The Apollo 13 malfunction was caused by an explosion and rupture of oxygen tank no. 2 in the service module. The explosion ruptured a line or damaged a valve in the no. 1 oxygen tank, causing it to lose oxygen rapidly. The service module bay no.4 cover was blown off. All oxygen stores were lost within about 3 hours, along with loss of water, electrical power, and use of the propulsion system.
The no. 2 oxygen tank used in Apollo 13 (North American Rockwell; serial number 10024X-TA0008) had originally been installed in Apollo 10. It was removed from Apollo 10 for modification and during the extraction was dropped 2 inches, slightly jarring an internal fill line. The tank was replaced with another for Apollo 10, and the exterior inspected. The internal fill line was not known to be damaged, and this tank was later installed in Apollo 13.
The oxygen tanks had originally been designed to run off the 28 volt DC power of the command and service modules. However, the tanks were redesigned to also run off the 65 volt DC ground power at Kennedy Space Center. All components were upgraded to accept 65 volts except the heater thermostatic switches, which were overlooked. These switches were designed to open and turn off the heater when the tank temperature reached 80 degrees F. (Normal temperatures in the tank were -300 to -100 F.)
During pre-flight testing, tank no. 2 showed anomalies and would not empty correctly, possibly due to the damaged fill line. It was decided to use the heater to "boil off" the excess oxygen, requiring 8 hours of 65 volt DC power. This may have damaged the thermostatically controlled switches on the heater, designed for only 28 volts. It is believed the switches may have welded shut, allowing the temperature within the tank to rise to over 1000 degrees F. The high temperature would have resulted in damage to the teflon insulation on the electrical wires to the power fans within the tank.
56 hours into the mission, at about 03:06 UT on 14 April 1970 (10:06 PM, April 13 EST), the power fans were turned on within the tank. The exposed fan wires shorted and the teflon insulation caught fire. This fire spread along the wires to the electrical conduit in the side of the tank, which weakened and ruptured under the nominal 1000 psi pressure within the tank, causing the no. 2 oxygen tank to explode. This damaged the no. 1 tank and parts of the interior of the service module and blew off the bay no. 4 cover.
The Damage to the Service Module
The Apollo 13 malfunction was caused by an explosion and rupture of oxygen tank number 2 in the service module about 56 hours into the mission. The explosion also ruptured a line or damaged a valve in the number 1 oxygen tank, causing it to lose oxygen rapidly. Within about 3 hours, all oxygen stores were lost, along with water, electrical power, and use of the service module propulsion system. Visual assessment of the damage could not be made until the end of the mission when the service module was jettisoned in preparation to reentry. Then it could be seen that the cover of service module bay number 4 had blown off and the equipment insdie was badly mangled.
Using the Lunar Module as a "Lifeboat"
With the service module damaged beyond use, the only source for power and consumables was the lunar module. The resources of the command module had to be preserved for the vital reentry operation. And so the lunar module became the lifeboat for Apollo 13. Fortunately, the loss of oxygen from the remaining tank was slow enough that the last fuel cell continued to supply power for about two hours. This was enough time to evaluate options and prepare for a quick return to Earth. The crew then powered down the command module and shifted to the powered up the lunar module for the trip home.
The major concern was, of course, did the loss of the service module leave Apollo 13 enough consumables to get home? The lunar module was built for a 45 hour lifetime and that had to be stretched to 90. It was determined that the oxygen supply was sufficient, and calculations indicated that there would be enought power in the batteries if all unnecessary electrical devices were turned off. Water looked like it was going to be a problem however. Water was used not only for drinking but for rehydrating food and for cooling onboard equipment. By restricting water usage as much as they could, the astronauts were able to stretch their supply for the rest of the trip.
The Fix for the CO2 Buildup Problem
One serious problem developed due to an oversight when making the original calculations. In normal operations, the lunar module was designed to support two astronauts for two days. However, in its "lifeboat" configuration, it would be supporting 3 people for four days. Part of the life support function was to remove carbon dioxide from the spacecraft. The system for doing this used lithium hydroxide canisters which had to be replaced periodically. In order to remove the carbon dioxide exhausted by three individuals, canisters from the command module would have to be used. The problem was that the canisters were would not fit into the lunar module environmental system equipment.
Fortunately, people on the ground had anticipated the problem and in a lengthy brainstorming session, had come up with an answer. They had designed a supplementary carbon dioxide removal system for the square canisters using only items that were available on the spacecraft. Following instructions from mission control, it took the astronauts about 1 hour to build the device out of plastic bags, cardboard, parts from a lunar suit and a lot of tape. In the words of Jim Lovell, "The contraption wasn't very handsome, but it worked."
The Coast Phase Activities
As soon as it became apparent that the mission had to be aborted, the spacecraft had been maneuvered back to a free-return trajectory. This assured that Apollo 13 would loop around the Moon and return to Earth. However, several additional maneuvers were required to make the fastest, safest trip possible. The first of these maneuvers was the transearth injection burn which shortened the return time by about 9 hours. Two more slight midcourse corrections were made during the transearth coast. These maneuvers were all performed with the descent engine or the reaction control system on the lunar module as the service module engine was not available.
All of this was accomplished in some very difficult conditions. With most of the electrical systems turned off to conserve power, they were not producing the usual amount of heat to help keep the spacecraft warm. The temperature dropped to around 40 degrees by the end of the trip. In addition, a considerable amount of moisture condensed on the interior of the spacecraft which added to the astronaut's discomfort.
The Reentry and Crew Recovery
Due to the unusual spacecraft configuration, resulting from retaining the lunar module, entry procedures had to be modified. First, came the separation of the damaged service module. Then the command module was powered up with the entry batteries which had been brought up to nearly full charge using lunar module power. Next, the crew transfered back to the command module, and undocked the lunar module. From this point, the mission was similar to previous flights, with a safe landing approximately 1 mile from the target point. An additional benefit of the transearth injection burn was that it shifted the the landing area back to the South Pacific where primary recovery support was located which may have contributed to the crew being recovered very quickly.
A Happy Conclusion
In spite of tha fact that the mission was aborted, Apollo 13 may have been the most carefully watched mission of the program. From the first news of the accident through the final tense moments of the reentry blackout and the worries about the heat shield, people all over the world followed the story on radio, television, and in their papers. For four days, it seemed everyone watched and waited to see if the crew would make it back safely.
During the emergency, the astronauts were unware of all the attention being given their situtation. They were focused entirely on the task of getting their crippled spaccraft home. Once safely back on earth, however, it didn't take long to get the full picture. The image shows commmander Jim Lovell reading one of the many news stories about the mission.