Amateur rocket structures are usually made of composite materials, wood or aluminium, their internal geometries and interfaces are usually restricted by the available manufacturing techniques. However, with the appearance of the additive manufacturing sector new possibilities arise for the design of the structures and its complexity. In this paper a PA-12 and glass fibre composite structure for the Phobos rocket is designed which the UPC Space Program aims to use to participate in the European Rocketry challenge. The Phobos rocket structure is designed and optimized to be fabricated using additive manufacturing by Hewlett-Packard. The structure is designed using a lattice approach to obtain a PA-12 skeleton which is then reinforced with a skin of glass fibre composite. Moreover, to obtain the desired structure an optimization methodology is set using a design loop in which the critical section of the rocket is parametrically optimized to reach the equivalent traditional structure performance. The structure is optimized in the size of the lattice geometry and in the thickness of the skin as parameters. To do so, the critical load during the flight of the rocket is identified and translated to the Nastran environment to run a parametric optimization of the structural model. The optimized geometry is then extended to the rest of the rocket to obtain the overall optimized structure. In addition, several analyses are conducted to validate the structure behaviour for the different load cases. Finally, both the optimized critical case and the overall optimized structure are compared to traditional design structures to obtain conclusive results about the use and limitations of the available additive technology and its materials