The driving goal of the HeHPV programme is the development and qualification of a lightweight, high-performance, reliable yet cost-competitive helium tank for application in future spacecraft programmes. The applied approach enabled qualification via similarity of a volume range of 50-75 l, where the tank diameter is common for the entire range. Leak-before-burst (LBB) and burst testing with dedicated qualification models confirmed predicted failure modes and high margins of safety.
Existing and future market needs show high potential for such tanks, in particular for satellite systems, where helium tanks are required for the pressurisation of chemical propulsion systems.
The key challenges of the HeHPV project was the achievement of a lowest possible tank mass while complying to the stringent ECSS damage tolerance requirements. This was successfully demonstrated via state-of-the-art fracture mechanics assessments including dedicated material testing, as well as LBB testing of a HeHPV with artificial defects to the titanium liner.
Qualification of the “tank family” with volume range of 50-75 l was achieved via selected similarity and scaling of driving loads for tank layout and dedicated testing.
In addition, the applied manufacturing processes were selected to maintain international cost competitiveness while maintaining high quality and reliability.
The qualified tank family offers explicit benefits, in particular for the European space industry market, as all tank components are ITAR-free. The tank mass of approximately 10,8-14,4 kg (including attachment bearings) of the volume range of 50-75 l is absolutely competitive with competing international tank designs yet offers full compliance with all relevant design regulations and standards, such as ESCC, MIL-STD-1522 and AIAA S-081.
In addition, the applied specifications consider enveloping of all relevant launch vehicle loads and environments.
Variation of the tank volume is achieved over the cylindrical portion of the metallic liner, whereas the liner diameter and wall thickness remain unchanged. The overwrap pattern remains identical, as do the remaining manufacturing, inspection and cleaning processes.
The utilised attachment bearings are industry standard, such to enable excellent exchangeability for satellite platform customers.
Despite the low mass and high reliability, the selected manufacturing processes were optimised to enable minimum recurring costs and hence maximise global cost-competitiveness.
The structural layout of the liner and composite overwrap with a nominal MEOP of 310 bar is driven by the need to ensure “leak-before-burst” structural integrity, as is demonstrated via qualification tests with the load cycles of four lives.
A state-of-the-art damage tolerance analysis (DTA) utilising representative material testing as well as leak-before-burst (LBB) testing with a dedicated tank unit – combined with qualified NDI methods - enables minimising the titanium wall thickness to ≤ 0,8 mm for the majority of the liner.
The high-strength, filament-wound carbon fibre composite overwrap carries the majority of the pressure loads and comprises approximately 2/3 of the net tank mass.
The demonstrated burst pressure of 633 bar offers high margin of safety and excellent compliance with performed predictions.
Standard, qualified spherical bearings on both tank ends, one free to slide in axial direction, offer the ideal interface to the customer satellite platform.
The HeHPV (helium high-pressure vessel) consists of a thin-walled (≤ 0,8 mm) titanium (Ti-6Al-4V) liner, composed of two domes and a cylindrical section of variable length. The liner is welded via TIG (Tungsten Inert Gas) state-of-the-art welding methods. The lower dome has an open port for filling and draining.
A filament-wound composite overwrap is applied to the metallic liner to realise a COPV (composite overwrapped pressure vessel), after which an autofrettage sizing step is performed to induce liner compression stresses and reduces the liner tension stresses in all subsequent pressure cycles up to the autofrettage pressure. The liner equivalent plastic yield at autofrettage is limited to avoid a change of the plastic yield in the compression mode after pressure relief, such that the liner behaves linear elastically in all subsequent pressure cycles.
Under the MEOP of 310 bar, the tank enables storage of up to 3,12 kg of gaseous helium or 21,7 kg nitrogen for the 75 l tank size.
The HeHPV project was initiated in the autumn of 2012 through early studies at MT Aerospace in advance of the formal contractual start of the programme within Artes 3-4, beginning with a first design and analysis loop as well as assessments of manufacturing technologies, such as welding and NDI.
The Preliminary Design Review (PDR) was successfully performed in April 2013, followed by CDR in July 2013, after which the design was frozen and respective test models were manufactured and tested.
Upon completion of all qualification testing in 2014, a Qualification Review (QR) was held in 2015 to formally conclude qualification.
Three dedicated development units were manufactured to validate performance and qualify the tank family range. An Engineering Model (EM) was utilised to demonstrate dynamic behaviour and rapid expulsion using thermal control hardware, where the thermal model was calibrated. A leak-before-burst (LBB) with artificial defects demonstrated over 8800 pressure cycles prior predicted, non-catastrophic leakage behaviour. Finally, a qualification model (QM) was tested, including vibration, pressure cycles and final burst pressure.
The qualified tank family is now in series production. To date, a total of 32 tanks have been delivered for the MTG and KARI (65 l HeHPV) and Spacebus 4000 (50 l HeHPV) platforms.
The 65 l version for storage of nitrogen (N2HPV) is moreover under production for application within the ESA Euclid space mission to explore the 'dark Universe'.