Category How Apollo Flew to the Moon

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."

Except for Apollo 11, surface crews had an important item of housekeeping to perfonn once they had returned to the relative safety of the LM’s cabin, namely to replenish their PLSSs if they had another EVA scheduled. Oxygen and water tanks had to be refilled – oxygen for breathing and water for cooling. Batteries for power and lithium hydroxide canisters for carbon dioxide removal had to be replaced. The Apollo 12 crew were the first to carry out these procedures and the engineers saw a chance for a data point.

The early missions tended to be pathfinders for the later extended missions and it was important to know just how– long the consumables in the PLSS had lasted in true lunar conditions as opposed to rehearsals on Earth. The crew could monitor oxygen consumption on their RCU displays and indications of a crewman’s metabolic output could be inferred from the biomedical data that was telemetered to Earth. What was missing was an indication of how much water had been consumed by the sublimator during the EVA to keep a crewman cool, so Conrad and Bean were tasked with weighing the remaining water after their first moonwalk. However, Conrad was none too enamoured with the equipment supplied for the job.

"Houston, you’ll never believe what wfe have been doing for the last 35 minutes.”

“Go ahead,” said Gibson, the Capcom in mission control.

“I am going to take this 35-cent scale that they sent out here to weigh these bags with and break it over somebody’s head!”

NASA had supplied an off-the-shelf spring scale designed for Earth’s gravity with the idea that a lunar measurement would require it merely to be scaled appropriately. But one feature had been overlooked, as Conrad explained after the flight. “The scale ought to have been set to zero. If anybody had thought about it. including myself, the spring tension in the scale itself was never zero in one-sixth g. As I unscrewed it to zero. I unscrewed it all the way, and the screw, the spring and everything disappeared into the bottom of the scale. We had some difficulty putting that baby back together again."

There was still enough momentum for science on Apollo 11 for NASA to begin a crash programme for tw’o simple experiments that the first crew’ could deploy in a few’ moments. This early Apollo scientific experiments package (EASEP) included a seismometer in a first attempt to investigate moonquakes. Solar panels powered the instrument throughout the lunar day and seismic signals were radioed to Earth in real time. It had small radioisotope heaters to provide warmth for the electronics during the intense cold of the long lunar night, but on the first attempt to reawaken it, engineers found it w’as malfunctioning and permanently switched off.

The second experiment still works today. It was the first of a series of laser retro

reflectors that used the fact that an internal corner of a cube mirror will reflect light directly back to its source; in this case, pulses of laser light from Earth. Apollo crews would eventually deploy two more reflectors at widely separated sites. Two further reflectors would be added by the Soviets mounted on their Lunokhod remote – controlled unmanned vehicles. This laser ranging retro-reflector (LRRR) allowed the distance between Moon and Earth to be measured to tens of centimetres accuracy, later to within millimetres. This elegant technique has become a powerful tool for understanding the relative motions of the Moon and Earth, and in turn, a wide range of subtle geophysical phenomena that include the tides, the structure of the Moon and even the move­ment of Earth’s tectonic plates.

A third experiment, not part of NASA’s EASEP, was the solar wind collector. This was essentially a sheet of foil that was left to bask in the sunlight and the solar wind.

It was designed by the University of Bern in Switzerland and jokingly referred to as the Swiss Flag. To­wards the end of the moonwalk, it
was rolled up and stored in a vacuum with the hope that it would contain embedded solar wind particles gathered from beyond Barth’s magnetosphere whose composition could be determined on Earth. Once returned to Earth, this experiment would provide direct information on the chemical composition of the Sun.

The various rescue plans for the CSM meant nothing if the LM could not get off the Moon and reach some kind of orbit. Should the ascent engine have underperformed for some reason and come up significantly short on velocity, the surface crew would be doomed within an hour of lift-off. If il failed to ignite, they w’ere doomed to expire on the surface. This was Michael Collins’s private fear as Neil Armstrong and Buzz Aldrin prepared to make their ascent in the top half of Eagle. “When the instant of lift-off does arrive, I am like a nervous bride. I have been flying for 17 years, by myself and with others; I have skimmed the Greenland ice cap in December and the Mexican border in August; I have circled the Earth 44 times on board Gemini 10. But I have never sweated out any flight like 1 am sweating out the LM now-. My secret terror for the last six months has been leaving them on the Moon and returning to Earth alone; now’ I am within minutes of finding out the truth of the matter. If they fail to rise from the surface, or crash back into it, I am not going to commit suicide; I am coming home, forthwith, but I will be a marked man for life and I know it.”

