Enabling & Support
09/03/2026
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The next generation of European launchers will demand lighter, stronger and more cost-effective designs. A recent ESA Discovery project led by TWI Limited and ArianeGroup has demonstrated a promising welding technique that could help meet these challenges, enabling the construction of internally stiffened cryogenic fuel tanks from advanced aluminium alloys.
As ArianeGroup’s launchers evolve towards reusability and increased flexibility, they require advanced materials and new tank designs, driving a shift from traditional smooth-panel cryogenic tanks to internally stiffened structures. These advanced designs, with the help of high-performance aluminium-copper-lithium (Al-Cu-Li) alloys, could reduce the dry mass of fuel tanks by 15–20%. However, the benefits of these lightweight alloys have been limited by manufacturing challenges.
“Traditional machining is costly and inflexible, whilst conventional fusion welding methods are prone to solidification defects in Al-Cu-Li alloys,” explains Dr Pedro de Sousa Santos, Principal Project Engineer at TWI Limited. “We needed to find an alternative approach.”
A solid-state solution
Internally stiffened structure using refill FSSW
The answer may lie in Refill Friction Stir Spot Welding (refill FSSW), a solid-state welding process that joins materials without melting them. Unlike traditional welding, which liquefies metal and may introduce solidification defects, refill FSSW uses friction and pressure to create strong bonds at temperatures below the melting point.
The process works by plunging a rotating tool into the materials to be joined. The friction generates heat that softens (but doesn’t melt) the metal, whilst the rotation stirs and mixes the plasticised material from both sheets together. As the tool withdraws, its independently moving components work together to refill the hole with this stirred material, creating a forged weld with a flush surface that requires no additional finishing. This automated process is energy-efficient, produces no added weight, and avoids the defects associated with fusion welding.
Preliminary studies on small-scale demonstrators had shown promise, but crucial questions remained. Could refill FSSW work with Al-Cu-Li alloys while also placing a cryogenic sealant layer between the metal sheets? And would the resulting joints meet the stringent requirements of cryogenic fuel storage?
Meeting the challenge
The ‘Development of Refill Friction Stir Spot Welding (RFSSW) for skin-stringer-frame stiffening of Al-CuLi cryogenic tanks‘ project set out to answer these questions. The team joined flat-sheet test samples of AA2050-T84 aluminium alloy with a cryogenic-compatible sealant interlayer, systematically testing different welding parameters to identify optimal conditions.
The welded joints underwent rigorous inspection using X-ray computed tomography and ultrasonic testing to identify any internal defects in the weld. To assess whether the joints could meet the stringent requirements of cryogenic fuel storage, samples underwent mechanical testing under tensile shear and cross tension loading conditions at room temperature as well as cryogenic temperatures (77 K and 110 K).
“After meticulously testing various welding configurations, TWI established a reliable operating window and repair capability that stood up to rigorous assessment,” says Dorick Ballat-Durand, R&T Project Manager for Metallic Materials at ArianeGroup. “The structural integrity tests showed the weld strength exceeded preliminary requirements derived from conventional fusion weld standards. While sealant management and tool service life posed some challenges, TWI was able to assure us that these would be actively addressed for future improvements. ArianeGroup congratulates the teams for such high-quality work and innovation.”
“The support from ESA Discovery and Preparation has enabled us to demonstrate feasibility and establish the fundamental building blocks for using refill FSSW in cryogenic products for space applications,” says Dr Pedro de Sousa Santos. “We’ve fulfilled the requirements for Technology Readiness Level 3.”
Building on success
The project has laid important groundwork, but further development remains necessary before refill FSSW can be implemented in flight hardware. Future work will need to address fatigue endurance, stress corrosion performance, and the influence of different welding tool profiles on quality. Testing of representative sub-scale or full-scale demonstrators under realistic loading conditions will be essential.
“TWI has gained valuable insight into the requirements for successfully deploying refill FSSW with a cryogenic sealant interlayer,” notes Dr Pedro de Sousa Santos. “This knowledge will help us support customers from various industry sectors in adopting the refill FSSW process for their products.”
The path from laboratory demonstration to operational launcher hardware is long, but this project has taken an important first step. By proving that refill FSSW can create reliable joints in advanced aluminium alloys at cryogenic temperatures, the team has opened the door to more efficient, cost-effective manufacturing of future launch vehicles.
“This research addresses one of the key challenges for becoming more competitive in the launcher market,” says João Gandra, ESA lead on the project and R&D coordinator for Materials, Manufacturing and Assembly. “We need cost-effective ways to build lighter, stronger tanks and structures, and understanding how new welding techniques perform is an essential step towards that goal.”
The project was submitted through ESA’s Open Space Innovation Platform, which seeks out promising new ideas for space research, and was funded by the Discovery element of ESA’s Basic Activities.
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