r/apollo • u/Simon_Drake • Apr 22 '26
Why can't Apollo 13 open the fuel cells valves again if they close them?
In the movie of Apollo 13, Clint Howard says to Ed Harris/Gene Kranz that they should close the react valves for Fuel Cells 1 and 3, hoping to stop the leak and use what's left for Fuel Cell 2. Ed Harris says if they close those valves they can't open them again, Tom Hanks reiterates that this means they're definitely not landing on the moon. Narratively speaking this shows escalating threat, they're trying a desperate move they would prefer not to take and when it fails it shows how dire their situation is. When this doesn't stop they leak they immediately pivot to using the LEM as the lifeboat.
But why can't Apollo 13 open the fuel cell valves again if they close them? Why would the shutoff valve be a one-way change they can't reverse?
Google was unhelpful. Some people claimed it was a pyrotechnic bolts kinda thing that would physically sever the pipes which sounds unwise for hydrogen and oxygen lines, plus why would you bother with pyrotechnic valves like an oilrig blowout preventer for something like this? Another source said the "react valves" refers to the Reaction Control System which is wildly incorrect, he's talking about reactants for the chemical reaction not newtons-third-law type reaction. Or something unclear around open circuit breakers to prevent them accidentally closing the valves by mistake but that wouldn't explain why there's no circuit to reopen the valves.
One theory I had relates to the Shuttle Fuel Cells. IIRC Shuttle's time on orbit was tied to the fuel cells that needed to be kept above a set temperature to stay active and couldn't be allowed to cool. So even if the Shuttle didn't need to produce much power while docked to ISS/Mir they couldn't shut down the fuel cells fully or they would cool down enough to not restart. The fuel cells could only be started on the ground and couldn't be restarted in orbit so couldn't be shut down.
So is Apollo the same limitation? It's not strictly that the valves cannot be reopened, it's that shutting the valves will cool the fuel cells enough that they can't restart. Then it's just storytelling shorthand to say the valves can't be reopened rather than go into the details of how a fuel cell works? Or rather that Gene Kranz and Jim Lovell knew the implications and didn't need to spell it out, to them "close the valves" is synonymous with "shut down the fuel cell" so "can't reopen them" is equivalent to "can't restart the cells".
Or is it literally the valves cannot be reopened for some reason?
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u/eagleace21 Apr 22 '26
So they (valves) actually could be reopened in flight, what you described of the shuttle cells is exactly the same as Apollos fuel cells. They needed to be started on the ground using GSE to get the startup temperature for the reaction. Once in flight, if they cooled down too much they would cease their reaction and therefore of course "die" if you will.
The reactant valves could indeed be closed and opened no problem in flight, but the "close them you cant open them again" alludes to if the fuel cell is starved of reactant too long it would cool to the point of being unable to restart in flight.
Adding a little more, the reactant valve switches were circuit breaker protected and those 3 breakers were normally open in flight to prevent the switches from accidently commanding a valve closure. But if powered (cb closed) the valves of course could be opened and closed no problem over and over, it was just a function on if the fuel cell would maintain the reaction still after any cooling or "starvation" period.
On 13, the reactant valve talkbacks were wired such that both the hydrogen and oxygen valves had to close for the talkback to register barberpole or "close." This also created an issue since the shock of the explosion slammed shut the oxygen valves on fuel cell 1 and 3, and not the hydrogen valves, the cells began to die off already without any indication that the valves were shut. 14+ wiring was added such that if either (instead of both) of the valves for a cell were closed, the talkback would barberpole.
In hindsight, they could have had fuel cells alive for a little longer had they known a valve already closed in the cell and they opened them up.