meiza - 28/2/2008 11:53 AM...2) thermionic (Russians did this, kinda in the middle, I don't really know what it is about)...
meiza - 28/2/2008 9:53 AMWell, as a broad view there are a few ways to convert the power of a reactor core to electricity.At least:1) thermocouple (really inefficient but reliable)2) thermionic (Russians did this, kinda in the middle, I don't really know what it is about)3) heat engines (moving parts, complex, expensive, error-prone, require big radiators, good efficiency)
...What feasible designs are on the table? What sort of size, mass, output are we talking about? What are the shielding/separation requirements?...
Kaputnik - 2/3/2008 10:07 AMThanks for the link.Starting to get some numbers now- good!However the information on the link about RTG power sources is very confusing: is the GPHS RTG a 290w, 2kw, or 100w system? All three figures are quoted. Also, what would the unit's complete mass be? The SRG appears to be more defined, at "100-120w", but it would be good to get figures on mass.
The Multi-Mission RTG (MMRTG) will use 8 GPHS units producing 2 kW which can be used to generate 100 watts of electricity and is a focus of current research.
Tom Ligon - 28/2/2008 9:36 AMI'm turning blue holding my breath, waiting for news from Santa Fe on Dr. Bussard's project. That's the "Interesting Fusion Talk on Google" thread of 2000+ posts below.If it pans out, and I'm really hopeful it does, it probably what you're looking for.
Sid454 - 3/3/2008 6:11 PMI feel compact nuclear power sources in the 30KW range and larger are more important then even heavy lift vehicles for a mars mission.ISRU and even just closed loop life support is going to need some serious KW let alone propulsion etc.But also remember it's gotta break down into easy to assemble parts that weigh less then 25 to 100tons so for near term fusion is out of the question.
khallow - 6/3/2008 8:07 AMOne thing about fusion is that it doesn't seem compact to me. I imagine the first viable, positive net power fusion reactors are going to be big. Even aneutronic reactors (which probably will be the first) will need space to absorb the energy of the charged particles emitted from the reaction. A larger volume means that the mass of the plasma to its surface area (and heat radiation) is larger (for a fixed plasma density). That makes it easier to keep the plasma hot. And if the reaction gives off neutrons, then a large reactor is going to keep a larger fraction of its neutrons in the plasma.Nuclear fission has already been made very compact. The real problem is the heat sinks.
neviden - 6/3/2008 10:24 AMNuclear power sources are good since they are independent from the environment. That means they would be good for Mars, but if we are talking about 10 - 100 KW of power, we could get that from solar power (maybe even from the wind). So that means that they are not absolutely needed if you can store enough power.
iamlucky13 - 6/3/2008 9:11 PMNASA's current analysis (referenced in ESAS) seems to indicate that for 100 kWe range on the moon, solar comes out heavier than nuclear due the need to oversize and have storage for overnight power. The balance would be even more heavily in nuclear's favor on Mars, both due to added panel size and higher gravity to support against.
iamlucky13 - 6/3/2008 2:11 PMQuotekhallow - 6/3/2008 8:07 AMOne thing about fusion is that it doesn't seem compact to me. I imagine the first viable, positive net power fusion reactors are going to be big. Even aneutronic reactors (which probably will be the first) will need space to absorb the energy of the charged particles emitted from the reaction. A larger volume means that the mass of the plasma to its surface area (and heat radiation) is larger (for a fixed plasma density). That makes it easier to keep the plasma hot. And if the reaction gives off neutrons, then a large reactor is going to keep a larger fraction of its neutrons in the plasma.Nuclear fission has already been made very compact. The real problem is the heat sinks.Right...Tokamaks (and stellerators) are huge. If Brussard's device actually is workable (far from proven), it might be a little smaller, but not overwhelmingly. Nothing else looks really promising for sustainable, net positive fusion at this point. However, anneutronic fusion is even further down the road than D-T fusion. It takes much more energy to start He-3 fusion.See below for a size comparison a human being, JET (currently the largest fusion reactor, but not capable of self-sustaining fusion), and ITER, just starting construction:http://www.jet.efda.org/images/level1/jet-iter/jet-iter-s.jpgQuoteneviden - 6/3/2008 10:24 AMNuclear power sources are good since they are independent from the environment. That means they would be good for Mars, but if we are talking about 10 - 100 KW of power, we could get that from solar power (maybe even from the wind). So that means that they are not absolutely needed if you can store enough power.NASA's current analysis (referenced in ESAS) seems to indicate that for 100 kWe range on the moon, solar comes out heavier than nuclear due the need to oversize and have storage for overnight power. The balance would be even more heavily in nuclear's favor on Mars, both due to added panel size and higher gravity to support against.
