You're right, of course. I was just referring to the 2-D brief span covered by the limited information we received from these stills, and thinking of it more in terms of a bullet, something propelled but no longer guided once launched.When I say "straight line", I mean in 3 dimensions. You have to balance gravity and lift (or up-vectoring) in order to keep that third dimension constant. Aerodynamics tends to be notoriously unstable, once you start pointing in the wrong direction, you tend to make yourself point more in that wrong direction, positive feedback ensues; sometimes your arse overtakes your head (c.f. rocket launch failures), sometimes you stall (c.f. a home-made Barnaby glider - make one and give it too much lift), it depends on the exact situation.