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For the first three landing missions, exploration of the lunar surface had been limited to how far a crewman could walk, then perform useful work and walk back to the LM (Lunar Module). Wide scale sampling of a region was impossible, given limited consumables and a tendency to get lost when crews ventured too far, so only the area around the LM and somewhere up to a kilometre away could be accessed. Since NASA wished to make science the cornerstone of their lunar exploration, they would have to increase the range of the astronauts' mobility.
The Lunar Roving Vehicle was the most important and visible part of NASA's intensification of its manned lunar exploration program. Powered by batteries and cunningly folded against the descent stage of the LM, the Rover would be deployed during the crew's first EVA (Extra Vehicular Activity). It would then transport the crew, their tools and instruments, and a miniature, interplanetary outside broadcast unit, up to 5 kilometres from the landing site and out of sight of the LM. To guide them, it carried a simple gyroscopic navigational unit that kept track of the distance and bearing to the LM. While taking the strain of transportation, it also gave the astronauts resting time between stops. All in all, an area of far greater geologic significance could be sampled and Apollo 15 would be first to put the little car through its paces, and so would effectively be the last engineering test flight of the program.
To exploit the Rover and allow effective exploration, three EVAs were planned for each mission. New suits and backpacks would let the crew stay outside for over seven hours at a time. The whole lunar excursion would last over three days. To support the needs of the crew for this time, more consumables; oxygen, water and food, had to be carried to the Moon, while greater quantities of rock were to be returned to Earth. The Lunar Module was improved to carry the increased payload of consumables as well as the Rover. Externally, the most noticeable of these changes was the lengthening of the engine bell on the descent stage of the vehicle and enlargement of the propellant tanks; but internally, the LM had an extra tank of water, extra batteries and improved systems throughout.
The CSM (Command and Service Module) was similarly upgraded. Extra consumables were added to support an extended mission, but, most significantly, a previously vacant bay in the Service Module now housed the SIM (Scientific Instrument Module). The SIM bay contained cameras and instruments to photograph, map and measure the lunar surface, sniff the tenuous lunar environment, determine the constituents of the rocks and, when the main mission was about to return to Earth, leave a small satellite to continue some of that work for the coming year.
To get this extra weight to the Moon, the Saturn V launch vehicle had to be extensively upgraded too, a process made simpler by the conservative margins built into the machine by its designers, led by Wernher von Braun. With a solid operational history behind them, they knew where improvements could be made. The F-1 and J-2 engines were uprated to add 13% to their performance. Some of the solid-fuelled ordnance around the vehicle, simple rocket motors that helped the stages to separate, was deleted or scaled back, saving weight in the process. A greater understanding of the utilisation of propellants aboard allowed each stage to be run nearer depletion, turning previously dead weight into useful energy. To further improve the lifting capability of the system, a lower Earth parking orbit was chosen to reduce the work which had to be expended in hauling the vehicle out of Earth's gravity well. The orbit was so low that the skin of the vehicle sustained significant heating due to friction with the tenuous air at only 170 kilometres altitude.
On 26 July 1971, Apollo 15 lifted into a clear, Floridian morning sky at 9:34 by local clocks, exactly on time. The ascent to orbit presaged the highly nominal nature of the whole mission. About the most remarkable aspect of Saturn V number AS-510, apart from this exotic vehicle's impressive performance, was a lack of distance between the first and second stages when the latter ignited, an anomaly of little consequence indeed.
Timely adherence to the checklist afforded the crew a few minutes sightseeing from Earth orbit before they committed the final stage of the launch vehicle to propelling them to the Moon. The burn of the S-IVB was accurate and no midcourse correction maneuver was required at the first opportunity. The S-IVB's penultimate act was to give up the Lunar Module to the nose of the Command Service Module. Then it set its own course for destruction in a high speed impact with the Moon, generating seismic tremors that would be registered by instruments from previous missions as the small planet reverberated for an hour from the shock.
