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On Tue, 7 Aug 2007 11:48:50 -0500, "Neil Gould"
wrote in : Recently, Larry Dighera posted: [snip] I see what you mean. Unfortunately, the highest power requirements of aircraft engines are during the takeoff and climb phases of flight. Power requirements are even greater when the ambient temperature rises resulting in less air density or a higher density altitude. That is when the most power is required for takeoff, but that would be a situation where the Stirling engine would have its minimum power production. If an engine's minimum power production is greater than the power required for takeoff, would it matter? Probably not, but it would mean you'd have significantly more power available at altitude if the Sterling engine were sized to provide takeoff power at high density altitudes. What I was getting at was the author of the articles emphasis on overcoming the reduced power output of IC engines at lower atmospheric pressure overlooks its possibly anemic performance (due to minimal air movement through the heat exchanger and higher ambient temperatures on the ground) when it is needed most, at takeoff. I find it revealing that the author failed to mention that point, and it reduces my confidence in the assertions he made in that article. It would seem that if this could be achieved, the operating conditions of the Stirling engine would be mostly understressed. I am unable to infer your meaning by that statement. Do you mean under emphasized or less mechanical stress on the engine, or what? I would also like to see a comparison of the efficiencies of IC and EC engines and their relative weight and size per horsepower compared. Unlike electrical motors, that must be constructed with heavy iron, IC and EC engines can be constructed of lighter materials like aluminum, but electrical motors are usually 80% to 95% efficient. With the Stirling aircraft engine there is a requirement for what I would imagine would be a large heat sink or heat exchanger located in the slip stream. The weight of this heat exchanger and its drag penalty must also be considered. Why couldn't the heat exchanger be an integral part of the airframe? Wings come to mind... ;-) I'm thinking there would be necessity for some means of conducting the heat from the engine to a remote heat exchanger, and the resulting complexity and weight increase would negatively impact the potential advantages of a Stirling aviation engine. In any event, in addition to the Stirling engine and its fuel, a heat exchanger of some type needs to factored into the weight, cost, performance, and efficiency equations. There might be one advantage to using Sterling external combustion engines for aviation: the use of atomic energy as a fuel source if the weight of the lead shielding were not too great. Imagine an aircraft that effectively never runs out of fuel! There'd be no more fuel exhaustion mishaps. One downside would be the hazardous materials that could be dispersed in a crash. There are a lot of down sides to atomic power, but NASA uses it to power Stirling engines in space. Here's some information about what NASA successfully has accomplished with nuclear power: http://www.grc.nasa.gov/WWW/tmsb/index.html The Thermo-Mechanical Systems Branch (5490) is responsible for planning, conducting and directing research and technology development to advance the state-of-the-art in a variety of thermal systems for space, aerospace, as well as non-aerospace applications. The systems of interest include thermal energy conversion for power systems and solar thermal propulsion systems. The effort involves working at the component level to develop the technology, the subsystem level to verify the performance of the technology, and the system level to ensure that the appropriate system level impact is achieved with the integrated technology. System analysis is used to identify high-impact technology areas, define the critical aspects of the technology that need to be developed, and characterize the system level impact of the technology. Specific technology areas of interest include: Dynamic Power Systems: Brayton, Rankine and Stirling Convertors, Solar Receivers and Thermal Energy Storage Primary Solar Concentrators: Thin film, SRP and Rigid Secondary Solar Concentrators: Refractive and Reflective Thermal Management: Radiators, Electronics Packaging, and Heat Pipe Technology http://www.grc.nasa.gov/WWW/tmsb/stirling.html Animation of a 55 We Stirling TDC (click on image to view) http://www.grc.nasa.gov/WWW/tmsb/sti...adisotope.html AVAILABLE TODAY FOR TOMORROW'S NEEDS NASA Glenn Research Center and the Department of Energy (DOE) are developing a Stirling convertor for an advanced radioisotope power system to provide spacecraft on-board electric power for NASA deep space missions. Stirling is being evaluated as an alternative to replace Radioisotope Thermoelectric Generators (RTGs) with a high-efficiency power source. The efficiency of the Stirling system, in excess of 20%, will reduce the necessary