In a small but important step towards activating a pad capable of launching the largest and most powerful rocket ever built, SpaceX has ‘sleeved’ one of its Starship-derived propellant storage tanks for the first time.

Starship is a fully-reusable, two-stage liquid rocket designed to ultimately cut the cost of orbital launch by at least one magnitude, opening the door for humanity’s sustainable expansion to Earth orbit, the Moon, Mars, and even beyond. To accomplish that lofty feat, it has to be a massive rocket. Measuring approximately 120m (~395 ft) tall and 9m (~30 ft) wide, Starship and Super Heavy will weigh on the order of 300 metric tons (~675,000 lb) when empty.

Once filled to the brim with cryogenic liquid methane (CH4) and liquid oxygen (LOx) propellant and gas, though, a two-stage Starship will easily weigh more than 5000 tons (11 million lb) shortly before and after liftoff. Further, SpaceX wants to be able to launch at least two Starships from Boca Chica in rapid succession. To meet the staggering needs of back-to-back Starship launches, SpaceX has thus had to design and build what will be the world’s largest launch pad tank farm.

Work on that tank farm is already well underway, though progress has been slower than expected. The site’s foundation and a few associated blockhouses were mostly completed by January 2021. By early April, the company had completed the first of at least seven steel propellant storage tanks at its Starship factory and rolled it to the launch pad for installation.

Notably, SpaceX chose to manufacture those storage tanks itself and ended up building structures virtually identical to the tanks that already make up most of flightworthy Starship and Super Heavy airframes. Depending on whether they’re meant to store liquid oxygen or methane, the seven tanks SpaceX is building are either 26 or 30 meters (85 or 100 feet) tall – though the concrete mounts they’re affixed to at the launch site are sized such that all storage tanks will have the same final height.

Of course, being made with the same tools and out of the same steel as Starship and Super Heavy, that means that SpaceX’s custom storage tanks are little more than 4mm (~1/6″) thick steel shells – about as bad as it gets for keeping cryogenic rocket fuel… cryogenic. If SpaceX were to simply use those unmodified tanks, it would be almost impossible to store Starship fuel for more than a few hours – and maybe just a few minutes – without it warming up past the point of usability.

As such, SpaceX’s final Starship tank farm design involves seven Starship-derived storage tanks and seven contractor-built tank sleeves. Measuring around 12m (~40 ft) wide and 40m (~130 ft) tall, those “cryo shells” will enclose all seven SpaceX-built tanks, allowing the company to fill the 1.5m (~5 ft) gap between them with an insulating solid, gas, or some combination of both. With those shells and insulation, SpaceX’s custom-built Starship tank form should be more than capable of storing cryogenic liquid oxygen and methane for days or even weeks.

As of August 5th, SpaceX has installed three of Starship’s custom ground supply equipment (GSE) tanks (with a fourth moved onsite on Thursday), moved two ‘cryo shells’ to temporary storage spots at the pad, and installed one cryo shell that actually turned out to be a million-gallon water tank. On Thursday, SpaceX ‘sleeved’ one of those storage tanks for the first time ever, marking an important milestone towards the activation of a tank farm capable of supporting Starship’s orbital launch debut. Another four sleeves are more or less complete, with the eighth and final sleeve likely just a week or two away from completion.

A fifth GSE tank is also more or less complete, leaving two more to go. However, with some basic math, it’s possible to determine that SpaceX’s orbital launch pad likely only needs five cryogenic tanks (three oxygen, two methane) – and possibly as few as four – to support Starship’s first orbital test flight(s). With SpaceX finally beginning to install tank sleeves, it’s possible that that four or five-tank milestone – and the first tests of SpaceX’s custom, unproven storage solution – are now much closer at hand.