Apollo Discussions > The Reality of Apollo
Apollo 13 questions
Willoughby:
It is my understanding that all the service modules burned up on reentry since they were jettisoned after being on course to reenter. This would of course include the Apollo 13 service module. There are fairly detailed explanations on what happened inside of the oxygen tank that exploded. I understand that a lot of this would simply come from telemetry data such as voltage, heat, etc., but the actual physical defect in the venting system that is described - is this just an assumption based on the data - because it couldn't be from physical examination.
Also, did mission control have the ability to remote fire the SPS engine? I wonder if there was ever a discussion of firing it after jettison to see if it was actually damaged - whether it would work or not or if it still would have been too dangerous in close proximity to the recently jettisoned command module.
Since they didn't fire the engine, do we know with any degree of certainty whether or not the SPS engine would have worked - or is that still as much a mystery now as it was when they made the decision not to risk firing it up? I ask this because I come across several people who make the claim that had the Apollo 8 astronauts experienced a similar event - obviously without the benefit of the LM's descent engine - then they would have surely died. I've made the argument that this isn't necessarily the case, that they just would not have had the option to avoid the risk. Of course the engine could have been damaged and exploded - or not fired at all which would have resulted in their deaths, but couldn't it have possibly worked and returned the astronauts safely? EDIT : They still would have run out of oxygen and power in the CSM, so nevermind the last part...
JayUtah:
--- Quote from: Willoughby on July 16, 2021, 10:56:40 AM ---I understand that a lot of this would simply come from telemetry data such as voltage, heat, etc., but the actual physical defect in the venting system that is described - is this just an assumption based on the data - because it couldn't be from physical examination.
--- End quote ---
They're inferences from telemetry, but pretty accurate and reliable inferences. The operation of an Apollo spacecraft was predicated on being able to infer likely causes and modes of failure based on telemetry, and instrumenting the spacecraft accordingly. If you plan to operate something based on on-site or remote inspections you have to plan for that in the design phase, and that would have been hard to do for much Apollo hardware. So while it was an unanticipated failure, the telemetry coupled with knowledge of the spacecraft design was the intended means for diagnosing failure.
However, the SM was photographed from the CM after separation. The resolution is not super, owing to the wide-angle lens and the distance. But a knowledgeable person can draw further conclusions from the disposition of the wreckage as seen in the photos.
--- Quote ---Also, did mission control have the ability to remote fire the SPS engine?
--- End quote ---
None whatsoever, after CM separation. And a fairly limited ability otherwise, in the normal configuration. For unmanned test flights, the software and hardware were configured to allow for it. In normal flight, the SPS would need to be armed (a physical switch in the cockpit) and the computer would need to be set to accept ground commands.
--- Quote ---...it still would have been too dangerous in close proximity to the recently jettisoned command module.
--- End quote ---
A very good point. The priority at that stage of the mission was the safe return of the astronauts. Engineers were lucky to get photographs.
--- Quote ---Since they didn't fire the engine, do we know with any degree of certainty whether or not the SPS engine would have worked - or is that still as much a mystery now as it was when they made the decision not to risk firing it up?
--- End quote ---
While the crew visually noted damage to the SPS nozzle, it remains unknown whether the SPS engine would have safely operated.
--- Quote ---I ask this because I come across several people who make the claim that had the Apollo 8 astronauts experienced a similar event - obviously without the benefit of the LM's descent engine - then they would have surely died.
--- End quote ---
A number of informed people have noted that had the accident occurred at most other points in the Apollo 13 mission, the crew would likely have perished.
--- Quote ---EDIT : They still would have run out of oxygen and power in the CSM, so nevermind the last part...
--- End quote ---
Right: the problem was consumables, not so much propulsion. A course correction had to be made, of course, to transfer from the hybrid trajectory to the free-return trajectory. The second major maneuver was to accelerate their return, but that too was a decision made on the basis of consumables budget. It could have been omitted. But the FRT correction was essential. Without it, they would have missed Earth and gone into solar orbit. So without the LM propulsion, they would have had little choice but to attempt the maneuver with the SPS.
smartcooky:
--- Quote from: JayUtah on July 17, 2021, 12:14:13 PM ---A number of informed people have noted that had the accident occurred at most other points in the Apollo 13 mission, the crew would likely have perished.
--- End quote ---
Two periods of time seem logical to me
1. Any time prior to LM extraction and docking would likely have been terminal because there would be no way to extract and dock quickly enough for the crew to board the LM.
2. Any time after LM separation in Lunar Orbit would also be terminal, because there would again, not be enough time to re-dock the LM and use it as a lifeboat
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?
JayUtah:
--- 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.
smartcooky:
--- Quote from: JayUtah on July 18, 2021, 03:13:01 AM ---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.
--- End quote ---
OK. Not so much a wiring fault as wiring damage caused by the use of incorrect procedures.
--- Quote from: JayUtah on July 18, 2021, 03:13:01 AM ---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 ---
So this could have gone boom at any time. Jack Swigert was just unlucky enough to be the guy who flicked the switch.
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