During the Two Betrayals level, the Master Chief comes upon a broken blast door and is "forced" to abandon his Ghost and face the Flood on foot. To get the Ghost through the door, line it up backwards with the hole in the door; allow the tail to stick out slightly through the hole. Leave it there. Now go back and get the Warthog, and use the Warthog to ram the Ghost through the crack. Hit it at a 45 degree angle to wedge it through.