Note: To celebrate the 40th anniversary of the Apollo 13 mission, for 13 days, Universe Today will feature “13 Things That Saved Apollo 13,” discussing different turning points of the mission with NASA engineer Jerry Woodfill.
The saga of the Apollo 13 accident actually began years prior to the launch of the mission. As Jim Lovell wrote in his book, “Lost Moon” the accident was “an accumulation of human errors and technical anomalies that doomed Apollo 13.” But had coincidences been just a little different Apollo 13 could have been an accident from which there was no rescue. NASA engineer Jerry Woodfill believes where Tank Two was positioned in the Service Module led to a successful rescue. “I contend that the crew would have died if the flawed O2 Tank Two had not been on the outer perimeter of the Service Module,” Woodfill said. “The position of that tank had much to do with the extent of the explosion’s damage. Had Tank One been damaged, no rescue would have been possible.”
Unfortunately there was another problem with the tank, that were it not for the fill tube damage, may not have been an issue. The oxygen tanks had originally been designed to run off the 28 volt DC power of the Command and Service modules. However, in 1965 the tanks were ordered to be refitted 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 26 degrees C (80 degrees F — Normal temperatures in the tank were -74 C to -174 C (-300 to -100 F.)
During pre-flight testing, Tank Two would not empty correctly, possibly due to the damaged fill line. The heaters in the tanks were normally used for very short periods to heat the interior slightly, increasing the pressure to keep the oxygen flowing. It was decided to use the heater to “boil off” the excess oxygen, requiring 8 hours of 65 volt DC power. This probably damaged the thermostatically controlled switches on the heater, designed for only 28 volts.
When the tanks were put into the Apollo 13 spacecraft, the damaged Tank Two was placed in the exterior position.
“Because the spark which ignited the oxygen in Tank Two was located at the top of the tank,” said Woodfill, the tank acted like a cork on a Thermos bottle. Since it was on the outside perimeter, it simply blew out into space along with the 13 foot panel covering the side of the service module. The oxygen tank shelf served to isolate the explosion from the hydrogen tanks below. But had the inboard oxygen Tank One 1 exploded, likely, this would not have been the case.”
Should the flawed tank have been the inner tank, Woodfill said, its explosive force would have taken with it the sister O2 tank amplifying the force of the explosion, just as using two sticks of dynamite instead of one, the destruction would be a magnitude greater.
“The added explosive force would have fractured the O2 tank shelf involving the fragile hydrogen tanks below,” Woodfill explained. “The volatile hydrogen gas now having a wealth of oxygen from the overhead tanks would surely have destroyed the entire spacecraft assemblage. Of course, the crew would have immediately perished as well. There would have been no clues, no telemetry data trace to explain what had happened.”
“Oxygen Tank One was given the inboard location adjacent to the flawed tank,” Woodfill continued. “Consider the likelihood of that placement. It is one chance in two. The odds for Apollo 13’s survival were fifty percent, a flip of the coin.”
Next: Part 10: Duct Tape
Other articles from the “13 Things That Saved Apollo 13” series: