The thruster plugin runs the thrusters for our simulated sub. It is responsible for:
When gazebo is run it starts both a server (gzserver) and a client (gzclient). Since model plugins are run on the server but visuals are run on the client gazebo messages must be sent from the thruster plugin (which is a model plugin) to the client in order to add visuals to the simulator.
To visualize each thruster a very narrow cylinder is defined for each. This cylinder is then scaled and positioned each time a new thruster message is recieved. To see how the visual messages are defined see here.
To see the actual scaling and resizing steps see UpdateVisualizers.