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With a recent decision to switch the Space Launch System (SLS) core from aluminum-lithium to non-lithium alloys, NASA has come full circle on a journey that started nearly twenty years ago with the development of Shuttle’s Super Light Weight External Tank (SLWT). The switch is not just about reducing costs, but it will also prove to be more optimal for the stresses that SLS will endure.
Meanwhile, a Shell Buckling Knockdown Factors (SBKF) project has been running at NASA Langley since 2007, funded by the SLS Advanced Development Office since Q3 FY12.
Launch vehicles need to allow substantial margins to avoid their tanks, intertanks and interstages buckling during launch. Rules for this were set by experiments in the sixties, but the state-of-the-art in analysis and construction has moved on a long way since then.
The engineering team are re-writing the rules for large vehicles via a combination of analysis and experimental verification, leading to a 2011 “can crush” test where they used a million pounds of force to buckle an “External-Tank-like Test Article”, which was 8.4m in diameter and 6.1m tall.
The team has already produced a first draft of their guidelines. In addition, since early 2012, they have been working closely with the SLS team on design of the core.
It is in this light that the SLS program recently reported a wholesale switch from Al-2195 to Al-2219 on the core. This “was based on a trade study that reduced payload mass by 3 t,” – taken from project reserves – “but that will result in approximately $30 million per flight savings.”
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