Like Getting Technical and Blood Bank, this Brotherhood of Steel quest is repeatable. Unlike those two quests, you cannot access Reactor Coolant until after you complete all of the main Brotherhood quests.
Go to the power armor bay in the Prydwen and talk to Proctor Ingram. She needs reactor coolant for the Prydwen's engines.
The challenge is that one of the four reactor coolant ingredients, three units of antifreeze, is one of the more uncommon junk items in The Commonwealth. The rest of the items — dirty water, nuclear material and a gas canister — can be found throughout the Prydwen and the Diamond City Market. One sure way to obtain antifreeze is to travel to the southeast corner of the map to the Atom Cats Garage. Look for a gal named Rowdy. She specializes in trading power armor parts, but she also carries antifreeze. Unfortunately, she only carries one unit at a time. If you don't want to search the rest of The Commonwealth for more antifreeze, you can simply rest at Atom Cats Garage for multiple 24 hours increments. It usually takes Rowdy three to four days to restock antifreeze.
When you have all the items, head to a chemistry station to brew the coolant. There's one such station on the upper deck of the Prydwen. Return to Proctor Ingram to complete the quest.