isotope inventory by a factor of at least 3 compared to RTGs. Stirling is the most developed convertor option of the advanced power concepts under consideration [1,2]. http://www.grc.nasa.gov/WWW/tmsb/sti...ng_bckgrd.html However, about this time NASA became interested in development of free-piston Stirling engines for space power applications. These engines use helium as the working fluid, drive linear alternators to produce electricity and are hermetically sealed. These 12.5 kWe per cylinder engines were intended for use with a nuclear reactor power system; the Space Demonstrator Engine (or SPDE) was the earliest 12.5 kWe per cylinder engine that was designed, built and tested by MTI. A later engine of this size, the Component Test Power Convertor (or CTPC), used a "Starfish" heat-pipe heater head, instead of the pumped-loop used by the SPDE. Recently, in the 1992-93 time period, this work was terminated due to the termination of the related SP-100 nuclear power system work and NASA's new emphasis on "better, faster, cheaper" systems and missions. http://www.spacedaily.com/news/outerplanets-00a2.html Europa Orbiter was replanned to use a new "Sterling" nuclear generator design which would use less plutonium http://www.cndyorks.gn.apc.org/yspac...heed_offer.htm Boeing, Lockheed Offer NASA Two Choices for Nuclear Power |
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Recently, Larry Dighera posted:
On Tue, 7 Aug 2007 11:48:50 -0500, "Neil Gould" wrote: Recently, Larry Dighera posted: [snip] I see what you mean. Unfortunately, the highest power requirements of aircraft engines are during the takeoff and climb phases of flight. Power requirements are even greater when the ambient temperature rises resulting in less air density or a higher density altitude. That is when the most power is required for takeoff, but that would be a situation where the Stirling engine would have its minimum power production. If an engine's minimum power production is greater than the power required for takeoff, would it matter? Probably not, but it would mean you'd have significantly more power available at altitude if the Sterling engine were sized to provide takeoff power at high density altitudes. Exactly, but I don't see that as a negative... ;-) What I was getting at was the author of the articles emphasis on overcoming the reduced power output of IC engines at lower atmospheric pressure overlooks its possibly anemic performance (due to minimal air movement through the heat exchanger and higher ambient temperatures on the ground) when it is needed most, at takeoff. I find it revealing that the author failed to mention that point, and it reduces my confidence in the assertions he made in that article. I understand your perspective, which is what prompted my reply. If there is sufficient power to take off, then the issue should be moot, unless I'm overlooking something. It should be reasonable to presume that any practical aircraft engine would have sufficient power to take off, right? ;-) It would seem that if this could be achieved, the operating conditions of the Stirling engine would be mostly understressed. I am unable to infer your meaning by that statement. Do you mean under emphasized or less mechanical stress on the engine, or what? Less mechanical stress due to operating well below maximum power settings under normal cruise. That should provide plenty of reserve power at altitude and increase the fuel efficiency as well. I would also like to see a comparison of the efficiencies of IC and EC engines and their relative weight and size per horsepower compared. Unlike electrical motors, that must be constructed with heavy iron, IC and EC engines can be constructed of lighter materials like aluminum, but electrical motors are usually 80% to 95% efficient. With the Stirling aircraft engine there is a requirement for what I would imagine would be a large heat sink or heat exchanger located in the slip stream. The weight of this heat exchanger and its drag penalty must also be considered. Why couldn't the heat exchanger be an integral part of the airframe? Wings come to mind... ;-) I'm thinking there would be necessity for some means of conducting the heat from the engine to a remote heat exchanger, and the resulting complexity and weight increase would negatively impact the potential advantages of a Stirling aviation engine. In any event, in addition to the Stirling engine and its fuel, a heat exchanger of some type needs to factored into the weight, cost, performance, and efficiency equations. Of course, but I don't see a lot of reason why that couldn't be incorporated into the overall design. My point is that heat exchangers need not be heavy, and could probably double as structural and/or aerodynamic components, further reducing (and possibly enhancing) their impact. There might be one advantage to using Sterling external combustion engines for aviation: the use of atomic energy as a fuel source if the weight of the lead shielding were not too great. Imagine an aircraft that effectively never runs out of fuel! There'd be no more fuel exhaustion mishaps. One downside would be the hazardous materials that could be dispersed in a crash. There are a lot of down sides to atomic power, but NASA uses it to power Stirling engines in space. Understandable, but their expectation is that catastrophic destruction would disperse the nuclear material harmlessly. That can't be presumed for light aircraft. Neil |
