By the end of November 2017, the PW-Sat2 team successfully conducted deployment tests of the deorbit sail in conditions of microgravity and decreased pressure in the Centre of Applied Space Technology and Microgravity in Bremen, Germany. The experiment was carried out as part of the 4th edition of the DropTES program organized by the Centre in cooperation with The United Nations Office for Outer Space Affairs and the German Space Agency (DLR).
The concept of the deorbit sail which is the main experiment on board the PW-Sat2 mission – was developed and tested starting from the beginning of the project in 2013. During this time many prototypes of the sail were designed and hundreds of unfolding tests were carried out. Conditions of microgravity were simulated by suspending the arms of the sail or by unfolding it on a flat surface covered in a thin layer of grease with a low friction coefficient to eliminate the impact of the surface on the movement of the arms. These tests however did not give the team complete certainty that the system will work according to expectations in orbital conditions.
Conducting the sail opening experiment during free-fall from the Drop Tower in conditions of decreased pressure (20 Pa) had important meaning for the PW-Sat2 team because it allowed for final verification of the efficiency of the sail opening system before delivering it to Earth orbit.
For the purpose of the experiment a 1:1 model of the PW-Sat2 satellite has been assembled. Due to sail opening dynamics and it’s dimensions, the model was dropped from a height of 8 meters in the lower, wider part of the tower, in the so called deceleration chamber. This gave around 1,2 seconds of free-fall. Due to such short time of the drop, the team had to give up on testing of the final sail opening system (based on burning of Dyneema wire). A faster approach was used – after detection of the freefall by the accelerometer placed on the model of the satellite and counting down 430 ms, power to an electromagnet holding the folded sail inside it’s tray has been cut off. This was followed by ejecting the sail from the tray on a conical spring in the same configuration as on the flight model of the PW-Sat2 satellite.
During the first week of tests the PW-Sat2 team in cooperation with the Centre of Applied Space Technology and Microgravity developed a system for ejecting the satellite model from the tower. After conducting many experiments it has been decided to place it on a moving supporting structure which was retracted from underneath the model a moment before free-fall. The next step was placing four high speed cameras inside the tower in such a way to cover the maximum length of the free-fall distance. The students also unfolded a special coating with a linear scale in the background which will help them verify the mathematical analysis with the test results.
Proper sail deployment tests were carried out in the second week of the experiment. Students had the possibility to carry out 4 drops from the tower. Each time the satellite model was placed on a supporting structure suspended 8 metres above ground. This was followed by pumping out the air from the deceleration chamber to a pressure of around 20 Pa. During all four drops the cameras registered complete deployment of the sail, which removed all doubt. The sail deployment system is effective in conditions of microgravity and reduced pressure.
Three accelerometers were placed on the PW-Sat2 satellite model designed for Drop Tower tests in order to deliver the acceleration values from all 3 axes during the drop.
The diagram below presents the accelerometer values during one of the drops. Just before the drop the Z-axis (vertical) accelerometer shows a value of -1, after that the satellite is released and accelerations on all axes drop to 0. After counting down 430 ms the sail is released from the tray, ejected on the spring and unfolded. After nearly a second the satellite comes to a stall on the protective mesh.
Videos from the cameras allow also for determining the angular speed with which the satellite rotates at the moment of sail deployment. This data will allow the students further analysis of the movement of the sail and it’s effect on the CubeSat 2U type satellite in a given configuration. The students want to check if the current sail deployment system will be useful for a sail of bigger dimensions.
The experiment carried out in the Drop Tower has proved the effectiveness of the sail deployment system used in conditions which are close to those that are present on Low Earth Orbit. The next time the sail will be opened will be in orbit around Earth and this time the tests will focus on the impact of it’s area on the time of deorbit of the Cubesat 2U satellite.
Translated by Tomek Rybarski. Thanks a lot for your help!