Category How Apollo Flew to the Moon

The second ground-based tracking system determined the range or distance to the spacecraft by measuring delay. Most people are familiar with the annoying delay introduced into television interviews carried out over satellite links. It takes light, and therefore radio about one and a quarter seconds to travel from the Moon to Earth, so the return travel time for a signal to a spacecraft at the Moon is about two and a half seconds. Engineers used this delay to measure range by putting a marker onto the radio signal which the spacecraft preserved and returned to Earth. The marker consisted of a digital code called pseudo-random noise, essentially a very large random number carefully chosen not to add undesirable artefacts to the radio spectrum. When the spacecraft synthesised the dowmlink carrier using the 240/221 relationship, it preserved this code, and sent it back to Earth. Engineers recovered the code and compared it with the transmitted code, ‘sliding’ one over
the other until they matched. The amount of ‘slide’ yielded a time for the round trip, and hence, knowing the speed of light, which is fixed, the distance. This technique was powerful enough to measure a spacecraft’s distance to an accuracy of better than 30 metres, and it could do so to a distance of nearly a million kilometres. For all of these ground-based systems, the movement of the ground station due to the rotation of Earth had to be taken into account before deriving measurements of the state vector.

The fuel cells were not the only source of power in the spacecraft. A collection of batteries were included and. despite their weight, there were good reasons for their inclusion. The fuel cells had a limited range of output power. They could not deliver more than 1.4 kilowatts at any one time, yet their power output had to be maintained above 400 watts at all times. The spacecraft’s requirements were much more variable, especially when motors had to operate. For example, the SPS engine had sizeable motors that gimballed its nozzle from side to side during a burn, and these placed heavy drains on the spacecraft’s electrical system. Batteries w ere a way of smoothing out the load on the fuel cells because they could supply extra power during the peaks in demand. At other times, their need to be recharged provided a convenient load for the fuel cells. At the end of the mission, after the fuel cells had departed along with the rest of the service module, the batteries were all that remained to power the command module as it streaked through the atmosphere during re-entry. They were therefore essential!

The CM carried a total of five silver oxide-zinc batteries mounted in the lower equipment bay below the navigation instruments. This battery technology had the highest energy to weight ratio at the time. Two of them were never recharged after launch. Their only use was to provide energy for the various pyrotechnic devices around the spacecraft. These devices performed a range of critical tasks; they separated the launch escape tower, the S-IVB and. at the end of its mission, jettisoned the lunar module. At re-entry, they separated the CM and the SM, jettisoned the upper heat shield from around the spacecraft’s apex and deployed the parachutes. Their health w’as checked regularly throughout the mission.

A further three batteries, each rated at 40 amp-hours, provided supplementary power during busy periods, but they became the sole source of power through re­entry, splashdown and post-landing operations. It was these that were recharged at times when the load on the fuel cells was low’. All five batteries were installed in separate pressure cases which, in case the batteries were to emit gases through failure or improper operation, could be vented to space to ensure that the gases did not enter the cabin. An additional 400 amp-hour non-recharged battery was added to the service module after the Apollo 13 incident.

In general, the batteries gave very little trouble. Only once, during the Apollo 7 Earth-orbital flight, were problems encountered when Walt Cunningham discovered that the batteries were recharging more slowly than expected. Then when the CM separated from the SM for re-entry, the voltage delivered by the batteries fell low’ enough to make the caution and warning lights "glow: yellow’ the rest of the w ay", as Cunningham put it. “This was a slightly traumatic experience at this point because we hadn’t expected anything like it,” he added. How’ever. the spacecraft’s systems operated satisfactorily with the slightly low operating voltage. Conversely, Apollo 12
relied on the CM batteries when its lightning strike incident took the fuel cells offline for most of the ascent to orbit.

During Apollo, colour television was still in its infancy and was still viewed as notoriously complicated technology. Conventional colour TV cameras of the time used at least three imaging tubes to generate simultaneous images in red, blue and green. The cameras were therefore large, heavy and required constant attention to keep the three images aligned in the final camera output. A simpler system was required and designers turned to a derivative of one of the earliest methods of gen­erating colour TV, the colour wheel.

