Kerbal Engineer reveals important statistics about your ship and its orbit during building and flight.
Learn about the HUD system with KSP-TV streamer "ShimmyTheJJ":
KSP Quick Tutorial - Kerbal Engineer HUD Config
Please note that by default KER runs using a career unlock system. This means that when in career mode it will require either an Engineer skilled Kerbal, an Engineer Chip/ER7500 part placed on the vessel or a level 3 tracking station to work in flight. Unless one of these three conditions is present, the KER icon will be disabled and greyed out in flight. This mode is completely optional though, and by clicking on "Settings" on the Kerbal Engineer Redux window in the editor, you may change its mode from "Career" to "Partless".
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Official Forum Thread
KSP-AVC Ready for Update Notifications
This mod includes version checking using MiniAVC. If you opt-in, it will use the internet to check whether there is a new version available. Data is only read from the internet and no personal information is sent. For a more comprehensive version checking experience, please download the KSP-AVC Plugin.
Completely voluntary, absolutely amazing, and really does help me out a lot!