neviden - 6/3/2008 12:49 PMQuoteiamlucky13 - 6/3/2008 9:11 PMNASA's current analysis (referenced in ESAS) seems to indicate that for 100 kWe range on the moon, solar comes out heavier than nuclear due the need to oversize and have storage for overnight power. The balance would be even more heavily in nuclear's favor on Mars, both due to added panel size and higher gravity to support against.As far as I am aware there wouldn't be much of overnight power need if Moon base would be on the Peaks of eternal light as it is currently planned. I am not 100% certain if the sun always shines there, but it shines most of the time (and when it doesn’t, don’t use any power hungry things). ISRU could be used as a power storage if it was designed with that in mind. You could build tents out of thin film solar panels on Mars. It is far worse energy wise, but it is at least possible to do it without nuclear reactor. Nuclear reactor would be nice, but is not that it is exactly cheap to design and build either.
iamlucky13 - 6/3/2008 2:11 PMRight...Tokamaks (and stellerators) are huge. If Brussard's device actually is workable (far from proven), it might be a little smaller, but not overwhelmingly.
publiusr - 7/3/2008 9:10 PMQuoteSid454 - 3/3/2008 6:11 PMI feel compact nuclear power sources in the 30KW range and larger are more important then even heavy lift vehicles for a mars mission.ISRU and even just closed loop life support is going to need some serious KW let alone propulsion etc.But also remember it's gotta break down into easy to assemble parts that weigh less then 25 to 100tons so for near term fusion is out of the question.I don't see that has to be the case. Use the heavy Lifter to launch to launch the nuclear power source.
iamlucky13 - 4/3/2008 1:26 PMBTW, the Exploration Systems Architecture Study, section 4, briefly discusses power options for a lunar outpost and their extensibility to a Mars mission. Their preliminary conclusions put solar and nuclear systems on similar feasibility levels, but favored a nuclear system for its ability to operate equally effectively over a 14 day-long night and for its extensibility to Mars exploration. In particular, they suggest that a 100 kWe reactor could be developed with a 10-20 ton mass. They don't discuss a buried reactor, but instead suggest placing it approximately 2 km away from the habitat.http://www.nasa.gov/mission_pages/exploration/news/ESAS_report.html
Kaputnik - 8/3/2008 12:07 PMQuotepubliusr - 7/3/2008 9:10 PMQuoteSid454 - 3/3/2008 6:11 PMI feel compact nuclear power sources in the 30KW range and larger are more important then even heavy lift vehicles for a mars mission.ISRU and even just closed loop life support is going to need some serious KW let alone propulsion etc.But also remember it's gotta break down into easy to assemble parts that weigh less then 25 to 100tons so for near term fusion is out of the question.I don't see that has to be the case. Use the heavy Lifter to launch to launch the nuclear power source.Yes, but if it's destined for the surface of Mars it hits a per-item mass limit- currently about 9t per piece and maybe up to 50t or so if radical new technologies for landing on Mars are developed.
Patchouli - 9/3/2008 3:21 AMA mars lander would have a much larger mass fraction then a lunar lander because you have an atmosphere to help you slow down.Of course the bigger you make these things the more mass efficient they get due to mathematics of scaling.We only tried I think one design of reentry vehicle so far on mars the viking system so there is a lot of room for improvement such as biconic and lifting body.Something lifting or even winged might have a much better mass fraction esp since it can be long and relatively narrow such as with DCY and the shuttle though no matter what you reenter with that finial descent will have to be under rocket power.
A_M_Swallow - 9/3/2008 7:55 PMIs there sufficient atmosphere on Mars that jet engines or propellers can provide say a quarter of the slow down delta-v?
Kaputnik - 9/3/2008 8:07 PMQuoteA_M_Swallow - 9/3/2008 7:55 PMIs there sufficient atmosphere on Mars that jet engines or propellers can provide say a quarter of the slow down delta-v?Delta-v for which phase of descent? The entry aeroshell- which has to be there anyway- does a good job down to about Mach 3, and parachutes are a very low mass way of slowing from there to less than 100m/s. So for the final decceleration we need the lowest mass solution to kill off tha final bit of speed. I doubt that anything is more mass-efficient than a rocket motor because the mass of the engine itself is nice and low and the propellant supply can also be quite small.
A_M_Swallow - 9/3/2008 8:29 PMDo rockets have to be switched off during the last few feet of the descent?100 m/s = 224 mph is well within the operating speed of helicopters and ordinary aircraft. Given an aircraft on a rocky place like Mars there are plenty of other journeys it can take.Bringing this back on topic, the nuclear powered aircraft may finally get built.
A_M_Swallow - 9/3/2008 2:55 PMIs there sufficient atmosphere on Mars that jet engines or propellers can provide say a quarter of the slow down delta-v?
clongton - 10/3/2008 1:13 AMQuoteA_M_Swallow - 9/3/2008 2:55 PMIs there sufficient atmosphere on Mars that jet engines or propellers can provide say a quarter of the slow down delta-v?Jet engine? There is no oxygen in the Martian atmosphere to speak of so such an engine couldn't function. So it doesn't even matter what the propellant is, you couldn't ignite it.
