• CubeSat
• green propellant
• Hydrogen Peroxide
• monopropellant
• propulsion system
• modular design
• HTP

Within the ever-expanding CubeSat Market, there is a strong drive to develop reliable, possibly green, and most importantly affordable propulsion systems. Today few nearly off-the-shelf European propulsion solutions can satisfy these requirements. The utilization of High Testing Peroxide (HTP) as a monopropellant can offer clear advantages, such as a long heritage for space utilization, relatively easy accessibility, provides a reduction of ground operation costs and has a relatively high volumetric specific impulse. Although over the years its competitor Hydrazine has been the default choice because of its superior performance, the use of Hydrazine is nowadays an additional and often unjustified cost given its complex handling; hence the market is shifting towards greener and more sustainable solutions.
The study describes the work carried out at the University of Pisa in order to develop an Engineering Model of a compact, low-cost and relatively simple CubeSat propulsion system based on HTP. This is a natural continuation of a previous study carried out in collaboration with ESA (CHIPS - CubeSat HTP Innovative Propulsion System) and represents the evolution and optimization of the system, currently at TRL 4-5, towards the development of the final flight model. The propulsion system in question is a H2O2 98% wt. monopropellant blow-down configuration. It operates at a MEOP of 24 bar and is capable of providing up to 0.5N of thrust and Isp greater than 160s, it has a dry mass lower than 3kg and occupies a compact 2U envelope in its largest configuration. It does so by maximizing the usage of the available volume thanks to its innovative cuboid design. The study tackled three main areas: the Positive Expulsion Device (i.e. bladder), the tank geometry and its manufacturing considerations, and finally the integration of the tank with the thruster-feedline assembly. After an initial assessment of the available COTS Bladders, two options were singled out and thus two tank configurations were determined. The former adopting a re-purposed COTS hydraulic accumulator bladder for a 1.5U Tank, and the latter developing a custom designed approach in collaboration with commercial suppliers for a 1U Tank.
One of the main challenges of HTP, is compatibility and reactivity with the wetted materials, therefore, in regards to the tank element itself, particular attention was devoted to the choice of proper materials which consequently fell on Aluminium alloys for the tank, Teflon seals and Fluorine-based plastics and elastomers for the bladder. The propulsion system was designed to allow easy interchangeability of the tank element allowing for different performance/envelope requirements. This was achieved through the design of an external “platform” housing: the feedline, the main propellant fill/drain valve, the electronic control system and, most importantly, the mounting points to the CubeSat chassis. The feedline system was redesigned in order to increase robustness against vibration whilst still allowing for thermal expansion during nominal operation, through the use of appropriate structural constraints. It was also designed with flexibility in mind such as the possibility of adding a RCS System utilizing the same pressurized HTP.