Soon after the dual spacecraft had separated from the launch vehicle, the crew encountered what was to be the most significant technical flaw of the flying portion of the mission. The Main Display Console is a huge semicircle of switches, knobs and instruments. Sited directly in front of the couches, it is the largest panel in the spacecraft and in the middle of it, a light had come on which should only have been energised when the large SPS (Service Propulsion System) engine was firing. Engineers feared that the energy which was powering the light could also open the propellant valves and thereby ignite the engine simply by arming it. An unplanned, undirected burn of such a large engine was not a scenario the crew, or Mission Control wished to deal with.
After isolating the control wiring from the engine valves, a series of rudimentary tests tracked down the problem to a short circuit in a switch in one of the two redundant control banks. The second midcourse correction opportunity was used to evaluate the performance of the engine when the faulty bank was driving it. Subsequently, procedures were worked out to operate the SPS for the rest of the mission using mainly the good control bank, bringing the other online only during long burns.
As an advanced mission, many of the procedures used to operate the spacecraft had been improved from previous flights and, occasionally, small operational bugs appeared when trying these out for real for the first time. During the coast to and from the Moon, all Apollo spacecraft were slowly rotated to evenly distribute the heat of the Sun and the chill of deep space over the surface of the Command Module's heatshield. Insiders called it PTC (Passive Thermal Control) but the media liked to use the term 'barbecue' mode when explaining it to the public. When Al Worden attempted to use a revised procedure to set up this rotation towards the end of the first day, an essentially routine operation, he needed three attempts to get it to work smoothly. It had not been realised that a switch had to be thrown quickly and cleanly through its central detent where, on previous flights, it had not mattered. Once the mechanics of the procedure had been sorted out, Al had no further problems with PTC and when he set it up late into the third day, Mission Control informed him that "it's one of the best they've ever seen."
Towards the end of the second day, Dave Scott and Jim Irwin began preparations for their lunar visit by entering the LM, powering it up and checking out its systems. Telemetry to Earth gave flight controllers in Houston a chance to monitor the lander's health, especially the batteries which would provide its only source of power for the three days of exploration. However, Dave Scott found something in the LM cabin which couldn't be detected by telemetry. The glass face of one of the craft's more important instruments, a tapemeter which displayed range and range rate, had shattered and glass shards, some a couple of centimetres long, were floating around the cabin. The activation of the LM's environmental control system began the process of clearing the glass from the LM cabin as air began to be drawn through filter screens which trapped wayward particles. Dave and Jim attempted to clean the filters with applications of sticky tape. They also searched all the little holes and crevices they could find where glass might have lodged, and later, brought a vacuum cleaner over from the Command Module which they taped to the ascent engine cover. It was hoped that leaving it running for a time would take the last fragments out of the air. Meanwhile, on the ground, tests were carried out on identical instruments to verify that, despite losing the internal helium atmosphere it had been designed with, the mechanism of the tapemeter would still operate properly, even in a vacuum.
Although Apollo 15 was considered to be a nominal mission, a rash of other small problems arose - something to be expected on machinery of such exquisite complexity. A blown capacitor in the mission timer popped a circuit breaker for lighting some of the CM (Command Module) displays. Mission Control did not dare reset the breaker, not knowing at the time what had caused it to pop, and it would be during lunar orbit that minor repercussions from the loss of this circuit would be felt.
The day after the episodes with the broken glass and the popped breaker, Dave Scott called the ground to say that water was leaking from the chlorination port and that they needed a way to stop the flow, quick! CapCom Karl Henize admitted that a similar leak had been noted on launch day and that someone had written up a procedure specially, in case of such trouble. He asked them how many drips per second the leak was, forgetting that drips don't happen in zero-g. After 15 minutes and a sizable ball of water had gathered around the port, the procedure was read up to tighten a retaining nut, leaving the crew to gently tease Karl about his drips query. It was not the last time the crew of Apollo 15 would be mopping up.
Science was to be the primary focus of Apollo 15 and the scientific work was begun almost as soon as the crew entered space. While still in Earth orbit, a program of ultraviolet photography was begun which would continue throughout the mission. To accommodate this, the rightmost of the Command Module's five windows had been fitted with UV transparent quartz panes and a UV focusing lens was available for the Hasselblad cameras carried onboard.
