The Kennedy deadline placed a number of constraints on the engineers and planners who had to decide how the landings would be accomplished and then had to design and build the actual flight hardware. The process of deciding how to get to the Moon has been thoroughly documented elsewhere - for example, in Roger Bilstein's Stages to Saturn and, at a less technical level, in NASA's Apollo Expeditions to the Moon, edited by Edgar Cortright, and in John Noble Wilford's "instant" New York Times book, We Reach the Moon. There were a variety of proposals floating around as to how one might accomplish a lunar landing but, in the end, only one concept offered any plausible chance of success by the end of the 60's. The technique was called Lunar Orbit Rendezvous and, in some senses, it involved the use of two separate sets of spacecraft.
The second largest component was the Command-and-Service Module (CSM) which carried the crew of three astronauts on the trip to lunar orbit and then, when the landing mission had been accomplished, the trip home to Earth. The Saturn V/CSM combination can be thought of as a system for the conduct of lunar orbit missions and, indeed, during the 1968 Christmas season, the crew of Apollo 8 performed just such a mission.
Conceptually, the hardware for the landing mission comprised a separate system - albeit one that was carried as cargo by the Saturn V/CSM. The landing systems allowed the Mission Commander and the Lunar Module Pilot (sometimes referred to as the CDR and LMP, respectively) to descend from lunar orbit to the surface while the Command Module Pilot (CMP), the third member of the crew, waited for their return. For Apollos 15, 16, and 17, there were four major elements of landing-mission hardware: (1) the Lunar Module (LM) Descent Stage which contained the engines and propellant for the actual landing, (2) the LM Ascent Stage which contained the crew cabin and the engines and propellant for the return to lunar orbit, (3) the suits which allowed the crew to work outside the pressurized cabin, and (4) the Rover which they drove to the various geology stops.
At the heart of the Descent Stage was the engine which the astronauts fired in lunar orbit to slow themselves through the twelve-minute descent to the surface. On Apollo 17, for example, at the moment Cernan first fired the Descent Engine, the total weight of the LM Challenger was 36,686 pounds and, of that total, slightly more than half (19,564 pounds) was descent propellant. The fuel and oxidizer were, respectively, hydrazine and nitrogen tetroxide. Because these burned on contact once they were introduced into the reaction chamber, there was no need for an ignition system. In addition, Cernan could control the fuel and oxidizer feed rates so that he and/or the on-board computer could control the thrust level and, therefore, fly a descent path tailored to minimize propellant use. At the moment of engine shutdown, Cernan had 1,315 pounds of propellant left.
In plan view, the descent stage had an octagonal cross section, with the engine, the associated propellant tanks, and also tanks of water and oxygen for use by the crew and the spacecraft systems occupying the central portion. Around the periphery, landing struts were attached at the center of every second section of the octagon. The intervening sections were used for stowage of the Rover, the scientific equipment packages, and such things as replacement batteries, spare lithium hydroxide canisters for the removal of carbon dioxide from the cabin and suit air, and even a two-day supply of food. Anything that could be stowed in the Descent Stage meant a savings in the weight and volume in the Ascent Stage and, thereby, a savings in fuel needed for the return to orbit. Stowage in the Descent Stage also gave the astronauts more elbowroom in the cabin.
When the LM was sitting on the Moon, the cabin was essentially a stubby cylinder lying on its side, with the flat ends pointed forward and aft. The accompanying 2002 photo shows Apollo Flight Journal Editor Frank O'Brien in a LM simulator. Note the LMP PLSS on the floor.
The cabin was surrounded laterally and at the back by tanks for propellants, water, and oxygen. There were hatches on the front face and at the top. The cabin had a diameter of 92 inches and an fore-to aft length of 92 inches, as well; however, much of the cabin volume was taken up by instrument panels, the environmental control equipment, storage compartments, and the ascent engine cover. In effect, the cabin was just barely big enough to hold two suited astronauts. When the LM crewmen were getting ready to go outside and had their suits pressurized and were wearing their backpacks, even turning in place required careful coordination.
