Once in the Tank Hanger, after going to the DARPA Chief, you will receive a level one keycard. Go into the Tank Hanger and climb up the stairs by the surveillance camera. There should be a door by the camera. Use the level one keycard to get inside, then find the cardboard box marked with "To Heliport". You can use the box in the missile hanger by getting into the back of the truck in back of the hanger. It will take you back to the heliport.