Marcus Young

An Overview of Advanced Concepts for Near-Space Systems

A brief review of both near-term and far-term platforms proposed for near-space operations is given. The primary focus of the paper is, however, a review of potential advanced propulsion systems for such long-duration near-space platforms. The basic requirements for near-space propulsion systems are defined. Low Reynolds number propellers, the current workhorse, are used as a baseline for comparison. Two broad classifications are identified as potential sources of force in near space: rarefied gas and electric propulsion. Radiometric force propulsion systems, the only candidate propulsion systems found in the open literature, suffer from both significant uncertainty in their underlying physics and from significant operational difficulties. Thermal transpiration propulsion systems were shown fundamentally incapable of providing the required performance. Air-breathing electric propulsion systems for long-duration near-space missions will be significantly different than their in-space counterparts with specific impulses likely under 100s. Electrohydrodynamics propulsion systems show some promise, but have thus far shown limited thrust efficiency at sea level operation, and the efficiency is only predicted to get lower at higher altitudes. The potential effects of systems based on breakthrough physics are also qualitatively discussed. All of the identified potential advanced propulsion concepts for long-duration near-space operations suffer from major technological challenges with significant advancements required for any of them to be viable.

Molten Boron Phase-Change Thermal Energy Storage to Augment Solar Thermal Propulsion Systems

Abstract : Solar thermal propulsion offers a unique combination of high thrust and high specific impulse levels that can provide competitive advantages relative to traditional satellite propulsion systems. In order to enhance the functionality of this technology, thermal storage combined with a means of thermal-to-electric conversion is suggested, with the idea of providing a dual-mode power and propulsion system based on thermal energy. A system including boron phase change material for storing energy, an insulating containment system consisting of boron nitride, carbon bonded carbon fiber, and vacuum gap insulation is proposed, with thermophotovoltaics used for electrical conversion. A laboratory solar concentration system has been constructed and experiments to directly heat small quantities of boron have begun, so that the nature and challenges of this system can be evaluated.

Phase-Change Thermal Energy Storage and Conversion: Development and Analysis for Solar Thermal Propulsion

Abstract : Solar thermal propulsion offers a unique combination of high thrust and high specific impulse that can provide competitive advantages relative to traditional satellite propulsion systems. Enhancing the functionality of this technology will require a robust thermal energy storage method that can be combined with thermophotovoltaic thermal-electric conversion. This combination creates a high performance dual mode power and propulsion system that can eliminate the traditional photovoltaic-battery combination on existing satellites. A thermal energy storage system based on the phase change of molten elemental materials is proposed as the enabling technology. Molten boron is identified as the optimal phase change material (PCM), but presents significant engineering challenges. Thus, molten silicon is proposed as a near term, moderate performance storage option. A systems level comparison against existing technologies shows that both materials present a performance benefit with current technological benchmarks, and with optimistic future assumptions, it appears that a more than 40 % _V improvement over chemical system is possible from boron based STP while maintaining high satellite maneuverability. An ongoing experimental effort is focused on producing a proof of concept thermal energy storage system. Materials testing has determined the stability of boron nitride in the presence of molten silicon in the short term, and solar furnace testing has resulted in silicon melting for the first time. Testing of the solar furnace using copper as a surrogate PCM has revealed experimental concerns with PCM heat transfer rates and has resulted in a design for a new full scale solar furnace. This furnace will operate at scales that are relevant to spacecraft development.

Molten Boron Phase-Change Thermal Energy Storage: Containment and Applicability to Microsatellites

Abstract : Latent heat thermal energy storage systems promise nearly constant temperature operation and greater energy storage densities than sensible heat energy storage systems, but they are not yet commonly used in practice due to limitations in material degradation and heat transfer rates. Systems employing particular elemental materials with high melting temperatures appear to overcome these limitations, yielding significant performance increases, particularly in bimodal (thermal and electric) solar thermal power systems. A review of candidate materials has concluded that silicon is an excellent candidate for near term, moderate performance systems, while boron, the primary focus of this paper, is an excellent candidate material for future high-temperature, high performance systems suitable for advanced microsatellite solar thermal propulsion and power systems. General considerations for systems employing such materials have been identified, the required support technologies, including high temperature thermal insulation and thermal to electric power conversion, have been evaluated, and a preliminary design for a general system has been completed. Several potential applications have been identified for this technology; one of them, a solar thermal power and propulsion unit for a 100kg microsatellite, will be described in this paper. The preliminary analysis indicates that such a bimodal system would enable large delta V maneuvers for 100kg microsatellites while also producing the required electrical power. A solar thermal test facility for further evaluating such systems is described along with initial results from the build-up phase of the facility.

