The Moon
Apollo 12
Apollo 14
| Apollo 13 Animations |
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The first two Apollo crews landed out on the smooth lunar mare, the lava seas that formed relatively late in the Moon's history. Both crews proved that, once the Commander took over control at an altitude of about 500 feet, he could move his landing spot by several hundred feet. After two landings out on the mare, NASA was ready for a visit to a site where landing accuracy promised insight into the history of the older parts of the Moon.
About half of the lunar Nearside - and virtually all of the Farside - is covered by heavily-cratered highlands, the light-colored regions which, as seen from Earth, contrast so strongly with the darker-colored mare. Because the mare rocks are relatively young and cover far less than half of the lunar surface, the scientific community needed highlands samples if they were going to understand lunar geology. To be sure, the Apollo 11 and 12 collections contained significant numbers of small rock fragments which differed markedly in composition from the bulk of the mare-derived materials; and there was every reason to believe that many of these "exotic" fragments represented ejecta from impacts in the distant highlands. However, while studies of these fragments produced plausible data on the general age and mineral content of highlands materials, there was no real substitute for taking a look at in-place samples of highlands bedrock.
Although NASA was not ready to commit a LM to a landing in really rugged terrain, the site selection committee had long been interested in a place called the Fra Mauro Hills, a small, relatively benign bit of highlands country sitting like a low island out in the middle of the Ocean of Storms. Of particular interest was a feature called Cone Crater, a comparatively fresh crater about 300 meters across. The Fra Mauro Hills are believed to be part of the extensive ejecta blanket laid down by the impact that formed the huge Imbrium Basin; and Cone Crater had been dug into a ridge of this material. Although the area is covered by materials derived from post-Imbrium events- such as ejecta from the large, young crater Copernicus some 500 kilometers to the north and, as well, materials dug out of the immediately surrounding mare by countless other impacts - Cone Crater was big enough that the astronauts would surely find Imbrium ejecta as they climbed toward the rim.
From an operational point of view, Fra Mauro offered some additional challenges and opportunities. Because the crew needed a relatively flat place on which to land, they would have to touch down more than a kilometer away from Cone and then walk to the rim. During the last half of the approach, the astronauts would have to climb a grade of about one in ten and the traverse promised to be a significant test of astronaut mobility. In terms of lunar surface activities, the Fra Mauro mission was far more ambitious than either of the other landings and, unfortunately, it took two tries to complete. Jim Lovell, Jack Swigert, and Fred Haise - the crew of Apollo 13 - set out on the first mission to Fra Mauro; but a spacecraft accident forced them to abort the attempt before they ever got to the Moon. Indeed, they were lucky to get home at all.
For years, NASA had concentrated its energies on the obviously critical stages of missions: the launch from Earth, the departure from low-earth-orbit, lunar-orbit insertion, the landing sequence itself, lift-off, rendezvous, the departure from lunar orbit, and the fiery plunge through the Earth's atmosphere toward splashdown. Ironically, it was during one of the quiet times, during the long outward coast, that something went wrong.
The people of Apollo took great pride in their resourcefulness and in their detailed understanding of the flight hardware. If there was a way to improvise and get the crew safely home, they intended to find it. And as they looked over the situation - flight crew and ground personnel alike - they realized that they had been very lucky. As desperate as the situation was, the accident had come early in the mission. They still had a healthy, fully-stocked lunar module. The margin of safety would be tight, but the LM had an engine to put the crew back on a homeward path, and it carried enough - not a lot, but enough - water, oxygen, and power for the four days. And there were plenty of lithium hydroxide canisters in the Command Module and, while they wouldn't fit directly into the LM ECS - being the wrong size and the wrong shape - surely a way could be found to put them to use.
Even in the earliest days of Apollo, during 1962 when NASA was still trying to decide on a basic mission mode, proponents of lunar-orbit rendezvous had argued that, in certain circumstances, the LM engines could be used as backups in case of a Service Propulsion System failure. These "LM Lifeboat" scenarios were never studied in great detail, but enough people had given the general idea some thought - even to the extent of having run some flight control simulations - that, within an hour of the accident, flight engineers were busily calculating trajectories and burn durations, figuring out new navigation and flight control procedures, and refining estimates of just how long the critical supplies would hold out.
