Nonisothermal Concentrator-Absorber System Performances for Solar Thermal Propulsion

The study examined solar thermal propulsion (STP) with nonisothermal concentrator-absorber system (CAS) as a power source with high energy efficiency. We used radial-type nonisothermal absorbers, where current temperature of propulsive mass (hydrogen) heating corresponds to radiant flux distribution in the focal light spot of parabolic mirror. We analyzed the thermal processes in the radial absorber of maximum nonisothermal type assuming no radial overflowing warmth, which is justified for mirrors with a diameter of more than 10 m. The study gives the results of numerical integration of absorber radius temperature distribution equation in the differential form for various accuracy parameter values. Moreover, we developed an algorithm for iterative calculation of hydrogen outlet temperature of concentrator-absorber system, determining the specific impulse of the engine. The paper presents the flowchart of the iterative calculation. We used numerical integration data for stating the regress relations, determining the nonisothermal CAS efficiency and updating the existing relations at high temperatures and accuracy parameter values, expedient for high-energy inter-orbital maneuvers. The study determined CAS rational performances in relation to the "solar" upper stage of "Soyuz-2-1b" launcher with the STP use in LEO-to-GEO mission within 60 days. Findings of the research show that if we select optimum performances of the extreme nonisothermal CAS in ranges of the concentrator accuracy parameter Δα= 0,8...1,1° and absorber temperature 3200...3400 K, the payload mass approaches to 2600 kg. At permissible decrease in payload mass (3...4%) the CAS rational characteristics correspond to accuracy parameter Δα = 1,3...1,5° and heating temperature 2800...3000 K; thus requirements to the CAS and its orientation conditions to the Sun are simplified. In this case the permissible angle to the Sun tracking corresponds to 2...2,2° and can be realized rather simple by the state-of-the-art technology. The chosen CAS performances make it possible to provide ballistic efficiency of the solar upper stage with such STP up to 20% higher compared to double-staged CAS use. As compared with the singlestaged STP, the payload mass increase up to 1000 kg, and exceeds modern liquid propulsion efficiency up to 1500 kg.

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