SpaceX’s fourth Starship test tank faces a devastating finale after a “cryosafe” pressure test on Thursday.
SpaceX’s newest starship test tank – SN7.1 – is the second in a series of two planned prototypes, both of which are designed to test the feasibility of using a new steel alloy to build future starships and super heavy boosters. According to CEO Elon Musk, SpaceX is technically adapting its own steel alloy for the production of Starship. However, Musk’s comments and the June 2020 SN7 test results suggest an offshoot of 304L with minor metallurgical improvements.
Prior to the SN7 testing campaign, Musk determined that the primary goal of the new alloy was to reduce the brittleness of Starship tanks and adjacent steel components under cryogenic conditions (i.e., extreme cold). Ultimately, SN7 seemed to confirm that the new alloy̵
SN7.1 follows in the footsteps of SN7 and is much closer to an actual Starship prototype.
“The SN7.1 is much more complex than its sibling and tests a ~ 304L Raptor mount (push puck) and a skirt section. The forces and general conditions that these new parts will be exposed to are vastly different from those that SN7 has been exposed to, meaning there is a likelihood that 304L steel is actually worse in some scenarios.
With a bit of luck, the SN7.1 test campaign, which is slated to begin on September 10th at 9 p.m. CDT (UTC-5), will be a flawless success and prove that SpaceX 301’s new steel alloy is superior for all Starship-related users Applications. In that case, Starship SN8 – the first full prototype of a new alloy – is likely to be fully equipped with a bow cone and collection tanks before acceptance testing begins later this month. “
Teslarati.com – September 10, 2020
For SN7.1, increased ductility could theoretically be a mixed bag. Assuming SpaceX also built the thrust puck out of 304L-contiguous steel, it could get too mushy under the extreme forces it will be subjected to. At full throttle, the thrust of three Raptor engines compresses the thrust puck – a cone roughly the size of a large round table – with the equivalent force of a weight of ~ 600 tons (1.3 million lb).
On September 10, SpaceX put the SN7.1 through its paces and performed a cryogenic proof test using liquid nitrogen (LN2) while the tank was still installed on the simple steel frame that supported it during production and transportation. This simple decision provides a glimpse into the extensive planning that enables SpaceX to optimize speed and efficiency while running successful tests. While SpaceX was able to technically install SN7.1 directly on a brand new launch bracket designed specifically for the anticipated testing, the company instead left the tank on its construction stand – much cheaper and far easier to replace than the former.
Technically, switching straight to the launch bracket would have made the testing process a little easier, but a tank breakage during a routine cryogenic proof test could have severely damaged or destroyed the bracket, which would take weeks of work to build a full replacement. After the SN7.1 successfully completed a cryogenic pressure test on September 10th, SpaceX simply lifted it from the work stand and installed it on a custom-made launch bracket.
SpaceX SN7.1 will be charged again with liquid nitrogen on September 14th at 9 p.m. CDT (UTC-5). This time the tank – after reaching a flight pressure of 7.5 to 8+ bar (110-120 + psi) – is subjected to the simulated thrust of three Raptor engines by a series of hydraulic cylinders. At least two tests are planned based on a public road closures schedule. The first will likely take SN7.1 through a number of Raptor thrust scenarios and profiles under the same tank pressures required for orbital spacecraft flights. If this test is successful, the SpaceX can bring the SN7.1 back to work before deliberately pressurizing the tank until it bursts sometime around September 17th.
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