Editing Apollo 13 (section)

== {{anchor|Review board report}}Investigation and response ==

=== Review board ===
[[File:Apollo13 tank2.jpg|thumb|Oxygen tank number 2, showing heater and thermostat unit]]
Immediately upon the crew's return, NASA Administrator Paine and Deputy Administrator [[George Low]] appointed a review board to investigate the accident. Chaired by NASA [[Langley Research Center]] Director [[Edgar Cortright|Edgar M. Cortright]] and including Neil Armstrong and six others,{{NoteTag|The others were Robert F. Allnutt (Assistant to the Administrator, NASA Hqs.); John F. Clark (Director, Goddard Space Flight Center); Brig. General Walter R. Hedrick Jr. (Director of Space, DCS/RED, Hqs., USAF); Vincent L. Johnson (Deputy Associate Administrator-Engineering, Office of Space Science and Applications); [[Milton Klein (engineer)|Milton Klein]] (Manager, AEC-NASA Space Nuclear Propulsion Office); [[Hans M. Mark]] (Director, Ames Research Center).{{sfn|Accident report|pp=1-1–1-4}}
}} the board sent its final report to Paine on June 15.{{sfn|Accident report|p=15}}
It found that the failure began in the service module's number{{nbsp}}2 oxygen tank.{{sfn|Accident report|p=4-36}} Damaged [[Polytetrafluoroethylene|Teflon]] insulation on the wires to the stirring fan inside Oxygen Tank{{nbsp}}2 allowed the wires to [[short circuit]] and ignite this insulation. The resulting fire increased the pressure inside the tank until the tank dome failed, filling the fuel cell bay (SM Sector{{nbsp}}4) with rapidly expanding gaseous oxygen and combustion products. The pressure rise was sufficient to pop the rivets holding the aluminum exterior panel covering Sector{{nbsp}}4 and blow it out, exposing the sector to space and snuffing out the fire. The detached panel hit the nearby high-gain antenna, disabling the narrow-beam communication mode and interrupting communication with Earth for 1.8&nbsp;seconds while the system automatically switched to the backup wide-beam mode.{{sfn|Orloff & Harland|2006|pp=372–373}} The sectors of the SM were not airtight from each other, and had there been time for the entire SM to become as pressurized as Sector{{nbsp}}4, the force on the CM's heat shield would have separated the two modules. The report questioned the use of Teflon and other materials shown to be flammable in supercritical oxygen, such as aluminum, within the tank.{{sfn|Accident report|pp=5-6–5-7, 5-12–5-13}} The board found no evidence pointing to any other theory of the accident.{{sfn|Accident report|p=4-37}}
{{multiple image
 | align = left
 | direction = vertical
 | width = 200
 | header =
 | image1 = Honeycomb panel test (S70-41982).jpg
 | alt1 = Panel in the process of being blown out
 | caption1 = 
 | image2 = Honeycomb panel test (S70-41983).jpg
 | alt2 = Further along in the process
 | caption2 = Panel similar to the SM Sector{{nbsp}}4 cover being ejected during a test performed as part of the investigation
}}
[[Shock (mechanics)|Mechanical shock]] forced the oxygen valves closed on the number{{nbsp}}1 and number{{nbsp}}3 fuel cells, putting them out of commission.{{sfn|Accident report|p=4-40}} The sudden failure of Oxygen Tank{{nbsp}}2 compromised Oxygen Tank{{nbsp}}1, causing its contents to leak out, possibly through a damaged line or valve, over the next 130&nbsp;minutes, entirely depleting the SM's oxygen supply.{{sfn|Orloff & Harland|2006|p=372}}{{sfn|Accident report|p=4-43}} With both SM oxygen tanks emptying, and with other damage to the SM, the mission had to be aborted.{{sfn|Orloff & Harland|2006|p=375}} The board praised the response to the emergency: "The imperfection in Apollo&nbsp;13 constituted a near disaster, averted only by outstanding performance on the part of the crew and the ground control team which supported them."{{sfn|Accident report|p=ii}}

Oxygen Tank&nbsp;2 was manufactured by the [[Beechcraft|Beech Aircraft Company]] of Boulder, Colorado, as subcontractor to [[Rockwell International|North American Rockwell]] (NAR) of Downey, California, prime contractor for the CSM.{{sfn|Accident report|p=4-2}} It contained two thermostatic switches, originally designed for the command module's 28-volt DC power, but which could fail if subjected to the 65 volts used during ground testing at KSC.{{sfn|Accident report|p=4-23}} Under the original 1962 specifications, the switches would be rated for 28 volts, but revised specifications issued in 1965 called for 65 volts to allow for quicker tank pressurization at KSC. Nonetheless, the switches Beech used were not rated for 65 volts.{{sfn|Orloff & Harland|2006|p=374}}