Oxygen turned out to be the least of the Apollo 13 worries. The LM carried generous stocks - including the backpack oxygen that Lovell and Haise were to have used for their first EVA at Fra Mauro. In order to conserve their own physical resources - and to minimize their carbon dioxide output - the crew would do their best to keep physical exertion to a minimum. Nonetheless, it was reassuring to know that they would only have to use about half of their oxygen stock in getting home. The power and water supplies were far more critical. A significant fraction of the electric energy stored in the LM batteries would have to be used during the engine burn and, if the astronauts were to survive the trip back to Earth, they would have to carefully conserve the remainder. All non-essential electronics would have to be turned off and that promised to make the trip home a damp and chilly one. Of even greater concern was the fact that, at first blush, there seemed to be no way to keep the Command Module batteries charged until they would be needed for re-entry. Under normal circumstances, the fuel cells in the Service Module were used to keep the CM batteries charged and then, only in the last few hours of the mission, once the Service Module had finished its job and was jettisoned, were they brought on line. Unfortunately, the accident had killed the fuel cells and, unless a way could be found to use the LM batteries to maintain a charge, the crew would have no way to control their re-entry and would perish just as surely as if they had crashed into the Moon.
The problem in maintaining a charge on the Command Module batteries was only one electrical path between the LM and the Command Module and that was a sensor circuit which, as Fred Haise remembers it, allowed the crew to monitor power usage by the LM systems. Because the LM and the CSM had no physical or electrical connections during launch from earth, the monitoring circuit was established only after the crew had docked with the LM, removed the docking hardware from the connecting tunnel, and plugged in an umbilical cable. According to Jack Schmitt, "After the explosion on 13, somebody started working the schematics of the two spacecraft and figured out that they could configure switches and circuit breakers so that current from the LM batteries could be trickled over along this sensor circuit. And that's what they did for five days: trickle this current over to the Command Module batteries. Without it, they could never have re-entered (Earth's atmosphere)."
By turning off as much of the electronics as they could, they not only saved power for the critical needs of the LM engine and the CM batteries, but also cut down on the use of water. Even with a normal ration of about a liter of water a day, the crew would have drunk less than ten percent of the 150 liters of water onboard the LM; however, even in a powered-down mode, virtually all of the 150 liters was needed for the sublimators that kept critical equipment cool, so the astronauts cut their daily ration to about a fifth of a liter apiece, a small glass full. They would be plenty thirsty when they got home, but at least they'd have a chance. As it turned out, they did a stunning job of conservation and got back to Earth with twenty percent of the LM power left and ten percent of the water. Indeed, in hindsight they may have done too good a job of water conservation. Lovell lost fourteen pounds (and Haise and Swigert another seventeen pounds between them) and they were all "tired, hungry, wet, cold, [and] dehydrated" when they landed. Because of the dehydration and other factors, Haise developed a prostate infection, a fever of 103 degrees Fahrenheit, and was seriously ill for two or three weeks after getting home. But all of that was of secondary importance. They had made it back alive.
In both spacecraft, under normal circumstances, the cabin air was fed continuously through environmental control equipment where, among other things, lithium hydroxide reacted with the carbon dioxide and trapped it. A single canister began losing its efficiency after about 40 person-hours of use and then had to be replaced. Unfortunately - and quite literally - the square CSM canisters wouldn't fit into the round holes of the LM unit; and, unless a way could be found to use the square ones, the carbon dioxide content of the cabin air would rise to poisonous levels long before the crew could get home. The advertised 60-person-hour combined lifetime of two LM canisters was, of course, a very conservative figure and, in fact, by allowing the carbon dioxide levels to rise above normal limits, they were able to keep them on line for 107 person-hours, or nearly a day and a half. And they had one other primary canister - 40 person hours design lifetime, 80 person hours at the higher CO2 levels - that they were holding in reserve in case it took extra time to devise a way to use the CSM canisters.