A_M_Swallow - 9/3/2008 10:20 PMQuoteclongton - 10/3/2008 1:13 AMQuoteA_M_Swallow - 9/3/2008 2:55 PMIs there sufficient atmosphere on Mars that jet engines or propellers can provide say a quarter of the slow down delta-v?Jet engine? There is no oxygen in the Martian atmosphere to speak of so such an engine couldn't function. So it doesn't even matter what the propellant is, you couldn't ignite it.Magnesium can actually burn the carbon dioxide in Mar's atmosphere.Propellers can be electrically powered.
Patchouli - 8/3/2008 9:21 PMA mars lander would have a much larger mass fraction then a lunar lander because you have an atmosphere to help you slow down.Of course the bigger you make these things the more mass efficient they get due to mathematics of scaling.We only tried I think one design of reentry vehicle so far on mars the viking system so there is a lot of room for improvement such as biconic and lifting body.Something lifting or even winged might have a much better mass fraction esp since it can be long and relatively narrow such as with DCY and the shuttle though no matter what you reenter with that finial descent will have to be under rocket power.
Kaputnik - 19/3/2008 6:10 AMSRGs have got me quite interested now. Cheaper to develop than reactors, safer, easier to package with no shielding worries.The big problem with them is getting hold of Pu238. I've heard that the manufacturing of this fuel has been stopped. Is it possible to re-start this, and is the stuff inherently very expensive? How much would we be talking about per kg, assuming quite large scale production?
khallow - 19/3/2008 12:26 PMThe method of production above seems to be to take neptunium 237, radiate it with protons, and extract the resulting plutonium 238. In turn. neptunium 237 appears to be a common byproduct of nuclear fissioning. I read here that out of 25 tons of spent nuclear fuel, the annual waste from a 1 GW fission plant, about 10 kg are neptunium 237. So plenty of stock to get plutonium 238 from.
vanilla - 19/3/2008 10:56 AMQuotekhallow - 19/3/2008 12:26 PMThe method of production above seems to be to take neptunium 237, radiate it with protons, and extract the resulting plutonium 238. In turn. neptunium 237 appears to be a common byproduct of nuclear fissioning. I read here that out of 25 tons of spent nuclear fuel, the annual waste from a 1 GW fission plant, about 10 kg are neptunium 237. So plenty of stock to get plutonium 238 from.You irradiate the neptunium with neutrons, not protons, to get Pu-238. Also, Np is NOT being extracted from spent nuclear fuel due to the US ban on reprocessing dating back to the 1970s. There is Pu-238 formed in conventional reactors, but it is mixed with Pu-239, Pu-240, and Pu-241 and is inseparable and worthless.The Np supply in the US, which is fairly small, has to be irradiated in dedicated reactors to form enough Pu-238 to make the separation worth the trouble. The problem is very much still there.
TyMoore - 19/3/2008 7:46 PMPu-238 is fine as an isotope heat source for RTG's but it is far easier and cheaper (safer too) to use ordinary fissile fuels to create a fission reactor for a nuclear power source. A fission reactor can be turned on and off, even throttled a bit for power production--while the isotope heat source cannot. Once formed, an isotope heat source is driven by the rate of decay and nothing else.
Actually, all isotopes of plutonium are fissile--even Pu-238. So a large RTG could actually go critical--if it were poorly designed! The reason why Pu-238 is not used at all for weapons is that a 10-15 kg core would produce 10 KW or so of heat--and it would literally be glowing red hot! The weapon core would eventually melt the weapon it was installed in!As far as safety--in terms of transportation of nuclear materials (i.e., launching them into space) then a compact nuclear reactor can be 'safed' or even 'inerted' by incorporating nuclear 'poison' rods which must be removed before the reactor can be 'switched on.' A couple of cadmium rods in a small reactor core will keep the thing from going critical under any circumstance...Also, look at the number of curies of radioactive materials being transported--because of Pu-238's short half-life of about 87.7 years, versus U-235's 700 million years, an RTG carrying 25 kg will have the same bulk radioactivity as around 211,000 tons of U-235 before the reactor is activated. Once the reactor activates, then fission products will begin to accumulate, of course...
And control is a real issue too--again, a fission reactor can be throttled, shut down, and restarted. An isotope heater cannot--once the isotopes a created and fabricated into heater units, their rate of heat production is set by the decay rate, period. For small devices like RTG's for deepspace missions where the total Pu-238 loading is relatively low (a few tens of kilograms) the total heat output is still only a few kilowatts--manageable for launch platform cooling systems. Once you start thinking in terms of hundreds of kilowatts of heat--then launch cooling is going to be a big problem. A reactor on the other hand will generate essentially no heat until the moment it is activated--and by then a mission can already be well on its way.