Called falcon wing doors because they are said to resemble that bird of prey in flight, they open upwards, hinging in two places – the somewhat similar gull wing doors only have one hinge, where they're attached to the roof – which allows them to be opened in tighter spaces than the sea bird design.
While the original prototype of the Model X had working wings, the engineering for a production model is no doubt a lot more difficult. Noted analyst Adam Jonas of Morgan Stanley said yesterday in a report that cut delivery estimates of the postponed car, "A door of that size and weight is going to be very hard to lift. Furthermore, having a hole of that size in the side of the car (that cuts deeply into the roof) could severely compromise structural rigidity impacting handling and safety."
While the challenges of the unique rear portals make some investors wish Tesla would just stick with traditional openings, Musk made it clear that the falcon wings are going forward. Taking to Twitter, he tweeted:
Rumors about canceling Falcon wing doors are false. At Tesla, the production car will always be better than the show car.- Elon Musk (@elonmusk) November 19, 2014