CBS, one of the United States’ three major TV companies at the time, initially developed the colour wheel camera in the days before a rival system was adopted for general use. The colour wheel camera had one great advantage that lent itself to use in space. Since the colour scans were expressed sequentially instead of simultaneously, only a single imaging tube was required and the camera could be made much smaller than conventional colour cameras of the time.

With only a single tube, the Apollo 12’s troublesome colour TV camera on

Apollo colour camera produced its tripod on the Moon. (NASA).

what was essentially a standard black-and-white signal at 60 fields per second, 262.5 lines per field, with about 200 useful lines per field. Directly in front of the imaging tube, between it and the lens, was the colour wheel. This had six filters as two sets each of red, blue and green. It was spun at 10 revolutions per second such that each field from the camera was an analysis of the image in red, then blue, then green, over and over. If viewed on a black-and-white monitor, the image would display a pronounced 20-Hz flicker because the field that represented green was brighter than the other two, but would only come around 20 times per second. The bandwidth given over to this television signal was increased to 2 MHz which overlapped other components in the Apollo S-band radio signal. Consequently, careful filtering was required to remove these from the TV image.

The flickering black-and-white signal received from the spacecraft, or from the Moon’s surface, had to undergo extensive processing at Houston. In television studios of the Apollo era, it was crucial that the timing of the TV signal was extremely accurate and stable. In other words, the pulses within the signal that define the start of a line or field should occur with extreme regularity and precision. In addition, all equipment dealing with the signal had to agree when the lines and fields began – that is, they all had to be synchronised. This was a problem for Apollo because there was no provision to send synchronising pulses to the camera. In fact, there would have been no point incorporating such pulses because the Doppler shift caused by the changing velocity of the spacecraft or of the landing site with respect to

the receiver on the turning Earth constantly altered the timing of the received TV signal anyway.

This was in the days before mass digital storage made the task of synchronising video simple. The engineers’ solution used two large videotape recorders to correct the signal’s timing. The first machine recorded the pictures coming from space and synchronised itself with the pulses that were built into the incoming television signal. However, instead of the tape going onto a take-up reel, it was passed directly to a second videotape machine which replayed its contents. This second machine took its timing reference from the local electronics so that when it reproduced the signal, it did so with the timing pulses synchronised with the TV station.

Once the timing had been sorted, a colour signal had to be derived from the three separate, sequential fields that represented red, blue and green. To achieve this, a magnetic disk recorder spinning at 3,600 rpm (once every 60th of a second) recorded the red, blue and green fields separately onto six tracks. From this disk, the appropriate fields could be read out simultaneously using multiple heads and combined conventionally to produce a standard colour television signal. Overall, the time required to undertake the processing put the image about 10 seconds behind the associated audio.

Apollo 10 proved that a colour camera worked within the overall Apollo system though Tom Stafford had to battle against a conservative NASA bureaucracy to get it there. On Apollo 12, colour TV was transmitted from the lunar surface for the first time – or at least it was until the camera was inadvertently aimed at the Sun. This destroyed part of the sensitive imaging tube and gave the commercial TV networks a headache while they scrambled for something to show the viewers!

The cameras for Apollos 11, 12 and 14 were merely placed on stands near the LM, which was acceptable as long as activity was centred around the lander, but when the Apollo 14 crew set off for their geological traverse they walked out of shot and left the audience watching an unchanging scene for several hours. It was clear that when lunar exploration stepped up a gear for the J-missions, the TV camera would have to be mounted on the lunar rover.

Early on the morning of 26 July 1971. the crew of Apollo 15 in the good ship Endeavour with its cargo (the lunar module. Falcon, and Lunar Rover-1) departed Earth-space for the Hadley-Apennine landing site on the Moon.

The expedition had actually begun 20 months previously and required more than 100.000 people to prepare the launch vehicle, prepare the three vehicles, prepare the spacesuits. gather up equipment, provisions, and instruments, and generally plan the three-day exploration of the mountains of the Moon.

Just over ten years had passed since May 1961. when President John F. Kennedy committed the United States to "landing a man on the Moon and returning him safely to Earth". During this period, tw’enty five US manned space missions had been flown (Mercury, Gemini and Apollo) nineteen in Earth orbit, three around the Moon, and three to the lunar surface.

