The "hot air balloon deployer" is a block that can be found in the construction menu under Air.
It looks like a metallic box with a hatch on top.
A hot air balloon deployer block deploys a balloon that creates upwards thrust. It can be used for flying vehicles.
The balloon is connected to a 5 meter long chain which fixates it to it's deployer block.
Since the balloon can move around freely, the chain can deform and allows the deployer blocks to be build in any wanted orientation. The balloon will always try to move upwards.
A balloon has the shape of a sphere with an approximated 5 meter radius. Upon deployment, the balloon is as small as the spawning deployer block but then grows to it's full size. During this short interval, the balloon fits through smaller holes which allows for a certain armor-plating around the deployer block. However the balloon can get stuck when contained in a narrow structure while still growing.
If a balloon gets stuck in a small room, it starts glitching and can suddenly penetrate blocks or wobble around strangely. This can cause the vessel to fall off balance.
A balloon takes 5 seconds to grow to its max size of 10m and produces 300 * Growth Percentage * Min(1, 150 / Max(Altitude,1)) kN of lift. That is, it produces 300kN of lift at sea level, halving the lift force generated every 150m.
It is wise to aim for the balloons when fighting an enemy airship in order to throw it off balance and to thereby impede the enemy's aim.