It\'s possible! And surprisingly easy! 1. Place the decoupler below any part. 2. Place the bottom of the RCS tank on the bottom of the decoupler so the tank goes inside the decoupler. 3. Place any part so that the top attaches to the top of the RCS tank. 4. Profit! Also, it will make the part below the decoupler look smaller, as a part of it is inside the decoupler and the RCS tank.