What is the content of the two little black side tanks at the SpaceX Starship SN3? And what it's used for?

Mk1 SN1 SN3 family photo

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.

Tanks on SN3


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.

COPVs by the dozen

So I extracted the label from the original resolution to show it clearly.

CNG Only label

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.

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    $\begingroup$ It's amazing what these photos tell you about the scale of the thing. In the OP's first photo, they look tiny, like maybe the size of an office fire extinguisher. But when you see them next to that crane and hangar in your shot you realize they're actually quite large - easily double an average human in height, which makes you appreciate the real scale of the Starship when you look back at the original photo... $\endgroup$ Apr 6 '20 at 13:30
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    $\begingroup$ @DarrelHoffman 50m tall, 9m wide is freaking big! The Shuttle ET is only 47m tall! And only 8.4m wide. (Ok .6m is 2 feet, but still!) Taller than the Statue of Liberty. And they have whipped out MK1, SN1, SN3 in the last few months. So they break in testing, no biggy next rev is due in a week... This is totally different than most any spacecraft building model we have ever seen, and I LOVE IT! $\endgroup$
    – geoffc
    Apr 6 '20 at 14:36

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