Previous crews who had travelled to the Moon had reported seeing intermittent light flashes. These had seemed to be internal to the eye and it was thought that they were caused by a particular form of cosmic ray interacting with the matter of the eye or the brain. The crew of Apollo 15 were to investigate the nature of this phenomenon. During the flight phase of the mission, three periods of an hour each were set aside when they would sit blindfolded in known positions within the Command Module, with their eyes dark-adapted, reporting each occurrence of a flash and describing its nature; a line or a point, in which eye and if it seemed to be moving.
Meanwhile, in light of the problem with the SPS, the backup Commander of the mission, Richard Gordon, had been practicing revised procedures for operating the engine with one good control bank. The faulty bank would be brought online after automatic ignition on the good bank, and then taken out again just prior to automatic shutdown. Mission Control read the procedures to the crew in plenty of time to allow them to consider and rehearse what they would have to do out of contact with the Earth - the Lunar Orbit Insertion burn.
As Apollo 15 accelerated in free fall towards the Moon, the door covering the SIM bay was explosively jettisoned. The spacecraft swung around the far side where the crew fired the SPS engine for 6 minutes and 38 seconds to slow them down and capture them in orbit around the lunar sphere. As they came into view of the Earth again, an exuberant Dave Scott shared his first impressions of lunar proximity. "Oh this is really profound; I'll tell you, fantastic!" While they coasted across the ancient face of the Moon, Mission Control informed them that their S-IVB had met its end in the interests of science by impacting, meteor-like, into the Moon.
Although there was a program of extensive lunar photography planned for the CSM, by the two converted reconnaissance cameras in the SIM bay and the onboard Hasselblads, the crew immediately became tourists during their first revolution across the sunlit half of the Moon. Dave Scott describes that first orbit as "mindblowing" and they snapped both planned and unplanned photos of the landmarks they saw. The SPS was fired again to bring the low point of their orbit down to a mountain skimming 17 kilometres. This would deliver the LM close to the Moon and minimise the fuel needed to achieve the landing. The crew bedded down for the night while their SIM bay began gathering data on the Moon and its environment.
The next day, the Sun had risen over the landing site and they found that the low point of their orbit had drifted even lower than the expected 16.1 kilometres to only 14.1 kilometres high because of the still poorly understood influence of the Moon's mascons; great concentrations of dense mass which perturb objects in lunar orbit. A burn of the RCS (Reaction Control System, 16 thrusters in 4 groups around the circumference of the spacecraft) was required to reestablish the orbit's intended shape. As they prepared for this burn, Al gave Mission Control a guided tour of the scenery from their window during a 10-minute TV show from the CSM. By the time they flew over the landing site, they were so close to the ground that it passed below then even faster than Al could get the camera trained on it.
The next two orbits were concerned with preparing the LM for its descent to the lunar surface. Ed Mitchell, the Lunar Module Pilot from the previous mission, Apollo 14, took over the CapCom position. While Dave took the opportunity to observe the landing site through the sextant for himself, they began equalising the pressures between the two craft prior to entering the Lunar Module. The relatively poor imagery of the Apollo 15 landing site had left some doubt in the minds of the mission planners about the suitability of the site; yet over the entire surface of the Hadley plain, Dave saw just one boulder of any significance. Hadley was a fine place to land.
Dave and Jim entered the LM and spent a complete orbit activating and checking Falcon's systems. While out of contact with Earth, Al threw the switch to undock the two craft - but nothing happened. A plug was loose leading to the docking equipment and the separation had to be delayed while Al re-entered the tunnel to reseat it. Though the problem delayed Al's subsequent activities, there was enough slack in the system to leave the landing unaffected. As the spacecraft eventually separated, Dave and Jim continued their preparations for the descent while Al returned to a roughly circular 110 km orbit.
Using the calibrated optics and the computer in the Command Module, Al took sightings of a crater near the centre of the landing ellipse, appropriately named Index Crater, to provide Mission Control with data which would improve their knowledge of exactly where the landing site was. Although he lost the first opportunity to do this because of the undocking problem, Al achieved it when Hadley next came around. Mission Control could then retarget the Falcon's computer to compensate for any errors in the LM's flight path.
