Added:

-cardboard
-background cardboard
-jet thruster
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Tweaks:

-Adjusted hitboxes for balloon, bomb, and propeller

-Changed propeller energy firing cost from 10 to 0

-Propeller (and jet thruster) now shoot air projectiles that can move other objects in the air.

-Slightly lowered the volume (sound) of the propeller
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Fixes:

-You are no longer able to hide a propeller protected inside your fort and expect it to thrust your base forward.