Begin the level Quarantine Zone, and continue past the first fight to the room with the Flood fighting the Sentinels trying to contain it. Once there, run forward underneath the table-like structure behind the wall when you first come in. There is a Warthog under it. Claim the Warthog and go back to the door you came in from. From there, aim straight at the door, and back up until it stops. Now go full throttle ahead, trying to ram the door before it opens. You will hit it and, when the door opens, you will be propelled into the air quite far.