The crew of Apollo 15 was now embarking on the next phase, the first extended scientific exploration of the Moon. NASA termed it "[one of] the most complex and carefully planned expeditions in the history of exploration." Only two more such missions were to follow, then there would be a hiatus lasting several decades, maybe even longer, before humans would onec again set foot on the Moon.

I was the commander of the Apollo 15 mission. Jim Irwin accompanied me down to the lunar surface in Falcon while A1 Worden looked after Endeavour in orbit. The planning and preparation for our mission had been so thorough that there was no doubt in our minds that we really knew "how to fly to the Moon" and to do so in any conceivable situation. But as "we” (all 400.000 people working on the Apollo program) had learned during the many preceding missions, flying to the Moon and returning to Earth (successfully, that is) is very, very difficult. So, just how did we actually plan and prepare for this extraordinary adventure – how was our Apollo to fly to the Moon?

lire Apollo program was implemented through five sequential tasks that evolved during two overlapping phases perhaps the "ABCs" of how to fly to the Moon:

Phase 1

A. Adopt a method by which men could fly to the Moon and return safely.

B. Build the spacecraft and ground facilities to implement the method.

C. Develop the techniques and procedures to accomplish the mission.

D. Select and train the astronaut crews (the vital link between В and C).

Phase 2

E. Upgrade the capabilities of the entire system to maximize the technical and scientific results of the Apollo phase of human lunar exploration.

In 1967 mission planners at NASA introduced an alphabetic nomenclature by which to describe where individual flights fitted into the overall scheme. This started at "A" with unmanned tests of the Saturn V launch vehicle and worked through a scries of missions of progressively greater operational capability, finally reaching "G", which was the Apollo 11 lunar landing, and "П" lor a number of follow-on missions. These constituted Phase 1. Missions flown during Phase 2 were intended to maximize the science objectives, and were designated "J“. Apollo 15 was the first such mission.

While the F-l used conventional kerosene-type fuel for brute force, the J-2 achieved almost double the efficiency through the use of relatively exotic liquid hydrogen. However, despite being more efficient, it could not match the raw power levels attained by the F-l, which made it more suitable for an upper stage. A single engine could balance over 100 tonnes and it could be restarted in space. It traced its origins to work done in the 1950s to create a hydrogen-burning rocket engine, but its development funding came solely from NASA who wanted the inherent benefits of hydrogen applied to its Saturn vehicles.

By concentrating almost single-mindedly on the goal of a manned lunar landing, secondary considerations like landing accuracy and science had taken a back seat. In the event. Apollo 11 had landed about seven kilometres beyond its planned site and for some time, no one knew exactly where they were. Not even Mike Collins had been able to see Eagle through his sextant – a powerful optical instrument built into Columbia’s hull. The science payload had been severely limited by mass constraints and lack of time. Future missions would make amends because the United States had invested heavily in the infrastructure to support Apollo and wanted to see results. It also demanded justification for the continuing costs. Fittingly, science became that justification.

To gain knowledge from the Moon. NASA had to go to sites where Earth-based and orbital imagery suggested that answers to questions might lie. However, given the limited walking range of an astronaut on the lunar surface, the ability to land "on target’ became paramount. Although Apollo 12 was not sent anywhere of particular geological importance, it was given a very small target to aim for. Specifically, it was to land within walking distance of Surveyor 3. a small robotic lander that NASA had sent to Occanus Procellar urn in April 1967.

The mission courted disaster in its first minute when the vehicle flew through a rain cloud and invoked a lightning strike. Regardless, the crew’ continued to the Moon amid fears that their command module may have been damaged by the surge of power that had passed through it. In the event, the CSM Yankee Clipper proved to be unharmed, and on 24 November 1969 Charles ‘Pete’ Conrad and Alan Bean landed their LM Intrepid some 1.500 kilometres west of where Eagle had landed and a mere 200 metres from Surveyor 3. This demonstrated that ground controllers and crew could bring a lunar module down exactly where they wished. Richard Gordon, orbiting overhead, confirmed their position by spotting both the LM and the Surveyor on the surface through his sextant.

