For my money I think it's a turbo fan, or directional RAM jet engine, they would not use prop vehicles, like we think, (helicopters and the like).

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Shovels down, Shovels up this trench better hold up as " Skull Legion" goes fighting along.

Our Motto: When in Rome do as the Legion does, kill, ravage, destroy; anything to make those Covenant SOB's forever sorry of the day they met the human race.