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Novel Manufacturing Methods for Titanium Tanks and Liners

Dr. W. Radtke* MT Aerospace AG, Augsburg / Germany

Traditionally, titanium tank and liner manufacturing is based on the forging route. Ever increasing tank size requirements force to leave that approach in favour of new methods, which present some cost reduction opportunity, too. Ti-15-3 sheet metal room temperature net-shape counter-roller spin-forming of domes has been qualified for the European ATV all-metal propellant tanks. The principle is under further development for the European Alphabus satellite propellant tanks. With the currently available equipment, spin-forming of domes up to 75 inches diameter is possible. Spin-forming of welded blanks has been shown to be feasible, too. Tank barrels are rounded and welded from sheet metal. For thick-walled domes, a near-net-shape forming method has been developed with Ti-6-4. Conventional EB as well as TIG methods for tank weld integration are applied. A completely over-wrapped liner with an integrally wound CFRP skirt is under development for the Alphabus propellant tanks, aiming at considerable mass reduction as compared to all-titanium tanks. Tank qualification results will be presented with respect to the ATV tanks together with development testing results with a CFRP over-wrapped propellant tank prototype and similar high-pressure vessel test items.

Nomenclature Ti-15-3 = Ti-15V-3Cr-3Al-3Sn ATV = Automated Transfer Vehicle Ti-6-4 = Ti-6Al-4V EB = Electron Beam (Welding) TIG = Tungsten Inert Gas (Welding) CFRP = Carbon Fibre Reinforced Plastic MON = Mixed Oxides of Nitrogen MMH = Monomethylhydrazine MEOP = Maximum Expected Operational Pressure

I. Introduction The traditional titanium tank and liner manufacture route is following the forging method. Here, as compared to

the final geometry a very high amount of rather precious material has to be forged and subsequently machined. Due to forging press force restrictions, this approach is limited to tank size dimensions, which are not longer appropriate to the future demand in satellites and upper stages. These considerations force to look for a new approach in tank and liner manufacturing. To reduce cost and lead time, it pays to leave the special forgings route, too, and to substitute it by a standard mill materials approach. As far as possible, a net-shape forming route shall be followed. Obviously, this implies to master the appropriate sheet metal forming and welding methods. With the steadily increasing demand for mass reduction, for propellant tank applications even high strength titanium alloys are not sacrosanct any more. In order to save mass and to keep the tanks helium tight and compatible to storable propellants as well, the titanium part is restricted to a load-carrying liner over-wrapped with CFRP. As compared to titanium, the carbon fibres exhibit an importantly increased specific strength. By virtue of that property and by an appropriate over-wrap design, the liner stresses and strains and hence the damage tolerance of the liner is set more or less arbitrarily without mass penalty. The paper reports on the actual development and qualification status. ______________ * Head of Manufacturing Development Dept., TFE, wulf.radtke@mt-aerospace.de

42nd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit9 - 12 July 2006, Sacramento, California

AIAA 2006-5269

Copyright © 2006 by MT Aerospace AG. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

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Figure 1. Near-net shape tank domes with EB-welded ports, final machined

II. Near-Net-Shape Manufacture of Domes In order to follow the standard mill products

approach even for tanks with smaller diameter and very thick polar interfaces, the domes have been hot press-formed from Ti-6-4 plate. The port interface, obviously not integral any more as with forged domes, has been machined from bar and electron beam welded to the domes. Non-destructive inspection and final machining led to the desired subassembly.

III. Net-Shape Manufacture of Barrels Ti-6-4 sheet metal is bent elastically or plastically

(depending on the wall thickness and the required diameter) and TIG welded to the barrel. Prior to bending, local thinning of the sheet by machining is an option in order to save mass.

