Apollo Discussions > The Reality of Apollo
Apollo 13 questions
bknight:
--- Quote from: JayUtah on July 18, 2021, 03:13:01 AM ---
--- Quote from: smartcooky on July 18, 2021, 01:11:40 AM ---I have a question though. I have read various articles that describe the wiring fault in No. 2 Oxygen Tank as making it effectively a bomb waiting to go off. Stirring the tanks was the trigger on A13, but could it had gone off at any time, were there other actions that the crew might have done which would trigger it, or was the stirring of the tanks the only possible trigger?
--- End quote ---
There was no wiring fault. There was no single fault, really. There isn't even a single root cause. This is why I teach this over and over to my junior engineers.
The precipitating event was the botched unshipping of the tank. They dropped it and damaged the purge system. This led to them trying a previously untested detanking procedure later using the tank heater on ground power. Ground power had been upped to 60 VDC from the standard 28 VDC used in most aeronautical systems. The belief was that the thermostat would work, but the testing to verify the thermostat at 60 VDC was incomplete. It was validated at 60 VDC with the contacts closed. Arcing was not part of the test plan, because contact separation was driven by thermal parameters, which did not change as a result of the modification to ground power. The thermostat tripped correctly at 80 F, and promptly arced and fused shut. That led to thermal runaway in the heater, which burned off key parts of the wiring insulation.
Once you have uninsulated wires, any energization of them has the potential to cause an arc in the presence of LOX. Stiring, sure. Heater activation, sure. Even the sensor transducers could have done it. It's all a matter of what newly-uninsulated wires are close enough to each other to allow for an arc.
--- End quote ---
Cascading errors, but what I would ask at this point:
If the ground power was configured to 28 vdc would there have been any likely errors to an including the venting of O2 through the bent valve?
nikolai:
--- Quote from: smartcooky on July 18, 2021, 08:20:27 AM ---Jack Swigert was just unlucky enough to be the guy who flicked the switch.
--- End quote ---
Or lucky enough, as he did it at a time when the resultant problem was survivable.
Obviousman:
--- Quote from: bknight on July 18, 2021, 01:44:10 PM ---If the ground power was configured to 28 vdc would there have been any likely errors to an including the venting of O2 through the bent valve?
--- End quote ---
I believe the procedure would have worked as expected, and Apollo 13 could have been just another moon landing.
JayUtah:
--- Quote from: smartcooky on July 18, 2021, 08:20:27 AM ---OK. Not so much a wiring fault as wiring damage caused by the use of incorrect procedures.
--- End quote ---
Maybe not even incorrect procedures. The procedure for removing the cryogenic oxygen tank was correct and well documented. But it was not correctly carried out: a bolt was left in place that, according to procedure, should have been removed. That's a human error, not an error in the procedure. The forklift operator who was to perform the final lift and extraction could have realized sooner that the tank was binding. But it wasn't supposed to be binding at all, so who was at fault? Would a more skilled operator have done better?
Once the tank had dropped two inches back onto the shelf, the correct procedure was to send it back to Beech Aircraft for inspection and requalification. Beech's inspection revealed a damaged purge assembly. But the purge assembly is not a flight-critical element. It's used only on the ground during detanking, which never happens in flight. The people who made those decisions did not consider the consequences of ad hoc procedures that might be employed later by others to compensate for the degraded purge assembly. To a certain extent, they are allowed to assume that all correct procedures will be followed from there on out.
The 60-volt qualification test was also done at this time. But that is an electrical test only. The engineers determined that 60 volts DC could be applied to the various components of the tank without any operation that exceeded the flight limits, including the thermostat (in the closed position). The thermal trip test is a different test. The thermostat doesn't have to be energized for that test, and it was not required for the 60 VDC qualification anyway. In hindsight we can certainly argue it should have been, so there's an example of an incorrect procedure.
A normal tank vent/purge cycle does not require the heater. You simply connect the ground purge assembly and open the purge valve. A technician monitors tank pressure on the ground service equipment (GSE) panel to verify when the tank has completed venting. Then you purge with something like gaseous nitrogen or ambient air through the fill valve for a prescribed amount of time to eliminate the possibility of an oxygen concentration in the tank. The tank is then considered "safed." The cycle has to complete before other operations near the SM can happen, such as those that might produce sparks. So there's some pressure (no pun intended) for it to happen according to schedule.
The decision to turn on the tank heater to speed up boil-off was approved by the relevant engineers, so by (pedantic) definition it was not an incorrect procedure. A significant part of my day on any given day is approving variances to established procedure. As soon as a procedure deviation bears my signature, it has just as much validity with respect to regulation as an approved standard procedure. The engineers who signed off on this procedure had every reason to believe that it was safe to do so because the Beech engineers had qualified the tank for 60 VDC GSE operation, and the procedure did not contemplate operating the tank outside the recommended limits. Sensibly enough, the tank temperature gauge on the GSE did not read any higher than the tank was designed to go, even when heated. Not incorrect procedure, but maybe short-sighted.
The concept at play here is something called "tolerance buildup." We approve decisions in engineering based on tolerances for error, safety factors, and so forth. A tolerance buildup occurs when individual components drift toward an out-of-tolerance condition in all the same direction, so that the result is an entire system that is dangerously close to an overall tolerance violation. At every step of the process in servicing and operating the cryogenic tank, a little step was taken toward making the tank a little less tolerant of heat and electrical conditions. Individually, most of them were innocuous. The only truly out-of-tolerance condition that was allowed was the runaway heater during the detanking. That resulted in a truly intolerable condition: uninsulated wires. But up until then, there was just a sequence of, "Yeah, I suppose that's okay."
--- Quote ---So this could have gone boom at any time. Jack Swigert was just unlucky enough to be the guy who flicked the switch.
--- End quote ---
Yup.
JayUtah:
--- Quote from: bknight on July 18, 2021, 01:44:10 PM ---If the ground power was configured to 28 vdc would there have been any likely errors to an including the venting of O2 through the bent valve?
--- End quote ---
No. The thermostat would have functioned as expected and prevented the heater from baking the tank.
Ironically the plethora of voltages used in the spacecraft and launch vehicle and the likelihood of human error in operating GSE connections was what motivated standardizing GSE DC voltage to 60 and validating the entire spacecraft to accept that voltage for direct DC connections. It's not clear from my reading that the GSE could even have been configured to supply a lesser voltage.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version