In a post-apocalyctic world such as Fallout 4’s, even junk can be useful, really useful. You can even make a launcher out of it! Notice we didn’t say what you launch with this weapon, as with it any item can reveal precious ammo for you to beat the hell out of your enemies. In order to get the Junk Jet, you will have to complete a few, not so challenging quests.
Travel Southeast from your starting location at Vault 111, towards the Cambridge Police Station until you come across the “Military Frequency AF95” radio station. Once you are listening to the radio station, you will notice there’s some help to be delivered at the station: there’s a ghouls attack in the making right there.
Talk with Paladin Danse after defending the station and go to ArcJet Systems right after
Follow Paladin Danse in there, and help him getting rid of all the enemies you will see around him.
Go into the rocket test chamber beneath ArcJet Systems, then reach the control room: on a table you will see there’s the Junk Jet. Grab it as soon as you can because, depending on your actions, there could be a chance of losing the opportunity to get it due to the control room’s door being shuttered.
Complete the quest for another great weapon, Righteous Authority (a laser rifle), provided by Paladin Danse as a reward. Be careful, by the way: you can’t access those latter specific areas going to ArcJet Systems before of this quest.