I just remembered Something in simpleplanes: If you build a VTOL engine separately from the aircraft, yet leave the thruster nozzles inside it, you will end up with a fully functional rocket engine. I cannot say that I discovered it, since it has probably been done before.

This is the Ragnarok hypersonic research craft, a plane inspired from the likes of the Bell X-1 Glamourous Glennis, X-15, and even the X-51 Waverider. This plane is the ultimate thrill ride: when you crank the throttle to the max, you will start to have a speed increase, and then, when the fuel gauge hits around 70-60%, the thruster will start to suddenly cut out, and your aircraft will turn into a glider, so make sure you know when to land! (or alternately, you can simply set the buoyancy rating to all parts to their maximum value, allowing you to waterland easily.)

See how fast you can go before that thruster stops!