I had a similar problem with the Komet last night.
On my second sortie in the thing I was cruising comfortably at around 18k and was gently banking when I suddenly got a "you have crashed" message. Wha happin'? I thought to myself.
I was in near level flight, had just a little under half of my fuel left (seems you watch that dial more then anything else in that thing). I did not experience any buffeting (although I did on my first sortie but didn't crash) so I have to wonder if this is a bug or a modeled feature.
Anyone else experiencing this?
Also, Vorticon, the rocket plane you are referring to that launched vertically was the Bachem Ba 349B-1 Natter.

