Excitation of Sail Motion
The baseline excitation method for the solar sail dynamics test used an electro-magnet mounted at each sail membrane quadrant corner near the mast tip (2 magnets per sail quadrant), for a total of 8 magnets. A side view of the mounting fixture is shown in Figure 2. The magnet is mounted on a vertical translation stage with a linear actuator for precise, remote in-vacuum positioning of the magnet.
Most of the dynamics testing effort was focused on getting the best quality data possible on a single quadrant in-vacuum. The quadrant that had the most pristine sail membrane surface with few flaws was selected. The quadrant test used only the magnets on the quadrant of interest for stimulating the dynamics. The quadrant test was followed by a full sail system test, in which one corner magnet on each quadrant is driven simultaneously.
This technique allowed for adequate excitation of the entire sail system and for the identification of major system level vibration modes. To reduce test time, the full sail system test only measured 5 sail membrane locations per quadrant and two mast tip measurements per mast. Since the test article configuration did not change from the quadrant tests to full sail system tests, the high spatial resolution quadrant test results with 44 measurements per quadrant could be compared with the lower spatial resolution system test results with only 5 measurements per quadrant.
Solar Sail Dynamics
The 1st fundamental system mode of the solar sail identified was a “Pin Wheel Mode” with all quadrants rocking in-phase (Figure 3) at a frequency of 0.5 Hz. In this mode all the mast tips are twisting in-phase and the quadrants follow the motion by rocking and pivoting about the quadrant centerline. The 1st sail membrane mode, that has low mast participation, is a breathing mode (Figure 4) at 0.69 Hz. In this mode, the sail quadrant undergoes 1st bending through its centerline. Other higher order sail dominant modes were also found in which the long edge of the quadrant is in 1st bending, but the centerline undergoes either 2nd or 3rd order bending. These test results are important for updating structural analytical models that can be used to predict the on-orbit performance of the solar sail, free of gravity, to aid in further design iterations.
Laser vibrometry was successfully used to identify the fundamental solar sail system modes for structural model correlation. Also, higher order sail membrane modes were identified through a combination of many tests on each quadrant. The methodology described in this article is being further utilized for other Gossamer test programs, such as the antenna technology development program to validate large space based communication antennas.