SpaceX Starship test tank survives first two nights of stress testing

SpaceX's newest Starship test tank has survived the first round of stress testing. SN2 - very similar to SN7.1 - is pictured here in March 2020. (NASASpaceflight - bocachicagal)

SpaceX’s newest Starship test tank has survived the first two nights of stress testing, pushing the steel tank one step closer to a destructive finale.

Known as Starship SN7.1, the new tank – aside from one critical difference – is similar to Starship SN2 (pictured above), a full-scale prototype SpaceX repurposed into a test tank in March 2020. SN2 served to test improvements made to the design of Starship’s “thrust puck,” a dense steel cone that must transmit the thrust of three Raptor engines through the rest of the rocket. Much like SN2, SN7.1 is a test tank with a focus on the behavior of Starship’s engine section under extreme loads at cryogenic temperatures.

Unlike SN2, however, SN7.1 is built almost entirely out of a new steel alloy – closer to 304L than the 301 stainless used on all previous prototypes.

SN2, July 2020. (NASASpaceflight – bocachicagal)
Design-wise, SN7.1 is almost identical to SN2. (NASASpaceflight – bocachicagal)

SpaceX rolled the tank to the launch site and pressurized it with cryogenic liquid nitrogen on September 10th as part of a routine “cryo proof” acceptance test. SN7.1 appeared to complete that proof without issue, exhibiting no leaks or unusual behavior, and likely reached pressures of 7.5-8 bar (~110-120 psi) before detanking.

Over the next three days, SpaceX inspected the test tank, relocated it to a more capable (and expensive) test stand, and connected hydraulic rams (used to mechanically simulate engine thrust) to its thrust puck.

While SpaceX never confirmed results, Starship test tank SN7 is believed to have broken pressure records before it burst, a strong sign that the new steel alloy is the superior choice for future prototypes. (NASASpaceflight – bocachicagal

Around midnight on September 15th, SpaceX kicked off the first round of SN7.1 stress testing, repeatedly loading and unloading the tank with liquid nitrogen. While it’s impossible to visually confirm the use of the stand’s hydraulic rams, it’s safe to assume that SpaceX used them to stress SN7.1’s thrust puck while chilled to cryogenic temperatures. The new steel alloy SpaceX is using on SN7.x and prototypes SN8 and beyond is designed to be less brittle at cryogenic temperatures, nominally ensuring that flawed or aged Starship tanks leak before they burst or explode.

Aside from the obvious triple-Raptor thrust simulation, SpaceX likely also simulated thrust from one or two Raptors to verify the new design’s ability to survive asymmetric thrust in engine-out scenarios. Ultimately, SN7.1 made it through the night without obvious issues and there have been no signs of leak-fixing today, suggesting that the tank performed well. SpaceX has a second SN7.1 test period scheduled to begin on September 17th, as well as backups on the 15th, 16th, 20th, and 21st. More likely than not, SN7.1’s next test will end when the tank is intentionally pressurized to failure.

Update: SpaceX has kicked off another night of SN7.1 stress testing, beginning almost as soon as the nine-hour window opened (9pm CDT (UTC-5) on September 15th). As of midnight, the company has already put the test tank through one cycle, rapidly filling and pressurizing it with liquid nitrogen before detanking. It remains to be seen if the company will continue testing this window, which closes at 6am on Wednesday. There is also a chance that SpaceX will intentionally pressurize SN7.1 to failure tonight, although it’s much more likely that the tank will be returned to a cheaper, simpler transport stand rather than risking damage to a new launch mount.

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Eric Ralph: I write about space, among other things.
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