When compared lo the fuss and bother of a launch from Earth, with its enormous launch gantries, heavy conerete pads, ground support equipment and launch control facilities, it is almost amusing to consider the relative ease with which two Apollo crewmen harnessed themselves into position, configured their spacecraft for launch and pressed a button to get themselves off the surface of another planet. The difference, of course, is that Earth is at the bottom of a very deep gravity well and every last item required for an Apollo voyage had Lo be lifted through a thick atmosphere and hurled towards the Moon. Such a feat required a vehicle of immense power and complexity the Saturn V and a cast of hundreds to send il on its way. The LM ascent stage, on the other hand, was a far simpler machine, made as light as could be and it would launch from an airless world whose gravity was barely one-sixth that of Earth. And given that its task was merely to rendezvous, it would require to provide only several hours of life support.

By the time the advanced Apollo missions got into their stride, NASA had enough confidence to partially power the LM down during the lunar stay, conserving battery power and permitting three days of exploration. In particular, the primary guidance and navigation system (PCNS) was turned off. Turning it back on involved a complete realignment of its guidance platform. As with platform alignments in space, Lite crew used the alignment optical telescope (AOT) mounted into the Lop of the LM to sight on a star. A major difference in the procedure came from the use of the direction of gravity as their second reference.

Earlier, the crew had temporarily depressurised the LM cabin to throw out any items not needed for the journey to orbit, especially the back packs that had kept them alive on Lite surface. They would keep their suits on until they returned to the CSM but would be connected directly to the LM’s supply of oxygen and coolant. Other equipment and samples had to be carefully stowed in predetermined positions around the cabin to ensure that the centre of mass of the ascent stage remained as near to ideal as possible the further it was from ideal, the more the RCS thrusters had to work during the ascent to maintain the stage’s attitude.

If the pressure of time allowed, the surface crew would try to test their rendezvous radar on the CSM as the mothership passed over the landing site one orbit prior to lift-off. The rendezvous radar worked with a transponder on the CSM to provide range, range-rate and direction to its quarry. Its dish was mounted above the LM’s front face and could move up and down or side Lo side as it tracked the CSM from a distance of up to 750 kilometres. At the same time, the CMP carried out a tracking program in his computer to help aim the 28-power sextant at the landing site. By taking marks on the LM centred in his viewfinder, he helped mission control to improve their reckoning of the LM’s state vector information that was loaded into the LM computer shortly before lift-off.

Mission control then read up a lift-off PAD to both the LM crew and the CMP that gave details of their rendezvous. On later missions, two PADs were sent covering two types of rendezvous – one as a fallback in case the other had to be aborted. With less than an hour to lift-off. the commander gave his RCS thrusters a cheek by firing them while still sitting on the surface. When Pete Conrad did this, he managed to blow over an umbrella-like dish antenna that he and Л1 Bean had deployed on the surface and the resultant loss of communications meant they had to switch over to the high-gain dish on the roof. Power was then switched away from batteries in the descent stage to a pair of batteries in the ascent stage. Flight control displays were set up for flight and the abort guidance system (AGS) was initialised to back up the PGNS for rendezvous guidance.

Proceeding on through the launch checklist, the surface crew donned their helmets and gloves. They were about to ignite the ascent stage’s engine while it sat on top of the discarded descent stage, which raised the possibility of the pressure wave from combustion compromising the LM hull. It was therefore wise to be fully suited for the ascent. Then with all checks completed and only a few’ minutes remaining to lift-off, the crew could make their final preparation to leave the surface.

‘ Stand by. You ready to watch the APS pressurise?’- Apollo 15’s David Scott was checking to make sure that mission control was going to watch the vital signs from the ascent stage’s propulsion system, the APS. Its tanks had remained unpressurised until this point.

“Okay, let’s let her go,” replied Ed Mitchell. It had become customary for the LMP from the previous mission to serve as Capcom for ascent because his awareness of what the LM crew were trying to do made him particularly suited to this role.