While the crew of Falcon received a Go from Mission Control for the descent, Al prepared for three days of intensive lunar science. Having delivered the LM to an orbit with a low point of about 16 kilometres, Endeavour's SPS engine was fired for four seconds to put the spacecraft in a roughly circular 110-km orbit. Throughout this solo mission, the orbit would gradually change; in size, as it was affected by the Moon's irregular gravity, and in groundtrack, as the Moon rotated beneath the spacecraft's inclined orbit.
Between the normal duties involved in keeping a one-man artificial planet operational, Al was either running the instruments and cameras in the SIM bay, photographing selected targets or making visual observations based on geologic knowledge taught to him by a remarkable tutor, Farouk El-Baz. With the acuity of the practised eye, he could observe, discuss, choose and describe features which would otherwise be difficult to detect or interpret from photographs. With a telephoto lens, he could aim a shot or two at those sites which caught his eye; looking at landslides around the far-side crater Tsiolkovsky, faulting across the spectacular ray crater Proclus or, significantly for the future Apollo 17 mission, identifying apparent cinder cones across the valleys on the eastern side of Mare Serenitatis, particularly in the valley of Taurus-Littrow.
Endeavour's package of instruments were a triumph of remote sensing and paved the way for sensors that would explore the solar system on later missions. A Gamma-ray Spectrometer, mounted on the end of an extendible boom, detected the specific emissions coming from the decay of heavy elements in the soil, identifying these in the process. It's results complemented those from the X-ray Spectrometer. This instrument detected the fluorescence in X-rays from the light elements in the soil. This fluorescence was stimulated by X-rays from the Sun and showed the concentration of the bulk constituents of the Moon; silicon, aluminium, magnesium, oxygen. An unexpected piece of serendipity was the correlation found between the aluminium results from the X-ray Spectrometer, and the altitude of the surface at a particular point as measured by the Laser Altimeter. This was an important finding in the search for the Moon's origins, contributed to by an instrument that failed partway through the flight.
The Laser Altimeter bounced pulses of laser light off the surface and timed their round trip back to the spacecraft. Knowing the orbit of the spacecraft, the altitude of the surface could thus be deduced to an accuracy of one metre. Before it failed, it provided measurements of the Moon's figure or shape with respect to its centre of gravity. These readings were encoded on the film of one of the two camera assemblies to help calibrate the mapping of the Moon. The Mapping Camera consisted of two cameras in one, as well as the Laser Altimeter. The largest of these two was the Metric Camera which took very accurate and often rather beautiful images of the Moon in a calibrated system. A Stellar Camera was included, exposing onto the same film, images of the star field at a known angle to the Metric Camera, providing accurate information about the camera's direction at the time of exposure. Complementing the Mapping Camera was the very high resolution Panoramic Camera, a modified version of an American spy-satellite camera from the early 1960s. This camera took images of a huge swathe of terrain by exposing a strip of film 1.2 metres long, with a resolution better than 2 metres.
There were two other instruments on board Endeavour's SIM bay; an Alpha Particle Spectrometer to detect radiation from gases which would be expressed during any possible lunar volcanism, and a Mass Spectrometer which, like the Gamma-ray Spectrometer, was mounted on its own extendible boom. This exquisitely sensitive instrument was intended to detect and characterise any trace of lunar atmosphere which might be present at the orbital height. An inlet to this instrument faced the same direction as the SPS engine bell so that by adjusting the spacecraft's attitude, any molecules present in the atmosphere could be rammed in or shielded. This would let analysts determine whether the results from the instrument were due to the atmosphere, or to gases routinely given off by the spacecraft. Results from the Mass Spectrometer failed to detect significant lunar atmosphere as local contamination tended to dominate the instrument.
Three days of orbiting the Moon alone was a first for an Apollo astronaut and Al Worden wanted to mark it in some way. With Farouk, he came up with the idea that when he came around from the far-side of the Moon every two hours, he would greet the Earth in a way that expressed the variation of human culture across the whole globe, not just in the English language. Using the welcoming phrase "Hello Earth, Greetings from Endeavour." he and Farouk phonetically wrote down how he could say this in a selection of languages and he used them regularly throughout his solo mission.
