Apollo 13

* Apollo 13 (April 11-17, 1970) was the seventh crewed mission in the Apollo space program and the third to land on the moon. The craft was launched from Kennedy Space Center (KSC) on April 11, 1970, but the lunar landing was aborted after an oxygen tank (tank #2) in the service module (SM) failed two days into the mission. The crew instead looped around the moon and returned safely to Earth on April 17.

* A routine stir of an oxygen tank (tank #2) ignited damaged wire insulation inside it, causing an explosion that vented the content of SMs oxygen tanks to space. Without oxygen, needed for breathing and for generating electric power, the SMs propulsion and life support systems could not operate. The Command Module’s (CM’s) system had to be shut down to conserve its remaining resources for reentry, forcing the crew to transfer to the Lunar Module (LM) as a lifeboat. With the lunar landing canceled, mission controllers worked to bring the crew home alive.

* Why did oxygen tank #2 explode? Oxygen tank #2 was manufactured by the Beech Aircraft Company of Boulder Colorado, as subcontracter to North American Rockwell (NAR) of Downey, California; prime contractor for the CSM. It contained two thermostatic switches, originally designed for the CM’s 28-volt DC power, but which could fail if subjected to the 65 volts used during ground testing at KSC. Under the original 1962 specifications, the switchs would be rated for 28 volts, but revised specifications issued in 1965 called for 65 votls to allow for quicker tank pressurization at KSC. Nonetheless, the switches Beech used were not rated for 65 volts.

* Oxygen tank #2 had orginally been installed in an oxygen shelf placed in the Apollo 10 service module, but was removed to fix a potential electromagnetic interference problem and another shelf substituted. During removal the shelf was accidentally dropped at least 2 inches.The probability of damage from this was low, but it is possible that the fill line assembly was lose 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 13, which was shipped to KSC in June 1969.

* The Countdown Demonstration Test took place with SM-109 in its place near the top of the Saturn V and began on March 16, 1970. During the test, the cryogenic tanks were filled, but oxygen tank #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 80 degrees F, but they failed under the 65-volt power supply applied. Temperatures on the heater tube within the tank may have reached 1,000 degrees F, most likely damaging the Teflon insulatiion. The temperature gauge was not designed to read higher than 85 degrees F, so the technician monitoring the procedure deteced nothing unusual. This heating had been approved by Lovell and Mattingly of the prime crew, as well as by NASA managers and engineers. Replacement of the tank would have delayed the mission by at least a month. The tank was filled with liquid oxygen again before launch. Once electric power was connected it was in a hazardous condition.

* A review board found that activation of the oxygen tank #2 fan at the request of Mission Control caused an electric arc that set the tank on fire. An investigative review board found fault with preflight testing of the oxygen tank and Teflon being placed inside it. The board recommended changes, including minimizing the use of potentially combustible items inside the tank; this was done for Apollo 14.



* Which supports and barriers were in play?

* What were the dynamics?

* Who, or what, won the Tug-of-War?

* Discuss the outcome with your friends and family.

* Use Post #4 as a reference for the dynamics, and the relationships, between supports and barriers.