I think this is like my 50th post today, but anyway, I made a custom jet engine! I know it's not very good, but it can be customized to be slightly better. It starts as a body:
If there's fuel inserted, sparking the green "O2" button releases o2. Sparking the red one stops it. Also if there's fuel inserted, sparking the green "Fuel and ignition" releases the main fuel, and lights it. The red one turns it off. To get fuel and nozzles go here:
Copy your fuel and nozzle from there, and place them in the directed locations in the body. I recommend Letting the o2 run for a little bit, then adding the fuel. About nuclear: Yes, there is fuel and a nozzle titled nuclear. It just a prototype though. I have been experimenting with it, and have been trying to improve it.