Direct printing of heating elements for thermal control systems of spacecraft

Heaters are a central element of a spacecraft's thermal control system. The state of the art is represented by polyi-mide foil film heaters with a structured copper film. The films resistance defines the thermal power of the heater. But these heaters have inherent limitations. They are limited to a maximum power density of 0.54 W/cm² in still air [1] and 3 W/cm² [2] and have an upper limit temperature for the polyimide material of 260 °C. Other techniques already used in commercial electronics have yet to be applied in space applications but could overcome the inherent limitations mentioned above. Here, an approach is presented which utilizes a process called dispenser printing. For that paste material is applied onto the surface of either of titanium or stainless steel flight propulsion tubing. The electrical insulation between substrate and heater is applied by thermal spraying of an aluminiumoxide layer. This heater printing method was able to produce resistances between 35-935 Ohm, resulting in power densities up to 40 W/cm² under vacuum conditions without active substrate cooling. The measured temperature at the mentioned 40 W/cm² was 330 °C. Different geometries, representing different resistances, were printed onto the tubes for two different paste com-pounds to test the effect of resistivity on the application and contact areas were printed using a paste containing silver and platinum to enable connection to a power source. The heaters were then tested in several conditons. With these tests we were able to investigate achievable power densities and also application failure mechanisms. Potential is seen in this technology, since it promises automatized application of heaters and possibility to expand the limitations of heater applications in the space industry [1, 2]. [1] ESCC Detailed Specification No. 4009/002, Issue 10 [2] S-311-P-079E Procurement Specification for Thermofoil Heater, Rev E