Toward the rear, the cabin was even more cramped than it was from side to side. At eye level, the cabin was a full seven and a half feet deep; however, from about knee height down, the aft portion of the cabin was filled with the ascent engine cover and, forward of that, the floor space was only three feet deep. In addition, the side bulkheads in the aft portion of the cabin were covered with storage compartments on the Commander's side and the environmental control equipment on the LMP's and, in effect, the aft portion of the cabin was only useful as a place for temporarily stowing such things as the suits and helmets and, at night, as a place to hang the Commander's hammock. Readers should also note that, contrary to what is shown in the accompanying drawings, the crews of the extended missions (Apollos 15, 16, and 17) did not wear their suits during the rest periods and, rather, slept in their underwear. The suits were stowed on the ascent engine cover and further reduced the usable space in the cabin.
From about waist height up, the forward wall and the bulkheads to either side of the forward section of the cabin were covered with gauges, switches, and circuit breakers. There was also a panel running across the front of the spacecraft at about waist height and, on it, each of the astronauts had a pistol-grip attitude controller at his right hand and a thrust controller at his left. In between these on the Commander's side, there was a manual start/stop button with which he could back up the computer and, in a similar position on the other side of the cabin, the LMP had readouts and a data entry pad for the Abort Guidance System (AGS), the back up navigation-and-guidance computer. On the waist-height panel between the two astronauts and above the hatch, they had a Display and Keyboard Assembly or DSKY - pronounced "dis-key"- which was used for input and output of the Primary Guidance-and-Navigation System or PGNS - pronounced "pings". Because the DSKY was over the hatch and extended about a foot out into the cabin, the astronauts had to be careful not to catch their backpacks on it as they crawled in and out of the spacecraft. The final piece of navigation instrumentation was a small, one-power, optical telescope or sextant with which the astronauts could make star sightings either while they were in orbit or once they were down on the surface. This Alignment Optical Telescope (AOT) was hung from the ceiling over the DSKY and had an aft-pointing eyepiece at about eye level.
Almost all references in the transcript to the Environmental Control System concern the settings of various ECS switches and valves. Consequently, in the next few paragraphs we will go into some detail about the relevant parts of the ECS.
At sea level, the Earth's atmosphere is a mixture of gases - primarily of nitrogen (78% by volume), oxygen (21%), water vapor (varying amounts depending on temperature and humidity), and traces of carbon dioxide and other gases. Oxygen is, by far, the most important component of what we breathe and, indeed, the Apollo astronauts breathed almost pure oxygen laced with controlled amounts of water vapor. With the nitrogen eliminated, the cabin pressure could be considerably less than sea-level pressure on Earth - about 4.8 psi (pounds per square inch) versus 14.7 psi - and, consequently, the cabin walls could be relatively thin and, therefore, light in weight.
Fresh oxygen had to be supplied more or less continuously because, as the astronauts breathed, they took in oxygen and exhaled carbon dioxide. Lithium hydroxide canisters (described below) were used to extract excess carbon dioxide from the cabin and/or the suits and, had fresh oxygen not been added, the cabin and/or suit pressures would have slowly decreased. Except when the astronauts were using the oxygen supplied from their backpacks, they used oxygen supplied to the cabin and/or the suits through a pair of oxygen demand regulators - usually referred to in the checklists and in the transcript as "Press(ure) Reg(ulator)'s A & B". Upstream of the demand regulators, various valves and switches allowed the crew to select the tank from which fresh oxygen would be drawn (two tanks in the descent stage and one in the ascent stage), allowed them to refill the tanks in their backpacks or, after a period of Extravehicular Activity (EVA), allowed them to repressurize the cabin. Also to be found upstream of the demand valves were various relief valves, burst diaphragms, and high-pressure regulators which protected the ECS from the high (2700 psi) pressures of the descent stage storage tanks.
The two demand regulators were hooked up in parallel so that the system would still function if one or the other of them failed. The controls were located on a panel behind the LMP's station. They were positioned at about waist height and each had four possible settings. With the control set in the Closed position, no oxygen would flow through the regulator while, in the Cabin position, enough oxygen was supplied directly to the crew compartment to maintain a pressure in the range of 4.6 to 5.0 psi. When set in the Egress position, the regulators maintained a pressure of 3.8 psi (plus or minus 0.2 psi) in the Suit Circuit described below; and, finally, the Direct O2 setting produced unregulated flow of oxygen into the Suit Circuit. Some readers may want to note that, with the demand regulators set in Direct O2, pressure in the Suit Circuit could, in principle, be controlled through use of the Suit Circuit Relief valve which is also described below.
As already mentioned, the ECS included a cabin repressurization valve. Its control was located on the same panel as the demand regulator controls and it had three settings. In the Closed position, there was no possibility of flow through the valve while, in the Manual position, oxygen would flow into the cabin until the setting was changed. With the control in the Auto position - and the repressurization circuit breaker closed - valve openings and closings were determined by a solenoid which was enabled whenever either of the demand regulators was set in the Cabin or Direct O2 position. With the solenoid activated and the repressurization control in the Auto position, a drop of pressure below 3.7 psi would activate the flow of oxygen until the cabin pressure built back up to about 4.5 psi
Although the repressurization valve never had to respond to an emergency during any of the Apollo missions, it was designed to maintain cabin pressure at 3.5 psi for at least 2 minutes following a 5-inch-diameter hull puncture. The only times when a puncture was at all plausible was during landing and during launch and rendezvous; and, at these times, the astronauts always wore their suits, albeit unpressurized so that they could move their arms with relative ease. Consequently, in the event of a puncture the astronauts would only need cabin pressure maintained for long enough that they could get their suits inflated. Two minutes gave plenty of margin. Because there was never any repressurization emergency during any of the Apollo missions, the repressurization valve was only put to use following the EVAs (three on Apollo 17) and various scheduled jettison operations (two on Apollo 17)
In order to depressurize the cabin for an EVA or to jettison trash (empty food packs, expended batteries, and the like), the astronauts could use one or both of two dump valves: one was built into the forward, or "EVA", hatch and the other into the overhead, or "docking", hatch. Each of the dump valves had two lever-type handles. One was on the outside of the spacecraft so that, if for some reason pressure built up in the cabin and was holding the hatch closed, the astronauts could use the external handle to dump the pressure. (There were never any unplanned cabin pressurizations during the Apollo missions and, consequently, the external handle was only used as a means of pulling the hatch partially shut for cabin thermal control during the EVAs.) The internal handle could be set and locked in any one of three positions. In the Closed position, no cabin gas could get through the valve. In the Open position, the valve was fully open and gases in the cabin could freely escape to the lunar vacuum. And, in the Auto position, the valve served as a pressure relief valve, and would have opened had the cabin pressure ever exceeded 5.4 psi. In the Open position, the either valve could reduce cabin pressure from 5.0 psi to 0.08 psi - a value low enough that an astronaut could pull the hatch open against the remaining pressure - in about 180 seconds. If, as was the case for the Apollo 11 EVA, a bacterial filter was installed on the forward valve, the time was 310 seconds. With both valves open and no filter installed, the time was 90 seconds.
As noted previously, oxygen was added to the Suit Circuit when either of the pressure demand regulators was set in the Egress or the Direct O2 positions. In Egress, enough oxygen was added to maintain the Suit Circuit pressure at 3.8 psi while, in the Direct O2 setting, the flow was unregulated, albeit at a modest 7 pounds of oxygen per hour. For comparison, the cabin repressurization valve supplied 6.6 pounds of oxygen in two minutes
Beyond the point where fresh oxygen was added to the Suit Circuit flow, the gas stream first passed through a heat exchanger where, depending on the setting of a control located behind the LMP's right foot, more or less heat was added.
