DEVELOPMENT of ALUMINIUM-TITANIUM TUBE JOINTS for SPACE PROPULSION SYSTEMS

Spacecraft propellant tanks require high levels of mechanical properties coupled with low mass, and are therefore manufactured using Ti-6Al-4V.  However, the ESA Directive “Design for Demise” has been introduced, which states: “What goes up must completely burn up – or ‘ablate’ – during uncontrolled atmospheric re-entry as a means of post-mission disposal”.  Although Ti-6Al-4V is an excellent material for the manufacturing of propellant tanks, titanium has a melting point of around 1600°C which is the same order of magnitude as the maximum temperature reached by the spacecraft during re-entry.  Therefore there is the danger that during uncontrolled re-entry, parts of the spacecraft including the propellant tanks may survive.

An alternative material to Titanium for manufacturing propellant tanks is aluminium, some of which have already been used for this purpose.  For example, the external oxygen tank of the Space Shuttle was manufactured from the Al-Li alloy AA2195.  If Aluminium propellant tanks are successfully introduced in space propulsion systems, it will be necessary to interface such tanks with the tubing and pipework surrounding the tank.  As such pipework will still be made from titanium alloys, usually Ti-6Al-4V or Ti3Al2.5V, it is necessary to consider how to join an aluminium tank with titanium tubing.

An ESA GSTP funded activity was started in 2011 together with Omnidea (Portugal), Omnidea-RTG (Germany), IWS (Germany), and TWI (UK), to focus on the problem of developing an Al-Ti tube-to-tube interface.  This paper will present the results of the Rotary Friction Welding activity which has resulted in the successful development of two tube-to-tube joints (¼" and ½").  The methodology used during the development programme will be presented together with selected data from manufactured samples.  Finally some data will be presented on a pre-qualification activity of the welded joints.