Upgrading your Nuclear Reactor will provide Cores for your planet.

For the game wallet, coins can be sent or deposited via the Wallet menu.

For rebel fights and star fighters, drones take the brunt of the enemy force, if your drones die and nothing is left to protect your star fighters then they will start dying instead.
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here is my reactor Nuclear Reactor    2/3    1 Droid, 400 Credits   N/A   won't upgrade? I need a Droid from somewhere?

Great info thanks for your time :-)
Cheers

P.S. I lost fighters today. actually gave me a -110 ? but didn't kill me off. must have been a big rebel group I just mindlesly fought.
I went and built more and lost the amount -110 in the process. ( bug Maybe to go - and not kill me off? )