Category How Apollo Flew to the Moon

In 1959, the Soviet Union continued a habit of achieving space firsts when they took the first images of the Moon’s far side. Though grainy and of low resolution, these were sufficient to show a dearth of mare landscape. They added to their tally when they softlanded a probe in early 1966 and returned the first picture from the surface.

However, it was the American unmanned missions that gained prominence in acquiring high-grade knowledge of the Moon prior to the Apollo programme. In the process, NASA learned how to design reliable spacecraft for the lunar environment and how to operate them from a distance. For a manned landing to be attempted, as Kennedy had directed in 1961, it was essential that the engineers designing the lunar module be aware of the nature of its surface. The best Earth-based images at the time showed features no smaller than about a kilometre across, and were hardly suitable for finding rocks and slopes that could topple a lander.

NASA initiated the Ranger project to take their first close look at the lunar surface. In its final form, Ranger was a simple probe; little more than a platfonn for slow scan television cameras that imaged the Moon as it fell at cosmic speed to its doom. Initi­ally, the series had little success because either the launch vehicle or the probe itself would fail before the target was reached.

NASA were on a steep learning curve about how unforgiving the practice of operating rockets and

spacecraft could be, but these The Ranger spacecraft. (NASA)
failures taught them about the need for extremely high reliability in the design and construction of spacecraft and their launch vehicles. Three space­craft in the series eventually met with success, beginning with Ranger 7’s impact on a patch of mare west of the centre of the Moon’s visible face. In view of our new knowledge of its surface, this previously unnamed area between the vast Oceanus Procellarum (Ocean of Storms) and Mare Nubium (Sea of Clouds) was renamed Mare Cognitum (Known Sea). Ranger 8 was targeted at another smooth area in the southern stretches of Mare Tranquilli – tatis, just east of the meridian, that planners believed might offer a good site for a future manned landing.

The final probe in the series, Ranger 9, was given over to the scientists who programmed it to dive into Alphonsus, a large, distinctive crater near the centre of the Moon’s disk. They were particularly interested in a number of unusual dark patches within the crater which appeared to be the result of volcanism. This mission was also of interest to commercial TV networks, which broadcast the spectacular live images streaming back from the spacecraft so that the public could watch its suicidal dive in real time. The final frames from these probes showed surface features as small as half a metre across and, to the relief of the lunar module designers, the presence of large rocks sitting on the soil strongly suggested that the surface would be able to support a LM.

Ranger’s plummet to the lunar surface could yield only limited coverage. Apollo’s planners wanted to survey large areas of the equatorial near side for possible landing sites and for landmarks to aid navigation. They also wanted high-resolution images of a number of sites to certify them for human visits. Meanwhile, scientists wanted imagery from across the entire Moon in order to improve their understanding of its complexities. The unimaginatively named ‘Lunar Orbiter’ series were able to achieve both of these tasks in a single year of operations. This was a much more sophisticated probe. It went into a controlled elliptical orbit around the Moon that had its perilune over the near-side equatorial zone. It photographed the surface with two cameras, one of which could capture surface details as fine as a metre across. Its imaging system chemically processed photographic film on board and later scanned and transmitted it to Earth. All five probes in the series were successful, although the first suffered operational problems that limited its usefulness. By the time Lunar Orbiter 5 was intentionally crashed to clear the way for Apollo, almost the entire surface had been mapped, with much of the near-side equatorial zone imaged at high resolution.

Forty years later, a team of engineers acquired one of the old tape machines that had archived the recordings from the Lunar Orbiter transmissions and they nursed it back to life. Once they could replay the archive tapes and digitise their imagery, an almost impossible feat in the mid-1960s, they were able to coax pictures out of them that hugely exceeded the originals. These can be compared to images taken in recent years to detect changes in the lunar surface across the decades.

Running concurrently with Lunar Orbiter were the last of NASA’s pre-Apollo probes, the Surveyors. Their primary task was to prove that a landing could be made using a leg technology similar to that being planned for the LM. Seven missions were launched, of which five were successful. Most were sent to characterise the surface near prospective Apollo landing sites along the equator. The final mission was given over to scientists, who sent the spacecraft well south to land in the highlands on the ejecta blanket of Tycho, one of the Moon’s most prominent craters.

Though these missions gave NASA a solid overview of the Moon’s topography, surface strength and texture in support of a manned landing, a deeper understanding of its composition and history had to await the results of the Apollo missions. Neither the Rangers nor the Lunar Orbiters went further than imaging the Moon in optical wavelengths. No spectral data, not even simple colour was obtained and therefore the composition of the lunar soil could not be studied. The last three Surveyors did carry small experiments to study the composition of the soil. These showed that, based on three sites, the maria soils were basalt-like and were richer in iron and titanium, while the highland soil near Tycho was richer in aluminium and calcium. This hinted at the bigger picture that would not become clear until lunar material returned by astronauts was analysed in terrestrial laboratories.

