tag:blogger.com,1999:blog-192883022973901942.post4170084711923286054..comments2023-10-08T02:46:24.823-07:00Comments on Bootstrapping Space: Interorbital Exchange - part 7, electric vehiclesChris Wolfehttp://www.blogger.com/profile/11247630943891521469noreply@blogger.comBlogger8125tag:blogger.com,1999:blog-192883022973901942.post-26185700882855228852016-03-19T19:17:55.654-07:002016-03-19T19:17:55.654-07:00Ah, that might be why. I'll send again :)Ah, that might be why. I'll send again :)Matter Beamhttps://www.blogger.com/profile/16721504049578296529noreply@blogger.comtag:blogger.com,1999:blog-192883022973901942.post-14664093532429825302016-03-18T06:36:22.785-07:002016-03-18T06:36:22.785-07:00Done, and following a few people now as well.
I ma...Done, and following a few people now as well.<br />I managed to mangle my own email address, the accurate one is christopher(dot)wolfe(dot)public(at)gmail(dot)com.Chris Wolfehttps://www.blogger.com/profile/11247630943891521469noreply@blogger.comtag:blogger.com,1999:blog-192883022973901942.post-80796392261954549402016-03-11T16:32:34.331-08:002016-03-11T16:32:34.331-08:00I refresh this page very often, don't worry :)...I refresh this page very often, don't worry :)<br /><br />Thank you for the reply, once again.<br /><br />Please do create a Google Plus account.Matter Beamhttps://www.blogger.com/profile/16721504049578296529noreply@blogger.comtag:blogger.com,1999:blog-192883022973901942.post-79165025513414789932016-03-10T16:39:13.920-08:002016-03-10T16:39:13.920-08:00The VASIMR engine heats its propellant with microw...The VASIMR engine heats its propellant with microwaves, for exxample, so yes it is possible. You could say that the distinction between electrothermal and electromagnetic is a bit fuzzy; I don't think there is a particularly sharp boundary between the two. In common use the term electrothermal usually refers to the simpler low-Isp devices.<br /><br /> The silicon cells of solar panels are covered in a 'front glass' layer for protection. Panels intended for space usually have this layer deposited directly on the silicon, so the thickness is tuned for the specific mission. Thicker glass (or alumina or sometimes silica gel) absorbs more radiation and allows for a slower decay of efficiency at the cost of more mass.<br /> Silicon is tougher than human flesh with regard to radiation. Even for fine details like in memory or processors it typically takes several damage events to disable a single feature (though high Z cosmic rays can definitely do real damage with just one impact). For bulk PV silicon the slow accumulation of lattice defects and implanted nuclei leads to a slow and even degradation of efficiency. A typical solar panel meant for GEO could survive a few months in the lower Van Allen belt and only lose perhaps 10% of its output, then lose another 10% over the following 10 years in GEO. Something built as a LEO to GEO tug would need much thicker protection to withstand the radiation for a few years.<br /><br /> You can reach me at christopher (dot) wolfe {at} gmail com. As you've no doubt noticed I can be a bit erratic with communication, but I would be glad to be of help.Chris Wolfehttps://www.blogger.com/profile/11247630943891521469noreply@blogger.comtag:blogger.com,1999:blog-192883022973901942.post-48924877920663736102016-03-04T22:54:27.118-08:002016-03-04T22:54:27.118-08:00Its always fascinating to have estimates based on ...Its always fascinating to have estimates based on real world numbers, not only idealized equations, so thank you for that. I'm surprised that you've taken a look at ToughSF too.<br /><br />Three questions:<br /><br />-Could there be a contactless version of an electrothermal engine, such as one where the propellant is heated by radio or by magnetic induction?<br />-You mentioned crossing the Van Allen belts. Are modern solar panels for space use specially vulnerable to that sort of radiation? Can they be made to resist damage?<br />-How or where should I contact you to discuss a near future setting that would deal with the initial steps to colonize the solar system?<br /><br />Matter Beamhttps://www.blogger.com/profile/16721504049578296529noreply@blogger.comtag:blogger.com,1999:blog-192883022973901942.post-36727670419102886952016-03-04T13:23:33.723-08:002016-03-04T13:23:33.723-08:00Then I see you've previously linked that same ...Then I see you've previously linked that same project rho page on your blog. Excellent.Chris Wolfehttps://www.blogger.com/profile/11247630943891521469noreply@blogger.comtag:blogger.com,1999:blog-192883022973901942.post-28886798983796094982016-03-04T10:01:06.725-08:002016-03-04T10:01:06.725-08:00Greetings, and thanks.
