posted 10-01-2020 09:59 AM               

Saturn V would have been bring back in the VAB and the Apollo capsule disassembled by launch vehicle. Then? The investigations would have been done in the factory or in VAB? Which changes were required?

Which the next launch window for the mission (November 1970)?

posted 10-05-2020 01:09 PM               

quote:

---

Originally posted by Fra Mauro:
The problem is the wiring in the O2 tank.

---

posted 10-06-2020 01:35 AM               

quote:

---

Originally posted by Spacepsycho:
...the O2 tank explosion was caused by Beechcraft's failure to upgrade the internal thermostat from 28v to 65v.

---

Referencing the accident investigation, Beech manufactured the tanks to the design specifications. The investigation determined that several flammable materials had been specified for use in an oxygen-rich environment, including the aluminum tank and Teflon insulation on the electrical wiring.

The O2 tank had been removed from the Apollo 10 SM (as a part of the shelf assembly) to incorporate design improvements to fix a potential electromagnetic interference problem and also incorporate changes for the upgrade 65 volt ground electrical system (incorporated to facilitate faster tank pressurization at the launchpad). During removal, the shelf was accidentally dropped at least 5 centimeters, reported to be because a retaining bolt had not been removed. Damage to the bottom of the tank was identified but considered acceptable continued for use (there was no specification to fill the tank with oxygen as a part of the post maintenance tests).

The tank was returned from the contractor and installed within the Apollo 13 SM (as a part of the shelf assembly). There was a string of inspections, documentation checks, ground testing, and the countdown demonstration test which began 16th March 1970, none of which identified any issue with the O2 tank. All tests of the system passed, however, the Oxygen Tank 2 could not be emptied through the normal drain line.

After a discussion between NASA and the contractors (both Lovell and Mattingly were aware of, and included in the technical discussions), attempts to empty the tank resumed on 27th March. When it could not be empty normally, a decision was made to use the heaters in the tank to boil off the oxygen. The heaters in the tanks were normally designed for very short periods of use, 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.

The investigation deduced that this probably damaged the thermostatically controlled switches on the heater, designed for only 28 volts. It is believed the switches welded shut, allowing the temperature within the tank to rise to over 1000 degrees F. The gauges measuring the temperature inside the tank were designed to measure only to 80 F, so the extreme heating was not noticed. The high temperature emptied the tank but also resulted in serious damage to the Teflon insulation on the electrical wires to the power fans within the tank.

This anomaly and the subsequent procedures were investigated by NASA and the associated contractors and deemed to be acceptable, the question over how Beech did not upgrade the tank electrical system to the 65 volt specification was considered during the investigation, however, it was impossible to determine if this alone was the reason behind the Apollo 13 accident. (Had the heaters not been used to boil off the excess oxygen for such a long time, would the wire insulation ever have been damaged?)

I believe that the cryogenic stir carried out when the O2 tank ruptured was the third such stir for that mission.

As with all other serious accidents that NASA have experienced with manned spaceflight, there was a chain of events that led up to the accident.

It should be noted that the cryogenic fuel cells were rated to produce between 400 and 1420 watts and between 27 and 31 volts DC (for the 28 volt DC batteries and electrical system) and inverted into 115/200 or 400Volts three phase AC where required. The O2 fan motors used "Two parallel d-c heaters in each tank supply the heat necessary to maintain design pressures. Two parallel 3-phase circulating fans circulate the fluid over the heating elements to maintain uniform density and descries the probability of stratification” (SM2A-03-Block II-(I) Apollo Operations Handbook).