Patent Number: 063114763
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to FIG. 1 of the drawings, a solar thermal engine for propelling and powering a spacecraft constructed in accordance with the teachings of a preferred embodiment of the present invention is generally indicated by reference numeral 10. Engine 10 is shown to include a movable thermal shield 11 and a housing 12 having an optical cavity 14, a heat exchanger 16, a propellant manifold or annulus 18, a plurality of static power converters 20, a heat pipe evaporator 22, a plurality of heat pipe condensers 23, a propellant nozzle 24 and an electrical energy storage device 26. Optical cavity 14 is operable for receiving the concentrated beam of sunlight 30 through an opening 32 from a mirror assembly (not shown) and converting it into ambient thermal energy. The term "ambient thermal energy" is used herein to designate thermal energy that has been converted from solar energy for immediate use in either propelling or powering a spacecraft. In this regard, ambient thermal energy is distinct from stored thermal energy (i.e., thermal energy that is stored and accumulated in a thermal energy storage device and removed as necessary for use in propelling and/or powering a spacecraft). Heat exchanger 16 surrounds optical cavity 14 and is thermally coupled to propellant annulus 18 and the plurality of static power converters 20. Propellant annulus 18 is formed between an inner pressure boundary wall 16a and an outer pressure boundary wall 16b. Propellant annulus 18 includes at least one fluid conduit 36 that for distributing a flow of propellant 38 through heat exchanger 16. In the particular embodiment illustrated, fluid conduit 36 is a spiral groove formed in propellant inner pressure wall 16a. Propellant annulus 18 is selectively operable in a heating mode wherein a flow of propellant 38 is introduced to propellant annulus 18 and directed through fluid conduit 36 to permit at least a first portion of the ambient thermal energy to be transmitted to the flow of propellant 38. Suitable propellants include hydrogen, ammonia or methane, but other types of propellants may also be used. The heated flow of propellant 38 is then directed to the converging portion of a converging/diverging rocket nozzle 24. The flow of propellant 38 is then expanded and accelerated through the converging/diverging rocket nozzle 24 to provide thrust. Those skilled in the art will understand that most or nearly all of the ambient thermal energy may be consumed in the heating of the flow of propellant 38 depending on the rate with which the propellant flows through the propellant annulus 18. Ambient thermal energy not consumed in the heating of the flow of propellant 38 is directed through heat exchanger 16 to the plurality of static power converters 20 for use in generating electric power. In the particular embodiment illustrated, the plurality of static power converters 20 are arranged in a single cylindrical array that is concentric with and spaced radially outwardly from the propellant annulus 18. This arrangement provides a thermal energy flow path that maximizes the amount of ambient thermal energy that may be transmitted to the propellant annulus 18 while still providing a highly efficient thermal energy flow path to the plurality of static power converters 20. The plurality of static power converters 20 may include any suitable device that converts thermal energy into electrical energy such as alkali metal thermoelectric converters, thermoelectric power converters and thermophotovoltic converters. Preferably, the plurality of static power converters 20 includes thermionic converters as these devices are operational at the elevated temperatures which are expected to be generated by the optical cavity 14, thereby reducing concerns for overheating the plurality of static power converters 20 during the times at which the propellant is not being heated. In the embodiment illustrated, static power converters 20 are thermionic converters having an emitter 20a that is coupled to the outer pressure boundary wall 16b and a collector 20b that is coupled to a cold wall 22a that is adjacent heat pipe evaporator 22. The portion of the ambient thermal energy that is not consumed to either heat the propellant or generate electrical energy is received by heat pipe evaporator 22 where it can be transmitted to the plurality of heat pipe condensers 23 and rejected by engine 10. Alternatively, heat pipe evaporator 22 may be configured to redistributed heat in a desired manner. For example, the residual portion of the ambient thermal energy may be used in a cascaded manner wherein it is used, for example, to preheat propellant or as a source of thermal energy for another type of static power converter that operates using lower temperature thermal energy. A layer of insulation 42 is disposed between the heat pipe evaporator 22 and the outer wall 44 of the housing 12 to ensure that the conversion of ambient thermal energy to electrical energy takes place at a relatively efficient rate. Movable thermal shield 11 is coupled to housing 12 and movable between a first position wherein thermal shield 11 is adjacent housing 12 and a second position wherein thermal shield 11 is spaced apart from housing 12. Positioning of thermal shield 11 in the first position permits thermal shield 11 to insulate heat pipe condensers 23 to thereby retard or inhibit the ability of heat pipe condensers 23 to reject heat, permitting solar thermal engine 10 to conserve heat when performing operations such as heating the flow of propellant 38. Positioning of thermal shield 11 in the second position permits heat pipe condensers 23 to reject heat in an unincumbered manner, as when solar energy is being used by solar thermal engine 10 only for the production of electrical energy. Electrical energy produced by the plurality of static power converters 20 is directed to an electrical energy storage device 26 for subsequent use. This configuration is highly advantageous over systems that store heat for subsequent conversion to electrical energy due to the relative inefficiency with which thermal energy is converted to electrical energy. Consequently, the configuration of the present invention permits a substantial reduction in the mass of the engine 10 as compared to engines that store thermal energy for use in propelling and powering a craft. Preferably, the electrical energy storage device 26 includes an electrochemical device 50, such as a lithium ion battery. The electrical energy storage device 26 may additionally or alternatively include an electromechanical storage device 52, such as a flywheel. One suitable flywheel device is disclosed in U.S. Pat. No. 4,668,885 entitled "Flywheel Energy Storage Device", the disclosure of which is hereby incorporated by reference as if fully set forth herein. Electromechanical storage devices having flywheels may be advantageously employed to impart rotational energy to a spacecraft to aid in the repositioning of the spacecraft in a known manner. While the plurality of static power converters 20 have been described thus far as being arranged in a cylindrical array that is generally concentric with the optical cavity 14, those skilled in the art will appreciate that the invention, in its broader aspects, may be constructed somewhat differently. For example, the plurality of static power converters 20 may be arranged in several arrays as shown in FIGS. 2 and 3. In this arrangement, the engine 10' is shown to include a plurality of static power converters 20 that are arranged in a plurality of cylindrical arrays 70 with each of the plurality of cylindrical arrays 70 being disposed radially outwardly of the propellant annulus 18. Construction in this manner provides a thermal energy flow path that maximizes the amount of ambient thermal energy that may be transmitted to the propellant annulus 18 while providing a slightly less efficient thermal energy flow path to the plurality of static power converters 20 as compared to the arrangement illustrated in FIG. 1. This arrangement is advantageously employed, for example, where less ambient thermal energy is needed to operate the static power converters 20, as when alkali metal thermoelectric converters, thermoelectric power converters or thermophotovoltic converters are employed. Another arrangement of the static power converters 20 is illustrated in FIGS. 4 and 5. In this arrangement, engine 10" is similar to engine 10' except that the plurality of static power converters 20 are arranged in a plurality of cylindrical arrays 80 with each of the plurality of cylindrical arrays 80 being disposed radially inwardly of the propellant annulus 18. Construction in this manner provides a thermal energy flow path that maximizes the amount of ambient thermal energy that may be transmitted to the plurality of static power converters 20 while still providing a highly efficient thermal energy flow path to the propellant annulus 18. While the invention has been described in the specification and illustrated in the drawings with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as defined in the claims. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out this invention, but that the invention will include any embodiments falling within the foregoing description and the appended claims.