During the EBEX2013 long duration flight the payload was free to rotate in azimuth. The observed azimuth motion consisted of a superposition of full rotations with a period of 10—30 min and oscillatory motion with an amplitude of tens of degrees, average period of 79 s, and period dispersion of 12 s. We interpret the full rotations as induced by slow rotations of the balloon and the shorter period oscillatory motion as due to torsional oscillations of the flight line. We derive the torsional stiffness of the flight line using the bifilar pendulum model and apply it to the flight line of the EBEX2013 payload. We find a torsional spring constant of 36 kg m 2 /s 2 corresponding to a period of 58 s. We conclude that the bifilar model, which accounts for the geometry of the flight line but neglects all material properties, predicts a stiffness and period that are 45% larger and 25% shorter than those observed. It is useful to have a simple, easy to use, coarse approximation for the torsional constant of the flight line.
Bibliographical noteFunding Information:
Support for the development and flight of the EBEX instrument was provided by NASA grants NNX12AD50G, NNX13AE49G, NNX08AG40G, and NNG05GE62G, and by NSF grants AST-0705134 and ANT-0944513. We acknowledge support from the Italian INFN INDARK Initiative. We also acknowledge support by the Canada Space Agency, the Canada Research Chairs Program, the Natural Sciences and Engineering Research Council of Canada, the Canadian Institute for Advanced Research, the Minnesota Supercomputing Institute, the National Energy Research Scientific Computing Center, the Minnesota and Rhode Island Space Grant Consortia, our collaborating institutions, and Sigma Xi the Scientific Research Society. Didier acknowledges a NASA NESSF fellowship NNX11AL15H We very much thank Danny Ball and his colleagues at the Columbia Scientific Balloon Facility for their dedicated support of the EBEX program.
- Bifilar pendulum
- Gondola motion