Second part where you will learn how to use SDF Ruby Macros, with a totally parametrised model making infinite number of elements: http://answers.gazebosim.org/question/16715/how-can-i-represent-a-anake-robot-with-many-identical-segmens-in-sdf/?answer=18303#post-id-18303

Q:How can I represent a snake robot with many identical segmens in SDF ?
A:We used Ruby code in the sdf to create a macro style sdf: http://www.stuartellis.name/articles/erb/