There is a push for smaller and cheaper satellites, especially with the commercialisation (r)evolution currently going on within the space industry. Cheaper and smaller means a higher level of integration, i.e. increase the efficiency of each component by combining functions.
One such an example is storing propellants inside the satellite structure rather than in a separate propellant tank.
The current project aims at identifying and demonstrating technologies for storing propellants in the (primary) structure of the satellite. This includes propellants for chemical as well as electric propulsion systems in the liquid as well as the gaseous state.
The project on one hand investigates which combination of propellant, propellant carrier and structure benefit most from integration. On the other hand, the influence on the mechanical behaviour, primarily the response of the structure to vibrations, is assessed. This is done experimentally as well as numerically.
The main challenge is to combine the, often contradictory, requirements for propellant storage, expulsion and load carrying elements in a complete and sufficient set of requirements for one (sub-)structure/component fulfilling multiple functions. At the same time, provided solutions should be such that industry may adopt the technology. This requires the solutions to be adaptations of existing designs, rather than revolutionary new approaches to satellite layout and manufacturing.
The benefits can be summarised as follows:
- Reduced total required volume of the satellite as the propellant tank and associated feed lines are no longer required. A certain amount of volume increase of the structure is to be expected though.
- Reduced amount of mass as the function of containing propellant and providing stiffness is now combined in a single element.
- Potential reduction of radiation level inside the satellite due to absorption of certain types of radiation by the propellant. Of course, an important requirement for the propellant is to be unaffected by radiation.
The main product feature is the combination of propellant storage and load carrying functions into a single element. On one hand, this changes the dynamic characteristics of the load carrying parts. It is believed that the propellant can positively contribute to the dynamic features. On the other hand, it makes the propellant tank redundant.
Propellant is stored in the side panels of a satellite. To this end the side panel forms a gas-tight storage container with specially manufactured inner structure to withstand the pressure inside the panel. These inner structures can be best thought of as struts that resemble a three-legged tree on both sides. They are designed in such a way that the maximum load can be transferred with the minimum amount of mass. Hollow glass microspheres (HGMs) are used to limit the pressure the panel is experiencing, while still providing sufficient pressure for the propulsion system. HGMs are tiny spheres (mean diameter of 16 µm) that can be pressurised well over 900 bar. For effective use, pressurisation of up to about 750 bar is sufficient, leaving enough margin. Pressure release is controlled by adjusting the temperature.
The project consists of two phases. In the first phase a detailed analysis is made of possible combinations of propellant, propellant carrier and structure. The approach will be to start from an existing design and investigate possible changes.
A trade-off will be made and the most promising combination will be experimentally investigated in the second phase of the project. Two types of experiments will be conducted: immersion and expulsion tests will verify the principles of the proposed technology. Vibration testing will shed light on the influence on the satellite structure.
Phase 1 of the project is completed. An analysis of potential mission scenarios, suited propellants and structures as well as the structural design was completed. Preliminary charging and discharging tests with HGMs and different gases was performed.
It was concluded that the current state of the art technology is not developed far enough to enable the proposed technology with reasonable financial means. The resulting mass saving is 2% at most. Currently available HGMs do not satisfy degassing requirements that would suit the typical satellite lifetime. For this reason, the project was ended.