Apollo 13

:This article is about the Moon mission. There is also a movie by the name of Apollo 13.

Mission highlights

When Apollo 13 was 321,860 kilometers from Earth, an oxygen tank in the service module exploded. The only solution was for the crew to cancel their planned landing, swing around the Moon and return on a trajectory back to Earth. However, because their command/service module "Odyssey" was severely damaged, the three astronauts had to use the lunar module "Aquarius" as a crowded lifeboat for the return home. The four-day return trip was cold, uncomfortable, and tense. But Apollo 13 proved the program's ability to weather a major crisis and bring the crew back home safely.

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Problem

As the spacecraft was on its way to the Moon, the number two oxygen tank in the Service Module (SM) exploded when Mission Control requested that the crew perform a "cryo stir", in which the oxygen "slush" is stirred to prevent it from stratifying. Damaged electrical wires powering the stirrer motor caught fire when power was applied. The fire caused a pressure increase above the tank's nominal 1,000 lbf/in² (7 MPa), and the tank exploded. This explosion damaged other parts of the service module, including the number 1 oxygen tank. At the time of the explosion, however, the true cause was not known; one conjecture was a meteoroid impact. The loss of both oxygen tanks in the service module and thus the oxygen required to create electrical power for the Command/Service Modules (CSM) meant that the CSM had to be completely shut down. The Command Module (CM) contained batteries for use during re-entry, after the Service Module was jettisoned, but these would only last about ten hours, and needed to be saved for re-entry. The crew survived by using the Lunar Module (LM, still attached to the CM) as a "lifeboat".

Related Topics:
Spacecraft - Service Module - Mission Control - Meteoroid - Command/Service Modules - Command Module - Lunar Module

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The damage to the CSM meant that the Moon-landing mission (originally intended to land at the Fra Mauro Highlands) had to be aborted; a single pass around the Moon was made and the spacecraft returned to Earth. Considerable ingenuity under extreme pressure was required from both the crew and the ground controllers to figure out how to jury rig the craft for the crew's safe return, with much of the world watching the drama on television. One of the major stumbling blocks in this was that the LM "lifeboat" was equipped to sustain two people for two days, and it would now have to sustain three people for four days. One of the most critical problems was that the carbon dioxide filters in the LM would not last for all four days, and the CM's spare filters were the wrong shape for the LM's filter receptacle; an adapter had to be fabricated from materials in the spacecraft.

Related Topics:
Fra Mauro - Earth - Jury rig - Carbon dioxide

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To accomplish a safe return to Earth, a significant course correction to place the spacecraft on a free return trajectory was required. This would normally be a simple procedure using the service module propulsion engine. However, the mission's ground controllers did not know the extent of the damage the service module had suffered and did not want to risk firing the main engine. Instead, the course correction would have to be performed by firing the lunar module's descent engine. After extensive discussion, engineers on the ground found it was possible. The initial maneuver to change to a free return trajectory was made within hours of the accident. The descent engine was fired again after passage around the Moon in order to accelerate the spacecraft's return to Earth, and later for a minor course correction. Although the LM was designed to be fired once and permanently shut down, all engine firings worked as hoped.

Related Topics:
Service module - Propulsion

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As re-entry to Earth's atmosphere approached, NASA took the unusual step of jettisoning the Service Module first, while the Lunar Module was still attached to the Command Module. The LM thrusters were used to maneuver the CM/LM stack to point its windows at the departing SM, and photos were taken. When the crew saw the damaged service module, they reported that the access panel covering the O2 tanks and fuel cells had been blown off.

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There was some fear that the extensive condensation in the CM, due to reduced temperatures during the return leg, might have seriously damaged the electronics of the Command Module, which would become apparent upon activation. But the equipment worked perfectly when activated, at least partly due to the extensive design modifications made to the CM after the Apollo 1 fire.

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The crew returned unharmed to Earth, although Haise had a urinary tract infection resulting from the scarcity of potable water on the damaged ship and the difficulty of disposing of urine, and had to be treated in an infirmary.