Each evening, before he slept, he spent a complete near-side pass out of communication with Mission Control as the radio signals from his spacecraft were bounced off the Moon, to be received on Earth and the information within them about the surface and subsurface structure teased out of the complex reflections.
As Dave Scott and Jim Irwin finished their exploration of Hadley/Apennine, Al had to prepare for their return by altering the plane of his orbit. The rotation of the Moon during his crewmates 3-day stay had taken his orbit away from the landing site so he fired the SPS engine for a calculated 18-second burn to restore it in time for Falcon's lift-off from the Moon. Their method of rendezvousing was the preferred direct ascent trajectory where the two spacecraft are brought together in a single orbit, a technique which is more efficient but more challenging to perform. Prior to docking, the LM crew inspected the SIM bay to see if they could find reasons for a problem in the Panoramic Camera, but to no avail.
After docking and the transfer of themselves and their haul of samples to the CM, the crew prepared to jettison the ascent stage of the LM, a procedure which included the firing of a mild detonating fuse around the periphery of the tunnel to physically sever it and LM from the apex of the Command Module. In light of a recent tragedy, when less than a month before the flight of Apollo 15, the crew of Soyuz 11 were killed when their spacecraft depressurised just prior to re-entry, the Flight Plan had been changed to have the Apollo 15 crew wear their suits during jettison. The dirt on their suits, however, seemed to be taking its toll of the seals and they had difficulty in getting a good pressure integrity in the suit circuit. They thought they had overcome this problem when they came upon another, namely, they didn't appear to be getting a good seal on the forward hatch, the one that led to the tunnel and thence to the, now unneeded, LM. Since only the forward hatch would be between them and space, it was vital that this provide a good seal. Mission Control and the crew backed out of their readiness to jettison and delayed it until the next orbit to give the crew a chance to check the seal on the hatch and, because the suit circuit seal had been broken, reverify it also. A poor glove connection compromised the second suit integrity check but eventually, this too was corrected and the time for jettison finally arrived.
The jettison of the LM was to be one of the most confused and "out-of-sync" episodes of this most nominal of flights. By the time Falcon was finally released, they had passed the ideal point in their orbit by ten minutes, altering the geometry of the situation substantially. So when Dave consulted the computer for details of the burn to take the CSM away from the LM, he was surprised to find a component of the burn was towards the discarded spacecraft. Mission Control was worried by their perception of the crew's tiredness, shaken by the problems with the pressure integrity checks, and now confused by the geometry of the two spacecraft with respect to the Sun and Moon. Unclear calls to the crew served to hamper a resolution until a straight call came to point at the LM and burn to move away. The crew was unflappable and rode out the situation despite having had a long day. Now they could settle down for some final orbital science and a good rest.
The concern of Mission Control for Dave and Jim's physical condition after an extremely strenuous stay on the surface came to light when Deke Slayton, their boss, came on to suggest they take a sleeping pill to try and get some rest. What Deke and the other managers knew, but chose not to tell the crew, was that the Flight Surgeons had noticed irregular heart rhythms from Dave and, especially, Jim. Subsequent to the flight, Jim continued to have heart problems and died of a heart attack twenty years after the flight. As commander of the mission there is a good argument that Dave should have been told of this condition, for later in the mission, Jim would, once more, be out in space in a pressure suit in a role that could just as easily have been filled by Dave. Had something happened to Jim then, the results could have been disastrous. For their final, full day in lunar orbit, Dave, Jim and Al got down to some intensive science in lunar orbit. Al continued with the operation of the SIM bay while they all got snap happy with much of the remaining film for their 70-mm cameras. The advancing daylight and their shifting orbit were revealing new vistas to the eye of humans, most particular, the area around the brilliant ray crater Aristarchus provided an endlessly spectacular variation of landscapes for their cameras, both volcanic and impact in its genesis.
