tag:blogger.com,1999:blog-7598615455712402973.post6852722979621667731..comments2024-03-25T11:14:45.840-04:00Comments on Polymath: Nuclear powered VASIMR and plasma propulsion doable now, Page 2.Robert Clarkhttp://www.blogger.com/profile/16114043697010364282noreply@blogger.comBlogger8125tag:blogger.com,1999:blog-7598615455712402973.post-52896599733857582672015-10-27T15:31:50.811-04:002015-10-27T15:31:50.811-04:00A key distinction not being considered is the poss... A key distinction not being considered is the possibility of running the reactor at an <em>intermediate</em> power level. They are either run at full power for propulsion of at 1/1,000th power for electricity generation.<br /> <br /> It is the greatly reduced power that allows them to run for years, in the range of 5 years in this report, in the electricity generation mode. But for the plasma propulsion we only need it to run for a few days. In this case the power level may only need to be reduced by ca. 1/10th. This results in a much higher specific thermal power.<br /><br /> About the conversion to electrical power the recent advances discussed in Part 1 plus cryogenics can give specific electrical power also above the 1,000 W per kg mark.Robert Clarkhttps://www.blogger.com/profile/16114043697010364282noreply@blogger.comtag:blogger.com,1999:blog-7598615455712402973.post-14980384392205435232015-10-24T02:37:02.937-04:002015-10-24T02:37:02.937-04:00The last link I provided is a study of 1mwe cargo ...The last link I provided is a study of 1mwe cargo transport. http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120001636.pdf<br /><br />On the page 13 there is a list of masses of individual components of power supply. Reactor core itself makes only 6% of that, a turboalternator (im assuming its turbine compressor and an alternator) only 1.3%. <br /><br />Turbine temperature at 1150k and heat exchanger at 450K. Im assuming the designers are competent and designed the least heavy system they could. <br /><br />As for low output temperature, in case second law still holds, and you use a liquid droplet radiator. The boom that dispenses this liquid and collector and the liquid in question still have some weight. Physics of it is similar to a normal radiator, just that the surface are is made of individual droplets. Rokhttps://www.blogger.com/profile/01594627296669652306noreply@blogger.comtag:blogger.com,1999:blog-7598615455712402973.post-45234415402390070222015-10-23T19:57:08.820-04:002015-10-23T19:57:08.820-04:00Thanks for that link:
Conceptual Design of a CERM... Thanks for that link:<br /><br />Conceptual Design of a CERMET NTR Fission Core Using Multiphysics Modeling Techniques.<br />http://enu.kz/repository/2011/AIAA-2011-5947.pdf<br /><br /> For a 512 megawatt thermal reactor it gives the reactor mass as 2761 kg. You are quite correct the total reactor mass, with the containment vessel, CERMET (ceramic-metallic) high temperature materials, and control rods is much more than just the uranium fuel. For this reactor it is at about 185,000 watts thermal per kg specific power.<br /><br /> It turns out this would still work to provide the needed 1,000 watts electric per kg specific electric power needed for the plasma propulsion IF you dispensed with the used up fuel elements. This is for the scenario I mentioned where you run continually at full power, while getting a longer run time by replacing the used fuel elements with new stored ones. I mentioned in the blog post you could improve the average specific power by jettisoning the used fuel elements. The average specific power then, even with the ten times higher reactor mass, calculates out still to give the high specific power needed.<br /><br /> And the situation becomes even better when you take into account you actually only have to run the reactor a few days, not the full 39 days, to get the speeds needed for the fast trip, due to the low habitat mass when you have a fast trip.<br /><br /> If it is the case you can have almost indefinite lifetime when run at low power then that would be actually better since I would not need to keep unboard the extra fuel elements so the overall specific power would be even higher. However, there should still be the problem of "burn-up" that should limit lifetime.