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While the crew was unfortunate to have this kind of major malfunction, they were still extremely lucky that it occurred on the first leg of the mission when they had a maximum of supplies, equipment, and power to use in the emergency. If the explosion had occurred while in orbit around the moon, or on the return leg after the LM had been jettisoned, the crew would probably not have survived.

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After the completion of the mission, there was a full investigation of the incident and the craft was modified to prevent future occurrences of the fault.

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Jim Lovell and Jeffrey Kluger's book about the mission, Lost Moon, was later turned into a successful movie, Apollo 13, starring Tom Hanks, Bill Paxton and Kevin Bacon as the Apollo crewmen.

Related Topics:
Jim Lovell - Apollo 13 - Tom Hanks - Bill Paxton - Kevin Bacon

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Cause of the accident

The explosion on Apollo 13 led to a lengthy investigation of the underlying cause. Thanks to detailed manufacturing records and logs of mission problems, the failure of the faulty oxygen tank was

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tracked to multiple faults that were not problems individually, but nearly led to disaster on this mission.

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Liquid gases are very difficult to handle, and most storage containers holding them are unsealed so that pressure from expanding gas will not cause the container to fail (much like freezing water in even the strongest sealed container will shatter it). Apollo's liquid oxygen tank was a marvel of engineering, able to hold several hundred pounds of highly pressurized liquid gas to supply the craft with oxygen, fuel for electricity (along with hydrogen) and water from the by-product of the fuel cells. Left alone, the tank was capable of safely holding liquid oxygen under high pressure for years before it evaporated because of its design and insulation. Unfortunately, the very characteristic that made the tank useful made internal inspection impossible.

Related Topics:
Engineering - Electricity - Water - Fuel cells

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The tank was made of several basic components that were relevant to the accident:

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• A thermostat to control the heater within the tank that sped the evaporation of the liquid into gaseous oxygen;
• A thermometer to determine the temperature of the heater;
• Valves and piping that were designed to allow the tank to be completely emptied of liquid by forcing gas into the tank;
• An interior coating of teflon that protected the wiring from the extremely cold gas; and
• An internal fan to stir the liquid oxygen (liquid oxygen will turn into a "slush" at these pressures if it is allowed to sit for a long period of time).

These were the basic design, manufacturing and operational problems that led to the accident.

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• The thermostat was originally designed to handle the 28 volt supply that would be used in the command module. However, the specification for the tank was changed so that it had to handle 65 volts on the launch pad. Most of the wiring was changed to handle the higher voltage, but the thermostat was not.
• The thermometer was designed to read out at the highest operational temperature of the heater, about 100 degrees Fahrenheit. Higher temperatures registered at 100 °F, but the thermostat was supposed to cut out at 80 °F (27 °C), making higher temperatures impossible.
• During assembly, the structure carrying the tank that failed was dropped about 2 inches (5 cm). The exterior was undamaged, but the pipes that directed flow within the tank became misaligned.
• For ground testing the tank was filled. However, when it came time to empty it, the problem with the piping was discovered. As such, the tank could not be properly emptied except by running the heater to evaporate the liquid gas. Not using this tank would have delayed the mission and there was no alternative tank available. Lovell was aware of the decision to use the heater to evaporate the oxygen, which was calculated to take a few days at the highest operational temperature of 80 °F (27 °C).
• However, when the heater was turned on continuously, the higher voltage fused the thermostat, which allowed the heater to keep heating up. Because the thermostat did not register temperatures higher than 100 degrees Fahrenheit (38 degrees Celsius), the monitoring equipment did not pick this up. The current recorder in the power supply showed that the heater was not cycling on and off, but no-one noticed it at the time. Instead of taking several days, the gas evaporated in hours, and the interior of the tank kept heating up, reaching an estimated 800 degrees Fahrenheit (430 °C). This burned off the teflon coating, leaving the wires inside the tank exposed.
• The rest was inevitable. When the tank was refilled with oxygen, it became a bomb waiting to go off. The order to run the "cryo stir" to run the fans set off sparks inside the tank which led to the explosion.
• However, disaster could have been averted if it had not been for the fact that both oxygen tanks were next to each other in the assembly. Although the second tank survived the explosion, its valves were damaged which allowed the oxygen within to leak out. In future Apollo missions, the two oxygen tanks were put in different parts of the craft.