A 25 kW Solar-Stirling Concept for Planetary Surface Exploration
- Paper number
IAC-05-C3.P.05
- Author
Dr. Henry Brandhorst, Auburn University, United States
- Year
2005
- Abstract
Modelling studies have shown that the implementation of mitigation guidelines, which aim to reduce the amount of new debris generated on-orbit, is an important requirement of future space activities but may be insufficient to stabilise the near-Earth debris environment. In fact, even if there were no further launches into space it is suspected that the space debris population would continue to increase due to persistent collision activity involving Earth satellites already on-orbit. The role of a variety of mitigation practices in stabilising the environment has been investigated over the last decade, as has the potential of active debris removal (ADR) methods in recent work by Liou and Johnson (2007). However, given the technological, financial and political constraints on the implementation of these schemes it is important to identify effective mitigation and removal strategies, based on reliable, robust criteria and appropriate performance metrics. In previous works, the selection of these has been accomplished on a relatively ad-hoc basis, dependent upon advances in simulation capability, and potentially optimal solutions may still remain unidentified. Consequently, we present a theoretical approach to the analysis of the debris environment that is system-led and aims to address some of the existing limitations in this domain. The new approach is based on the study of networks, composed of vertices and edges (or nodes and links), which describe the dynamic relationships between Earth satellites in the debris system.
The objective is to provide a network representation of the debris environment that can be described using statistical measures and to explore the function of mitigation and removal in this context. Future projections of the 10 cm and larger satellite population in a non-mitigation scenario, conducted with the DAMAGE model, are used to reconstruct a network in which vertices represent satellites involved in collision events and edges encapsulate information about the events, such as the kinetic energy, collision probability and longevity of the collision pair. The network is then quantified using statistical measures such as transitivity (grouping), assortativity (similarity) and betweenness (centrality), providing a numerical baseline for this future projection scenario. Finally, the impact of mitigation strategies and active debris removal, which can be mapped onto the network by altering or removing edges and vertices, can be assessed in terms of the changes from this baseline. The paper introduces the network methodology, highlights the ways in which this approach can be used to formalise criteria for debris mitigation and removal, and then summarises changes to the adopted network that correspond to an increasing stability and changes that represent a decreasing stability of the future debris environment.
In the 1990’s, NASA developed a 25 kW free piston Stirling convertor for testing under the SP-100 space nuclear power program. This convertor was successfully tested before the program ended. The 25 kW convertor consisted of two 12.5 kW convertors connected through the central heat source, resulting in a dumbbell-shaped system. Later the system was disassembled into two separate convertor units. The goal was a system operating at TH of 1050 K and TC of 525 K and a ratio of 2. The convertor was built from Inconel 718 and was operated at a hot end temperature of 650 K and a cold end of 325 K to save time and costs. It was operated for 1500 hrs of essentially unattended operation with efficiency above 25
The purpose of this paper is to propose a new lightweight solar-powered system concept that uses the updated 25 kW convertor, an inflatable Fresnel lens solar concentrator and an updated liquid sheet radiator. Inflatable Fresnel lens concentrators have been produced in a 5 m diameter at a specific mass of 0.5 kg/m2. The new version of the liquid sheet radiator adapted for planetary surfaces is essentially a fountain enclosed in a transparent envelope. The liquid that flows down the inside of this envelope is thick enough to have high optical emissivity for the system. Past studies using silicone oil have shown that a liquid thickness of only 300 micrometers is sufficient to achieve an optical emissivity of 0.85 at a temperature of 373 K. Theoretical calculations indicate further increases in emissivity with temperature up to at least 400 K. One additional characteristic of the liquid sheet radiator concept is that it is exceptionally stable and does not require special machining to achieve its performance. Additional advantages of the Stirling system compared to solar arrays is that it produces alternating current and provides waste heat that can be used for habitat heating. This heat can also supply part of the thermal energy needed for in-situ resource processing. Based on existing data, it appears likely that this system could achieve a specific power equivalent to or better than the best solar arrays of today.
This system represents a novel, updated concept for planetary surface power systems in the 25 kW range. The system can also accept other forms of energy from a laser or a radioisotope heat source for night time use. From the use of lightweight Fresnel lenses and enclosed liquid sheet radiators coupled with recent advances in Stirling convertor technology, a new, space tolerant planetary power system may emerge.
- Abstract document
- Manuscript document
IAC-05-C3.P.05.pdf (🔒 authorized access only).
To get the manuscript, please contact IAF Secretariat.