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In the Everyday Astronaut video A conversation with Elon Musk about Starship after about 13:00 Musk shows Dodd a cell phone video that he shot in the top(?) of the SpaceX Starship and explains that there are four 100 kWh Tesla batteries welded to the header tanks.

The batteries power Tesla Model 3 motors which are currently used to pump hydraulic fluid that's used to actuate the flaps.

Per the video, the header tanks contain oxygen and fuel as a "mini version" of the main tanks.

Question: But what are the header tank and what do they provide these liquids(?) for?

A conversation with Elon Musk about Starship

This video shows the tank location and the way they are depleted:

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  • $\begingroup$ Is there any relationship to How will SpaceX's Starship's future pressure-fed thrusters work at “any gee’s, any attitude”? $\endgroup$ – uhoh Oct 9 '19 at 16:04
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    $\begingroup$ Not in my wheelhouse, but i.imgur.com/tJ1XMTl.jpg $\endgroup$ – Organic Marble Oct 9 '19 at 16:35
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    $\begingroup$ @OrganicMarble okay that's interesting! I've added a (?) to the word "top" in the question now. $\endgroup$ – uhoh Oct 9 '19 at 17:08
  • $\begingroup$ Related: space.stackexchange.com/questions/18768/… $\endgroup$ – Organic Marble Oct 9 '19 at 21:51
  • $\begingroup$ The edit with video was originally rejected by the community probably because the comment giving the edit reason wasn't explanatory and so it looked like it was trying to answer the question by editing it. However the edit itself does explain the reason "This video shows the tank location and the way they are depleted:" and I think it's a good reason, so in this case I overrode the reject. Thanks to community members for being cautious and to the editor for bringing the video to my attention; now I have some idea where it might be and what it might look like (roughly at least). $\endgroup$ – uhoh Oct 26 '20 at 23:23
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This has been answered before, but I chose not to just mark it a duplicate because there is one new reason for the header tank unique to Starship.

In a Reddit AMA in 2017 where Elon Musk answered questions about the original ITS design, he answered a question about the header tanks:

Those are the header tanks that contain the landing propellant. They are separate in order to have greater insulation and minimize boil-off, avoid sloshing on entry and not have to press up the whole main tank.

Elon Musk Explains reason for the header tank in the old ITS design

In other words, if you just leave enough propellant for landing in the main tank, it will slosh around the mostly empty tank, screwing up the handling of the vehicle. And to pressure feed the engines they would be required to keep the main tank pressurized right until landing.

Much better to move the fuel for landing to a smaller tank, run the main tank dry on ascent, and then you have a completely full, much smaller tank to deal with for landing. That strikes me as a much better way to go, and clearly SpaceX feels that way as well.

Musk also mentions that the smaller tank can be better insulated to avoid boil-off of the cryogenic fuel. This is important for the landing fuel, because unlike the main fuel which is gone in a few minutes after takeoff, the landing fuel has to remain in the vehicle until the mission is over and it returns. Why insulate a gigantic tank to protect 10% of the fuel, when you can just insulate a separate header tank?

Now, the new reason for the Starship header tank is that by putting it in the very tip of the nose of the Starship you move the center of mass higher on the rocket - especially during reentry. If the fuel was in the main tank, the weight and balance would be too far aft for the 'skydiver' horizontal return method. So putting a separate tank in the nose of the Starship solves a whole lot of problems at once.

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    $\begingroup$ Curiously I think for landing stability you would prefer a center of mass closer to the ground though, right? Great answer! $\endgroup$ – mothman Oct 10 '19 at 0:24
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    $\begingroup$ Well, by the time they land the header tank should be nearly empty, so only the weight of the tank itself will be left. But yeah, on the ground it would be better lower down. Everything in engineering is a tradeoff... Thanks for the nice comment! $\endgroup$ – Dan Hanson Oct 10 '19 at 1:53
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    $\begingroup$ Great answer! I was surprised to see that the've been moved from inside the larger tanks to the top of the rocket, but your explanation of this change is really helpful. Thanks! $\endgroup$ – uhoh Oct 10 '19 at 3:28
  • $\begingroup$ Can the header tanks also lose propellant due to the boiling? $\endgroup$ – Joe Jobs Dec 10 '20 at 19:03
  • $\begingroup$ @JoeJobs If I understand the post correctly, yes they could, but they are much better insulated (because they are there not only for some minutes). $\endgroup$ – peterh - Reinstate Monica Jan 20 at 16:47

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