SpaceX has destroyed the third Starship ‘test tank’ prototype since it began building and testing the stout steel structures earlier this year.
Much like the third test tank SpaceX built was fashioned out of in-work hardware originally destined to become the full-scale Starship SN2 prototype, the latest tank was built with parts that may have initially been meant for Starship SN7. While SN7’s purpose – stress-testing new designs and manufacturing techniques – was largely identical to its predecessors, the test tank was by far the most radical departure from past hardware yet. The reason: SN7 was built entirely out of a different stainless steel alloy.
Deemed 304L, the type of steel is still readily available off the shelf and only 10-20% more expensive than the 301 alloy SpaceX has used to build all Starship prototypes up to SN7. The biggest change it brings to the table is improved ductility (malleability), particularly at the cryogenic temperatures Starship’s tanks will often be held at. By reducing brittleness, Starships built out of 304L steel should be able to fail far more gracefully by developing stable leaks instead of violently decompressing. In fact, the very same test tank destroyed on June 23rd demonstrated that capability perfectly when it sprung a leak during its first pressure test on June 15th.
During its first cryogenic pressure test with liquid nitrogen, SpaceX CEO Elon Musk revealed that the SN7 test tank managed to reach 7.6 bar (~110 psi) before it began to leak – technically satisfactory for orbital Starship launches with an industry-standard 25% safety factor. Thanks to the general flexibility of steel, including the new 304L alloy SN7 was built with, SpaceX was able to simply repair the leak it identified, readying the test tank for a second cryogenic pressure test barely a week later.
The tank’s second test was all but identical to the first up to the end, where its lower dome appeared to more or less unzip from the steel ring it was welded to. It remains to be seen if SN7 was able to beat its previous record during the second test but the failure mode was quite a bit different from any previous test tank, meaning that SpaceX has gathered useful new data regardless. If the 304L steel tank matched or beat a 301 tank’s current record of ~8.5 bar (~125 psi), SpaceX will very likely build all future Starship tanks out of the material.
According to Musk, another 304L test tank is already in the works and should be able to take SN7’s place very shortly. At the same time, SpaceX appears to finally be ready to roll the next full-scale Starship prototype (SN5) to the launch pad as early as June 24th to kick off cryogenic proof testing, Raptor static fires, and (hopefully) flight tests.
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