The protagonist in this one is a young man hoping to become a space pilot. Early on in the story, he clashes a bit with one of his professors over the physical stress induced by takeoff and reaching orbit. He argues that the force required to escape Earth's gravity would render any man unconscious. The professor dismisses him in class, and there are a few chuckles. Later on, the professor meets with the student privately. He admits that the student is right, and because of his understanding of things (along with his talent), he is offered the chance to become a candidate for the program.
The protagonist joins in the space program, and it is there we learn that the solution to the problem of being knocked unconscious is that a second personality is created that only manifests itself when a person is unconscious. This personality is called [pilot name]-2. Thus, if the pilot's name was David (I'm can't remember his actual name in the story), the subconscious personality would be David-2. David-2 would actually handle all of the flight operations, and the "real" David wouldn't be aware of the flight at all.
The twist in the story comes when the spacecraft has an emergency situation. A small (centimeter across or so) meteorite punctures the hull and it is decompressing. David-2 is unable to deal with the situation, due to his seat restraint buckle jamming, and all seems lost. It is at that point we learn there is a third personality, David-3. David-3 is an emergency backup that was created without the pilot's knowledge. This personality is programmed to do what is necessary for survival, at all costs. David-3 forces open the heavy metal release, with his fingers, breaking one after the other until the clasp gives way. He is then able to seal the breach and save the day.
It sticks in my mind that this one is very old - 1930's or 1940's. The central theme of Man not being able to endure a launch into space seems very dated and before the days of the "Space race" of the 50's and 60's.