For the rocket thrusters on Spaceport, you don't actually need to do anything specific beyond go to a location. Even though the objective says you need to "scope out" the rocket thrusters, all this means is you need to ARC Raiders Items hang around there for a few moments until the task is complete.

The location you must visit is actually closer to Control Tower A6 than Rocket Assembly. Just north-east of Control Tower A6, you'll spot some rocket thrusters mounted on trailers. Approach the thrusters and stay in the area until the task is complete. Use the thrusters as cover to avoid getting spotted by other raiders and Arc enemies like the pesky Rocketeer that killed me shortly after I completed the mission.

Finally, you're tasked with visiting a "barricaded area" on the sixth floor of the Space Travel building on Buried City. The most important thing here is making sure you're in the Space Travel building, and not the Research building found right next to it.

You can either take the stairs or the zipline up the elevator shaft to reach the sixth floor (the zipline will take you to the roof though, so make sure to go back down one floor). Head down the corridor until you find a barricaded door. Breach it and head inside to tick off the buy ARC Raiders Items objective. If the door is already open by the time you get there, like it was for me, just enter each room for a couple of seconds until it completes.