Following my July 21 blog entry about the near-miss of the International Space Station by orbital debris, I thought you would be interested to know how an ISS crew would respond to a rapid depressurization of the Station. A depressurization would certainly result from a breach of the hull if hit by a piece of debris. A leak could also occur following failure of a vent valve or a seal. Whatever the cause of the depressurization, the efficient execution of the rapid depress procedure by the crew is critical since the time to act would be limited.
The loss of internal pressure would initially be detected by Station sensors or even by the crew. In fact, a popping sensation in our eardrums may be the first clue that the Station is losing pressure. Automated software then reconfigures some of the onboard systems. For instance, certain valves and overboard vents are closed and ventilation is shut down.
The crew would muster (gather) in the Russian Service Module. The commander will ensure that all crew members are aware of the emergency condition and are safe. She or he will inform Mission Control of the situation and coordinate the crew’s and the ground’s emergency responses.
The crew’s first action in the rapid depress response would be to calculate the ‘reserve time’. A key instrument that helps us with this calculation is the Manovacometer. This is a handheld, analog gauge that measures ambient pressure. It is quite precise. With a bit of mental math it can be used to determine the pressure drop rate and the amount of time remaining until cabin pressure is expected to reach 490 mm hg – the reserve time. If the reserve time ever reaches 10 minutes or less, we would stop searching for the leak and take refuge in the portion of the Station already confirmed to not be leaking or, in a worst case scenario, our Soyuz vehicles with hatches closed.
The Manovacometer used by the crew to locate a leaking ISS module
The crew then enters the docking module connected to our respective Soyuz vehicles and performs leak checks of the combined Soyuz/docking module volume. We need to know that our Soyuz vehicles are not the source of the leak since they function as our lifeboats and are our potential means of escape.
If those volumes are intact (good news!) and if the location of the leak is still unknown, the crew would then begin a search throughout the Station.
The Station is assembled as a series of interconnecting modules – one module connected to another – something like a Lego block structure. For example, the US Lab module is connected to the Node 1 module on its aft end and to the Node 2 module on its forward end. A module has a hatch on each end that connects to another module. Some modules can have up to six hatches! On a nominal day when there is no emergency situation, the crew leaves the interior hatches open so that we can easily fly through the hatchways as we move about the Station.
To determine the location of the leaking module, the crew will effectively divide the Station into thirds by closing hatches. It is important that all crewmembers stay together and remain on the Soyuz side of a hatch whenever a hatch is temporarily closed. If for some reason we could not re-open a hatch, we wouldn’t want to strand a crewmember on the side of a hatch away from their escape vehicle!
First, the Russian segment is cut in half by closing a hatch between the Service Module (SM) and the Functional Cargo Block (FGB). We then look at the Manovacometer. If the Manovacometer shows that the pressure is holding steady, then the leak must be on the opposite side of the hatch (i.e. in the forward portion of the Russian segment or somewhere in the US Segment). If the Manovacometer shows a dropping pressure, then the leak must be on the same side (i.e. aft portion of the Russian segment) of the hatch.
The International Space Station is comprised of interconnecting modules and hatches
If the leak is forward of the SM, then the crew collectively moves forward, temporarily closes the hatch that separates the Russian and US segments (i.e. between FGB and Node 1) and monitors the Manovacometer.
This methodology allows the crew to determine which third of the Station to focus on for the remainder of the search. Once the affected third of the Station is known, this hatch closing procedure is performed over and over via a ‘divide and conquer technique’ until the leak location is narrowed down to a specific module.
If the hole in the leaking module can be seen and accessed, then the crew will apply a patch and stop the leak. If it can’t be isolated, then the crew will remove much of the important equipment from the leaking module and close its hatch. The leaking module will eventually depressurize down to vacuum so equipment left in the module must be remotely powered down to avoid additional damage.
On a later date it may be possible for two crew members to perform a spacewalk to locate the hole in the hull and repair it from the outside. Once repaired, the module could possibly be repressurized and the hatches reopened.
The crew may not always need to perform this manual procedure using the pressure gauge and progressive hatch closings to locate a leak. If the leak is in the Russian segment, then we may be able to use an automated method that relies on airflow sensors and an algorithm to locate the leaking module.
I hope you found this explanation of the crew response to a rapid Station depressurization interesting. Writing this blog entry brought back fond memories of the training that was provided preflight to my ISS Expedition 20/21 crew. Our NASA trainers Scott Segadi and Valori Leinmiller trained my crew well so that we would be fast, effective and confident in our responses to on-orbit emergency situations.
Now wouldn’t life in space be a lot less complicated for crews if orbital debris was not a problem?