To pressurise the tanks, explosively actuated valves from two very-high-pressure helium tanks were operated to release the gas into the propellant tanks and bring them up to their working pressure. Mission control checked each tank in turn, fuel and oxidiser, to ensure that if there were any signs of a leak, lift-off could be carried out as soon as possible in order to minimise propellant loss. “Okay, here comes tank 1.” announced Scott. “And w’e‘11 stand by for your call for tank 2.”

“Roger.” said Mitchell. After a brief pause for flight controllers to monitor tank I’s pressure, Mitchell gave the go-ahead for the second Lank. “Okay. Go with tank 2. looks good.”

“Okay. Tank 2 coming now.”

After another pause, Mitchell confirmed that all wtis well. “Looks good down here.-’

“Okay, thank you. Looks good up here,” replied Scott.

“And, Dave, you’re Go for the direct rendezvous. Both guidance systems look good; PGNS is our recommendation.” Mitchell w’as letting Scott know’ that, of their two practised methods for rendezvous, the flight controllers recommended that they use the planned-for quick technique called direct rendezvous, and that, of their two guidance systems, they should rely on the primary.

“Roger. Go for direct on the PGNS.”

“Okay, loud and clear, Dave, and you’re Go for lift-off.” Then Mitchell reminded the crew’ that it was time for another transition in their roles: “And I assume you’ve taken your explorer hats off, and put on your pilot hats.-’

“Yes sir, we sure have. We’re ready to do some flying.” replied Scott.

“Standing by for one-minute,” prompted Jim Irwin whose primary task was to

look after the AGS and see that its knowledge of the ascent matched that of the PGNS. “Guidance steering is in." was his next call as he commanded the AGS to take its guidance information and generate steering commands for the RCS to use in case of an emergency. With a normal ascent, the guidance mode control switch would route steering commands only from the PGNS, blocking those from the AGS. "

Prcloadcd with the data from the ascent PAD. the PGNS was nearly ready to ignite the engine. “Okay, Master Arm is On; I have two lights," called Scott, as he armed the pyrotechnic system that would sever the two halves of the LM. and saw an indication that the circuits were good.

“Average g is on…” The DSKY display had blanked to show that the PGNS was now calculating the average acceleration the LM would experience as it flew. In other words, it was now guiding the LM. It just had not ignited the engine yet. On the right side of the cabin, Irwin started a 16-mm movie camera aimed out of the window to film the view of the ascent.

Scott continued with his steps prior to ignition.

“Abort stage." Pressing the ‘abort stage’ button caused the ascent and descent stages to separate, using explosive bolts to sever the four attachment points holding them together. At the same Lime, explosive charges drove guillotine blades through the bundles of wiring and plumbing to sever those connections also.

“Engine arm to ;ascent’." Arming the ascent engine allowed the engine controller to open the valves on the engine. Then at T minus five seconds, the DSKY displayed Verb 99, which was its way of asking the crew if they washed it to proceed with engine ignition.

“99, Pro." intoned Scott, at which point he pressed the ‘Proceed’ button on the DSKY, essentially replying, “Yes, please."

Television viewers on Larth were given a ringside seat at the launch of the last three LM ascent stages. Before he entered the LM for the last time, the commander parked his rover 100 metres east of the LM. From this vantage point, this miniature interplanetary outside broadcast station, remotely controlled from Larth via its own radio link, provided coverage of the lift-off itself, and the quiet, still and desolate scene that followed for as long as the rover’s batteries and equipment continued to operate. When Apollo 17’s ascent stage lifted off. Ed Fendell, who operated the TV camera, managed to follow the early stages of Challenger s ascent to orbit, despite a З-second delay between his command to Lilt and seeing the result on his monitor. It showed how7 the ascent stage went straight up for just 10 seconds – yawing a little as it did so to aim the vehicle towards the flight azimuth then promptly pitched nose dow’n by a little over 50 degrees in order to start adding horizontal speed. This was very different to a launch on Earth, where a streamlined rocket has to rise essentially vertically during the first few minutes to escape the bulk of the atmosphere before it can ramp up its horizontal speed to reach orbit. The lack of an atmosphere on the Moon allowed the LM to start to gain horizontal velocity almost as soon as it left the ground w hich permitted a more efficient flight profile.

