TyMoore - 21/11/2006 4:18 PMI didn't know about the Federal Law Mandating highly enriched cores--I just assumed that the use of 90-95% Enriched Uranium was more engineering necessity to get the most watts/kg of mass than the Law.Interesting...
Marcus - 21/11/2006 3:59 PMQuoteAvron - 11/1/2006 7:29 PMI have two issue, that someone could provide some insights...
Avron - 11/1/2006 7:29 PMI have two issue, that someone could provide some insights...
Carl G - 6/11/2006 7:30 PMWhoa, old thread, but this is a good one. This would be an amazing ship to build.
James (Lockheed) - 27/11/2006 6:30 PMSure, but those days of this being possible, due to budgets, are over.
vanilla - 27/11/2006 8:09 PMQuoteJames (Lockheed) - 27/11/2006 6:30 PMSure, but those days of this being possible, due to budgets, are over.If the AG-NEP vehicle comes out to be a lower overall cost than a chemical or a nuclear thermal option, then I don't see why that would have to be the case. Such an total cost analysis should also include the costs of microgravity adaptation research and countermeasures that would not need to be done if the crew could get the gravity they need for health.
Chris Bergin - 9/1/2006 10:45 AMWe're placing another Powerpoint presentation on to the download section - this is from slightly earlier (2002) but has good explantatory background.58 pages.
SteveMick - 21/11/2006 2:15 PMThe numbers you used for solar cell power density are way off. Triple junction PV for use with concentrated sunlight made by the "Solarex" co. currently approach 1KW/kg. The concentrator can easily have a factor of 10KW/kg. and as a result, mass goes up very little as sunlight intensity drops as Mars is approached. Since this tech is developed and at least two orders of magnitude cheaper, I am puzzled as to why NEP would ever be considered for this role. Also, a solar electric rocket can operate as a solar thermal rocket to acheive Earth escape from LEO much faster. It really is the best of both worlds and has other advantages besides. The concentrator mirrors can double as communication and/or radar antennas and the intense heat at the concentrator's focus can be used for direct ISRU. Please use this as the "competition" for NEP and not a straw man system. I think you'll come to agree that NEP is impractical or at least inferior for Mars transit.Steve
SteveMick - 29/11/2006 2:47 PMConcentrator cells are a real breakthrough and by using the concentrators for solar thermal and antenna duty magnifies this advantage.
SteveMick - 29/11/2006 2:47 PMWere you using concentrator type cells from Spectrolab and if so, at what concentration factor? What kind of concentrator did you use - one from L'Garde? No one argues that traditional non-concentrating type arrays have a mass that is closer to NEP they are so relatively massive. Concentrator cells are a real breakthrough and by using the concentrators for solar thermal and antenna duty magnifies this advantage. Steve
vanilla - 29/11/2006 6:51 PMQuoteSteveMick - 29/11/2006 2:47 PMConcentrator cells are a real breakthrough and by using the concentrators for solar thermal and antenna duty magnifies this advantage.Using concentrators doesn't do anything about the basic problem of thermal rejection. If you want to generate 4 MW of power, and you've got 33% efficient cells, assuming everything else is perfect, you need to collect and focus 12 MW of heat energy and then reject 8 MW of it to space. The radiator will be sized according to the temperature at which you do this rejection, but as a general rule, solar cells don't like to get hot. Let's assume you have some hot-shot cell that will go to 150 C and still run at 33% efficiency. Even with a perfectly emissive radiator, you still need to reject 8 MW of heat at something less than 150 C. But let's be kind and assume you've got a perfectly emissive radiator (e = 1.0) and you've got isothermal heat transfer from the back of your concentrator arrays to your radiator. You're looking at 6600 square meters of radiator, or a square 80 meters on a side. If you have to reject at a lower temperature, it gets worse with the fourth power of the temperature.One of the basic advantages of a nuclear-electric power system is that it is a very dense heat source. You could couple it to a power conversion system like a potassium-Rankine cycle and reject waste heat at much higher temperatures than solar cells like.It is also conceivable that you could run a K-Rankine cycle off concentrated solar energy--for typical conversion efficiencies of 20% or so, you'll need 20 MW of input heat to drive the cycle at generate 4 MW of electricity. That will require ~15000 m^2 of solar collection area (at a flux of 1300 W/m2 and perfect reflectivity) and 430 m2 of radiator area (assuming 16 MW of rejected heat at 900 K emission temperature and perfect emission).
wingod - 29/11/2006 9:30 PMOr you use non concentrating cells and the equlibrium temperature is about 60-70c.
vanilla - 29/11/2006 11:31 PMQuotewingod - 29/11/2006 9:30 PMOr you use non concentrating cells and the equlibrium temperature is about 60-70c.Yes but SteveMick was pretty emphatic that the concentrating cells are the ones that should be compared to NEP, and that beyond that even, that concentrating surfaces (inflatable mirrors and so forth) could offer even greater advantages. I enjoy a good solar dynamic system just as much as the next guy, but I don't think it can credibly compete with NEP for a human Mars mission.