The more powerful of the two cameras in the SIM bay was the panoramic camera, which was also derived from contemporary secret reconnaissance cameras. Far from conventional, this large camera produced enormous negatives 114 mm wide and 1.15 metres long. An exposure was made by rotating a large lens of 610-millimetre focal length from one side to the other while simultaneously winding film through the camera. The long axis of the resultant image was perpendicular to the spacecraft’s orbital track and documented a swathe of terrain 330 kilometres from end to end.

The imagery at the centre of the frame showed the ground directly beneath the spacecraft and could resolve features as small as two metres across. Included with the camera was a sensor that measured how rapidly the ground was moving past the camera in order to compensate for motion smear during the exposure. Additionally, the entire optical assembly could be pivoted forwards and backwards to facilitate stereo imaging of the same landscape with every fifth exposure. This huge camera was fed by a cassette that held two kilometres of film, sufficient for over 1,500 exposures during a mission.

Once the spacecraft had begun its long coast back to Earth, the CMP made a brief spacewalk down the side of the service module to retrieve the cassettes for both the panoramic and the mapping cameras.

Remote sensing

Lunar scientists took the opportunity, and the flowing money associated with Apollo, to endow the SIM bay with other capabilities in addition to its photographic coverage. These allowed measurements of the Moon’s composition to be taken across a wide area from orbit. These could be calibrated and contextualised by the ‘ground truth’ provided by the surface crews.

Techniques for determining the composition of distant astronomical bodies were worked out by astronomers in previous centuries. In simple terms, they relied on the property of substances to radiate or absorb light in precise wavelengths, or energies. To the eye, each substance appears to have a characteristic colour which, when spread out into a spectrum by a spectrograph, reveals patterns of lines that act as a fingerprint of that substance. Spectra for common chemicals can be obtained in a laboratory. When the same patterns are observed in the light from distant bodies, researchers can be certain of the chemical constituents in that body. All you need is a spectrograph to break light into its separate colours and you can see the patterns of radiation or absorption that correspond to each substance.

Using appropriate instruments, this basic technique can be expanded beyond the narrow range of light wavelengths that we see with our eyes to include the wider electromagnetic spectrum and the various particles associated with ionising radiation. As the CSM flew over the Moon, instruments in the SIM bay took

A frame from Apollo 15’s panoramic camera showing Hadley Rille.

" (NASA)

advantage of the complete lack of a worthwhile atmosphere to determine the makeup of the surface using a varied suite of techniques.

The final major manoeuvre of the rendezvous was braking. Since the TPI burn, the LM had been coasting on an intercept trajectory that was essentially part of an orbit. The apolune of that orbit was a kilometre or more higher than the altitude of the CSM and. without braking, the LM would have sailed by in front of its target. When at a distance of about three kilometres, the commander began a series of manoeuvres to reduce the closing speed of the two spacecraft. Each was pre-planned to occur at ever narrowing ranges to the CSM. and although the checklists gave suggested approach speeds, the commander used his piloting instincts to achieve the actual braking thrust at each. As explained by John Young, w’hen he talked about his approach on Apollo 16. the approach speed was determined by other factors. If the LM thrusters stopped working, could the CSM finish the job? Young was aw’are of the difference between the light LM with its effective thrusters and the heavy CSM, which was still loaded with propellant for the burn home.

”As opposed to the usual Kamikaze brake that I usually make, we kept it very conservative. We decided that we w’ould ahvays keep the braking within something that the [CSM] could do. This means that, contrary to the braking gates that w;e use in the LM. you sort of have to lead them. In other words, at the range that you want to be at, you almost have to be at the braking velocity to give the [other spacecraft] a fighting chance in case it has to do it. I never had any doubt that we would do it all ourselves because that machine was working so beautifully. We just closed in and it was so good I wanted to do it again. It was really slick.”

Buzz Aldrin was struck by the responsiveness of the lightweight LM. “Each time you hit the thrust controller,” he explained, “the vehicle behaved as if somebody hit it with a sledge hammer, and you just moved. There is no doubt about the fact that the thrusters were firing. It was sporty; there’s no doubt about it.”

“It’s a very light, dancing vehicle,” agreed Armstrong.

