Mars and nuclear power

[krrr]

<font color="yellow">NEP is inferior to SEP below ~1 Megawatt.<br /><br />?One mWe? Supporting data with links please. [...] When the scale of energy starts to climb into the tens of kilowatts range nuclear just completly runs away from solar-power in the ratio of power/mass. The 100 kWe power SAFE-400 reactor masses less than one tonne.<br /></font><br />Modern solar array technologies such as the SLA are supposed to have a specific power of 330 W/kg or better, which means that a 1 MW power plant would mass less than 4 tonnes. I'm not disputing the fact that at some point nuclear becomes more mass-efficient than solar, but that point is in the 1 MW region (for in-space applications).<br /><br /><font color="yellow">Nuclear-thermal is woefully inefficient (yes, one could go to Mars in 4 to 5 months instead of 6 to 8, but 60% of a long time is still a long time)."<br /><br />Woefully inefficient? How do you get that?<br /></font><br />Well electric propulsion produces an Isp of 4000 sec or better, while nuclear-thermal seem to be limited to less than 1000 secs. I agree that this is still better than chemical, I'm merely pointing out the wastefulness and clumsiness of that technology. <br />

 

[john_316]

Someone please correct me if I am wrong but I think I recall a study done by NASA comparing Chemical, Nuclear Gas Core and Nuclear Solid Core rocketry Specific Impulses for a manned mission to Mars and for return.<br /><br />I believe correctly that the Gas Core NCR was the better deal of all 3 designs and I also think they said it could also offer a 72 day trip to Mars and the same back compared to the 120+ days chemical would provide. The Solid core I also believe offered a 90 day to less than 120 day travel span.<br /><br />My point is this. What happens when you expand a base? Are you going to keep on delivering Solar arrays to Mars Surface possibly with erecter sets for wind power too? Or would you preferr a system that is self contained and can at least give you 5-15 years worth of power and that could be ranged higher in Kw if need be compared to the fraction of solar power.<br /><br />Solar Power conversion isnt that proficient and wont hit 75% for some time to come. Its possible solar may reach 75% effeciency by 2020 but I think thats unlikely. And by the time it reaches that mark I think we will have another alternate power source. But Fusion isnt it...<br /><br />You can use solar powered rockets for cargo carriers etc etc but to get there fast and back fast would be better with a NCR. Even a 5-10 year reactor for Mars is sufficient until any type of solar arrays and batteries can be supplied to the Martian surface.<br /><br />I support the idea of using both types of power sources but I would rather have something that takes up less space and is practical for time immerse travel and power. I just cant see Solar power doing it. <br /><br />I also support the idea of using ion engines for small spacecraft exploring the outer planets but something like JIMO could be built and launched faster and get there before a Ion built one would be. It would still need some chemical push to get it started...<br /><br />Lets Go Nuke...<br /><br /><img src="/images/icons/smile.gif" /><br></br>

 

[krrr]

As I said, for a Martian base nuclear is probably the most robust choice.<br /><br />By the way, solar power conversion (by solar cells) will never be much better than today's ~30%. So one is restricted to ~400 W/m² near Earth and 200 W/m² near Mars. However, this conversion efficiency is much better than fission-to-electric (or fission-to-"kinetic").<br /><br />As for human transportation, I think nuclear-thermal is just not necessary. Some of the Isp advantage will be offset by increased mass for shielding etc. The trip to Mars and back will be long and tedious in any case.<br />

 

[gunsandrockets]

"Nuclear-thermal is woefully inefficient ..." <br /><br />"... electric propulsion produces an Isp of 4000 sec or better, while nuclear-thermal seem to be limited to less than 1000 secs. I agree that this is still better than chemical, I'm merely pointing out the wastefulness and clumsiness of that technology."<br /><br />Woah woah woah, hold it. You are switching gears here. Your original post claimed manned NTR was clumsy and inefficient compared to CHEMICAL propulsion. Here is your original quote, "* Human transport: Chemical propulsion plus solar power generation. Nuclear-thermal is woefully inefficient..."<br /><br />And now your talking about comparing NTR to solar-electric-propulsion? If you want to compare solar-electric-propulsion to nuclear-electric-propulsion, fine. That's a case where nuclear is superior to solar as well. <br />

 

[gunsandrockets]

"Modern solar array technologies such as the SLA are supposed to have a specific power of 330 W/kg or better, which means that a 1 MW power plant would mass less than 4 tonnes."<br /><br />Thanx for the link. The hour is late and I'll need some time to digest the info.<br /><br />Finally some new data. It's refreshing to see input other than the more common, "no it isn't" "yes it is" "no it isn't" "yes it is" variety.

