Houston ? Astronauts tinkered Sunday with a troublesome piece of equipment designed to help convert urine and sweat into drinkable water, which is vital to allowing the international space station crew to double to six.

Station commander Michael Fincke and space shuttle Endeavour astronaut Donald Pettit changed how a centrifuge is mounted in a urine processor, which is part of the newly delivered $154 million water recovery system. The centrifuge is a spinning device that helps separate the water from urine.

The astronauts have been trying to get the system running for four days, but the urine processor has worked for just two hours at a time before shutting down. A normal run is about four hours.