On completion of a successful rendezvous, it was normal for the two vehicles to spend some time station-keeping – that is, floating next to each other – to give each a chance to inspect the other. For example, on Apollo 15, Scott and Irwin were asked to look at the SIM bay in the side of Endeavour’s service module. While Worden had been operating the cameras and instruments mounted in the bay, mission control had noticed that the output from a sensor was not as expected. It was designed to measure how fast the landscape below passed by, to enable a mechanism in the panoramic camera to compensate for image motion. Scott was being asked if he could see any obstruction in front of the sensor, which he could not. The problem lay in its optical design.

On Apollo 16, as Orion lifted off the Moon in front of the rover’s television camera, flight controllers noticed that the launch appeared to disrupt the skin at the

rear of the ascent stage, the part that faced the camera. This unpressurised section housed much of its electronics, including the electric control assemblies (ECAs), which were part of the spacecraft’s electrical supply system. Ken Mattingly in the CSM Casper was asked to describe what he saw as Young made Orion perform a pirouette in front of his camera.

"Okay, on back side, it looks like some of the thermal blanket around the ECAs on the back end there is pretty badly chewed up,” radioed Mattingly. "A couple of panels are torn off. And some of the stripping in between, it looks like it was struck by something, but it looks like all the Mylar blankets underneath are still intact.”

Mission control were keen to know the depth of the damage. Jim Irwin, Capcom for the rendezvous, enquired further. "Ken, can you observe whether it’s possible for sunlight to directly impinge on portions of the spacecraft equipment?”

"No, sir,” replied Mattingly. "It’s not possible from the back; I can’t tell about the bottom; but, on the back side, the Mylar blankets are still intact – it’s only that outer covering that’s broken.” Orion’s damage did not prove to be a problem for the rest of its short life.

On all return flights from the Moon, the CMP w’as kept busier than his crewmates. because of his role as the navigator as w’ell as the keeper of the CSM. He regularly realigned the guidance platform to keep it orientated with whatever REFSMMAT was in force at that point of the mission. He was also responsible for maintaining an autonomous ability to navigate the spacecraft to an accurate splashdow’n in the event of a loss of communications with Earth. All the way out to the Moon, he had kept his techniques and skills of cislunar navigation up to date with a series of practise sessions w’hereby his measurements of the angle between the Moon or Earth and a star were processed by Program 23 to yield the spacecraft’s state vector as a basis for calculating its trajectory. Periodically, however, mission control would replace the state vector with one derived from radio tracking.

From Apollo 14 onwards. NASA decided to simulate this ‘no-communications’ scenario by having the CMP maintain an entirely independent state vector. There were two areas of the computer’s memory where the numbers relating to the state vector were kept: the CSM slot and the LM slot. During the coast home, mission control left the state vector in the CSM slot alone for the CMP to refine during his navigation exercises. The only time they touched it was to take account of any engine burns. They would then download it to Earth, bias it with the effect of the burn, and reinstate it for further use by the CMP. Of course, the crew were never expected to rely solely on their state vector. As a precaution, the ground’s version was always kept in the LM slots, since there was no longer a LM attached.

As Apollo 15 hurtled home, its CMP A1 Worden was continuing this practice. At the Capcom console. Bob Parker reported the results of Worden’s P23. navigation sightings: "15, Houston. Looks like a good set of P23s again. Al. And your gamma, right now, on your vector, is 6.5." In the clipped, economical parlance of NASA’s astronauts, what he meant was that calculating Worden’s state vector forward would result in the spacecraft meeting Earth’s atmosphere at an angle of 6.5 degrees. This was the ideal value a point not lost on Worden.

“It sounds like, after a while, we might get along without you, huh. Bob?’’ After all, this was the object of the exercise.

“No comment,” returned Parker.

Worden continued to rub it in. “As a matter of fact, if you guys keep working on your ground [calculated] vectors, they might even converge to the onboard vectors pretty soon."

The crew’ spent the next few’ hours of their approach by making extensive checks of their electrical power, environmental and propulsion systems as well as their caution and warning system. If a final mid-course correction was required, this was the time to make it, and if so then additional onboard navigation sightings w’ould be taken in case communications w’ere lost in the final hours.

Before they reached the waiters of the Pacific Ocean, a coordinated sequence of events had to occur in the correct order. A number of those events included explosive devices. These would separate the command and service modules before re-entry and operate the various subsystems of the ELS during the final drop through the denser layers of the atmosphere. This included the jettisoning of the forward heatshield or apex cover to uncover the parachutes, and their subsequent deployment. These complex events were under the control of the sequential events control system (SECS), whose status could only be verified via telemetry. To give these circuits an early check, the crew7 temporarily armed them and asked the flight controllers to inspect them.