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On Tue, 07 Aug 2007 17:51:27 GMT, "Neil Gould"
wrote in : Recently, Larry Dighera posted: [...] I'm thinking there would be necessity for some means of conducting the heat from the engine to a remote heat exchanger, and the resulting complexity and weight increase would negatively impact the potential advantages of a Stirling aviation engine. In any event, in addition to the Stirling engine and its fuel, a heat exchanger of some type needs to factored into the weight, cost, performance, and efficiency equations. Of course, but I don't see a lot of reason why that couldn't be incorporated into the overall design. My point is that heat exchangers need not be heavy, and could probably double as structural and/or aerodynamic components, further reducing (and possibly enhancing) their impact. How would you get the heat from the Stirling engine to the heat sink? If you use liquid coolant, it would be heavy and prone to leaks. There might be one advantage to using Sterling external combustion engines for aviation: the use of atomic energy as a fuel source if the weight of the lead shielding were not too great. Imagine an aircraft that effectively never runs out of fuel! There'd be no more fuel exhaustion mishaps. One downside would be the hazardous materials that could be dispersed in a crash. There are a lot of down sides to atomic power, but NASA uses it to power Stirling engines in space. Understandable, but their expectation is that catastrophic destruction would disperse the nuclear material harmlessly. That can't be presumed for light aircraft. If the rocket detonated in the atmosphere, it might not be so harmless. I would guess the reactor is jacketed with sufficient strength to preclude its destruction. Presumably, that could be done for a Stirling aircraft engine also. |
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Recently, Larry Dighera posted:
On Tue, 07 Aug 2007 17:51:27 GMT, "Neil Gould" wrote in : Recently, Larry Dighera posted: [...] I'm thinking there would be necessity for some means of conducting the heat from the engine to a remote heat exchanger, and the resulting complexity and weight increase would negatively impact the potential advantages of a Stirling aviation engine. In any event, in addition to the Stirling engine and its fuel, a heat exchanger of some type needs to factored into the weight, cost, performance, and efficiency equations. Of course, but I don't see a lot of reason why that couldn't be incorporated into the overall design. My point is that heat exchangers need not be heavy, and could probably double as structural and/or aerodynamic components, further reducing (and possibly enhancing) their impact. How would you get the heat from the Stirling engine to the heat sink? If you use liquid coolant, it would be heavy and prone to leaks. I'm not a Stirling engine designer, so I can't answer that factually. I have been reading up on it a bit since the article was referenced in this thread, but I haven't seen such things as the required rate of dissipation for the engine to work efficiently. If the heat sink needs to be large and close to the engine, perhaps a design where the engine is mounted on or even incorporated into the wing is a way to go. There might be one advantage to using Sterling external combustion engines for aviation: the use of atomic energy as a fuel source if the weight of the lead shielding were not too great. Imagine an aircraft that effectively never runs out of fuel! There'd be no more fuel exhaustion mishaps. One downside would be the hazardous materials that could be dispersed in a crash. There are a lot of down sides to atomic power, but NASA uses it to power Stirling engines in space. Understandable, but their expectation is that catastrophic destruction would disperse the nuclear material harmlessly. That can't be presumed for light aircraft. If the rocket detonated in the atmosphere, it might not be so harmless. I don't see why it would be nearly as bad as a "dirty bomb" would be, as the material would be dispersed over a pretty large area. I would guess the reactor is jacketed with sufficient strength to preclude its destruction. My guess is that NASA et al are just hoping for good fortune. Having a reactor land from orbit intact in the middle of a city wouldn't be all that desirable. ;-) So, my bet is on there being no good plan for dealing with such a catastrophe *other* than wide dispersal of the nuclear material or the luck of landing in the ocean. Not that *that* outcome is desirable either... Neil |
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