The Fuel Rod is what you need to make run the Excalibur Spaceship and start your quest in Space! This why you need the other items and get it from the robot in jail.
How to get the Fuel Rod
Go back down the trap door in the Ye Olde Rumour Mille and then walk left. Pick up the book that is sitting on the chair. This is Mordred’s Journal. Then walk all the way to the left and keep pushing against the wall. After a few seconds it will crumble into pieces. Go all the way to the left and you will see a small hole. (You need to make this open on the hidden jail in the library castle.) Go through it to enter the cell where you found the robot. He will self-destruct and a fuel rod will fly to the left beyond the bars. You can’t reach it by yourself, but your new friend the owl can! Click on the owl and then click on the green rod and the owl will retrieve it for you. Click on the owl and then click on yourself and your owl will give you the rod.