At about the time the PAD was being passed to the crew’, flight controllers were granted direct uplink access to the onboard computer’s memory to change the

REFSMMAT to that required for re-entry. With an hour and a half remaining, the guidance platform was given its final realignment to this REFSMMAT.

It was usual throughout a flight that, when the orientation of the guidance platform had to be changed, it would be realigned twice. The first would be according to whatever orientation had been in use up to that point, which, prior to re-entry, was likely to be the PTC REFSMMAT. This was simply to determine how far the platform had drifted since its last realignment. It gave engineers an additional ‘data point’ in their record of the mechanical characteristics of the IMU in case this data had a bearing on subsequent flights. The second realignment swung the platform around to its new orientation as defined by the entry REFSMMAT. *

As a backup, the same alignment was passed to the spacecraft’s secondary attitude reference, which consisted of the body-mounted gyroscopes and their associated electronics system, the giro display couplers (GDCs). This just required a press of the ‘GDC Align’ button, and although this backup system was more prone to drift, if all went well the spacecraft would be in the water before it became an issue.

Because he had received most training for the re-entry procedures, the command module pilot took the left couch. None of the Apollo crew s w ore their pressure suits for re-entry after the Apollo 7 commander rebelled, citing his head cold as a reason not to have to wear the bulky garment. The CMP manoeuvred the spacecraft to an attitude that put the heatshield forward with the crew hcads-down and looking back. It wasn’t their final entry attitude; a further pitch-up w;ould be required for that a manoeuvre they would execute a few minutes before reaching entry interface. But in this attitude, they could make checks of their attitude control and trajectory, and it w as a starting point for the procedures to jettison the service module.

The first of these checks required that the sextant be aimed at an angle given in the PAD, with the expectation that, if their attitude was as it should be. a specified star would be visible in the instrument’s narrow field of view-. With that check made, the spacecraft’s optics had performed their final task, and were sw ung to a shaft angle of 90 degrees and powered down. Earlier flights also included an additional check of attitude using the COAS – an optical sight, similar to a gun sight, mounted in one of the rendezvous windows. Again, with the sight mounted at specified angles, a named star was expected to be visible through the instrument.

For the first three lunar landing crews, the task of getting out of the CM and back to the ship was further complicated because, up to that time, no one really knew whether or not the Moon might be harbouring some exotic form of life that could threaten Earth’s biosphere. NASA found itself in the unfortunate position of being unable to prove a negative when government advisers and exobiologists raised the question of possible lunar life. It did not matter that the Moon had already shown itself to be an incredibly hostile, dry, irradiated vacuum. Indeed, some pointed out that its surface had the makings of quite a good steriliser. In the event, what had begun as a small laboratory to handle the lunar samples, evolved into a hugely expensive facility surrounded by difficult procedures for the protection of the home planet.

Naturally, the precautions extended to the recovery process. NASA and the other interested parties considered the contamination question for some time before

agreeing that the crew would spend 21 days in quarantine from their first exposure to lunar soil – this being longer than the incubation period for terrestrial bugs. The first few days, of course, were spent in the command module returning to Earth. Soon after splashdown, a frogman opened the spacecraft’s hatch a little and threw in three coveralls and masks, then disinfected the surround of the hatch. In the cramped confinement of the spacecraft’s cabin, and with Earth’s gravity further reducing their room to move, the weak-muscled crew wrestled to get into these biological isolation garments (BIGs).

In the heat of the equatorial Pacific Ocean, Mike Collins found the BIGs to be dreadfully hot and uncomfortable, increasingly so as the long, drawn-out procedures for appearing to care for the safety of the world were acted out. "We put the BIGs on inside the spacecraft. We put them on in the lower equipment bay. Neil did first, then I did after him. Buzz put his on in the right-hand seat. We went out; Neil first, then me, and then Buzz. It’s necessary, at least the way we had practised it, for us to help one another in sealing the BIGs around the head to make sure the zipper was fully closed.” The BIGs were dropped after Apollo 11, and the 12 and 14 crews were only required to wear masks.

The team of frogmen inflated a life raft alongside the flotation collar around the spacecraft. Once again the door was opened and, one after the other, the crew were helped into the raft. The Apollo 11 crew, saddled with wearing BIGs, proceeded to douse each other with disinfectant. ‘‘We sprayed one another down inside the raft,” said Collins during the crew’s debriefing. ‘‘There was some confusion on the

One of the Apollo 14 crew is winched off the life raft in a ‘Billy Pugh’ net. Note the lack of inflated uprighting bags. The recovery team had arrived quickly before the crew had a chance to inflate them. (NASA)

chemical agents. There were two bottles of chemical agents. One of them was Betadyne, which is a soap-sudsy iodine solution, and the other one was Sodium Hypochlorite, a clear chemical spray.” Collins also wondered what was to stop an alien life form from getting washed into the fertile ocean.