There was, of course, a fix; and it came in the form of an ingenious combination of suit hoses, cardboard, plastic stowage bags, and CSM canisters - all held together with a liberal application of grey duct tape. As was usual whenever the Apollo team had to improvise, engineers and astronauts on the ground got busy devising ways around the problem and then checked out the new procedures. A day and a half after the Apollo 13 accident, the ground teams had designed and built a filtering device that worked to their satisfaction. They promptly radioed instructions to the crew, carefully leading them through about an hour's worth of steps. As Lovell wrote later: "the contraption wasn't very handsome, but it worked." And that was all that mattered.
include a drawing and photo of the canisters from the Apollo 13 mission report
The first burn went smoothly and there was every reason to believe that the LM engine would function perfectly the second time as well. However, for this second burn, the crew had to take particular care to make sure that the engine was pointed in just the right direction and, because of the accident, they had to deal with some unusual navigation problems. Because they were traveling through vacuum, there was nothing to disperse the cloud of Service Module debris that enveloped the spacecraft. As they peered through the LM's one-power telescope trying to make star sightings, glints of sunlight reflecting off the debris made the job all but impossible. And to add to their difficulties, they were all exhausted and kept making uncharacteristic mistakes. But they kept at it and, with help from the ground, figured out how to use sightings of the Sun and the crescent Earth to verify their alignment and then did the job over and over to make sure that they had it right. When the time came, the burn was perfect.
As we now know, the temperature-sensitive switch was not designed to operate at 65 volts. During normal operations, the heater was on for only brief periods and the switch never opened. However, during what proved to be a lengthy process of emptying the tank using the internal heater, the switch opened but, then, was immediately welded shut again by an electric arc driven by the high voltage. Indications that the switch had closed were missed. Subsequently, whenever the CSM was powered up, the heaters went into operation without the protection normally provided by the switch; and, at some point during pre-launch activities, the whole assembly reached a temperature of over 500 degrees Centigrade (1000 F). This was a high enough temperature to cause severe damage to the Teflon insulation protecting the fan-motor wiring and, as the Apollo 13 Review Board later concluded, "from that time on the oxygen tank was in a hazardous condition when filled with oxygen and electrically powered." The stage was set for the accident.
Despite all the rattling that must have gone on during the launch and subsequent engine firings, nothing untoward happened inside the tank until fifty-five hours, fifty-five minutes into the mission. At that moment, at a quiet time and, undoubtedly as a result of something so simple as the start up of the fan, the wires arced and the insulation caught fire. It was the 1967 Apollo launch-pad fire all over again, only this time it was a fire fed by a superabundance of pure oxygen, a fire that wouldn't quickly go out. The heat of the fire began boiling the liquid oxygen that mostly filled the tank and the pressure began to rise. Within a half minute, the pressure was too high for the tank's thin walls and they burst. The explosion wreaked havoc throughout the innards of the Service Module, rupturing the other oxygen tank and blowing out the side of the spacecraft.
From a purely engineering point of view, the Apollo 13 accident didn't reveal any fundamental flaws in the Apollo design concept. In any project of such size and complexity, unforeseen problems are to be expected and, what the accident did was to underline the lessons of the Apollo fire: NASA needed to do a better job of identifying problems before they could happen. The agency needed to conduct another thorough review of designs and procedures, particularly with regard to components that came in contact with the oxygen supply and, in the future, to pay closer attention to design changes, manufacturing quality, and the implications of anomalous test data. But there was no need to go back to square one. From an engineering point of view, once the problem had been identified, the fixes were easily accomplished. True, the accident had cost NASA one of its now limited opportunities to complete a landing and had almost cost the lives of three astronauts; but when measured in terms of the engineering goals that had been set in the early sixties, NASA was still well ahead of the game.
The main trouble with such accidents is that they have the potential of causing real political mischief. Although the death warrant for Apollo had already been signed in January 1970 - with the assembly lines shutdown and one of the remaining missions cut from the schedule for lack of funds - it is not coincidental that two more missions were dropped in the interval following the accident. Congressional support for Apollo had been weak for years and now there was a new President who was less than an ardent fan; and, while prediction of the political impact of serious accidents is far from an exact science, any accident was bound to raise questions about the credibility of NASA and its programs. As both the Apollo fire and the Challenger accident of the Shuttle era indicate, serious accidents do not necessarily doom a program but, given the expense and high visibility of such enterprises, the political risks are considerable.