IV. Near-Net-Shape Liners for High-Pressure Vessels

By combination of the above explained domes and barrels, tanks and liners of arbitrary length and diameter may be TIG welded. Once qualified for a special shape, the degree of forming will be independent of the respective tank diameter. Hence any tank diameter different from the qualified one will not need further qualification as it would be the case with forged domes. An appropriate heat treatment leads to the desired mechanical properties and reduces welding related residual stresses. Partial or full CFRP over-wrap imparts the required vessel pressure rating. Depending on the application and some design details, the liner and the over-wrap may have to be adhesively bonded together in order to preclude the liner from buckling. Development status: The liner has been completely CFRP over-wrapped, but not tested yet. Applicability: Pre-stressed high-pressure vessels for He or Xe service with polar mounting (integrally wound CFRP skirt possible), potential replacement of ARIANE 5 GAT and GAM high pressure vessel liners made up to now from grade 250 maraging steel. The titanium liner would be less prone to SSC and present a lightweight solution. The GAT and GAM tanks store hydraulic oil and helium for the high-pressure hydraulic oil supply to the actuators of the booster nozzles and the main engine, respectively.

V. Net-Shape Manufacture of Domes By virtue of its body-centred cubic crystal structure, the high-

strength Ti-15-3 alloy is formable at room temperature. This is required for a net-shape titanium forming approach, because hot forming would compromise precision in shape and thickness, surface quality, and freedom from alpha case. With the hexagonally structured Ti-6-4 alloy, forming at room temperature is impossible. However, due to both the high strength even in the

Figure 2. High-pressure vessel liner, TIG welded from near-net shape domes and sheet metal barrels

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Figure 3. Pre-contour machined flat blank for Alphabus propellant tank domes

annealed condition and the very low Young’s modulus, a very high elastic range results, which makes simple forming operations even with Ti-15-3 not easy to perform. With the help of a special counter-roller spin-forming technique, this obstacle can be circum-navigated successfully. The computer-controlled motion of the roller pair allows for the prescription of the correct meridian path and an appropriate continuous feed, while the work-piece, clamped at its rim, is rotating. The forming process has to be recurred in several steps leading to the incremental shaping of the dome. No forming die is necessary – the control of the dome shape and, in case of need, its correction, is a matter of software modifications only. The counter rollers, acting with opposing force vector on the same spot at the work-piece, superimpose compressive stress and thus enhance room temperature formability. By virtue of the com-

pressive stress superposition, welded blanks may be spin-formed, too. This is useful especially in case of raw material shortage. The rim clamping method means, that the work-piece area enlargement during forming is at the expense of the blank gage. Use is made of a flat disk as blank material. The required final shape of the dome and the mechanical properties of the material under consideration, respectively, directly influence the thickness reduction over the meridian during forming. If this is not commensurate to the required thickness distribution, a so-called pre-contour machining in the flat condition prior to forming allows for the forming related thickness reduction. Machining of the shaped dome is not necessary, expensive clamping jigs are superfluous.

Figure 4. Counter-roller spin-forming of an ATV propellant tank hemisphere

Figure 5. Spin-formed dome, Alphabus propellant tank

The cold forming operation imparts very high residual stresses, which have to be relieved by annealing. The currently available spin-forming machine is restricted to form domes with a diameter of 75 inches (1905 mm). In principle, there is no diameter limit. Hence, tank domes with even larger diameter are formable in this way, too. Once qualified for a special geometry, standardized sheet metal fabrication and forming strategy as well are to scale arbitrarily to the respective tank diameter requirements.

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VI. Net-Shape All-Metal Titanium ATV Tanks

Figure 6. ATV propellant tank

The principle outlined above applies to the eight spherical propellant tanks for storable propellants (MON & MMH) of the European Automated Transfer Vehicle (ATV). This vehicle will enter service next year. Two spin-formed domes are EB welded with a seamless equator ring. The port interfaces and a polar pick-up are EB welded as well. Apart from the domes, all other elements are made of Ti-6-4 alloy. Each tank has a volume of 850 l, a MEOP of 25 bar, a proof pressure of 1.25 x MEOP, and a burst pressure of 1.5 x MEOP. The tank is qualified to the following pressure cycle requirements: 3 x proof pressure, 15 x MEOP, 180 x (18 to 25 bar), 1000 x (18 to 24 bar) with a scatter factor of 4 applied on all cycles. The minimum wall thickness of the net-shape spin-formed domes is 1.2 mm, driven by damage tolerance considerations. The real burst pressure was 48 bar.