Since Apollo 12 was an II-mission, Conrad and Bean made two moonwalks. On the first they laid out an ALSEP which was an autonomous scientific station that operated on the lunar surface for many years after they left. The next day they hustled across the surface taking a circular route of over a kilometre, pausing at preplanned points of interest on the way to visit the Surveyor probe. After examining

and photographing the probe, they removed pieces to enable researchers to study how well the hardware had survived 31 months of exposure to the lunar environment. In terms of public relations, the low point for this fun-loving crew was when their television camera was ruined early in the first moonwalk by being inadvertently pointed at the Sun. TV networks struggled to provide a visual accompaniment to the crew’s voice communication and audiences quickly became bored of listening to indistinct and often arcane yakking by the two guys on the surface. Nonetheless, like every crew after them, Conrad’s and Bean’s two joyous forays out on the surface yielded samples of greater bulk than the previous mission, and the scientists were more than happy with what they brought back. In particular, tiny grains of a very slightly radioactive rock type began to lift the lid on important aspects of the Moon’s early history.

Despite concern that it may have been damaged during launch, Yankee Clipper’s successful splashdown concluded a successful, if charmed 10-day mission that was marred only by the loss of the TV coverage.

At 8.9 seconds to lift-off, a command was sent to the Saturn V to begin the ignition sequence for the five F-l engines at the base of the first stage. The Saturn’s instrument unit then sent start commands to each engine, their timing slightly staggered in order to prevent a single jarring ignition transient being imposed on the launch vehicle. First to be commanded was the centre engine, followed at quarter-second intervals by diagonally opposed pairs of engines. Each engine then went though an elaborate sequence that was carefully choreographed to minimise rough starting, with, if all went well, all engines attaining full thrust by T-l second.

A description of the astonishing F-l engine is necessary before going through its ignition sequence. The most prominent component of the engine was the bell or nozzle, usually seen with an extension added to improve its performance. This tapered to the throat and a cylindrical space, not quite a metre across, called the combustion chamber. At the far end of the chamber was a thick steel injector plate with hundreds of slightly angled holes like a giant shower head. Alternate rings of these holes sprayed jets of fuel or oxidiser that impinged and burned together. The walls of the chamber and nozzle were constructed of piping through which the

kerosene fuel was circulated to cool the structure, prior to it being sprayed through the injector plate.

As is to be expected for any fluid system, the propellants arrived at the engines with a pressure that depended on the height of the fluid above – its head – and any added by the pressure of the gas in the top of the tank. This was not nearly enough to inject fuel and oxidiser directly into the chamber. The huge internal pressures from their combustion would simply have forced the propellants back through the holes in the injector plate. Each engine was therefore provided with a high-pressure pump arrangement to force propellants into the combustion chamber. This dual turbopump was mounted to the side of the combustion chamber and was driven by burning some of the propellants. In an action somewhat similar to that in a jet engine, the hot gases from this combustion forced a turbine to spin a shaft which drove the pumps. The final task for the turbopump’s exhaust gases was to be expelled at the join between the engine bell and the nozzle extension via a large wrap­around manifold. Although the turbopump exhaust was hot, the combustion gases coming from the chamber were far hotter and by forming a thin film of relatively cool gas, it served to protect the extension from erosion. Four pipes, two each for fuel and LOX, led from the pumps to the injector via valves that controlled the engine.

The ignition sequence for the F-l began with firework-like igniters going off, some of which burned to ignite the turbine propellants, others to ignite its fuel-rich exhaust gases when they reached the engine bell. They also burned through electrical links to provide a signal to begin to open the LOX valves and pour LOX into the combustion chamber. This in turn, caused another valve to open to send propellant

to power the turbopump. As the turbopump accelerated, the pressure in the fuel lines rose and burst a cartridge of hypergolic[1] fluid. As its contents were injected into the chamber followed by fuel, engine start-up was ensured by its spontaneous ignition with the LOX already in the bell. When combustion was detected in the chamber, the fuel valves opened, flushing ethylene glycol out from the cooling pipework and into the chamber where it helped to soften the thrust build-up as the engine strove to assume its steady-state condition.