One by one, the three astronauts were winched on board a helicopter via the ‘Billy Pugh’ net, then flown to the recovery aircraft carrier. "The helicopter pilot was real good,” said Collins. "You put one hand or foot anywhere near that basket, though, and they start pulling; they don’t wait for you to get in and get all comfortable before they retract. Just like a fisherman, they felt a nibble on the end of that line, and he started cranking.”

The short ride to the carrier tested the endurance of Armstrong, Aldrin and Collins inside their sealed garments. "Aboard the helicopter, we started storing heat,” related Collins. “For the first time I became uncomfortably warm during the helicopter ride. This is the time when the crew is really starting to get uncomfortable. If the crew has to stay in that helicopter 15 or 20 minutes longer than we did, I guess the hood on the BIG would come off.”

Armstrong agreed. "I think we were approaching the limit of how long you could expect people to stay in that garment.”

On reaching the ship, the Apollo 11 crew were not allowed to leave the helicopter. They had to wait while an elevator lowered it to the hangar deck where many of the ship’s crew were waiting, along with President Richard Nixon. Still cocooned in their BIGs, they strode across the deck into the mobile quarantine facility (MQF) that was

to be their home for the next few days. The MQF was carried to Hawaii, offloaded and driven to Hickam Field, loaded into a C-141 Starlifter and flown to Fllington Air Force Base, offloaded and driven to the lunar receiving laboratory (LRL) in Houston where the remainder of the quarantine and debriefing was carried out.

When the first three erews to walk on the surface of the Moon failed to show any sign of illness, the entire quarantine procedure was dropped for Apollo 15 and subsequent flights. Their recovery was a much quicker and easier affair without the worry of planetary contamination weighing down the procedures. But these flights had a particularly heavy workload and. upon reflection. David Scott wished that his crew had also been quarantined in order to give them time to wind down and go through a more rigorous debriefing.

Taking a lunar module to the Moon was not like jumping into a boat, casting off and sailing away. This was an extremely complex, diverse and exotie machine, perhaps even more so than the CSM, and one whose many capabilities were pushed to the limit in order to save weight. The machine had already been given a preliminary check on the coast out from Earth, but now, every system was going to be tested as far as possible while they were still attached to a good CSM.

First of all. the three crewmembers had to put on their suits. At this point there was no need to wear helmets and gloves and this made it much easier to operate equipment and talk to each other. Next, they checked to ensure that it was safe to open the two hatches that separated the spacecraft – after their earlier inspection the hatches had been closed so as to ensure that a failure of the thin-skinned LM, perhaps through meteoroid impact, would not have a catastrophic effect on the command module’s atmosphere. Once they had checked a pressure gauge to confirm that the tunnel was at the same pressure as the CM cabin, the forward hatch was removed, followed by the probe and drogue assemblies that had brought the two craft together. Having gained access to the tunnel, the LMP opened the LM’s upper hatch and floated through into the lander’s cabin. NASA even dreamt up an acronym for this; IVT. for inlravehieular transfer. When the LM’s battery supplies had been brought on line, the umbilical that fed power from the CSM could be disconnected.

Numerous items were transferred across for use in the time the LM would be operating independently. These ranged from pens and books to the helmets and gloves that they would wear on the Moon. Valves were opened to enable the ascent stage to access water and oxygen supplies in tanks contained in the descent stage. The reason these were stored in the descent stage was that they would be left behind on the lunar surface in accordance with the philosophy of discarding dead weight prior to major manoeuvres. Communications, cooling, caution and warning, guidance and navigation, environmental control all the systems that make a spacecraft fit to carry a human – were turned on, tested and checked. Rows of circuit breakers similar to those found on contemporary aircraft were opened or closed as required, based on diagrams in the checklist that gave the LMP a quick method of checking their state by simply scanning his eyes across and comparing the patterns of white or black dots. White meant the breaker should be pulled to reveal a white ring indicating that it was open.

Ever since the lunar module had been installed within the shroud at the top of the Saturn V. its landing gear had been Lucked tight against its descent stage. Explosive devices were fired to deploy the gear to give the LM its familiar form with out – splayed legs. At the same time, long probes that had been folded up against three of the legs were released. These probes, which extended 1.7 metres below the landing pads, would reach the surface shortly before touchdown proper. Their purpose was to provide a cue for the commander to shut down the engine while the LM was still a short distance above the ground. From there, it could gently drop under the Moon’s weak gravitational attraction. The lip of the engine nozzle was only about 30 centimetres above the plane of the landing pads, and planners feared that if a small bump in the surface were to even partially plug the nozzle opening, it could result in a dangerous backpressure within the engine.

