NASA has been developing large ultra-lightweight structures commonly referred to as Gossamer space structures for many years to reduce launch costs and to exploit the unique capabilities of particular concepts. For instance, dish antennas are currently being pursued because they can be inflated in space to sizes as large as 30 meters and then rigidized to enable high data rate communications.
Another example of a Gossamer structure is a solar sail that provides a cost effective source of propellantless propulsion. Solar sails span very large areas to capture momentum energy from photons and use it to propel a spacecraft. The thrust of a solar sail, though small, is continuous and acts for the life of the mission without the need for propellant. Recent advances in materials and ultra-lightweight Gossamer structures have enabled a host of useful space exploration missions utilizing solar sail propulsion.
Testing of solar sails on the ground presented engineers with three major challenges:
- Measurements on large area surfaces thinner than paper;
- Air mass loading under ambient conditions was significant, thus requiring in-vacuum tests;
- High modal density required partitioning of the surface into manageable areas.
Laser vibrometry has proven to be a critical sensing technology for validating the dynamical characteristics of these Gossamer structures, due to its precision, range, and non-contacting (zero-mass loading) nature.
The team of ATK Space Systems, SRS Technologies, and NASA Langley Research Center, under the direction of the NASA In-Space Propulsion Office (ISP), has developed and evaluated a scalable solar sail configuration (Figure 1) to address NASA’s future space propulsion needs. A Polytec Scanning Laser Vibrometer system (PSV-400) was the main instrument used to measure the vibration modes. The laser scan head was placed inside a pressurized canister to protect it from the vacuum environment. The canister had a window port from which the laser exited, and a forced air cooling system prevented overheating.
A Scanning Mirror System (SMS) was developed and implemented that allowed full-field measurements of the sail from distances in excess of 60 meters within the vacuum chamber. The SMS was mounted near the top of the vacuum chamber facility and centered over the test article, while the vibrometer head was mounted above the door frame of one of the large chamber doors. The SMS contained a stationary mirror that reflected the Polytec laser beam to a system of two orthogonal active mirrors. These mirrors were used to scan the surface of the sail to find retro-reflective targets previously attached to the sail surface. These targets were essential to getting a good return signal and overcoming the specular nature of the reflective sail surface. A specially developed target tracking algorithm enabled automatic centering of the laser beam on each retro-reflective target.
The initial laser system alignment, target tracking process, and entire data acquisition procedure were automated using the Microsoft Visual Basic (VB) programming language. Polytec’s VB Engine and PolyFileAccess allowed the program to control all the functional capability of the Polytec system. This fully automated test procedure was considered critical, since many tests could take over 5 hours to run.