For a full investigation I ...Greetings, and thanks.<br />For a full investigation I strongly recommend Atomic Rockets as a reference. For example:<br />http://www.projectrho.com/public_html/rocket/enginelist.php<br /><br /> The reactor design presented in the paper I referenced is scaled up to 5 MW in their graphs. I think we can take that as a high-confidence estimate. Terrestrial nuclear reactors range up to about 1.3 GW (many examples at this power level and a small number higher), so the potential to go bigger is certainly there. I think it would be fair to scale up the Promethius design to perhaps 50 MW with an alpha of 18 kg/kW including the reactor, shadow shield, power conversion equipment, radiators and thrust structure. That would be 900 tons, so we're talking about a very large craft.<br /> Their thruster design is also scalable, but it looks like they chose to use an optimized thruster design and simply used enough of them to provide sufficient thrust, spares and propellant throughput. A 50 MW JIMO-like spacecraft should have enough area at the back to mount enough thrusters to maintain desired acceleration and lifespan.<br /><br /> Electrothermal drives are basically rocket engines that get their heat from electricity instead of combustion. A cryogenic engine burning liquid hydrogen and liquid oxygen is able to cool its combustion chamber and nozzle by passing the liquid propellant through cooling channels, so these engines can have extremely high combustion temperatures. By contrast, the electrothermal engine is typically using hydrazine or a pressurized gas at ambient temperature. Cooling becomes a serious problem at high temperatures.<br /><br /> State of the art electrothermal devices do have slightly higher Isp than direct nuclear-thermal engines because the filaments or electrodes can be made of tungsten, which can handle temperatures over 3200 °C. Nuclear-thermal engines have to be kept below the melting point of the fuel elements, cladding, etc., so zirconium oxide and uranium oxide are the typical limiters to around 2400 °C.<br /><br /> You are correct that using the reactor for NTR would be simpler in some ways, but it would be less efficient. This would be offset somewhat by the much higher levels of thrust, gaining some Oberth benefit and much shorter trip times. An NTR cargo tug would be viable and is a core option in NASA's current Mars mission plan. Still, using the same power output to drive thrusters with an Isp of 6000 or so yields far more delta-V for the same mass.<br /><br /> The crossover point varies by mission and by vehicle size. At Earth's orbit, tens to hundreds of tons of cargo and for missions of 10-20 years solar power is going to win probably every time. One possible exception would be a dedicated heavy-cargo LEO to GEO tug that crosses the Van Allen belts multiple times per year; the added mass needed for radiation shielding might be beaten out by a very efficient reactor. If you want to build a 50k ton GEO power satellite then you will probably want a few heavy nuclear cargo tugs to move components out of LEO and EML1/2.<br /> Mars missions for reusable craft that need to make 5 or more round trips with tens to low hundreds of tons of cargo are in the right ballpark for crossover. Disposable ships should probably just use solar. Heavier cargo favors nuclear.<br /> Main Belt bodies like Ceres make nuclear even more competitive even for single-shot missions, while the gas giants and outer bodies require nuclear for almost any useful mission.Chris Wolfehttps://www.blogger.com/profile/11247630943891521469noreply@blogger.comtag:blogger.com,1999:blog-192883022973901942.post-26757660977478765052016-03-03T11:26:40.331-08:002016-03-03T11:26:40.331-08:00A very interesting write up. I wish I had as much ...A very interesting write up. I wish I had as much technical knowledge for use in my own sf blog.<br /><br />A couple of questions:<br /><br />How scalable is the design?<br /><br />Why are electrothermal drives limited to such low Isp?<br /><br />If you have 6MW of onboard thermal power, why not use it directly as a nuclear thermal rocket, with possible simpler design?<br /><br />At what power output do you think nuclear designs would become more efficient than solar designs, in your example?<br /><br /><br />Matter Beamhttps://www.blogger.com/profile/16721504049578296529noreply@blogger.com