Harry Markos 
Gilbert’s Universe [PDF ebook] 

Gilbert is a genius who loves to build things using all kinds of rubbish. With his friend and neighbour Ginny they build ‘Victoria, ’ a space ship that they can use to explore the Universe. On their maiden voyage Victoria is damaged by a giant space dragon, which causes them to crash land on the moon! How will they get back home to Earth? And what will the giant space dragon do to them now that it’s also landed on the moon?