So Pvt. Grichman's answer got me in the right direction. I didn't succeed with his design, though; I always ended up having somehow unstable airplanes. Pretty often, the tail "wiggled" and/or the plane became totally unstable when I fired the rocket boosters.

So I took a slightly different approach:

The main body is a fuel tank with a Swivel engine at the end. I start with the normal jet engines (which takes almost the complete runway). Once it's in the air, it's stable and easy to fly and manages to fly at altitudes between 11k – 12k without problems.

Once I'm near or in the target area, I set the throttle to 0 and start the Swivel. Then I increase throttle until about 2/3 or even full, climbing with a high inclination (it can also go straight up). Immediately starting rocket engines at full throttle turned out to make most of my aircrafts totally unstable, turning them up gradually worked way better.

With this thing, I can easily reach 20k.

I also added a parachute and decoupler since I find it too hard to correctly land this thing yet ;-) I managed to land in the water once, though.

So Pvt. Grichman's answer got me in the right direction. I didn't succeed with his design, though; I always ended up having somehow unstable airplanes. Pretty often, the tail "wiggled" and/or the plane became totally unstable when I fired the rocket boosters.

So I took a slightly different approach:

The main body is a fuel tank with a Swivel engine at the end. I start with the normal jet engines (which takes almost the complete runway). Once it's in the air, it's stable and easy to fly and manages to fly at altitudes between 11k – 12k without problems.

Once I'm near or in the target area, I set the throttle to 0 and start the Swivel. Then I increase throttle until about 2/3 or even full, climbing with a high inclination (it can also go straight up). Immediately starting rocket engines at full throttle turned out to make most of my aircrafts totally unstable, turning them up gradually worked way better.

With this thing, I can easily reach 20k.

I also added a parachute and decoupler since I find it too hard to correctly land this thing yet ;-) I managed to land in the water once, though.