 

[krrr]

<font color="yellow">Your original post claimed manned NTR was clumsy and inefficient compared to CHEMICAL propulsion.</font><br /><br />Well I said it was inefficient (Period). But I realize it's one of my weaker arguments. More an aesthethic dislike of an inelegant technology.<br /><br /><font color="yellow">If you want to compare solar-electric-propulsion to nuclear-electric-propulsion, fine. That's a case where nuclear is superior to solar as well.</font><br /><br />As discussed, SEP is superior to NEP because a solar array is lighter than a nuclear reactor, for power levels under ~1 MW. <br /><br /><br />

 

[ldyaidan]

Considering that our solar technology still has a long way to go in order to be efficent here on earth, it would not be suffcient on Mars or the moon. Now, if that technology improves greatly, which it is getting better, and it could be build using ISRU, then it might be a way to go. But, as it stands now, I can't see solar being used except for as an additional energy source. Especially considering that Mars is further away from the sun than the earth, thus not getting as much sunlight as we do, and considering the dust storms that would a)damage the solar arrays and b) make them useless during the storms as well as until they are cleaned up afterwards. Personnally, I would not want to be dependant on them for my survival. I would want something that I know is going to be able to provide my life support systems regardless of the weather outside.<br /><br />Rae

 

[scottb50]

I haven't mentioned batteries because they are not needed. Solar would be used exclusively to hydrolize water and consummable power would be produced by fuel cells. <div class="Discussion_UserSignature"> </div>

 

[mlorrey]

Scott, not to be rude, but you have a backaswardy way of doing things. Why, if I've got solar power panels up in the sun 12 hours a day, would I use that solar power to electrolyse water, at an efficiency loss, then recombine the two in a fuel cell at another efficiency loss? <br /><br />Admittedly, you want to store energy for night time, and this is likely the best option (though the lithium ion battery technologies may have a better energy density than O2/H2, particularly considering the energy losses in compressing/decompressing fuel and oxidizer. Neither is a perfect gas.) However, electroylsis is nowhere near as efficient for real time energy generation/transmission/consumption.<br /><br />Given the calcs we've made on the mass of solar panels needed to properly power an agricultural facility for ONE person's dietary needs, there is no way that this can be done without nukes providing heating and some of the lighting. As solar flux is 44% on Mars as on Earth, then you need about 3 times as much area of light collection vs agricultural area, if you use light piping. If you use PV collection and LED illumination, you will need collection area between 20-50 times larger than the agriculture area you are illuminating, without even considering heating requirements, depending on your PV and LED efficiencies.

 

[josh_simonson]

Those SLA cells are neat, but they require accurate pointing at the sun (which cells without concentrators don't need). This wouldn't be so much of a problem in space, but on moon/mars it would need to be supported by a stand with the ability to point it directly at the sun. <br /><br />Also, on a manned spacecraft with high-thrust propulsion, the arrays would need to be sufficiently strong to not sag under the acceleration. This can add a lot of weight. <br /><br />Aerocapture is also impractical if you use solar panels for power. Using aerocapture results in nearly 50% fuel savings, so that must be counted against a solar option as well.

 

[mlorrey]

"high thrust" in interplanetary missions generally means .10-1.0 G. <br /><br />A very good point about aerocapture: if you can't fold and stow it before you hit atmo, hasta la vista, one reason nuke has a significant delta-v advantage.<br /><br />Nuke can also give high thrust, lower than avg Isp via LOX injection into the nozzle, which solar-electric just can't do (though I have some ideas on this, one of which being the combining of ionized O2 and H2 plasma streams)....

