John Travolta and wife Kelly Preston are going to try for another baby.The 'Pulp Fiction' star and the actress - who already have two children, Jett, 14, and seven-year-old Ella - have revealed they are keen to add to their family and have set themselves a fertility timetable. John, 53, said: "I was told by Kelly that at the end of the summer we're going to try."Kelly, 44, added: "We'll try after we finish the movie we're doing together,'Old Dogs', with Robin Williams and our daughter Ella. So after that!"The 'Saturday Night Fever' actor has made no secret of his desire to have more children and recently spoke about the joys of parenthood. He said: "I really like having kids. I enjoy their energy and it keeps everything wonderful and hopeful. I grew up with six kids in my family. I thought I would have five or six."It's fascinating watching your kids growing up, because they're yours and they're beautiful and they're constantly changing and offering you new thoughts and ideas, so I love watching them grow up. They're your future,really."