The whole launch was so exciting to Homer because his father had finally shown up to watch and take part in this thing that had consumed his life. He had always felt like his father loved Jim more than him because he was so into the football thing and spent so much time and effort worrying over it. So when he showed up and they got him to launch the last one, it was an incredible moment for Homer.
When the rocket landed, Homer's dad was excited but then was overcome by one of his coughing fits. Homer puts his arm around him and says "You did really good Dad. Nobody ever launched a better rocket than you."