SpaceX CEO Elon Musk has shown off a rarely-seen phase of Starship construction, further confirming that the next full-scale rocket ship is rapidly coming together after operator error destroyed its predecessor on April 3rd.

Around 5 am local time (10:00 UTC) on April 5th, Musk shared a photo of a smaller secondary tank’s installation inside Starship SN4’s full-size liquid methane tank. The photo is the first time SpaceX or its CEO has offered a glimpse inside this lesser-known part of Starship production and assembly and simultaneously offers insight into a design that’s been a mystery for months. Over the last few weeks, local residents-turned-unofficial-SpaceX-photographers have captured several photos of an internal Starship header tank design far removed from the nosecone tank Musk has previously discussed.

Those header tanks are a necessity to enable Starship’s ability to quickly, reliably, and safely reignite its Raptor engines for recovery and landing-related burns. For a ship like Starship SN4, those header tanks are a sign that SpaceX may want to use the ship to perform Starship’s most crucial test short of orbit — a ~20 km (12.5 mi) flight test meant to demonstrate a skydiver-style landing maneuver. While that skydiver landing test remains several consecutive milestones distant, Musk’s April 5th photo confirms that Starship SN4 is making significant progress towards final assembly.

Known as header tanks, SpaceX’s large Starship launch vehicle upper stage and orbital spacecraft requires smaller, secondary tanks aside from the main liquid oxygen and methane propellant tanks that make up the bulk of its body. In these early stages of prototype development, those tanks serve one main purpose: reserve a small portion of pressurized propellant for Starship landing burns.

While Starship’s main tanks still need to be pressurized at all times to ensure the rocket’s structural integrity, smaller header tanks make it much easier to safely feed Raptor engines fuel during even the most chaotic of aerial maneuvers. For rocket engines, even the slightest introduction of pressurization gas or voids into the combustion process can lead to immediate destruction — a bit like how a tiny air bubble can be almost instantly fatal for humans. Starship header tanks thus ensure that only a fraction of the overall tank volume is in play during the ship’s most critical maneuvers.

Requiring both a fuel and oxidizer, Starships thus need two header tanks. Currently, Starship’s design places the liquid methane header tank directly inside the main methane tank itself. The liquid oxygen header tank, however, is situated in the very tip of Starship’s nose section, a location chosen to optimize the vehicle’s center of gravity for stability during a radical skydiver-style landing maneuver.

Musk’s April 5th photo and caption revealed that SpaceX began installing Starship SN4’s methane header tank just an hour or two after it had flipped the ship’s partially-completed liquid methane tank dome. Thanks to SpaceX’s more efficient use of a common dome design in their Falcon and Starship rockets, that dome also serves as the upper dome of the ship’s larger liquid oxygen tank. After the methane header tank is installed, a funnel-like sump will be the last addition needed to finish the section.

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With the majority of Starship SN4 already in work around SpaceX’s Boca Chica, Texas rocket factory, the ship could be just a week or less away from kicking off the stacking phase of assembly. Stay tuned!