Re: wild rotation?
The design can be widely adjusted for reaction effects and responsiveness. If there is no blast plate at all, there is a small drag force (from friction) in the direction the plane launches. Installing a blast plate across the exhaust would capture some of the thrust energy and redirect it perpendicularly, canceling it out.
Another option is to enclose the exhaust gases in a piston-launcher, which turns almost 100% of its initial thrust to pushing on both the launcher and plane; this makes a much stronger reaction force. That kicks against the launcher and violently jolts it — which gives a launch pad that retains stability in free-fall, and also protects nearby delicate bits extremely well.
I have photos that demonstrate the thrust containment; I have built a black-powder piston launcher made entirely of combustible materials. It's untouched (save the blast containment tube, which is a bit cooked) after three launches. It launches 1:48 scale F-104 plastic models with Estes engines.