First, join the Brotherhood Of Steel, which can be done immediately after the game starts. You can join by going to Cambridge Police Station or just following the main story. Either way triggers the "Reveille" and "Fire Support" quests. Follow the Brotherhood's quest line until they make you a member. Then, complete their "Tour Of Duty" quest, where you will meet everyone on the airship. You can then pickpocket a fusion core (make sure you do not get caught) from one the Knights inside The Prydwen (their airship headquarters). Make sure to do this in a secluded area. Once you pickpocket the fusion core from the Knight, he or she will come out of the powersuit, replenish their fusion core, and re-enter the powersuit. Once they are inside their powersuit again, pickpocket the fusion core again. Repeat this process as many times as desired. Note: Quicksave after each successful attempt, just in case you get caught.