For about a second after full thrust had been achieved, great flames roared from below the static spire of the Saturn V while sensors measured each engine’s perfonnance. In that second, and every subsequent second of the S-IC’s powered flight, each engine consumed nearly one tonne of kerosene and almost two tonnes of LOX – 13 Vi tonnes across all five engines – as the vehicle sat at full power, waiting

Graph of thrust build-up of the five F-l engines on Apollo 8’s S-IC. Note the staggered start of the engines and the hiccup as the four outboard engines ingested helium from the pogo suppression system. (Redrawn from NASA source.)

Diagram of the linkage arrangement of a hold-down arm. (NASA)

for the confirmatory signal that they had achieved the required thrust and the Saturn V could be released.

A sense of the prodigious power that was being expressed by this machine can be gained from a little maths. One of the most basic equations in physics is that for kinetic energy. A mass that is moving has a quantity of energy that would be expressed if it hits something stationary – think of a car hitting a wall. While carefully avoiding an equation in this book, kinetic energy can be worked out by taking half the mass and multiplying it by the square of the velocity. To apply this to the Saturn V, during each second of operation, the energetic chemical reaction in the combustion chambers of the five F-l engines made 13.4 tonnes of mass leave the engine at almost three kilometres per second. Therefore we multiply 13,400 by 0.5 and further multiply it by 3,000 squared. The answer we get is 60 billion joules of energy. If we express that as energy per second, in other words, power, then we find the output power of the Saturn first stage was 60 gigawatts. This happens to be very similar to the peak electricity demand of the United Kingdom.

As soon as the escape Lower was jettisoned, the rules changed again on what to do in the case of emergency. The vehicle was now being flown in abort mode two, which took account of the fact that, to all intents and purposes, the remaining stack was in space and catastrophic break-up from aerodynamic forces was no longer a concern. This abort scenario called for the CSM to detach itself from the rest of the stack and use either the service module’s main engine or its small RCS thrusters to increase its distance from the failing launch vehicle. Once clear, the CM would detach from the SM and descend on parachutes to a normal splashdown in the Atlantic at some point downrangc. These abort rules applied until the S-II was spent.

As the stack passed over NASA’s network of communication stations around the world, its orbit was carefully measured and intensive calculations were performed to enable FIDO to choose exactly when and how the TLI burn should be made. This information was radioed up to the Saturn’s instrument unit, which would control that burn. In particular, the computed lime of ignition was back-timed to a moment 9 minutes and 38 seconds earlier, when the instrument unit needed to begin Timebase 6. a choreographed sequence of events that would lead up to ignition and through the burn.

The start of Timebase 6 was indicated to the crew when a lamp on their panel came on for 10 seconds. This was one of the cluster of lamps that had informed them of the status of the launch vehicle throughout its flight. At this point, the Saturn’s computer checked the state of a switch in the CM to verify that the crew really did still want to go to the Moon. This switch was provided so that further preparations for ignition could be terminated if a problem surfaced that necessitated cancelling the lunar phase of the mission. If all was progressing well, valves were closed to stop the S-IVB’s tanks from venting, and a burner was ignited to heat helium gas that would repressurise the tanks, to prepare them for operation.

At 100 seconds before ignition, the computer display blanked to let the crew know that the guidance system had begun to measure whatever acceleration the S-IVB was about to imparl. With 80 seconds to go before ignition, the aft-facing ullage motors within the APS modules filed to push the fuel and oxidiser to the base of their tanks in order to settle them and provide a little head of pressure into the engine. The crew still had options to stop the S-IVB from starting up. If the)’ did so earlier than 18 seconds prior to ignition, the inhibit switch would work; otherwise, an adjacent switch, one which normally made the second and third stages of the Saturn separate, would have to he used.

At eight seconds prior to ignition, valves were opened to route hydrogen fuel through the engine to chill its pipes and ducts. As this was a restartable engine, the ‘start’ tank had been refilled with hydrogen during the first burn. At the calculated time of ignition, the contents of this tank were discharged through the pump turbines, spinning them up and increasing propellant pressures in the pipes that led to the core of the engine. The propellant valves, which had been cracked open slightly at this point, then began to fully open, allowing a rush of fuel and oxidiser into the combustion chamber where an ASI initiated full combustion and the engine brought itself up to full thrust.