Originally a probe had been attached to all four footpads but Neil Armstrong had pointed out the possibility that his descent down the ladder might be impeded by a large length of metal probe that had been bent in some unpredictable way during the landing. The probe below the ladder was therefore removed from Eagle and all subsequent landers.

In the continuing spirit of NASA’s defence-in-depth philosophy, a series of PADs were read up to the crew that not only told them exactly when they were going to start their descent to the surface, but also what to do in the event of an abort being necessary at various times before, during and after the descent – in case the radio link were to fail and prevent the provision of the information at the time of the energency. As the programme progressed and planners made their procedures more elaborate, these PADs increased in complexity. All these PADs were based around a single event, powered descent initiation (PDI) which was the moment the main engine

was ignited to start to slow the spacecraft and take it out of orbit and on to the surface. Some PADs told the crew what to do if the landing was aborted before PDI, others were relevant if PDI did not occur. Yet more had details of the bums to make if an abort were required during the descent – one relevant to the first six minutes, the other relevant to an abort between six minutes and touchdown. All these PADs were copied down by the LMP onto forms in the checklist.

With all the preparations done, they were nearly ready to go to the Moon and pick up some rocks.

Over the decades, many stories have been told about why. when they already had enough to contend with just to land Eagle on the Moon. Armstrong and Aldrin found themselves dealing with what appeared to be a balky computer. The rendezvous radar became deeply implicated in the problem and stories abounded about whether or not it should have been powered at this point. Some said it was a procedural error, or a crew7 error. In fact, there was no problem with it being powered. It was the mode that it was in that was related to the actual problem.

In the 2006 documentary In the Shadow of the Moon, Aldrin. who w ent by the nickname ‘Dr. Rendezvous’ by virtue of his thesis on the subject, saw it as a conflict between the checklist and his operational needs. “Being Dr. Rendezvous,

… I was going to leave the rendezvous radar on and active so if we had to abort, it was on and working and we could reacquire Mike as soon as possible if we had to go up."

Probably the best account of the alarms is told by Don Pyles, then a young engineer at MIT who specialised in the new7 field of software. lie was an integral part of the team who w7rote the computer code for the landing. Given the close relationship the computer had with the spacecraft’s other systems, he knew a lot about them too.

Eyles agrees with Aldrin. "Many explanations have been offered for why the [rendezvous radar] was configured in this way for the lunar landing. For example, a fanciful scheme for monitoring the landing by comparing [radar] data to a chart of expected readings may have been considered by some people in Houston. However, a simpler explanation is sufficient to explain the facts: The [radar] was on for no other purpose than to be warmed up if there were an abort.”

The tale of the program alarms that Eyles tells is quite nuanced. It centres on a little electrical funny associated with two of the LM’s major systems, the computer and the АТС A (attitude and translation control assembly), ‘flic latter mediated between the spacecraft’s controls and the computer. These systems synchronised themselves by way of 28V. 800 IIz AC signals which were meant to match in frequency, which they did. However, the relative phase of the signals was not defined and w7hen the computer was powdered up, which was after the ЛТСЛ had been powered, the phase angle between the two systems could be of any value. If it was near 90 or 210", there were odd consequences. In this condition, data about the pointing angle of the rendezvous radar would make no sense to the rest of the G&1S system if it were in either its Slew or Auto modes (the third mode, ‘LGC. was not implicated). The result of this circumstance was that counters in the computer were continuously being incremented or decremented, an operation that took up valuable cycles of processing time. Unfortunately, upon power-up, this was the situation that Eagle found itself in.

During PDI. the computer was already very busy with all that it had to do to keep the LM on a safe flight path to landing. Ever)’ two seconds, it would run through its list of jobs. These included updating the state vector, controlling the LIVTs attitude, controlling the descent engine’s throttle, adjusting the engine’s gimbals to maintain its aim through the centre of mass, and flying the desired trajectory to the surface. The extra cycles required to deal with the errant counters took the computer very near to the end of its available time before it ran out.

What Look it over the edge in the first instance was a task that Aldrin had to perform when he instructed the computer to display delta-II. the difference between what the computer thought their height was, and the accurate value determined by the landing radar. This task caused the computer to run out of time before it could complete all its allotted jobs, at which point it threw’ up an error code and performed a reset. Thankfully, the software had been written in such a way that, after each reset, it could pick up the threads of all the tasks it had been executing and then continue as if nothing had happened. ‘This resilient approach to software writing is attributed to Hal Laning, another of the software engineers at MIT.

