In an earlier blog about the Apollo 1 fire, my former neighbor Space Shuttle Astronaut Mike Mullane and I talked about the Apollo astronauts breathing a 100% pure oxygen atmosphere during their missions. NASA was concerned the crew could get the bends from breathing a nitrogen / oxygen mix as the cabin pressure was reduced from 16.7 psi on the launch pad to 5 psi in orbit. This led Mike and me to another discussion about how Space Shuttle and International Space Station astronauts avoided getting the bends (technically, decompression sickness) during an EVA, i.e. a spacewalk.
Jack, as I said, the Space Shuttle atmosphere protocol was to fly with a 14.7 psi oxygen/nitrogen atmosphere. There was no change in atmosphere or pressure during a mission with no spacewalks. So, on those missions, the crew breathed sea level Earth-type air from launch to landing. On spacewalk missions the risk of bends did become an issue since the spacewalkers would be going from a 14.7 psi oxygen/nitrogen atmosphere to an EVA suit with a pure oxygen atmosphere of about 4.3 psi.
To limit the possibility of the bends for those spacewalkers, the cabin pressure of the Shuttle was reduced to 10.2 psi about 24 hrs prior to the EVA. Now the entire crew would be breathing nitrogen at a lower pressure and thus cutting down on the saturation of that gas in the body tissue. Then an hour before getting in the suit, the spacewalkers would pre-breathe pure oxygen. This would help wash out the remaining nitrogen from the tissue. As far as I know, no astronauts experienced symptoms of the bends with this protocol.
I do need to add this caveat on all I’ve said above. This is the protocol I recall. I left NASA in 1990 after my last mission, STS-36, (which was the 34th mission). One-hundred-and-one more missions followed. There were not a lot of EVAs in these early Shuttle years. (I did not do an EVA.) So, the protocol might have changed as data from more EVAs was collected. And I have no clue what the protocol would be for the ISS. I seriously doubt they depressurize the entire ISS to a lower cabin atmosphere pressure 24 hours prior to a spacewalk, since that lower pressure could affect some experiments. You’d have to do more research on the ISS/spacewalk protocol.
Thanks, Mike. Interesting question about the ISS. I did some research and it seems through 2016, the ISS astronauts also breathed pure oxygen for several hours prior to doing a spacewalk. I found a 2009 NASA article that said ISS spacewalkers were using the several hour pure-oxygen purge then (which would imply Space Shuttle’s protocol was also the same as when you flew). Ditto for another report dated January of this year (“a two-hour pure oxygen purge”).
Using the airlocks designed for ISS, the crew doesn’t have to depressurize the rest of the ISS living & working space. The astronauts pre-breathe pure O2 for about an hour while riding a stationary bike to drive out nitrogen in their bloodstream. They then enter the airlock where the pressure is lowered to 10.2 psi. They don their suits and breathe pure oxygen for another 60 minutes while the pressure in the airlock is further reduced to 5 psi and the suit is at 4.3 psi.
In 2006, NASA tried an experiment by having two ISS astronauts sleep in the Quest airlock overnight while they gradually reduced the atmospheric pressure in there from 14.7 psi to 10 psi. The idea was to purge the nitrogen in steps, the same way scuba divers do when they ascend to the surface. NASA estimated it would save 30-60 minutes versus having to do a pure oxygen-based purge. That particular test had to be aborted because of some instrument metering problems. I couldn’t tell if they were successful later on, but based on the stuff above, they seem not to have used that approach very much, if at all.
Were airlocks used on the Space Shuttle from the beginning, Mike, or were they added later on in the program? Did the Shuttle airlock do any purging of nitrogen gas to help avoid the bends, like they did on Apollo?
All Shuttles had airlocks in case of emergencies that required an EVA. There was an airlock that filled the back of the “mid-deck” of the Orbiter allowing astronauts to enter from the pressurized living area of the mid-deck, get dressed in the suits, close the mid-deck hatch, depressurize the airlock, open the outer hatch and exit into the front of the cargo bay. On return from the EVA, the process was reversed. On some missions, the airlock was external to the mid-deck, placed at the front of the cargo bay. Shuttles never flew without an airlock.
As far as I remember, the EVA process from the Shuttle didn’t change the gas composition (O2 and N) in the airlock, just the pressure.
The EVA crewmembers would enter the airlock and get help from other crewmembers in getting fully dressed in the EMUs [the space suits]. At this point the whole Shuttle living area and the airlock would all be at 10.2 psi, oxygen/nitrogen. After the EVA’ers were in their suits with their helmets on, they would then be on 100% oxygen via an umbilical to the Shuttle. The other crewmembers would leave the airlock and close the inner hatch. After some period of the 100% oxygen purge of their bodies, the airlock would be depressurized to vacuum and the EVA’ers would detach their umbilical and exit the outer hatch in suits that were then pure oxygen at 4.3psi.
As far as I know, the atmosphere in the airlock was always two gases. It was never pure oxygen even during the EVA process.
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