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One of America's most famous adventures in space, the Apollo 13 mission was originally intended to include a lunar exploration, but catastrophic events led to improvisational changes to the ordeal that was eventually labeled a "successful failure" by NASA. Problems arose even before the April 11, 1970 liftoff. Command Module pilot Ken Mattingly was scratched from the mission because of the possibility of German measles and Jack Swigert replaced him at the last minute. Shortly after takeoff, an engine shut down early during the second-stage boost, but major problems arose when an oxygen tank exploded two days into the mission. The crew relayed the news in an understated message to mission control:
"Houston, we've had a problem."
The explosion caused damage to the outer shell and comprised the crew's life support systems, causing critical problems concerning communications, oxygen, and heating. The most serious problem involved the carbon dioxide filtration system, which the crew creatively solved through the "jury-rigging" of materials onhand. There were concerns that the craft might not be able to return safely, due to the damage to the heat shield and engine. But the problems were solved, and the crew--Commander Jim Lovell and pilots Fred Haise and Swigert--touched down six days after liftoff.
The mission began with a little-known smaller incident: during the second-stage boost, the center (inboard) engine shut down two minutes early. The four outboard engines burned longer to compensate, and the vehicle continued to a successful orbit. The shutdown was determined to be due to dangerous pogo oscillations that might have torn the second stage apart. The engine experienced 68g vibrations at 16 hertz, flexing the thrust frame by 3 inches (76 mm). The engine shutdown was triggered by sensed thrust chamber pressure fluctuations. Smaller pogo oscillations had been seen on previous Titan and Saturn flights (notably Apollo 6), but on Apollo 13 they were amplified by an unexpected interaction with turbopump cavitation. Later missions implemented anti-pogo modifications that had been under development. These included addition of a helium gas reservoir to the center engine liquid oxygen line to dampen pressure oscillations, an automatic cutoff as a backup, and simplification of the propellant valves of all five second-stage engines.
En route to the Moon, approximately 200,000 miles (320,000 km) from Earth, Mission Control asked the crew to turn on the hydrogen and oxygen tank stirring fans, which were designed to destratify the cryogenic contents and increase the accuracy of their quantity readings. Approximately 93 seconds later the astronauts heard a loud "bang", accompanied by fluctuations in electrical power and firing of the attitude control thrusters. The crew initially thought that a meteoroid might have struck the Lunar Module (LM).
In fact, the number 2 oxygen tank, one of two in the Service Module (SM), had exploded. Damaged Teflon insulation on the wires to the stirring fan inside oxygen tank 2 allowed the wires to short-circuit and ignite this insulation. The resulting fire rapidly increased pressure beyond its 1,000 pounds per square inch (6.9 MPa) limit and the tank dome failed, filling the fuel cell bay (Sector 4) with rapidly expanding gaseous oxygen and combustion products. It is also possible some combustion occurred of the Mylar/Kapton thermal insulation material used to line the oxygen shelf compartment in this bay.
The resulting pressure inside the compartment popped the bolts attaching the Sector 4 outer aluminum skin panel, which as it blew off probably caused minor damage to the nearby high-gain S-band antenna used for translunar communications. Communications and telemetry to Earth were lost for 1.8 seconds, until the system automatically corrected by switching the antenna from narrow-band to wide-band mode.
Mechanical shock forced the oxygen valves closed on the number 1 and number 3 fuel cells, which left them operating for only about three minutes on the oxygen in the feed lines. The shock also either partially ruptured a line from the number 1 oxygen tank, or caused its check or relief valve to leak, causing its contents to leak out into space over the next 130 minutes, entirely depleting the SM's oxygen supply.
Because the fuel cells combined hydrogen and oxygen to generate electricity and water, the remaining fuel cell number 2 finally shut down and left the Command Module (CM) on limited-duration battery power. The crew was forced to shut down the CM completely and to use the LM as a "lifeboat". This had been suggested during an earlier training simulation but had not been considered a likely scenario. Without the LM, the accident would certainly have been fatal.
En route to moon, the oxygen tank of the space craft ruptured. Due to a small spark, the tank exploded and destroyed one side of the spacecraft. there was a huge risk for the crew of not surviving while returning to the Earth. But they managed to return safely.
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