How accurate are this OTRAG parts. Are they balanced for the real solar system or the vanilla game?
The thing that interests me most in that rocket is the unique (?), dead simple, pressurization system it uses. The tanks are filled with 40 bar air, then emptied until the pressure is 15 bar (also serving as a test for the valves and such), then the fuel and oxidizer are injected till they fill 2/3 of the tank and the pressure go back to 40 bar. Then, during the course of the burn, the tank/injector pressure drops from 40 to 15 bar, and as consequence the chamber pressure drops along from 30 bar to 10 bar. That provides natural throttling to limit the Gs and at the same time does not cause problems like flow separation or too big of an isp drop because the outside pressure drops much faster than the internal one. That is a very interesting dynamic and is the main reason I want to fly one of those rockets.
In addition to that, the single valve controlling the fuel and oxidizer flow has 3 positions: 100%, 40% and closed. Those percentages are of the maximum thrust at that point in the burn, so in the end it can be throttled to 13.3...% of the initial thrust. The reason the third and last stage has 4 modules is because the pitch and yaw would be controlled by throttling or shutting down individual engines (for roll, they would use cold gas thrusters). I see that in the arrangement in your add-on the last stage is composed of a single module (to make it lighter to the physics engine?). How does your add-on handles pitch and yaw? Simple magic ksp reaction wheel? That is not actually that important for me, but would be cool to have as well as an realism option.
Finally, it would be cool to have different options on the length of the modules. I assume you have this material, but dropping the link anyway: PDF about OTRAG