To create a nuke, all you have to do is the following:
1.Make the biggest solid rocket fuel tank you can
1.1Make sure that the fuel tank is completely full
2.Use the tinker panel to make it smaller by using the part scale option
3.If you want to stick the nuke onto a craft, use the tinker panel once again and use the craft mass option and set it to 0
Spawn it and blow it up!!!