SLS takes on new buckling standards, drops Super Light alloy
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.
SLS Core Material:
In 1998 – after a challenging four year development program that kick-started NASA’s knowledge about Al-Li alloys – STS-91 flew the first SLWT.
This tank had been re-engineered to use Al-2195 and Al-2090 extensively, which were stronger and lighter than the Al alloys on LWT (Al-2219, etc).
Together with the first use of an orthogrid structure – on the LH2 tank’s barrel – this lightened the structure and improved payload, especially to the high-inclination orbit of the International Space Station (ISS).
However, those new alloys were also more brittle and difficult to weld, and experience showed a maintenance overhead. All of the dome and ogive sections were reverted back to aluminum over three subsequent revisions – first flown on STS -116, -119, and -130 respectively.
At the same time, SLWT experience allowed the LH2 orthogrid to be further optimized and lightened from STS-119.
But the Space Shuttle Program (SSP) wasn’t finished with problems with Al-Li – the intertank stringers which failed during STS-133 tanking and had to be reinforced, were Al-2090.
Notably, SLS will move completely away from the stringer design for its intertank, which carries much higher loads supporting a larger LO2 tank, an upper stage, payload and PLF – all at higher G’s, and with greater aero and bending loads than the ET.
The new design will have integrally machined stiffeners instead of riveted sheet metal stringers.
Similarly, the components which failed during a recent pressure test of the Exploration Flight Test -1 (EFT-1) Orion flight article were also Al-2195.
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.”
L2 sources confirm that Al-2195’s brittleness was the limiting factor when trying to beef up the structure for SLS.
Orthogrids – or possibly isogrid (as shown in the image to the left) – on the tank barrel are machined from flat plates, leaving stiffening ribs that ideally are tall (for strength, or more accurately, stiffness), allowing them to be thin (for lightness).
However, the ET was already using the thickest plate that could survive being formed to the tank’s 8.4m diameter.
Al-2219 is less brittle, so they can use thicker plate which can be reliably formed, and the thicker orthogrid actually results in a lighter structure overall.
Information also notes that the SBKF project has also been doing preliminary work on a new alloy. AL-2050 adds magnesium for an Al-Mg-Li mix, and “is already used extensively in several commercial aircraft”. This promises plates and orthogrids three times thicker than Al-2195 – up to six inches – with weight savings of as much as 20-30 percent.
This effort has been spun off into a Small Business Innovation Research project “to investigate material properties and structural design optimization for heavy lift LV cryotanks”.
It is possible that one day this could become the basis of a Super Light Weight upgrade to SLS.
(Images: Via NASA, TerraBuilder, Inc and L2 content from L2′s SLS specific L2 section, which includes, presentations, videos, graphics and internal – interactive with actual SLS engineers – updates on the SLS and HLV, available on no other site.)
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