 

[scottb50]

Scott, not to be rude, but you have a backaswardy way of doing things. Why, if I've got solar power panels up in the sun 12 hours a day, would I use that solar power to electrolyse water, at an efficiency loss, then recombine the two in a fuel cell at another efficiency loss?>><br /><br />Because the loses are immaterial when you consider solar power is free for the taking. The main concern with using fuel cells is the consistency of the power output, especially with electronics. Figuring you need the water anyway why add more weight with batteries? <br /><br />What I intend to do is begin stockpiling Hydrogen and Oxygen from the very beginning, when the moon and Mars Stations are fairly small, so adequate supplies are available before a major influx of people starts. <br /><br />Another factor is purification of waste water and recycling. <br /><br /> <div class="Discussion_UserSignature"> </div>

 

[josh_simonson]

Exactly. Solar is perfectly suited for slow (gentle) yet efficient SEP probes or pre-positioning cargo at mars. For people though, some ISP efficiency is sacrificed for thrust to make the trip shorter with NTP. This requires a more rugged spacecraft. Far out in the solar system, with un-manned probes, NEP is the only way to go. <br /><br />There is a large variety of system requirements and available light that determines what would be best, but there is a place for all three methods. <br /><br />Remember that for SEP you need electric thrusters as well as the solar panels. Using NTP, the reactor is the thruster. Both technologies will become more weight and ISP efficient in the future as they evolve. I'd dare say that there has been less work on lightweight reactors than solar panels, so there might be more room for improvement.

 

[CalliArcale]

<blockquote><font class="small">In reply to:</font><hr /><p>Because the loses are immaterial when you consider solar power is free for the taking. The main concern with using fuel cells is the consistency of the power output, especially with electronics. Figuring you need the water anyway why add more weight with batteries?<p><hr /></p></p></blockquote><br /><br />Yes, fuel cells are useful for storing power when the sun's not available, or even just evening out the available current. That's what the next phase of Helios would've attempted -- self-regenerating fuel cells, so that Helios could've stayed in the air 24-7.<br /><br />Now that might be a neat thing for DARPA to fund: a fuel cell that can be reversed somehow so that instead of joining hydrogen and oxygen, it splits them back up. It would make it rechargeable without actually opening it up. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[mattblack]

Didn't I once read somewhere about fuel cells that could use methane instead of hydrogen? But I imagine at an efficiency loss over hydrogen. Methane as we know can be made from Mar's atmosphere. <div class="Discussion_UserSignature"> <p> </p><p>One Percent of Federal Funding For Space: America <strong><em><u>CAN</u></em></strong> Afford it!!  LEO is a <strong><em>Prison</em></strong> -- It's time for a <em><strong>JAILBREAK</strong></em>!!</p> </div>

 

[mlorrey]

Yeah, and they don't last as long without maintenance. It is rather easy, though, to include a gas reformer that cracks the methane molecule prior to injection into the fuel cell.

 

[scottb50]

The only difference between a fuel cell and a hydrolizer is the direction of flow. The same cell could be used for either. What I am ythinking of would be alternate Hydrolizing and power production cells in a stack. This would make the unit more compact and ease the cooling requirements.<br /><br />The problem with reversing a cell would be contamination to the membrane which would probably lower reliability. Water would be highly purified, run through hydrolizers and fuel cells multiple times before it is used. Being a sealed system contaminents would be eliminated allowing long term trouble free operation. <div class="Discussion_UserSignature"> </div>

 

[josh_simonson]

Flywheels are pretty efficient at storing energy, and can be used as gyros too. They may be too heavy for mobile spacecraft though. I know NASA has invested in them.