On the tenth day, a final rush of science data gathering and photography preceded what would be the final burn of the SPS engine; a burn that would send the spacecraft, crew, samples and film out of lunar orbit and on a long, ballistic fall to Earth. This burn, Trans-Earth Injection, used up the final 30% of the SPS tanks' original load, leaving just a small amount for any major tweaks to their trajectory that might be required for their journey home. In the event, the burn was so perfect that no correction was ever needed until just before splashdown when the RCS thrusters made the midcourse-7 burn - and that was only because of some uncoupled RCS thrusting that pushed their flight path slightly away from the nominal.
As they lifted away from lunar orbit, they enjoyed a fine view of the rugged territory at the Moon's eastern limb and towards its south pole. The Mapping Camera, likewise, continued to photograph the view, producing some of the finest whole-moon view from the Apollo program.
The coast home was a time to rest, reflect and prepare for the end of the mission. The time, however, was punctuated by a number of important tasks and top of the list was Al Worden's opportunity to leave his spacecraft to retrieve film magazines from the Mapping and Panoramic Cameras within the SIM bay. Wearing his own pressure suit and linked to the spacecraft by a tethered umbilical, Al also wore Dave's LEVA (Lunar Extravehicular Visor Assembly) - to protect his eyes from the glare of the strong sunlight - and an unused OPS (Oxygen Purge System) which provided emergency oxygen should his suit become torn while outside. With Dave seated in the cabin, ready to accept the film cassettes and Jim standing in the open hatch watching in case of any problems, Al maneuvered along handrails attached to the hull of the Command and the Service Module, reaching a set of foot restraints which afforded him a secure foothold and a good view of the bay.
Between retrieving the heavy film cassettes, Al was able to have a close look at the bay and try to find possible causes of the problems he and Mission Control had experienced with some of the instruments. The Laser Altimeter had worked for only part of its planned lifetime; a sensor on the Panoramic Camera was failing to sense how much compensation should be made for the spacecraft's motion; the boom carrying the Mass Spectrometer instrument couldn't retract in some circumstances. Though there was no hurry at all, Al efficiently carried out his tasks and, after only 20 minutes, was back inside.
The instruments of the SIM bay could still be pressed into service even though they no longer looked at the Moon. In particular, the early days of gamma-ray and x-ray astronomy were fuelled by the results from the relevant spectrometers measuring the high-energy flux coming from deep space. The cloud of gases that accompany the spacecraft during coasting flight could be measured by the Mass Spectrometer. Within the cabin, the crew continued to take UV photographs of the Earth and Moon, and to probe the nature of the light flashed in their eyes.
During their final full day in space, they had the good fortune to be sited midway between the two components of the Earth/Moon double planet, just when Moon passed through the shadow of the Earth. Coasting off to one side of the Earth/Moon line, they could still see the major globe of the system as a thin crescent while the smaller body became lit in the glow of all Earth's sunrises and sunsets.
Throughout their journey home, Al worked independently from Mission Control to determine their position and velocity - the raw details of their trajectory - so that, if communication with Earth were lost, he would be able to guide Endeavour back into the atmosphere without help. To achieve this, he measured the angular position of the Earth with respect to the stars and so accurately did he achieve this, Mission Control were having trouble deciding whether their determination, or his was the most accurate.
On August 7, 1971, after 12 days of voyaging to a stunning bay, embraced by vast, rounded mountains on another world, Apollo 15 re-entered the Earth's atmosphere right on schedule. The final few thousand metres of their descent was punctuated by the loss of one of their main parachutes, likely due to the dumping of RCS fuel. Their splashdown was somewhat harder than usual but well within tolerance levels and the crew were subsequently spared the indignity of being placed into biological isolation on reaching home, though Dave certainly felt he could have used the rest.
Though subsequent missions travelled further on the Moon, brought back more samples and put the lessons of Apollo 15 into practice, this feat of unalloyed exploration still stands out as a great moment of human achievement. It is remembered still for its combination of competent enthusiasm, magnificent machinery, finely honed science and the grandeur of a very special site in the cosmos beside a meandering rille and graceful, massive mountains - Hadley Base.
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