ESA MLI Testing, Europe

With a Stirling Cryogenics SPC-1 and a CryoFan, the Netherlands based facility of the European Space Agency (ESA) is testing MLI for satellite insulation.

One of ESA’s research activities is to investigate the behavior of reflective foil for satellites insulation, named Multi Layer Insulation or MLI.
These are polyimide or polyester films, coated with aluminum or usually gold for the space industry.
The foils are combined to blankets of 5 to 30 layers, which are attached to the outside of the satellite. Their purpose is to reflect radiation from the sun to prevent heating of the satellite.

ESA requires to test the reflectiveness of these MLI blankets for which they have a vacuum chamber called a Calorimeter in which this can be measured.
In the past this was done by cooling the Calorimeter with liquid nitrogen to approximately 80K. So only at one temperature because ESA wanted to test over a temperature range, as a different cooling concept was required.

Stirling designed a system that flows Helium gas through the MLI heat-exchanger rather than liquid nitrogen. The helium gas is circulated using a CryoFan. Where liquid nitrogen has a limited temperature range between ice and gas, Helium gas can be cooled or heated to any temperature. This allows the MLI to be tested in the specified temperature range from -240 to +60°C.

Cooling is achieved by a Stirling cryogenerator adapted to the requirements of the cooling concept for ESA. The cryogenerator and the Calorimeter have been connected to a cryostat, in which the CryoFan, a heater and control valves for the helium flow where integrated. The control valves act as a thermal mixer tap for water: by directing the circulating gas through the cryogenerator and/or the heater in a certain ratio, any Helium gas temperature gas can be supplied to the Calorimeter.

On the system control screen, the customer can set the required stable temperature or a temperature ramp. The system control will then keep the Calorimeter at the set temperature, or raise or lower the temperature according the set ramp.

In this way, the MLI can be tested throughout the temperature range that it will meet in space under practical conditions.