This is a fork in the story and if you decide to do the quest with the Institute, the Brotherhood will become permanent enemies, and vice versa.
You need to be on the main quest Mass Fusion (if you decided to ally with the Institute) or the quest Spoils of War (if you are allied with Brotherhood of Steel). You need to modify your Power Armor’s torso and put the Jet Pack Mod in it (requires 7x Adhesive, 10x Aluminum, 9x Asbestos, 12x Nuclear Material for X-01 Power armor). Be level 41 so you can have all the necessary perks. You need to have power armor with the Jet Pack mod installed. To even consider beginning this quest, you will have to meet the following prerequisites: How to reach the 28th floor of Mass FusionHere’s a video guide showing the whole process, if you enjoy watching more than reading: Be warned that this is end game stuff and the following contains major story spoilers. In this guide, we’ll tell you all the necessary steps you need to take to obtain this rare piece of equipment without using cheat codes. For a bonus that does not seem that important, Bethesda put this piece of armor in a very inaccessible place that you can reach only by spending considerable resources and using glitches in the walls (as far as we were able to conclude). There are Right and Left leg pieces and when worn, they prevent falling damage. Freefall Armor is a unique item in Fallout 4 that is almost impossible to get from the 28th floor of Mass Fusion building.