A petty own and one born of my own foolishness, but a triumph nonetheless. With an altered tech tree and a collection of mods to encourage flight-first development, I carefully designed an early-campaign jet that had some minor potential stability issues. With this in mind, I tweaked the wings and ailerons to ensure that the center of lift was properly behind the center of mass, though the margin was less than I preferred. I then took it up to see what its performance was, conduct some SCIENCE!, and fulfill some hardware test and scientific observation contracts. Immediately, I noticed that it wasn't quite generating enough lift; I needed to pull at least 50-70 m/s of acceleration on the runway to get it off the ground, at which point it took off with a vengeance. I made a mental note to be careful landing and to upgrade the SPH to permit more parts, and carried on. Otherwise, the design impressed: at full throttle, it could easily hit the sound barrier if I gave it its head (and bounce off, but who's counting?), and it had a service ceiling of around 13 km (a bit shy of the edge of the "troposphere" at 16 km; I just realized I forgot to add the mod that makes atmospheric pressure non-linear).
As I started running low on fuel, I turned homeward. At this point, I realized a minor issue I had forgotten during development. When I adjusted for stability, I was doing so with full fuel tanks. With the fuel tanks half-empty, my lovely jet's center of mass moved, putting it right on top of the center of lift. By extension, running the fuel tanks all the way down put the center of mass *behind* the center of lift, setting me up for some nasty positive feedback every time I made a course adjustment. Of course, I didn't know this for sure yet, though I had my suspicions; what I knew was that when I turned around, the airplane started bouncing like a top in all three axes, lost all lift as it turned into a lawn dart, and dropped a couple thousand meters before I managed to pull it out of its tailspin. Fighting the plane the whole way, I managed to land it just outside the KSC. I landed it on fumes and I dented the nose as it came down, but I landed it nonetheless.
Valentina made it home safe and sound with the science, and won some nice ribbons out of the deal. To wit, she pulled a G-Force XII ribbon for sustained 12Gs of acceleration for 3 or more seconds, and a fuel ribbon for making it back on vapors without deploying a parachute. The only thing I'm worried about is that Jeb, whose chief experience is with turboprops, is going to feel the need to one-up her now. I think rockets might be safer than this.