An entirely different technique to determine position and velocity was brought to bear in the spacecraft which relied on sightings of the stars, Earth and the Moon. It was designed by MIT under the direction of Charles Stark Draper. To reinforce his faith that his team could successfully come up with an accurate system to navigate to the Moon and back, and somewhat to the mirth of folks at NASA, he put himself forward as an astronaut candidate. The MIT system was based on a computer, an inertial platform, and optical devices; one of which was directly descended from an instrument used by generations of sailors to navigate across the world.

Set into the hull of the command module, opposite the hatch, were two apertures that accommodated the spacecraft’s optics. The smaller was for a so-called telescope, although it hardly justified the name as it had only a ‘times-one’ magnification. Neil Armstrong later quipped, “NASA is probably the only organisation in history that’s been sold a one-power telescope.” Its function was to give the CMP a wide-angle overview of the constellations visible at that side of the spacecraft to assist in aiming the other instrument, the sextant.

The second aperture in the hull was a disk and slit affair that accommodated the

objective optic of the sextant, a 28-power device used by the CMP to measure angles. Like a mariner’s sextant, it had two lines of sight with the ability to move one with respect to the other. The version used by marine naviga­tors for hundreds of years works by viewing the horizon through a small telescope mounted on an arc which sweeps through one – sixth of a circle (hence the name ‘sextant’). A mirror arrangement on a radial arm permits the image of a celestial body (the Sun, Moon or a star) to be aligned with the view of the horizon. The

Apollo 16 command module Casper in lunar orbit showing the exterior apertures of the sextant and telescope. (NASA)

Line of to horizon

angle between the two could then be read off a scale at the circumference of the arc. If carried out when the Sun was at its highest point, this measurement would yield the ship’s latitude.

The role of the sextant on the Apollo spacecraft was similarly to measure angles, and it worked in much the same way, but with major refinements. It also had two lines of sight – one fixed, the other movable – both of which peered through the slitted disk in the spacecraft’s hull. The fixed line of sight, also called the landmark line of sight (LLOS), had to be aimed by controlling the attitude of the entire spacecraft. A dense filter was placed in its light path so that the relatively bright horizons of Earth or the Moon would not swamp the stars with which they were to be compared. The movable line of sight was usually aimed at a star, and was thus called the star line of sight (SLOS). It could be swung up to 57 degrees away from the fixed line of sight to bring the image of a star into alignment with the image of the horizon. It was important that the star image be placed on that part of the horizon that was nearest to or furthest away from the star, depending on which horizon was illuminated by the Sun. Because the computer was closely integrated with the optics, the angle between the two lines of sight could be directly fed to it and used in its calculations of the state vector. The entire optical head could be rotated about the fixed line of sight and, as it did so, the disk on the outer surface of the spacecraft also turned to accommodate it. A crude sextant had been tested during the Gemini programme with mixed results. Mike Collins had tried using two hand-held models without success on Gemini 10. Later on Gemini 12, Buzz Aldrin brought one into play to help with angular measurements during a rendezvous after the spacecraft’s radar had failed.

This ability to measure the angle between a planet’s horizon and a star was what enabled onboard determination of the state vector to work. As a spacecraft coasts from one world to another, the apparent position of either orb against the stars will change, and this change will reflect the progress of the craft along its trajectory. The angle between the planet and the star at a particular time can only be valid for a single trajectory given the laws of celestial mechanics and the layout of our solar system. It can therefore be used by an onboard computer to calculate their current state vector. Repeated measurements could be used to refine the state vector. Because Program 23 in the computer was being used for this task, crews referred to their navigation task as doing a ‘P23’.

During the system’s development, experiments carried out on Gemini flights revealed difficulties of knowing exactly where Earth’s horizon was. First, having selected a star, there was a 50:50 chance that the nearest point of a planet’s horizon would be in darkness. To work around this, the CMP had to tell the computer whether he was using the nearest point or, if it was dark, the furthest point of the horizon relative to the star he was using. The second problem was that optical navigation was most sensitive when the spacecraft was near the planet on whose horizon the CMP was trying to sight. Unfortunately, the nearer they were to Earth or to the Moon, the less well-defined was the horizon. Earth’s atmosphere blurred the precise edge of its limb and the Moon’s rough terrain could make its limb decidedly knobbly when observed up close. Based on the pioneering work of Jim Lovell, who gave the onboard guidance system a workout dur­ing Apollo 8, MIT set up a simulator to train the astronauts how to choose an appropriate horizon when trying to mark on a nearby Earth or Moon.

