Evaluation of Different Architectural Concepts for Huge Deployable Solar Arrays for Electric Propelled Space Crafts

Kurzfassung

As the technology for solar electric propulsion has been further matured in the past years, the number of current and planned electric propelled spacecraft is rising. This technology can be applied to different mission scenarios such as: Deep space probes, communication satellites (transferred from a GTO or LEO to a GEO) or even reusable Moon or Mars supply modules. Although electric propulsion requires only a relatively small portion of fuel it does demand a lot of power. For deep space probes or Moon and Mars supply modules constant power supply with some hundred kW and up to 1MW are realistic. State-of-the art solar array concepts could be adapted to such dimensions but the question needs to be asked if these architectures are the most suited for very huge arrays. Moreover, the availability and constant maturing of flexible photovoltaic cells raises the question if membrane based, gossamer structures could be a more weight efficient alternative. Based on current DLR technologies for deployable space structures two concepts for large solar arrays are examined. The analysis is done for a power spectrum of 2:5kW to 500kW and two types of solar cell technologies: thin-film solar cells (TF) applied to a thin membrane substrate and state-of-the-art high performance multi-junction cells (MJ). The first concept is based on the DLR GOSSAMER solar sail architecture while the second is a flexible blanket design similar to the ISS. To evaluate conceptual design changes several sub-configurations are examined as well by use of parametric finite element models. The configurations are analysed regarding their performance parameters specific power (power per array mass), stowage volume efficiency (power per stowed volume) and structural mass ratio (structural, mechanism and residual mass divided by photovoltaic blanket mass). For all configurations a partially dramatic decrease in performance with increasing power and size is observed. While in the small to medium power region (2:5.. 100kW) good performance is achieved for both configurations, in the high power region (100.. 500kW) especially stowage volume efficiency and stowed dimensions become a critical factor. The impact of array size can be observed when comparing both cell technologies. To compensate for the loss in cell efficiency, TF arrays are significantly larger. While in the small power regions performance values to MJ arrays show small differences this diverges strongly with increasing array power and thereby size. In case of structural mass ratio a difference up to a factor of 3 and in stowage volume up to a factor of 5 is observed. Comparing the concepts among each other the flexible blanket design outperforms the solar sail structure. Typical values for a 100kW-array of the best performing sub-configurations using MJ cells are 184W=kg specific mass, structural mass ratio of 0:5 and stowage volume efficiency of 35kW=m3 in case of the solar sail configuration. For the flexible blanket design 273W=kg in specific mass, a structural mass ratio of 0:2 and a stowage volume efficiency of 110kW=m3 is achieved. Furthermore critical components, development needs and recommendations for further enhancement of array performance are identified and a break-even cell efficiency for the thin-film photovoltaic technology is calculated.