Fallout 4: How to get the Junk Jet (the new Rock-It Launcher)
The world of Fallout 4 is packed with more post-apocalyptic junk than just about any game in history. Fortunately, the trash is actually really useful. Not only does it drive the game's engaging crafting sub-systems, you can also use it as a weapon -- provided you find the right weapon. Previous titles in the franchise had the Rock-It Launcher, which allowed you to pelt enemies with teddy bears and tin cans for some serious damage. Fallout 4 introduces the Junk Jet, and lets you load it with just about anything you can carry. Time to recycle...with extreme prejudice.
You'll have to complete a couple of minor quests to gain access to the Junk Jet, but fortunately there's nothing especially challenging standing between you and your dreams of garbage-driven mayhem. Here are the steps you need to follow to grab the Junk Jet as early as possible in your playthrough.
1. Travel Southeast from your starting location at Vault 111, towards the Cambridge Police Station until you come across the "Military Frequency AF95" radio station
2. Listening to the radio station begins a quest to help defend the station from an attack by ghouls. Go do that!
3. After defeating the ghouls, speak to Paladin Danse to move on to the next stage in the quest. Danse will lead you to ArcJet Systems.
4. Paladin Danse will lead the way through the facility, killing robots, turrets, and Synths. Follow along and help him out as much as you can (or don't help him at all, that part isn't really important).
5. Don't miss the magazine that will boost your energy weapon critical damage! This isn't related to the Junk Jet, but it will help you make the most of all those energy weapons the Synths drop.
6. Descend into the rocket test chamber beneath ArcJet Systems and head into the control room. There on the table, you'll find the Junk Jet!
7. If you don't take the Junk Jet now, be careful! Depending on what you do next (including shutting doors and turning off the power to the facility) the control room might become inaccessible later. At the very least, you should grab it and leave it in a workshop in case you want it later.
8. You're now free to go have fun with your Junk Jet! You might want to complete the quest with Paladin Danse first though, as he'll give you a pretty fantastic laser rifle ("Righteous Authority") as a reward.
And that's it! Both the Cambridge Police Station and ArcJet Systems are fairly close to Vault 111, and neither ghouls nor Synths are particularly hard to kill -- especially when a member of the Brotherhood of Steel AND Dogmeat helping you out -- so you can make an early run for the Junk Jet and Righteous Authority without too much trouble.
Note that you CAN'T skip the Paladin Danse section and just head right to ArcJet to grab the Junk Jet on your own. The interior of the location will actually be slightly different before you arrive with the Paladin, and the lower area with the rocket and the control room will be inaccessible without him.