What is the content of the two little black side tanks at the SpaceX Starship SN3? And what it's used for?
The picture is of Mk1 then SN1 (tanks on left side) then SN3 (tanks on middle front).
Here is a clearer view of the tanks mounted on SN3.
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They are COPV (Carbon Overwrap Pressure Vessels). They are high pressure gas tanks. Basically a steel tank with carbon fiber on top to make them stronger so they can hold very high pressure gasses.
People have gotten close up 'spy photos' of the label which suggests the latest ones (used on SN3) say CNG only (Compressed Natural Gas, aka Methane, aka CH4).
Here is the image (courtesy of BocaChicaGal at nasaspaceflight.com) of a rack of COPV's with one being removed to be installed on a vehicle. I had to resize it down, so I am not sure you can zoom in and see the label or not.
So I extracted the label from the original resolution to show it clearly.
They are used to hold gas at very high pressure (the mounting bracket on a similar tank inside the Falcon 9 second stage failed and vented on the CRS-7 mission that led to a loss of vehicle. When filling one of these the piping got so cold that the liquid oxygen solidified and then ignited when the pipes flexed, led to the loss of the AMOS-6 vehicle on the pad before launch during a test fire).
They are used to pressurize something. In this case, SpaceX plans to use autogenous pressurization, where hot gas from the engines will be used to pressurize the fuel tanks as needed. However it appears that they are not yet at that stage of development and for these test vehicles are using COPV's to hold pressurizing gasses.