Looks good!
But the second stage looks really tiny in comparison to the first stage. You could probably transfer some of the fuel from the first stage to the second one. That rocket should be able to carry way more than 13 tons by the way.
Also, you could consider making the "boosters" detachable. As is, they are part of the core and the whole thing behaves as a single stage.
The boosters help in the beginning of the flight because the rocket would be too heavy otherwise. But as and when it burns fuel, the rocket alleviates itself, so at some point you don't need that much power. Dropping the boosters allows to remove some dry mass, which increases your rocket's efficiency.