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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
    Commented Oct 9, 2019 at 16:04
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    $\begingroup$ Not in my wheelhouse, but i.imgur.com/tJ1XMTl.jpg $\endgroup$
    – Organic Marble
    Commented Oct 9, 2019 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
    Commented Oct 9, 2019 at 17:08
  • $\begingroup$ Related: space.stackexchange.com/questions/18768/… $\endgroup$
    – Organic Marble
    Commented Oct 9, 2019 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
    Commented Oct 26, 2020 at 23:23

1 Answer 1

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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
    Commented Oct 10, 2019 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
    Commented Oct 10, 2019 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
    Commented Oct 10, 2019 at 3:28
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    $\begingroup$ @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
    Commented Jan 20, 2021 at 16:47
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    $\begingroup$ @mothman Actually, the weight at the top makes it more stable. Try balancing a stick with weight on top vs not and you will see it in action. The steering force is applied at the bottom of the rocket, so you want that part to be the easiest to move back and forth to steer, which means least mass on the lower part of the rocket, especially during the biggest steering maneuvers, like belly flop and landing flip. $\endgroup$
    – The2x4
    Commented Feb 8, 2021 at 21:49

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