<br /><br /> BTW, there was recently published a new proposal for a space nuclear reactor that has even better specific power:<br /><br />Innovative concept for an ultra-small nuclear thermal rocket utilizing a new moderated reactor.<br />Nuclear Engineering and Technology.<br />Volume 47, Issue 6, October 2015, Pages 678–699<br />http://www.sciencedirect.com/science/article/pii/S1738573315001540<br /><br /> It would give 100 MWt at a reactor mass of only 180 kg, for a specific thermal power of 550,000 watts thermal per kg.<br /><br /> About the poor specific power of the proposed Prometheus nuclear reactor I suspect the reason it had such long lifetime was specifically because it ran at greatly reduced power. For instance, in that "ultra-small" reactor discussed in the above paper, it has a low power electricity generator mode at only 100 kWt, 1/1000th the power of full power. So when you take this into account the Prometheus reactor would have a much better specific power if it had been run at full power, though then at much shorter lifetime.<br /><br /> In regards to the higher radiator mass for high efficiency conversion, due to the low output temperature, in a follow up blog post I'll show you can have actually zero radiator mass due to the low output temperature. This is possibility that does not obtain for the low efficiency, high output temperature scenario.<br /><br /> Robert Clarkhttps://www.blogger.com/profile/16114043697010364282noreply@blogger.comtag:blogger.com,1999:blog-7598615455712402973.post-80266555240766229182015-10-19T18:18:00.561-04:002015-10-19T18:18:00.561-04:00Reactor with Cermet fuel tubes has maximum tolerab...Reactor with Cermet fuel tubes has maximum tolerable temperature of 3000k. Below that, its almost indefinite. Reason for lower power level is that radiator is too small or not efficient enough. Operating as NTR there is no problem rejecting this much heat with hydrogen exhaust. <br /><br />Your fuel canister idea wouldnt fly, as canister is basically the whole reactor, and 200kg of uranium is enclosed in cermet matrix, which is about as heavy. Not to mention it could be used for more than one mission. <br /><br />You might have noticed that 1MWe module had a mass of about 25t, which is about 40w/kg. <br /><br />http://enu.kz/repository/2011/AIAA-2011-5947.pdf<br /><br />In the prometheus concept, they limited the temperature to the turbine to 1150K. Limited by material. Maybe in the last 10 years the state of the art moved to 1300K, maybe. That still leaves the reactor waay hotter than needed. Lowering the temperature in the reactor would not reduce its power output. It could in fact increase it as the temperature difference between uranium fuel core and coolant is greater. <br /><br />http://www.osti.gov/scitech/servlets/purl/881290<br /><br />Heat rejection in prometheus was at 500K. That gives the temperature range. Lower heat rejection temperature means bigger radiator. Higher turbine temperature is limited to material durability. There seem to be commercial gas turbines that operate at higher temperatures, but they have their blades cooled by channels inside them. <br /><br />As for supercritical fluids: http://web.mit.edu/rsi/www/pdfs/papers/2005/2005-ianr.pdf<br /><br />This assumes heat rejection at 800+K. Minimising radiator mass while keeping efficiency at somewhere around 20%.<br /><br />To make the powerplant specific power greater than 1kw/kg means all the parts that pass power has to have greater specific power. And even higher to compensate for structure, coolant, and shielding.<br /><br />For a bonus: http://web.mit.edu/22.33/www/thesis.pdf<br /><br />A rankine cycle power concept, 1mwe, up to 95% efficient turbine while cycle efficiency is 11%. No weight citations sadly. <br /><br />Basically most of what you mentioned on these 2 blog posts is just a miss in terms of physics limits. <br /><br />For last: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120001636.pdf<br />A detailed case, 17kg/kw or 59w/kg power system and comparison with hall and vasimir. In which vasimr loses...Rokhttps://www.blogger.com/profile/01594627296669652306noreply@blogger.comtag:blogger.com,1999:blog-7598615455712402973.post-42810472712488396752015-10-19T12:29:47.052-04:002015-10-19T12:29:47.052-04:00As you indicated in your follow-up comment, the id... As you indicated in your follow-up comment, the idea was for this to be closed-loop reactor system. So you would have a working-fluid for the nuclear reactor system that went through a heating and cooling cycle. But separate from that you would have a propellant that got accelerated by the electric propulsion system for thrust.