How to Charge and Fuel an Iron Jetpack

Charge it with an Energy Cell

A Jetpack stores energy and must be charged before it can fly. The Energy Cell is the item that holds and transfers that energy. Craft an Energy Cell of the same tier as your Jetpack, then charge the Energy Cell using your mod's energy source. Once the Energy Cell holds energy, use it to charge the Jetpack so the Jetpack's internal buffer fills up.

Keep a charged Energy Cell in your inventory as a spare. When the Jetpack runs low mid-flight, top it up from the Energy Cell rather than returning to base.

Match the tier

Energy Cells, Capacitors, and Coils are tiered: Basic, Advanced, Elite, and Ultimate. A higher-tier Energy Cell holds more energy and charges a higher-tier Jetpack. Pair a Basic Energy Cell with a Basic Jetpack, an Advanced Energy Cell with an Advanced Jetpack, and so on up through Elite and Ultimate.

If your Jetpack will not accept a charge, check that the Energy Cell is not empty and that its tier matches or exceeds the Jetpack's tier. Recharge the Energy Cell whenever it drains, then recharge the Jetpack from it again. Capacitors of the matching tier also store energy and can serve as additional reserves.