‘The load on the computer increased again when P64 began to run. It had the additional task of calculating LPD angles for the commander and with the errant counters still eating up computer cycles, the time ran out once more. The crew were presented with their second series of alarms. Although the numerical error code was different, the guidance officers recognised it to be of the same basic type as the first, and therefore harmless so long as it did not become continuous.

In 2004, Eyles summed up Laning’s achievement thus, “When Hal Laning designed the Executive and Waitlist system in the mid 1960s, he made it up from whole cloth with no examples to guide him. The design is still valid today. [It] still represents the state of the art in real-time GN&C computers for spacecraft.’’

Since the computer was still doing its primary job flawlessly, despite the alarms, the crewr returned to their roles; Armstrong looking out, and Aldrin keeping him abreast of the numbers. “35 degrees. 35 degrees. 750 [feet]. Coming dowrn at 23 [feet per second].”

“Okay.”

“700 feet. 21 [feet per second] down. 33 degrees.”

“Pretty rocky area," said Armstrong. The erratic LPD angle had swung by a huge amount to 33 degrees and it w’as indicating that they were heading towards an area just outside a large crater known informally as West Crater. It was so named because it was situated on the western end of the landing ellipse. It w:as common for the ejecta blanket around such a crater to include a scattering of large blocks. ‘This

did not look like a place he wanted to set down. Armstrong never got to use the ability of P64 to redesignate his landing site. He was too preoccupied with computer alarms and by the inability of the LPD to give him a trustworthy idea of where the computer was aiming. Instead, he took control, made his decisions and carried them out.

"PICKING UP SOME DUST”: P66 "600 feet, down at 19.” Aldrin continued his litany of data while Armstrong weighed up his prospects. The computer was still behaving and otherwise the descent seemed to be going well. But he had to decide what to do about the block у ejecta around West Crater.

“I’m going to…” he told Aldrin. and assumed manual control of the LM’s attitude by changing to P66. He then pitched forward to an almost vertical attitude that allowed Eagle to maintain its horizontal speed and let him fly over the boulder field of West Crater. Once clear, he pitched the LM backwards again to resume cancelling the craft’s horizontal speed, and he searched for somewhere safe to bring it dowm.

P66 looked after the LM’s vertical speed, also known as its rate of descent (ROD), by adjusting the throttle to maintain a desired value. The commander had a ROD switch that he could flick up or down momentarily to increase or reduce the rate of descent by fixed increments. At the same time, his hand controller let him adjust the vehicle’s attitude, which gave him control of horizontal speed, very much in the manner of a hovering helicopter, ‘l ilt to the left and the engine would aim slightly to the right, pushing the LM towards the left.

“100 feet, 7>Vi down, nine forward.’’ called Aldrin. “Live per cent. Quantity light.” he added.

A light had come on to indicate that they had only 5.6 per cent of their propellant remaining. From pre-flight analysis, planners had decided that, from this point, they could fly safely for only another 114 seconds before they must either land the LM or abort. A 94-second countdown began in mission control that would lead to a call for the crew either to abort or land. If the commander felt he could get the ship dowm within the remaining 20 seconds, he could continue, otherwise he had to get out of there by punching the abort button.

However, Apollo ll’s slosh problem had fooled them again. By triggering the quantity warning light early, it made them believe they had less propellant than was actually available and it came very near to causing an unnecessary abort. A set of fold-out baffles were retro-fitted to Apollo 12’s LM but they were not very effective. It wasn’t until Apollo 14 that the slosh problem was resolved.

Tindallgrams

The manner in which the team decided how to deal with this low-level quantity warning light taps into one of Apollo’s most interesting side stories, because it illustrates the management style of How ard Wilson (Bill) Tindall, one of the senior
engineers. He was an expert on the subtleties of rendezvous and trajec­tories, and became head of the Mission Planning and Analysis Divi­sion. In the hectic days that led up to Apollo’s successes, he coordinated the planning process that threaded together the disparate systems and people to create the bureaucratic edifice that was an Apollo mission.

His method of decision making touched just about every facet of a flight, from the dumping of urine to the position of the Navy’s recovery force or any other thing that was intertwined with the trajectory, and he is considered by many to be a major reason for the success of the programme.

There were two sides to his style.