The oxygen stream next entered two Suit Isolation valves - one for each crewman. The controls for these valves were located behind the LMP's station at about knee height and each had two settings. In the Suit Flow position, the valve let oxygen flow from the Suit Circuit into the hoses and then back again. In the Suit Disconnect position, the valve was closed to flow. The prime position of the valve was Suit Disconnect and, indeed, the valve was equipped with a solenoid-activated, spring-operated Override which would cut off the flow if a sensor detected a suit circuit malfunction. If the actuator had ever been tripped, the astronauts would then have hooked up to the backpacks or the Oxygen Purge Systems described below.
Beyond the Suit Isolation valves, the oxygen flow next encountered the Suit Circuit Relief valve. This control was located on the inboard face of the ECS cabinet, a position which was relatively hard to reach when the astronauts were wearing their backpacks and had their suits pressurized. Indeed, the only time the J-mission crews had to change the Relief valve setting while they were pressurized was just prior to the backpack jettison at the end of the last EVA - at a mission elapsed time of 171:48:42 in the case of Apollo 17. The transcript indicates at that point, with his backpack off, Lunar Module Pilot Jack Schmitt had enough freedom of movement that he could reach the control without notable trouble. The Relief valve had three settings. In the Open position, oxygen flowed freely out of the Suit Circuit into the cabin; while, in the Closed position, no flow could occur. Because both the Open and Closed positions represented a slightly increased risk in the operation of the Suit Circuit, microswitches were used to produce a telemetry signal whenever the control was in either of those settings. The normal setting was Auto. Usually, this meant that the valve was closed but, when the Suit Circuit pressure exceeded 5.3 psi, the valve would open to relieve the excess into the Suit Circuit. The only time during Apollo 17 when the Suit Circuit Relief valve was not in the Auto position was during a brief period prior to depressurization for the backpack jettison. For these few minutes, the Relief valve was Closed and one of the Pressure Demand Regulators was set to Direct O2, thereby permitting suit/suit circuit pressurization for an integrity check. Once the suits had been pressurized, the regulator was reset to Egress; and, then, a short while later when the integrity check was complete, the Relief valve was reset to Auto. Thereafter, pressure in the Suit Circuit was maintained at 3.8 psi by the Demand Regulators
Next downstream from the Suit Circuit Relief valve was the Suit Gas Diverter Valve - located behind the LMP's station, just to the right of his head. This control was a push/pull-type handle. The fail-safe position was Pull/Egress which closed the valve and prevented the flow of oxygen out of the suit circuit, through the valve, and into the cabin. The alternative position was Push/Cabin which permitted free flow of oxygen out of the suit circuit into the cabin. Under normal conditions, this setting was used only when the astronauts were out of their suits and wanted to maximize the interchange of air between the Suit Circuit and the cabin and/or they wanted to dry the suits. Had the cabin pressure ever fallen with the Suit Gas Diverter set in Push/Cabin, a solenoid would have released a spring and returned the valve to the Egress position
Oxygen flowing out of the Suit Gas Diverter valve returned to the suit circuit through the Cabin Gas Return Valve. This control was also located on the inboard face of the ECS cabinet. As with the Suit Circuit Relief valve, its setting was never changed when the astronauts had their suits pressurized. The control could be set in one of three positions: Open, Auto, and Egress. The Open setting, of course, permitted a free return of cabin gas to the suit circuit and the Egress setting prevented such flow. In Auto, the flapper-type valve was allowed to respond to pressure differences between the cabin and the suit circuit - closing when the cabin was at lower pressure than the suit circuit and opening in response to the reverse condition
Beyond the Return valve, oxygen in the suit circuit flowed through one or the other of two carbon-dioxide/odor-removal canisters, the choice of canister depending on the settings of selectors mounted on the in-board face of the ECS cabinet. The canisters each contained beds of lithium hydroxide (LiOH) for carbon dioxide removal and beds of activated charcoal for odor control. The larger Primary canister had a capacity of 41 person-hours while the smaller, Secondary canister, identical to the canisters used in the backpacks, had a 14-hour capacity. Replacement LiOH cartridges - along with backpack replacements - were stored outside the cabin in the Modular Equipment Storage Assembly (MESA). On Apollo 17, replacement of the Primary ECS cartridge was done after wake-up on Day 2 of the lunar surface stay and then, a second time, about 35 hours later at the end of EVA-3. We note that the astronauts were outside the spacecraft during two 8-hour EVA periods during the time between the first changeout and the second. Consequently, the cartridge was used for about 38 person-hours, a figure close to the design lifetime. As a back up, there was a carbon dioxide sensor in the Suit Circuit which, if the CO2 partial pressure exceeded 7.6 mm of mercury, would have turned on warning lights. And, finally, because of the heavy - if relatively brief - dust load anticipated following the EVAs, the Primary canister was protected with a dust filter which, if it became clogged, could be bypassed by changing a valve setting.
