A wholly owned subsidiary of the University of Bristol, NCC is a non-profit organisation that transforms scientific and technical research into industrial impact. With industry guidance from Thales Alenia Space, the organisation has manufactured an all-composites linerless (Type-V) tank demonstrator that is representative of those used for launch vehicle and satellite propellant tanks, delivering an estimated weight saving of 30% compared to conventional metallic tanks in current use. NCC has demonstrated this to engage UK interest in the manufacture of composite tanks for space propulsion needs of the future. This project demonstrates how advanced composite technology can play a vital role on future engineered structures in space propulsion.
Composite materials in space
The overall aim for the SpaceTank project was to develop UK-based research and development tools necessary for the manufacture and inspection of Type V pressure vessels. In turn, the project would position NCC to better support the growth of the UK space launcher and satellite propulsion economy. With an industry that is price-driven by launch mass, and where a 1 kg reduction in weight can return thousands of pounds to the launcher and satellite manufacturer, NCC is showcasing how composites can play a critical role in reducing the structural weight of a tank to help realise lower costs for satellite launch.
Tanks are important in the space industry for holding the many mission-dependent liquids and gasses, such as helium, liquid hydrogen, liquid oxygen and RP-1. Making these tanks as lightweight, strong and durable as possible is integral to successful space missions. The one-year SpaceTank project focused on developing a new manufacturing solution for tanks to hold typical propellants and pressurants.
Some companies are already exploring the use of composite materials in the design of space tanks. NCC cites, for example, Virgin Orbit (United States) and RocketLab (New Zealand), both of which have developed and tested composite tanks as an alternative to metal for their Launcher One and Electron rockets, respectively. Similarly, in Australia, Omni Tanker and its partners are seeking to develop and commercialise a composite liquid hydrogen tank without an internal coating. In Europe, NCC also notes that MT Aerospace is developing materials and manufacturing methods and conducting tests for a new rocket fuel tank. The NCC’s SpaceTank project is thus enabling the United Kingdom to position itself as a key player in this field.
Design and performance features
NCC’s SpaceTank demonstrator is 750 mm long and has a diameter of 450 mm, with a fluid storage capacity of over 96 l.
This SpaceTank has been designed and manufactured with a nominal wall thickness of 4.0 to 5.5 mm, which would allow for the tank to withstand 85 bar of pressured propellant. The carbon composite main body comprises of only 8 kg of material mass, but as with all engineered structures, there are further opportunities with SpaceTank to optimise this weight reduction further. The pressure rating of NCC SpaceTank can be increased with the use of higher strength carbon fibres and by increasing the composite thickness. Alternatively, the SpaceTank could be optimised for weight and cost for much lower pressure applications by using a thinner and lower stiffness composite material.
During the SpaceTank project, NCC’s team developed a method for including the metal fluid valve ports to the washable tool, to remove the need for any secondary assembly and bonding operations needed for the final product. These fluid valve ports were held in the tooling, enabling them to be co-bonded to the carbon composite during the latter stage of the manufacturing process.
“Lightweight fuel tanks and systems will be an important contributor to future Sabre (motor) powered vehicles. Reducing the mass and weight of components is a significant challenge and we are excited about the NCC’s innovative SpaceTank project which has shown the potential benefits of composite propellant tanks for space applications,” stated Richard Varvill, chief technical officer at Reaction Engines, a British company that has worked for decades on an air-breathing rocket engine for spaceplanes and other hypersonic vehicles.
A combination of expertise
For the carbon composite main body, NCC used MTC510 epoxy carbon prepreg (supplied by SHD Composites) in 300 mm tape width. MTC510 is an epoxy resin system that is specifically designed to cure between 80°C and 120°C and is toughened to increase its damage tolerance.
This material was prepared for the SpaceTank manufacturing process by Bindatex who performed narrow width precision slitting of the tape and returned 22,000 m of the material in a 6.35 mm format prepared perfectly in accordance with NCC’s requirement for use in its Coriolis Automated Fibre Placement (AFP) manufacturing cell.
The slit-tape was deposited onto the washable tooling using the tensioned filament winding operation of the Coriolis AFP system. A combination of helical and hoop-winding designed by NCC engineers using patented Material’s Cadwind filament winding software was used to deposit over 24 layers of the material to a nominal 5.5 mm thickness. The thickness and direction/angle of winding used here was for demonstration only; it is possible for NCC to either increase or decrease this composite wall thickness and alter the winding angle and ply structure to fully optimise the tank for a specific pressure or loading requirement.
After the material deposition, NCC’s SpaceTank was inspected for defects and thickness variation. It was then autoclave cured at 100°C and re-inspected afterwards. Ultrasonic C-Scanning and thermography Non-Destructive Test (NDT) inspection techniques were used after curing, and these methods compared and contrasted for suitability of inspecting future tanks for defects such as delamination and porosity. Finally, the internal tooling core was washed out with pressurised cool water to leave the inner tank cavity empty.
The tooling technique for the linerless NCC SpaceTank proved to be difficult, which led to NCC developing a casting process in partnership with AeroConsultants utilising their Aqua water-soluble core material. The finished tool features a centralised male core with a nominal 30 mm wall thickness which is cast in 2 parts and then bonded. The tool has 3 washable internal stiffening rings which were designed and manufactured to help withstand the torsion loading expected during the automated composite layup and the pressure exerted during the autoclave curing.
“Composite tanks are important for us because they have the ability to drastically reduce the cost and weight for future satellite systems. With the NCC helping to increase the performance capabilities of our systems, we can provide the best service possible to the range of governments, institutions and companies who rely on us,” concluded Roger Ward, chief technology officer at Thales Alenia Space.
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