Progress has been slow. I've been working on making the turrets aim:
After experimenting with projectiles that are influenced by gravity I'm now experimenting with ones that aren't. The goal of the turret is to find a spot along the rocket's projected trajectory where a bullet would arrive at the same time as the rocket. The turrets in the picture are assuming that the rocket undergoes constant acceleration. They use the sum of the rocket's current thrust acceleration vector and the gravity vector at the midpoint between the rocket and the turret. This gets them pretty close in the picture shown (the endpoints of the white lines) but often it's a lot worse. There are two reasons for this: the main one is that the gravity vector's strength changes with altitude and its direction changes as you go around the planet. A second inaccuracy is that when the player is thrusting without steering, the thrust vector is automatically rotated slightly as the rocket goes around the planet. (This essentially makes the thrust direction view-relative, since the view rotates in this way.)
I'm currently replumbing things so that the turrets can examine the plotted rocket trajectory instead. It is built using forward integration (albeit with a much larger time step than the game update) so it predicts where the rocket will go almost exactly. This should allow them to aim very accurately. In the past when I've worked on aimy-shooty things I've found that it's best to make them perfectly accurate and then work from there toward something that is inaccurate in a fun way.
The exact role of the turrets is not yet clear. It's no fun when you get shot with no time to react, so they need to be ineffective at close range. Probably they will avoid shooting at all at very close range (due to fear of unwanted splash damage, let's say). Making them turn slowly, and need to sync up to tracking the rocket's trajectory before firing, should also help.
I think I will plot the intercepts of all flying projectiles with your projected trajectory, although I don't yet know exactly what that ought to look like. Ideally it would give you an idea of the best evasive directions. It's possible that having all these trajectory plots will rob the game of any required skill, but my hope is that it will instead allow players to play a much denser game. I've noticed with the trajectory plots that I fly more aggressively than I ever would without them, and I feel much more in control as a result. Without plots I fly defensively and do a lot of waiting.
Ultimately you'd be able to destroy turrets to make things easier for yourself. (There'd probably be a bombing trajectory plot as well!) I've also been thinking about having turrets drop in from space over the course of the mission; maybe key the drops to astronaut pickups so that the difficulty ratchets up as you go.