Oxygen flow through the Suit Circuit was achieved primarily through the operation of one or the other of two suit-circuit fans. The circuit breaker for fan No. 1 was located on breaker panel 11 on the Commander's side of the cabin while the circuit breaker for the other fan was located on panel 16 on the LMP's side. A selector switch was located on a panel on the forward cabin wall, right next to the ECS caution lights and just to the LMP's side of the centrally-mounted Alignment Optical Telescope (AOT). Generally, Fan No. 1 was used during the landing and launch phases while Fan No. 2 was used during the pre-and-post EVA periods. A sensor called Suit Fan Delta-P measured the pressure differential across the fan assembly. It was operated with a circuit breaker on the LMP's panel and, when the breaker was closed, lit a warning light if the pressure difference across the fan assembly was less than 9.0 inches of water (about 0.3 psi). Obviously, the warning light would come on as the fan was turned on and, by going out, would indicate that the fan had come up to speed. Backflow across an inoperative fan was prevented by flapper-type check valves
Beyond the fans, oxygen in the suit circuit flowed through a sublimator where excess heat was removed and then through one of a pair of water separators where excess water was removed and transferred to the waste management system. A selector switch was located near the LiOH canisters on the in-board face of the ECS cabinet.
Next to his skin, the astronaut wore a Liquid-Cooled Garment (LCG), a pair of 'long johns' embedded with a network of thin plastic tubes. (Long Johns were long, winter underwear issued to U.S. soldiers during World War II. 'John' may be a reference to boxer John L. Sullivan, whose boxing costume the underwear resembled.) During the EVAs, water was pumped through the tubes where it picked up excess body heat. The water then flowed out of the LCG, through a connecting hose, and into the backpack where it was cooled before flowing back to the LCG. This supply of LCG cooling water flowed in a closed-loop system and there was never any appreciable loss during the course of the mission. Inside the backpack, there was another supply of water called the feedwater; this was fed at a controllable rate through a heat exchange coil where it extracted heat from the LCG water. After being heated in the coil, the feedwater was then allowed to sublimate into the lunar vacuum and, thereby, carried away the excess heat. Each of the backpacks could hold about twelve pounds of feedwater, enough to provide cooling for about eight hours of fairly strenuous activity
The suits were made of an inner bladder covered by several layers of insulating aluminized Mylar which, collectively, not only held pressure but also provided thermal protection when the astronauts were out in the direct sun. In addition, the outer layers of the suits were strong enough to resist tearing and abrasion as the astronauts crawled in and out of the LM, carried equipment, jumped onto their Rover seats, brushed up against rocks, and sometimes even fell. At stomach and lower chest levels, the suit was equipped with connectors for water hoses, oxygen hoses, and a communications cable. The hose connectors each had a ring lock and, embedded in it, a ring-locking mechanism ingeniously called a "lock-lock". There were also dust covers over each of the connectors
The hoses ran from the connectors around to the backpack or PLSS (Portable Life Support System) - pronounced "pliss" - which contained communications and telemetry gear, tanks of oxygen and feedwater, a fan to move the oxygen through the suit, a pump to move the closed-loop water through the Liquid Cooled Garment, a lithium hydroxide canister to remove carbon dioxide, a battery for electric power, and assorted subsystems which allowed the astronauts to recharge and refill the tanks for multiple forays out onto the lunar surface. For Apollo 17, each PLSS could hold enough oxygen (1.8 pounds), feedwater water (12 pounds), and battery power (25 amp hours) to see an astronaut through seven to eight hours of Extravehicular Activity (EVA). The exact length of an EVA depended on the astronaut's level of exertion and, therefore, the rate at which he used his supplies of "consumables". The nominal eight-hour capacity - with about an hour's worth of cooling water in reserve and even more of oxygen and battery power - meant that the astronauts got back in the LM at about the time when prudence would suggest getting out of the suits for a period of rest anyway. Indeed, the astronauts could divide their lunar stays into something resembling three normal 24-hour days - four hours of EVA preps, eight hours of work outside the spacecraft, four hours of post-EVA activities, and eight hours of rest - and, thereby, minimize the fatigue and stress with which they had to cope
