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Linear Friction Welding Tabs to Satellite Fuel Tanks

Introduction

TWI has been leading a new programme in partnership with Airbus UK and ArianeGroup, funded by the European Space Agency (ESA). The main objective of the programme is to lower the cost of manufacturing large structures for spacecraft and launch vehicles.

Complex critical aerospace structures are often machined from solid plates or forgings. Consequently, any external features like reinforcement ribs, bosses or mounting brackets expand the envelope of the structure, significantly increasing the raw material costs. This is especially important when considering large structures like launch modules or satellite fuel tanks. Designers may choose to attach such features using mainstream joining techniques (e.g. fusion welding, mechanical fastening or adhesive bonding). However, these processes will either add weight or require increasing joint thickness to compensate for an inherent local reduction of material strength.

ESA sought to explore if friction welding could be a more suitable alternative, as this family of processes is renowned for producing high quality joints, with low distortion and while minimising damage to the parent material structure.

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Satellite Fuel Tank Construction

One of the solutions under development regards the connection of tabs on satellite fuel tanks by Linear Friction Welding (LFW). Airbus UK is currently working on Eurostar NEO as part of the next generation of telecommunication platforms. This platform will incorporate a new propulsion system based on a cluster of two large tanks (Figure 1) for storing MMH (Monomethyl Hydrazine) and  MON (Mixed Oxides of Nitrogen). Each tank will have a maximum volume of 1630 litres and an external diameter of approximately 1.4m.

Propellant tanks for satellites are usually made of Ti6Al4V. Most tanks comprise two domes and a cylindrical section (often comprising multiple segments), often joined by electron beam welding.

These propellant tanks are integrated in the satellite structure using an interface of tabs arranged either in a polar, equatorial or pedestal configuration, depending on the requirements. Equatorial interfaces are the most complex in term of manufacturing, as they involve machining an integral ring comprising all tabs from a solid forged billet. This ring is subsequently electron beam welded to the tank dome (Figure 2). Consequently, equatorial interfaces require a dedicated forming and welding process, in addition to those necessary for producing the tank domes and cylinders.

The Eurostar NEOSAT propellant tank has been proposed as a relevant case study to demonstrate this novel concept (Figure 3). This is a large tank equipped with an equatorial skirt produced via the traditional route.

Figure 1. Next Generation Eurostar NEO Telecommunication Platform (full chemical diagram, part of the service module)
Figure 1. Next Generation Eurostar NEO Telecommunication Platform (full chemical diagram, part of the service module)
Figure 2. EBW end skirt lugs to a generic satellite propellant tank
Figure 2. EBW end skirt lugs to a generic satellite propellant tank
Figure 3. Tank end-skirt mounting interface used in the Eurostar NEOSAT design (dome section)
Figure 3. Tank end-skirt mounting interface used in the Eurostar NEOSAT design (dome section)

Benefits of Linear Friction Welding

The current manufacturing route is an expensive and time-consuming process. However, using LFW to join independent tabs machined from stock plate is an attractive alternative with four main advantages:

  • Improved buy-to-flight ratio, as dedicated forging for the tab ring will no longer be required. Instead, independent tabs will be machined from stock plate and then individually friction welded in place. This creates substantial machining and cost saving, while avoiding the lead times associated with forging procurement.
  • Improved joint performance due to replacing fusion welding with a solid state joining process, which will potentially allow for better weld quality, reduced distortion and lower residual stresses.
  • Ability to tailor grain direction of connection tab according to load path (instead of being fixed to a grain orientation in a forging).

First Technology Demonstrator and Next Steps

TWI has successfully produced the first technology demonstrator for this application. Three triangular tab pre-forms were joined to a representative full size section of the tank dome (Figures 4-6). Experiments proved that it is possible to prevent impingement of the weld flash onto the dome surface or the adjacent tabs. A section of the final assembly was also machined to obtain the finished tab and wall thickness of the dome.

The two-year programme still has further work to complete before it finishes in the first quarter of 2020. Preliminary structural design criteria will need to be completed following tensile, fracture toughness, fatigue and stress corrosion cracking testing. This testing will aid the life cycle estimation. An economic assessment will also follow. TWI is currently benchmarking strategies of machining the weld flash at the interface of the tab and tank dome.

This programme is classed by ESA as a basic technology research programme, aimed at developing new technologies or processes up to technology readiness level 3, according to (TRL3).

To find out more, please contact the programme manager, Dr João Gandra, Principal Project Leader at the Friction and Forge Processes section at TWI.

Figure 4. LFW demonstrator in Ti6Al4V showing the addition of multiple connection tabs to a full-scale fuel tank dome section - general view
Figure 4. LFW demonstrator in Ti6Al4V showing the addition of multiple connection tabs to a full-scale fuel tank dome section - general view
Figure 5. Interface between dome and tab
Figure 5. Interface between dome and tab
Figure 6. Individual tab in as-welded condition
Figure 6. Individual tab in as-welded condition
Avatar João Gandra Principal Project Leader – Friction and Forge Processes

João specialises in friction welding processes, including Friction Stir Welding. His current role is to support TWI Member companies seeking to adopt these technologies to manufacture new or existing products. He acts as a consultant during product development, design-for-manufacturing, prototyping, technology transfer and continuous improvement. Most of his experience was gathered in the aerospace, rail and automotive sectors. Before joining TWI, João completed a PhD in Manufacturing and Industrial Management at the Technical University of Lisbon, where he also worked as part-time lecturer and researcher. He has published over 20 peer-reviewed publications and conference papers, actively participating in international standards committees like the ISO 25239 for Friction Stir Welding.

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