The first was the manner in which he handled large meetings that involved engineers, programmers, mathematicians, crews or whoever in order to get this diverse mass of people to reach a decision – “knocking people’s heads together”, as one engineer described it. David Scott attended lots of these meetings and shares the admiration that many have for his abilities. “Tindall would control the debates in terms of giving people the opportunity to talk, and then mix and match and make the trades. Then he would make a decision and say, ‘I’m gonna recommend this to management. Anybody have any really strong objections?’ And the guy who lost the debate may say, ‘Yeah, it won’t work!’ And Tindall would say, ‘OK, fine. We’ll go this way and if it won’t work, we’ll come back and re-address it, but we’ll make a decision today.’ They were good debates and anybody could stand up and debate the issue. But he kept it moving. He didn’t get bogged down because he himself was a brilliant engineer. I think Tindall was a real key to the success of Apollo because of how he brought people together and had them communicate in very complex issues. He was very good at it. He’d have them explain it, and in front of all their peers.”

The second side to Tindall’s ability was in the extraordinary memos he wrote, now fondly called Tindallgrams. NASA often displayed the formal stuffiness of a government bureaucracy, yet the memos from this particular senior engineer not only showed how he tied the project’s final stages together, but they revealed a unique chatty, easy to understand style that historians thought was quite remarkable. For example, a memo that discussed the possible reasons for Apollo ll’s overshoot had as its subject line, ‘Vent bent, descent lament!’ Another, written
before the Apollo 11 mission, concerned the LM’s low-level warning light, and was sent to a large list of addressees. It had this wonderful section:

‘"I think this will amuse you. It’s something that came up the other day during a Descent Abort Mission Techniques meeting.

“As you know, there is a light on the LM dashboard that comes on when there is about two minutes’ worth of propellant remaining in the DPS tanks with the engine operating at quarter thrust. This is to give the crew an indication of how much lime they have left to perform the landing or to abort out of there. It complements the propellant gauges. The present LM weight and descent trajectory is such that this light will always come on prior to touchdown. This signal, it turns out. is connected to the master alarm how about that! In other words, just at the most critical time in the most critical operation of a perfectly nominal lunar landing mission, the master alarm with all its lights, bells and whistles will go off. This sounds right lousy to me. In fact. Pete Conrad tells me he labelled it completely unacceptable four or five years ago, but he was probably just an ensign at the lime and apparently no one paid any attention. If this is not fixed, I predict the first words uttered by the first astronaut to land on the moon wall be ‘Gee whiz, that master alarm certainly startled me."’

Sheer engineering magic.

By the time the Apollo missions arrived at the Moon, scientists knew that the Moon was a rock-strewn, battered world where very little happened. Sunlight, unfiltered by an atmosphere, irradiated its surface making it hotter than any landscape on Harth, and after sunset this heat was quickly radiated to space, making it colder than the depths of the Antarctic. Scientists had established that the surface was basically a rubble layer, called the regolith. that had accumulated over aeons by the incessant pounding of incoming hypervelocity meteoroids ranging in size from sub­microscopic dust to mountain-sized asteroids and comets. They were sure that volcanism on a large scale had created the maria but didn’t know whether it had also occurred in the highland regions. They had some theories, largely unsupported by hard data, to account for the Moon’s existence and for why Earth should have such a large satellite in comparison to its size.

“Apollo 8, Houston. What does the ole Moon look like from 60 miles?” Capeom Gerry Carr could not suppress his desire to ask the obvious question when the crew of Apollo 8 came around from behind the Moon on their first pass in orbit. CMP Jim Lovell had dreamed of this day from childhood, and took the opportunity to reply. It seems unsurprising now. but what he saw was very similar to the view anyone can see through a telescope, only from a much closer perspective. “Okay, Houston. The Moon is essentially grey, no colour; looks like plaster-of-paris or sort of a greyish beach sand.’’

Bill Anders later spoke of his impressions of the Moon’s far side. “The back side looks like a sand pile my kids have been playing in for a long time. It’s all beat up. no definition. Just a lot of bumps and holes.”

They were not telling the scientists anything that they did not already know from the pictures sent by Lunar Orbiter. Apollo 8 added little to our understanding of the Moon, as would be expected of a brief pioneering reconnaissance mission. Its role during the 20 hours it spent in lunar orbit was to give its crew and the mission control team experience of operating a manned spacecraft in the lunar environment. While they were there, they could also inspect two possible landing sites on the southern plains of Mare Tranquillitatis. Planners were keen for the crew to study them visually from orbit and to inform future crews of what to expect, given that they had arranged for these sites to have the same early morning illumination that the landing missions would expect. Apollo 10 likewise concentrated on operational matters, rehearsing the steps that would lead to a landing. Both crews had Hasselblad cameras, and obtained many photographs on 70-mm film of selected swathes of the lunar surface. Although these covered areas already imaged by Lunar Orbiter. Apollo had the great advantage of returning its film for processing.

With Apollo 11, a crewman was left alone to look at the lunar landscape while the focus of exploration moved to the surface.