For its time, the PLSS design represented an amazing engineering achievement: a complete life-support system in a reliable, manageable, 80-pound package. However, the fact that crew safety was so dependent on PLSS operation dictated that backup systems be available
On top of their PLSS's, each of the astronauts wore a forty-pound Oxygen Purge System (OPS) which provided backup oxygen. The purge system contained about a thirty-minute supply which could be used in the event of a PLSS failure, a suit puncture, or exhaustion of the primary oxygen supply. Because the OPSs could not be recharged and, in addition, were intended to serve as sources of emergency oxygen in case the astronauts could not dock with the Command Module and, therefore, had to make a spacewalk, there was never any thought given to use of the OPSs to extend an otherwise normal EVA. However, if oxygen were needed in an emergency, the astronaut could pull an actuator mounted on the Remote Control Unit (RCU) on his chest and the OPS would supply whatever oxygen was needed to keep the suit pressure at 3.8 pounds per square inch. In the case of a suit puncture, oxygen would flow out of the OPS, into the suit, and out through the hole; and, if it was a PLSS failure that caused the problem rather than a suit puncture, the astronaut could open a purge valve fitted on the right side of his chest in order to create a pressure difference and, thereby, initiate the emergency flow. The purge valve had two settings. If the LCG/PLSS cooling system was still working, the astronaut would put the purge valve in the Low Flow setting (4 lbs/hour) and would be able to get oxygen for about an hour. If the cooling systems was inoperative, he would use the High Flow setting (8 lbs/hour) and would have about 30 minutes to climb back in the cabin and get hooked up to the LM oxygen supply
Because the astronauts planned to drive for up to an hour to get to their most distant geology stations, if they were going to make it back to the LM in the event of a PLSS failure they had to be able to operate the OPS in Low Flow. Consequently, it was essential to have a backup to the cooling system. This was provided by the Buddy/Secondary Life Support System (BSLSS) which consisted of a set of water hoses and fittings which would allow the astronauts to share cooling water from the good PLSS. Because the astronauts could not have walked very far or very quickly tied together with hoses, the BSLSS was only of practical value if the Rover was still working. However, the risk of a simultaneous PLSS/Rover failure was considered to be acceptably low
The various PLSS controls were located on either the chest-mounted Remote Control Unit (RCU) or, the lower, right corner of the front of the PLSS. Switches for the oxygen fan and the LCG pump were located on the RCU along with the comm switches, a volume control, an oxygen quantity gauge, and an assortment of warning flags which would indicate PLSS malfunctions. The gauge and the flags were located on the top surface so that, in principle, the astronaut could merely look down and see them. Sun glare and dust sometimes made the reading a little difficult. The switches on the bottom of the PLSS were located far enough forward that, in principle, the astronaut could reach back and turn the sublimator on, switch from the primary feedwater tank to the auxiliary tank, or adjust the amount of cooling he was getting by changing the rate of feedwater flow through the sublimator. Each of the switches was shaped differently from the others so that they could be distinguished by feel. However, the stiff suits made the act of reaching the switches a bit difficult. When the astronauts were inside the LM, reaching the switches was particularly difficult - especially for the shorter-armed astronauts - and many had to help each other get their cooling water turned on or off. Outside, the stiff suits forced most of them to first move their right arms up and forward so that they could get some momentum in a downward swing as they reached back for the control.