W-Band Free-Space Dielectric Material Property Measurement Techniques for Beamed Energy Applications

Free-space techniques in W-band (75-110 GHz) using a vector network analyzer and millimeter-wave lenses are applied at room temperature to measure the intrinsic electromagnetic properties of materials, including dielectric constants and dielectric loss factors to characterize millimeter-wave absorption and related electromagnetic interactions with the material. The measurement techniques from the dielectric property measurement experiments will be performed at a range of higher temperatures (up to 2300K) to identify materials of interest for scaled beamed energy heat exchanger measurements and to provide inputs into a coupled analytical model for a scaleable design of a millimeter-wave absorbing heat exchanger for high temperature aerospace and industrial beamed energy processes.

Numerical Analysis of a Single Microchannel Within a High-Temperature Hydrogen Heat Exchanger for Beamed Energy Propulsion Applications

The requirement that the propellants used in launch vehicle systems must also provide the thermal energy to be converted to kinetic energy in the rocket nozzle has plagued rocket designers since the dawn of the space age. Beamed propulsion systems, however, avoid this constraint by placing the energy source on the ground and transmitting the energy to the spacecraft via microwaves. This computational work evaluates three different heat exchanger channel designs for use in a beam propulsion spacecraft. The working fluid was hydrogen and the input energy was 1.3 kW. The increase in axial temperature along the 0.1 m long channel was as high as 2000 K. In addition, it was found that despite the very small diameter of the minichannels, 1 mm, each design produced extreme temperature gradients across the channel cross section. These temperature gradients affected the velocity profile; the maximum velocity was not located at the channel center.© 2013 ASME

Experimental Investigation of Latent Heat Thermal Energy Storage for Bi-Modal Solar Thermal Propulsion

Abstract : A bi-modal solar thermal system capable of providing propulsive and electric power to a spacecraft has been identified as a promising architecture for microsatellites requiring a substantial _V . The use of a high performance thermal energy storage medium is the enabling technology for such a configuration and previous solar thermal studies have suggested the use of high temperature phase change materials (PCMs) such as silicon and boron. To date, developmental constraints and a lack of knowledge have prevented the inclusion of these materials in solar thermal designs and analysis has remained at the conceptual stage. It is the focus of this ongoing research effort to experimentally investigate using both silicon and boron as high temperature PCMs and enable a bi-modal system design which can dramatically increase the operating envelope for microsatellites. This paper discusses the current progress of a continued experimental investigation into a molten silicon based thermal energy storage system. Using a newly operational solar furnace facility, silicon samples have been melted and results indicate that volumetric expansion during freezing will be the primary difficulty in using silicon as a PCM. Further experimental studies using different materials and test section fill factors have identified potentially reliable experimental conditions at the expense of energy storage density. In addition to conducting experiments, a concurrent computational effort is underway to produce representative models of the experimental system. The current models generally follow experimental results; however, difficulties still remain in determining high temperature material properties and material interactions. This paper also discusses the future direction of this research effort including modeling improvements, analysis of convective coupling with phase change energy storage and potential facility improvements.

Thermal Modeling and Performance Measurements of Radiometric Arrays for Near Space Propulsion

Abstract : A propulsion system based on an array of radiometer vanes has been proposed for wind disturbance correction on near space vehicles. The radiometric force is produced by a vane due to a temperature gradient between a surface heated by insolation and a convectively cooled surface. Previous research has identified several key parameters that might affect the thrust produced by a radiometer array. These parameters include the heat transfer mechanisms on the individual vanes and between nearby vanes in an array, the proximity or spacing of radiometer vanes in an operational array, the effects of the vanes attached to a near space vehicle operating in the boundary layer near the surface of the vehicle, and the effects of attaching the vanes to a substrate on the vehicle itself. A combined numerical and experimental approach was used to investigate these effects. Numerical codes were used to assess the heat transfer mechanisms and the boundary layer flow. Experiments were designed and carried out to measure the force produced by a small array of radiometers. These results were combined to assess the performance of a notional array of radiometer vanes for the purpose of counteracting drag on a near space vehicle operating at an altitude of 60 km. The array performance was assessed based on the maximum force that can be produced as a function of array mass and area.

Wind compensation by radiometer arrays in high altitude propulsion

Numerical analysis has been conducted to assess the feasibility of using radiometer arrays mounted on a near-space vehicle, for wind disturbance compensation. The results indicate the possibility of using radiometric force for that purpose for altitudes of 80 km and smaller, and head winds up to 30 m/s.