From the earliest part of the quest to build rockets, Ike had been an integral part of it. Homer struggled with some of the more technical aspects of the building and as they had just started to get some fuel together, Ike helps to teach Homer how to do some basic things, but of course ends up doing most of the machining himself. He also helped teach Homer some of the basics about rocketry in the more practical sense, like rocket fuel burning too hot for solder.
When he is killed in the mine, Homer feels it is his fault because it was helping Homer with the rockets that got him sent down into the mine instead of working in the machine shop. It isn't until Ike's widow tells Homer that he ought to keep working on the rockets that he is able to go back and start building again.