The Rover was a lightweight vehicle, weighing only 462 terrestrial pounds empty - 70 pounds on the Moon. It was battery-powered and had a potential range of about 50 kilometers. On level ground, the Rover was capable of about 10 - 12 kilometers per hour and, when one added to the driving time the time spent on Rover deployment and housekeeping chores, the savings over a walking traverse was not particularly impressive. However, because the astronauts used far less oxygen, feedwater, and physical energy when they were riding than they did when they were walking, use of the Rover meant that they arrived at the geology stops rested and refreshed and with a far greater stock of consumables than they would have had otherwise. In addition, the Rover could carry up to 970 terrestrial pounds of payload and, when one subtracted about 600 pounds for the astronauts and their life-support gear, there was still plenty of capacity for communications equipment and a generous assortment of scientific paraphernalia
The four Rover wheels were each made of a strong wire mesh and had a chevron-shaped, metal tread that covered about fifty percent of the traction surface. When fully loaded, the Rover chassis had a clearance of about 14 inches. Rocks bigger than that had to be avoided, of course and, indeed, Cernan tried to avoid as many rocks as he could to avoid damage to the wheel. Inevitably, he couldn't avoid all of the six-inch rocks and ran over quite a few and, by the end of the third EVA, had collected a number of golf-ball-sized dents in the mesh. The dents did not noticeably degrade the Rover's performance and, indeed, because each of the wheels was driven by a separate motor and because the vehicle had, as well, both front and rear steering systems, there was little chance of a complete Rover breakdown
The Rover was equipped with a navigation system which used counts of wheel rotations to measure the distance driven since the system was last reset. More importantly, when the wheel-rotation data were combined with heading information provided by an internal gyro, the Nav system provided a range and bearing to that last point of initialization - always a point near the LM
The "problem" of getting back to the LM from some distant geology station was actually a trivial one. There were outbound Rover tracks to follow and, even if the astronauts weren't retracing their outbound course, there were plenty of landmarks to lead them close enough to see the 23-foot-tall LM. For obvious reasons, the astronauts always landed in the middle of a large area of relatively flat terrain and, even if the LM was in a local low spot, all one had to do was find a bit of high ground and the glint of sunlight off the LM's foil covering would immediately catch the eye. Certainly, there was no chance of mistaking the LM for a rock and, if it was in the line of sight, it could be seen from a considerable distance. The only exception to this general rule was when the astronauts had to look directly into the low, early-morning Sun - in the direction called "up-Sun". In such a case, even had they been able to see into the bright glare, they would have been looking at shadowed surfaces on the spacecraft and might have had trouble spotting it. However, it wouldn't have taken much of a detour to get a good viewing angle and, all in all, there was virtually no chance of getting lost
Because even the "flat country" is rolling and hummocky and everything but the LM is some shade of grey, the problem of finding anything else - say a specific crater - could be challenging. As an extreme example, on Apollo 14 Alan Shepard and Ed Mitchell walked to within twenty meters or so of the rim of Cone Crater and then, after a tiring climb and a fruitless search, eventually had to head back toward the LM without ever having actually looked into the crater. With the Rover, the problem of finding geology stops was much less dramatic
Because the Nav system measured distance and bearing relative to the LM, finding a specific crater meant that the astronauts had to know exactly where they had landed. On each of the three Rover missions, there was some initial confusion about the absolute location of the LM; but, after a while, the crews were able to unambiguously identify a crater or two. Navigation readouts at those places then helped the support people back in Houston to pin down the landing site and, thereafter, the crews could use the Nav systems to drive with confidence from one place to another. The Nav system was only accurate to about 100 meters; but, if there was a little remaining confusion when the crew got close to their target, they could always spend a minute or two driving around, seated comfortably, until they found the crater they were looking for.
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