VII. Net-Shape Titanium Liners for High-Pressure Vessels

Load carrying liners with minor gage are to fabricate by means of the net-shape approach, too. The barrel, if any, is made of Ti-15-3 as has been explained for barrels of Ti-6-4 above. After machining of the port interfaces from Ti-6-4 plate or bar, they are EB welded into the domes. TIG welding completes the liner. An appropriate heat treatment imparts the desired mechanical properties and reduces the welding related residual stresses. Depending on the ratio of liner thickness and diameter, there may be required an adhesive bond between the liner and the composite over-wrap. Pre-stressing of the system enhances its structural efficiency. A test tank with 660 mm diameter and a liner thickness of 0.7 mm, welded and completely over-wrapped, has been equipped with a fibre-optical sensor system. By virtue of this system, the fibre strains in the thick-walled over-wrap are measured. Testing of this tank is imminent.

Figure 7. High-pressure vessel liner with 660 mm diameter

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VIII. Net-Shape Titanium Liners for Storable Propellant Tanks

Figure 8. Propellant tank liner with a 1310 l volume

Currently, there are two different tanks for storable propellants under development. The first one has an internal cylinder diameter of 1140 mm and is equipped with Cassini-shaped domes. Its volume is 1310 l. The polar interfaces are EB-welded, whereas the hoop welds are TIG welds. The liner is completely over-wrapped. The over-wrap and the liner are bonded together to preclude the latter from buckling during negative pressure loading. The minimum dome thickness is set to 0.7 mm and the minimum barrel thickness to 0.5 mm, respectively. The MEOP is 22 bar, proof pressure is 1.25 x MEOP, burst pressure is 1.5 x MEOP. The tank fixation interface is a CFRP skirt, integrally wound onto the pressure vessel CFRP over-wrap in order to avoid any mismatch. There have been introduced some artificial bonding defects between the thin-walled liner and the over-wrap in order to confirm finite element modelling results on their negligible effect on potential buckling of the liner and to demonstrate the appropriate NDI capabilities to reliably find those defects as well. Pressure cycling including a negative pressure check and a final leak check was successful according to the following steps: 4 x (27.5 bar + 31.6 bar + 35.8 bar + 39.9 bar) + 14 x (22 bar + 25.3 bar + 28.6 bar + 31.9 bar). Up to now, burst test results are not available.

The second tank is identical to the first one in terms of materials, fabrication, and dome shape. However, the Alphabus propellant tank is considerably larger. The liner inner diameter is 1583 mm and the volume, depending on the cylinder section length, is 1610 or 1910 litre. The tank MEOP is 24 bars with the common safety factors. The minimum liner wall thickness is set to 0.7 mm, driven by damage tolerance considerations together with the available NDI capabilities. The preliminary design status is current. The volume of the 1910 litre tank is in the range of launcher upper stage tanks as for example the ARIANE 5 EPS tanks. Hence additional applications are conceivable.

IX. Non-Destructive Inspection and Component Testing

The qualified NDI methods for the liner piece parts are automatic ultrasonic and eddy current inspection. The methods utilized for the welds are x-ray and eddy current inspection. With thicker gage, control of full weld penetration is by ultrasonic inspection. Complex geometry inspection is with eddy current by hand or with the penetrant method. Both ultrasonic and shearography inspection of the adhesive bond between the liner and the over-wrap is state-of-the-art. Ultrasonic inspection applies to the composite, too. For pressure testing, strain gauge and displacement sensor arrangements are current. In special cases, especially for pre-

Figure 9. Wrapped 1310 l propellant tank with integrally wound CFRP skirt

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Figure 10. Alphabus propellant tank with integrally wound CFRP skirt and titanium attachment insets

stressing and proof pressure testing of thick-walled pre-stressed composite high-pressure vessels, acoustic emission and fibre-optic sensors are available.

X. Conclusion The replacement of the traditional forging

route for titanium tank domes and barrels has made considerable progress. Apart from the already qualified all-metal ATV propellant tanks, an important application will be the very large storable composite over-wrapped propellant tanks for the European Alphabus satellites, which are currently under development. Additionally, MT Aerospace proposes the qualification of the described pre-stressed composite pressure vessels.

Acknowledgments

The very helpful support of DLR and ESA is to acknowledge gratefully. The valuable contribution of Deutsche Titan is highly appreciated. The friendly cooperation with EADS and Alcatel is to mention thankfully.