A comprehensive investigation has been undertaken to study the effects that hypervelocity impact of micrometeoroids may have on GEO solar generators, in particular, whether and under which conditions permanent short-circuits may be induced by impact-generated plasma at the impact location. Highly instrumented experiments have been performed using a light-gas gun at Fraunhofer EMI and a plasma-dynamic accelerator at Technische Universität München. Astrium GmbH built solar array samples were set-up under representative electrical conditions for test. Unique test data has been acquired and the team succeeded in identifying the threshold conditions for impact-induced arcing and interpreting the underlying failure mechanism.
The objective of this project is to study and test the susceptibility of modern solar array designs to micrometeoroid and space debris impact. This includes:
- Establishing understanding of failure mechanisms and influencing factors,
- Experimentally simulating hypervelocity impacts under various conditions on operated test samples representing state-of-the-art solar arrays for high-power telecommunication platforms,
- Determine threshold conditions above which impact induced permanent short circuits may occur,
- Define appropriate design approaches to mitigate the effects of micrometeoroid impacts.
Picture: Impact plasma and temporary sustained discharges at a solar panel sample perforated from its rear side by a 1.6 mm particle impacting with 7.3 km/s.
The key issue of this project was to evaluate the characteristics and effects of the impact generated plasma on an operating solar array and to quantify the related failure onset conditions.
The main challenges of the project were to experimentally reproduce impact induced failures on solar arrays on ground on the one hand. This required considering all aspects relevant to the failure process as well as their interactions. On the other hand, it was demanding to gain an experimental insight into the failure mechanism due to the temporally and spatially limitation of impact plasma related processes.
The project yield an understanding of impact induced discharges on solar arrays and determined the failure onset conditions. This provides a benefit for solar array designers by identifying and understanding causative processes of power degradations on orbit. This finally allows for:
- Quantifying the risk posed by micrometeoroids and space debris impacts on solar arrays in GEO.
- Defining appropriate mitigation measures to improve solar array robustness against impact effects while at the same time avoiding unnecessary design limitations.
The project substantiated that severe damages by impact induced discharges are only to be expected for current-voltage conditions that significantly exceed the existing threshold levels of electrostatic induced sustained discharges. Therefore, the power generating network was found to be robust against impacts, if ESD safe levels are included in its design. However, a potential risks was found for the transfer harness bundles on solar array rear side. Impact protection measures have been identified and respective technology development activities are recommended.
In-depth theoretical analyses have been performed to identify relevant failure scenarios and conditions for testing. This included a solar array design review, orbit environment simulations and hypervelocity impact effects evaluation.
More than 60 hypervelocity impact experiments have been performed using the fastest two-stage light gas gun at Fraunhofer EMI and the plasma-dynamic accelerator located at the Technische Universität München. Those facilities complement each other with regard to the projectile size from the micron to the millimetre range, while both provide hypervelocity capabilities that are unique in Europe.
Representative solar array test samples have been designed and manufactured at Astrium in Ottobrunn. Components and layout represent state-of-the-art solar arrays of European GEO telecommunication platforms. This includes GaAs multi-junction cells and modern substrate technologies.
Representative operational conditions have been established by a dedicated test set-up, which simulated the electrical output and dynamic behaviour of a solar panel at test sample level. In addition, high vacuum conditions have been established in the target chambers of the accelerators in order to allow for neglecting plasma interactions with the residual atmosphere.
Comprehensive diagnostics have been implemented in the test set-up to monitor sample operational parameters and acquire data of impact processes and effects for scientific exploitation. In view of the importance of the impact plasma for the failure processes, special plasma diagnostics have been applied, comprising emission spectroscopy, charge yield measurements, electrical plasma probing and high-speed imaging.
The contract was concluded at the beginning of 2010, the test campaigns accomplished at the end of 2011 and the final deliveries completed and the final review held in September 2012.
Within that time period the following work packages and accompanied milestone meetings have been completed:
WP2000: Literature/technology review & assessment of failure mechanisms
WP3000: Sample design and test plan definition
- Test plan review
WP4000: Sample manufacturing & test apparatus preparation
- Test readiness review
WP5000: Test execution
- Test review board
WP6000: Test results evaluation
- Final review
The project has ended. All study objectives have been successfully achieved:
- More than 60 hypervelocity impact experiments have been performed under representative conditions.
- Comprehensive diagnostic data on impact plasma and failure processes have been acquired and evaluated.
- An understanding of impact induced failure mechanism has been established.
- Threshold conditions above which permanent failures may occur have been determined.
- The criticality of impact effects on GEO solar generators has been identified and quantified.
- Mitigation measures against impact effects have been proposed.