The way I choose to interpret the jet engine is -- I'm struggling to find another reference that parallels it, so maybe like one of those old cartoons where we see someone going off on a ludicrous rant about impossible adventures they have and then in the last five seconds you see some physical evidence of the thing's real existence appear in the real world. You know what I'm talking about -- this is how Kristin Wiig's "Penelope" sketches on SNL tend to end. It's Donnie getting the gift of having his craziness intrude into the real world, become realized, become inarguably true, and for him to pay the price for this gift by having it also end his life.

That's my hyper-abstract interpretation, anyway. This is how I read The Philosophy of Time Travel, too -- as an actual science-fiction explanation of the events of the story it's really annoyingly convoluted and unnecessary, but as a sort of twisted metaphorical take on why, perhaps, real schizophrenic people develop suicidal tendencies, it makes sense -- the "Tangent Universe" is Donnie's own fucked-up crazy take on the world, the "Primary Universe" is the real world, the two can't coexist, the "Artifact" is the one heroic act that Donnie can take to acknowledge this dichotomy, assert power over it, and by doing so become its master rather than its victim (suicide).

Taken in that light and as a hyperliteral thing it becomes an actual exhortation to alienated mentally ill teens to slash their wrists in the bathtub, which is why I don't try to take it too literally, and why the surrealistic mechanism of having it be Donnie's own madness literally killing him (something appearing from nowhere above his head) is so much better than having Donnie actually shoot himself or something.