At NAR's facility, Oxygen Tank&nbsp;2 had been originally installed in an oxygen shelf placed in the [[Apollo 10]] service module, SM-106, but which was removed to fix a potential [[electromagnetic interference]] problem and another shelf substituted. During removal, the shelf was accidentally dropped at least {{convert|2|in|cm|0|order=flip|sp=us}}, because a retaining bolt had not been removed. The probability of damage from this was low, but it is possible that the fill line assembly was loose and made worse by the fall. After some retesting (which did not include filling the tank with liquid oxygen), in November 1968 the shelf was re-installed in SM-109, intended for Apollo&nbsp;13, which was shipped to KSC in June 1969.{{sfn|Accident report|pp=4-19, 4-21}}

The [[Terminal countdown demonstration test|Countdown Demonstration Test]] took place with SM-109 in its place near the top of the Saturn&nbsp;V and began on March 16, 1970. During the test, the cryogenic tanks were filled, but Oxygen Tank&nbsp;2 could not be emptied through the normal drain line, and a report was written documenting the problem. After discussion among NASA and the contractors, attempts to empty the tank resumed on March 27. When it would not empty normally, the heaters in the tank were turned on to boil off the oxygen. The thermostatic switches were designed to prevent the heaters from raising the temperature higher than {{convert|80|F|C|order=flip}}, but they failed under the 65-volt power supply applied. Temperatures on the heater tube within the tank may have reached {{convert|1,000|F|C|order=flip|-1}}, most likely damaging the Teflon insulation.{{sfn|Accident report|p=4-23}} The temperature gauge was not designed to read higher than {{convert|85|F|C|order=flip}}, so the technician monitoring the procedure detected nothing unusual. This heating had been approved by Lovell and Mattingly of the prime crew, as well as by NASA managers and engineers.{{sfn|Chaikin|1995|pp=330–331}}<ref>{{cite web|url=http://nssdc.gsfc.nasa.gov/planetary/lunar/ap13acc.html|last=Williams|first=David R.|title=The Apollo 13 Accident|website=NASA Space Science Data Coordinated Archive|access-date=December 31, 2012}}</ref> Replacement of the tank would have delayed the mission by at least a month.<ref name = "mission summary" /> The tank was filled with liquid oxygen again before launch; once electric power was connected, it was in a hazardous condition.{{sfn|Orloff & Harland|2006|p=375}} The board found that Swigert's activation of the Oxygen Tank{{nbsp}}2 fan at the request of Mission Control caused an [[electric arc]] that set the tank on fire.{{sfn|Chaikin|1995|p=333}}

The board conducted a test of an oxygen tank rigged with hot-wire ignitors that caused a rapid rise in temperature within the tank, after which it failed, producing telemetry similar to that seen with the Apollo&nbsp;13 Oxygen Tank&nbsp;2.{{sfn|Accident report, appendix F–H|pp=F-48–F-49}} Tests with panels similar to the one that was seen to be missing on SM Sector{{nbsp}}4 caused separation of the panel in the test apparatus.{{sfn|Accident report, appendix F–H|pp=F-70–F-82}}

=== Changes in response ===
[[File:Apollo 14 redesigned oxygen tank (S71-16745).jpg|thumb|Redesigned oxygen tank for Apollo{{nbsp}}14]]

For [[Apollo 14]] and subsequent missions, the oxygen tank was redesigned, the thermostats being upgraded to handle the proper voltage. The heaters were retained since they were necessary to maintain oxygen pressure. The stirring fans, with their unsealed motors, were removed, which meant the oxygen quantity gauge was no longer accurate. This required adding a third tank so that no tank would go below half full.{{sfn|Gatland|1976|p=281}} The third tank was placed in Bay{{nbsp}}1 of the SM, on the side opposite the other two, and was given an isolation valve that could isolate it from the fuel cells and from the other two oxygen tanks in an emergency and allow it to feed the CM's environmental system only. The quantity probe was upgraded from aluminum to stainless steel.{{sfn|Apollo 14 Press Kit|1971|pp=96–97}}

All electrical wiring in Bay{{nbsp}}4 was sheathed in stainless steel. The fuel cell oxygen supply valves were redesigned to isolate the Teflon-coated wiring from the oxygen. The spacecraft and Mission Control monitoring systems were modified to give more immediate and visible warnings of anomalies.{{sfn|Gatland|1976|p=281}} An emergency supply of {{convert|5|USgal|L|order=flip}} of water was stored in the CM, and an emergency battery, identical to those that powered the LM's descent stage, was placed in the SM. The LM was modified to make transfer of power from the LM to the CM easier.{{sfn|Apollo 14 Press Kit|1971|pp=96–98}}