLC 2011 antenna on Earth had reacquired a signal. This preliminary information included what the spacecraft’s computer believed to be the perilune and apolunc of the orbit. Now that they had a radio signal to work with, the engineers at the remote station began to track the spacecraft in order to provide an independent determination of its orbit. If all was well, their figures would compare closely to those from the crew and show an orbit whose low point around the far side was 110 kilometres in altitude, but which, on the near side, rose to around 300 kilometres.

The crew of Apollo 12, the chummiest ever to fly to the Moon, didn’t hold back their awe once they had entered lunar orbit. Although Alan Bean possessed a talent for artistic expression that would come to the fore later in his life, his ’gee whiz – look at that’ reaction matched his friends’ simple ebullience. "Look at that Moon.” he said in awe. His commander Pete Conrad agreed: "Son of a gun. Look at that place.”

"Gosh! Look at the size of some of those craters.” continued Bean, as their conversation descended into something akin to a B-movie script. The direction of the Moon’s lighting on this flight was quite different from that encountered by previous crews. As the landing site was well to the west of the near side, the Moon appeared to be nearly full to observers on Earth. As a result, the far side was largely unlit and the crew’ wouldn’t get to glimpse of the Moon close up prior to LOI.

After they had cleared up more of the spacecraft’s configuration post-LOl, Bean took time to look out the window again: "Man! Look at that place.” Lunar module pilots always seemed to have a little more Lime available to them when in the CSM – although, of course, the situation would change when they were in the lunar module. Bean was later reminded of old science fiction serials when thinking about how his friend, Richard Gordon, had looked after the CSM that had brought them from the Earth: "Outstanding effort there. Dick Gordon. Flash Gordon pilots again!”

Among the crews, there was a fascination with the Moon’s colour up close, as if it would be any different from being viewed across 400,000 kilometres of hard vacuum.

"Look at that Moon bugger! I’ll tell you.” said Bean. "I may be colour blind, but that looks grey as hell to me.”

Conrad chimed in again: "Good Godfrey! That’s a God-forsaken place; but it’s beautiful, isn’t it? Look how black the sky is." With his eyes adapted to the brightness of the lunar limb, the blackness of space stood out in stark contrast.

"That’s grey and something else,” said Bean. In his later years, he would confess to a fascination with colour and would spend much of his artistic life imbuing an otherwise colourless Moon with a vast range of hues.

Conrad expanded on the description: "Chalky white those craters have been there for…” Bean interrupted him, "a few days."

"Yes."

At last, Gordon threw his opinion into the pot: "Man, this is good to be here – is all I can say.”

Most of the LM’s descent stage was taken up with the descent propulsion system (DPS). In the parlance of Apollo, the DPS was always pronounced dips’. The designers had come up with a simple cruciform structure for the descent stage which held a propellant tank in each of the four box-shaped bays around a central space where the engine was mounted.

This engine was remarkable for its time because it could be throttled, i. e. its thrust was variable, which was a major technical achievement. The basic idea of the throttle mechanism was to alter the area of the injector plate in use at any one time – similar

to an adjustable shower head. At its theoretical full power, the DPS could generate a thrust of 47 kilonewtons, about half of one engine on a Boeing 727 airliner. In operation, it could be throttled smoothly between 10 and 65 per cent full thrust or run at a steady 92.5 per cent. This ability to be throttled was essential to enable the computer to optimise the vehicle’s descent to the surface and to hover in the final moments before touchdown.

As in the SPS engine, propellants were forced into the combustion chamber by pressure alone. There were no pumps to fail. This pressure was provided by two helium tanks, one of which stored the gas at ambient temperatures; the other tank stored it as supercritical helium (SHe), a strange phase of the gas brought about by a combination of very high pressure and extremely cold temperatures. By using SHe, more of the gas could be crammed into a much smaller tank, thereby eliminating over 100 kilograms of weight from the vehicle. Care had to be taken, however, to carefully handle the heat in the overall system. In steady operation, the warmth in the fuel would be used to heat the SHc via a heat exchanger. But at engine start, before the fuel got flowing, there was a possibility of it being frozen by the SHe. This was the main reason for using the ambient Lank, which would not freeze the fuel while it provided the pre-start pressurisation. First, three explosively operated valves were opened by command from the crew7 to release the ambient helium into the tanks. This was part of the preparatory checklist. Later, once the engine was actually running, another explosively operated valve would automatically open to release SHc into the propellant tanks.

In view’ of the severe weight restrictions imposed on the LM. and given that propellant w-as a major fraction of the spacecraft’s weight, typically about 70 per cent, it wras vital that the tanks, which were somew hat larger than they wrere required to be, were loaded with only as much fuel and oxidiser as would ensure a safe landing. It was equally vital that a system be in place that would allow’ the crew and flight controllers to monitor the remaining quantity, especially because the levels would get low just when the commander w-as likely to be hovering, looking for a safe place to set down.

The tanks of the DPS had two independent systems for measuring propellant quantity, either of which could be monitored by a gauge in the cabin and both of which could be monitored from Earth. As the descent progressed, flight controllers closely watched how each system responded to the falling propellant levels and decided which one seemed to be more trustworthy and appropriate for the crew to monitor.

”Eagle, Houston,” said Duke from his Capcom seat. “It’s ‘Descent two’ fuel to monitor. Over."