<br /><br /> However, I'm looking at the possibility that we could use a single fluid for both. We would have to optimize the heating through the nuclear thermal component and the electric acceleration component.<br /><br /> Bob ClarkRobert Clarkhttps://www.blogger.com/profile/16114043697010364282noreply@blogger.comtag:blogger.com,1999:blog-7598615455712402973.post-20869489037943792382015-10-19T10:59:32.868-04:002015-10-19T10:59:32.868-04:00Thanks for the response. Ten years for the Prometh... Thanks for the response. Ten years for the Prometheus would be a long time for a nuclear reactor. In the reference I cite in the post, "A One-year, Short-Stay Crewed Mars Mission Using Bimodal Nuclear Thermal Electric Propulsion (BNTEP) - A Preliminary Assessment", to get long lifetime in the range of years they have to run the reactor at greatly reduced power level. This reduces the power-to-weight ratio.<br /><br /> In my proposal since it only has to run for days, you run the reactor at an intermediate power level. This means the power to weight ratio is much higher, in fact higher than the 1kW/kg level required for VASIMR and other plasma propulsion methods such as Hall effect thrusters. <br /><br /> Actually in an upcoming blog post I'll discuss that since the time it needs to run is shorter than 39 days, you can run it at even higher power level so the power-to-weight ratio is even higher than I indicated here.<br /><br /> I cited the case of the SSME hydrogen turbopumps because they had high power-to-weight and high efficiency of 80%. However, in looking up other turbine generators I found that even 90% efficiency is not uncommon. So this high efficiency is actually a common occurrence for long running power turbines.<br /><br /> In regards to getting lightweight turbines, recent research shows supercritical CO2 for the working fluid gives turbines a fraction of the size for steam turbines.<br /><br /> I'll discuss these factors in an upcoming blog post.<br /><br /> Bob ClarkRobert Clarkhttps://www.blogger.com/profile/16114043697010364282noreply@blogger.comtag:blogger.com,1999:blog-7598615455712402973.post-37105793765984098152015-10-18T05:08:38.274-04:002015-10-18T05:08:38.274-04:00To expand on ma previous comment.
I know the sys...To expand on ma previous comment. <br /><br />I know the system you were thinking of would be closed loop. But afaik for efficiency reasons Brayton systems are designed for lower pressure and temperature ratios. <br /><br />Like for example project Prometheus, where the system is designed to operate at full power for 10 years. Power to weight ratio is low, 30w/kg. Which includes 500m2 of radiators for 1mw of thermal output from the reactor. Generator efficiency is about 20%.<br /><br />SSME turbopump is bad comparison for these systems as it was only supposed to operate for 7 hours before overhaul. And it ran cooled by cyrogenic propellants. Btw, turbine in the case of Prometheus was suposed to be also 85+% efficient.Rokhttps://www.blogger.com/profile/01594627296669652306noreply@blogger.comtag:blogger.com,1999:blog-7598615455712402973.post-22875398273151412222015-10-17T00:56:48.280-04:002015-10-17T00:56:48.280-04:00Im not sure you got this right.
You are heating ...Im not sure you got this right. <br /><br />You are heating some mass of hydrogen to maximum temperature allowed by the wall of the reactor. Then trough the turbine to near vacuum. Electrical power produced is thermal energy at the start minus losses. Then use that energy to give a small amount of hydrogen alot of kinetic energy, trough vasimir system. <br /><br />Mass of hydrogen going trough the turbine is many times greater than that being expelled trough vasimir. And 30 to 40% of the reactor energy still has to be expelled with radiators. As if you are expelling hydrogen at the end of turbine cycle, ISP goes down down down...Rokhttps://www.blogger.com/profile/01594627296669652306noreply@blogger.com