During the flights, the CMPs made it a matter of pride to excel in their navigation exercises, even though, in most cases, their results were only meant as a backup in case communications were lost. Nevertheless, a friendly rivalry existed between some crews and the trajectory experts on Earth as to whose evaluation of the state vector was the most accurate. When Lovell put the onboard navigation system through its paces for the first time, there was a lot of interest in his results. Two days out from Earth on Apollo 8, and one day

from the Moon, Lovell informed mission control of his progress with the P23 navigation work. *Tt might be interesting to note that after sightings, we ran out P21, and we got a pericynthion of 66.8 [nautical] miles.”

What Lovell had done was to use P21 in the spacecraft’s computer. This program’s task was to deter­mine the spacecraft’s path across a planetary surface. If the crewman entered a time, it used the current state vector to return three values; the spacecraft’s latitude and long­itude directly below the ship at that time and its altitude, also at that time. As he knew roughly when they should arrive, he tried entering times at 10-minute intervals around their expected closest approach. With each advancing time entered, he noted how their predicted alti­tude above the lunar surface dropped, reached a minimum value, and then began to rise again. The point where it reached a minimum was their pericynthion – the spacecraft’s closest approach to the Moon. What Lovell was saying was that his predicted value for the pericynthion was very near the ideal of 60 nautical miles (110 kilometres). Bill Anders’s wit intervened. "I knew if he did it long enough, he’d finally get one that was close.”

Lovell continued to make P23 measurements and checked his resultant state vector once again with P21. Frank Borman informed Mike Collins in Houston of his results. ‘‘Mike, we ran the latest state vector we have through the P21, and it showed the pericynthion at 69.7 [nautical] miles. We’ve got the navigator, par excellence.” This may have been a gentle dig at Collins, who had been CMP on the Apollo 8 crew before standing down to undergo surgery. Nevertheless, the flight controllers were impressed. “You can tell Jim he is getting pretty ham-handed with that P21,” congratulated Collins. “He got a perilune altitude three-tenths of a mile off what we are predicting down here. Apparently, he got 69.7 [nautical miles], and the RTCC says 70.” The RTCC was the real-time computer complex, a bank of huge IBM-360 mainframe computers at mission control that were processing the radio tracking data.

Thus, at the first test of the Apollo navigation system, two entirely different systems were coming up with determinations of the spacecraft’s position that agreed to within 500 metres at a range of 300,000 kilometres out from Earth. It was a huge

confidence boost, proving that the engineers had done their work well. Procedure dictated that Lovell’s determination would be noted, but the crew would be instructed to place a switch into the correct position to accept data uplinked from the ground, w’hereby the Earth-based solution w’ould be sent up by radio and loaded directly into the onboard computer’s memory, supplanting Lovell’s effort. Apollo 8’s navigator saw the opportunity for a little one-upmanship.

“Houston, Apollo 8,’’ called Lovell.

“Apollo 8. Houston,’’ replied Collins.

Lovell then jokingly reversed the usual procedure. "Roger. If you put your [telemetry switch] to Accept, we will send you our state vector.’’ Mission control had no such switch and the request was in jest. But Lovell knew’ his state vector was as good as theirs and Collins knew’ it too. “Touche.’’ Collins responded.

Later, as Apollo 8 coasted back towards Earth, Lovell continued his P23 navigation exercises. As he did, mission control still found it hard to say w’hether his solution or the one from Earth was better. Gerry Carr informed the commander: “Frank. Let him know the state vectors have converged. They are very, very close now.”

“Is that right, Gerry?’’ replied Borman. “Okay. I’ll tell him. Thank you.”

“Don’t let his head get big. though," suggested Carr.

“You guys arc going to make it impossible to live with him.” moaned Borman. “It always was pretty hard.”

A day later. Lovell was doing even better. Carr brought the bad news. “I hate to tell you this, Frank, but that last set of marks put your state vector right on top of the [ground’s] state vector.” Borman returned with a mock plea. "Come off that, Gerry. Come on; you promised.”