 The Command and Service Modules were mated in the KSC altitude chamber in September , and combined system testing was performed . Altitude testing was performed first unmanned , then with both the prime and backup crews , from October 10 through December 30 . During this testing , the Environmental Control Unit in the Command Module was found to have a design flaw , and was sent back to the manufacturer for design changes and rework . The returned ECU then leaked water / glycol coolant , and had to be returned a second time . Also during this time , a propellant tank in Service Module 017 had ruptured during testing at NAA , prompting the separation of the modules and removal from the chamber so the Service Module could be tested for signs of the tank problem . These tests were negative . 