As the LM pitched over, the crew’ gained a view7 of the landscape they had previously been exploring. As Aldrin related during his debrief, they were also able

to watch the after-effects of their lift-off. “I could see radiating out, many, many particles of Kapton and pieces of thermal coating from the descent stage. It seemed almost to be going out with a slow-motion-type view. It didn’t seem to be dropping much in the near vicinity of the LM. I’m sure many of them were. They seemed to be going enormous distances from the initial pyro firing and the ascent engine impinging upon the top of the descent stage.”

“I observed one sizeable piece of the spacecraft flying along below us for a very long period of time after lift-off,” noted Armstrong. “I saw it hit the ground below us somewhere between one and two minutes into the trajectory.”

Aldrin was fascinated by the physics demonstration he was seeing: "It’s very difficult to conceive of such lightweight particles like that just taking off without any resistance at all,” he continued during his debrief. "It’s easy to think back and say

that they would do that. But it just seems so unnatural for such flimsy particles to keep moving at this constant velocity radially outward in every direction that I could see out the front window. I don’t recall seeing any impact with the ground, but there were sizeable pieces."

Just behind the two crewmen in the centre of the cabin was the cylindrical cover of the ascent engine. Many have remarked on how close two astronauts stood to an operating rocket engine. As he and Irwin flew over the meanders of Hadley Rille, Scott noted how the sound from the engine was audible through his helmet. “Both guidance systems are good, Dave." called Mitchell as the flight controllers closely watched the numbers coming up on the PGNS and the AGS.

"Okay, looks good up here." returned Scott. Then he remarked to Irwin. "It almost sounds like the wind whistling, doesn’t it?’’

Scott was impressed by the sound, and the ascent in general. “Truly amazing," he described decades later. "The LM launch and ascent were so quiet, especially when compared to Titans and Saturnsl It was almost peaceful – some vehicle oscillation, periodic at a couple of degrees, and the periodic sound of a slight wind, pulsing at about two to three seconds in frequency. And. of course, the view of the Rille as we flewr right along its course, face down. Just spectacular. Could not have been a better farewell. Most pleasant and certainly indelible.”

The gentle wobble imparted on Falcon as it soared through the lunar sky was caused by the periodic thrusting of the RCS jets. For simplicity, and to reduce the weight, the ascent engine was fixed. It could not aim its thrust anywhere but straight down along the spacecraft’s, v axis. As a result, the RCS had to do all the steering by turning the entire spacecraft and therefore aim the ascent engine in the right direction, which it did at each two-sccond cycle of the computer’s tasks. Only the downward-facing thrusters were used, there being no need to fire the upward-facing thrusters as they would be thrusting counter to the ascent engine.

The ascent was a critical event. If the APS engine were to underperform, which it never did, there was a real possibility that the LM would not achieve a stable orbit and instead would crash after less than half a revolution. The crew watched the velocity and altitude readings from both of their computers and compared them to charts, looking for any deviations that might indicate a problem. One remedy for an underperforming ascent engine was to augment its thrust with the four RCS jets that were aimed in the same direction. Because the ascent stage was relatively light and the Moon’s gravity relatively weak, the ascent engine’s rated thrust was only 15.6 kilonew tons. about the same as the first jet engines introduced during World War II. Four thrusters could provide more than one-tenth of that at 1.8 kilonewtons total a thrust that could make a difference in the later stages of a problematic ascent. With such emergency contingencies in mind, the LM’s designers had arranged the RCS plumbing so that, if their own tanks ran dry. they could draw propellant from the ascent engine’s tanks.

Once the PGNS had determined that the ascent engine had added enough velocity, it commanded a shutdown. Immediately, the crew quizzed the computer on the size of the orbit they had achieved.

“PGNS says it’s in a 40.6 by 8.9.’’ reported Scott as soon as Falcon had entered

orbit. The numbers showing on the DSKY represented the altitudes of the apolune and perilune respectively, given in nautical miles. ‘I’heir orbit appeared to be 75.2 by

16.5 kilometres, which was only slightly lower than desired.

“Roger, we copy," replied Mitchell in mission control; then reassuringly said, "the guidance still looks good to us.’’

"Okay."

Within a minute, mission control had their orbit, as determined by radio tracking, to hand. "Falcon, Houston. We have you at a 42 by 9,’’ announced Mitchell.

"You’re looking good."

"Okay. 42 by 9," confirmed Scott.

Although their tracking data put the orbit a little higher than Falcons at 77.8 by 16.7 kilometres, the trajectory experts were satisfied that the rendezvous should go ahead as planned.