 

[priusguy]

<b>I also think, with enough storage capability, solar would work just fine on Mars, not as easily as in orbit or Space, but not too difficult.<br /><br />I don't doubt the capability of Nuclear all I doubt is the viability in the proposed environment, ridding a reactor of excess heat, not involved with developing power is a major problem in our atmosphere. How do you control it in Space or the minimal atmosphere of Mars?</b><br /><br />Mars' atmosphere is too thin to support life, but is more than enough to carry away heat -- which is why all rovers need plutonium heating units. And Mars' <i>ground</i> will leach away the heat of any structure sitting on it, lt alone buried in it. So no, getting rid of excess heat will not be a problem -- and is the main reason solar panels will never be adequate on Mars. They could provide enough power for all electric devices, but not for heating.<br /><br />In fact, I suspect Mars EVA suits will need radioactive heating units to keep them warm. Loss of heat will be much more of a problem than overheating.

 

[mlorrey]

Quite right. Mars needs as much heat as we can dump into it. We should be shipping oodles of old reactor rods there and just piling them in craters, let the piles go super critical and just steam the CO2 out of the ground. I really want to terraform Mars as fast as possible. The only other option is dropping a few comets on it.

 

[josh_simonson]

Heating mars would only accelerate the boil-off of it's atmosphere, and probably cook off most of the remaining water too. Mars needs to be considerably heavier before it is capable of holding lighter gasses like N2, O2 and H2O. It'll also need a magnetic field so that the solar wind is prevented from stripping away the upper atmosphere. Until we have the ability to hold an atmosphere to mars, there's no point in trying to encourage one.

 

[JonClarke]

I have been effectively offline for most of this interesting discussion, so rather than responding to specific posts I will try and make some general summary comments.<br /><br />First, I think the supposed dichotomy between nuclear and solar options for Mars missions completely artificial. The issue is not some sort of ideological commitment to one or the other, but what is the best option for a particular mission. Often what is best will not be decided only by some simple technical parameter, such as Isp or kW per tonne, but by a complex of associated issues such as development costs, reliability, repairability, and safety, issues that are difficult to quantify, especially by none specialists such as ourselves at the best of times.<br /><br />Second, I find division of people into pro solar or pro nuclear camps inaccurate. As far as I can see, everyone in this discussion here recognises that only nuclear options are viable for power supply to missions to Jupiter and beyond. Similarly, everyone accepts that solar power as the best for powering satellites in earth orbit. When it comes to Mars what we are debating is which is best for a particular mission scenario. Spacster’s comments are spot on.<br /><br />Third, thus comments such as (my apologies to John_316) “The idea of being anti-nuke is what other's say it is. Anti-advancement in technology and is all just a POLITICAL sham from the crazies on the far left...” or Skyone’s (again apologies) “Don't forget the bible thumping anti-evolution, anti-science, anti-advancement, anti-technology, anti-nuke sham from the crazies on the far right…” are utterly unhelpful and counter productive. So would comments at the other extreme such as accusing those advocating nuclear space technologies of being techno-cranks and suffering from strangelovian obsessions. As john_316 pointed out in the same post, advocating nuclear space technology does not mean that you are, ipso facto, a project orion enthusiast.<br /><br />I suggest <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>

 

[JonClarke]

“Nuclear propulsion is essential for Mars missions” is a common assertion. It is wrong. There have been many Mars mission proposals that have not relied on nuclear propulsion. For example:<br /><br />Project Deimos (~1965) http://astronautix.com/craft/proeimos.htm <br />NASA 1971 study http://astronautix.com/craft/nasn1971.htm <br />Planetary Society (1983) http://astronautix.com/craft/plan1983.htm <br />Case for Mars II (1984) http://astronautix.com/craft/casarsii.htm <br />Pioneering the Space Frontier (1984) http://astronautix.com/craft/piontier.htm <br />Ride report (1986) http://astronautix.com/craft/rideport.htm <br />90-day study (1989) http://astronautix.com/craft/90dstudy.htm <br />1989 Study http://astronautix.com/craft/marion89.htm<br />Mars direct (1991) http://www.marsinstitute.info/rd/faculty/dportree/rtr/ma20.html and http://astronautix.com/craft/marirect.htm <br />Boeing STCAEM Cryogenic aerobrake (1991) http://astronautix.com/craft/stcbrake.htm <br />Two studies by Grover (1996) http://www.aa.washington.edu/research/ISRU/ARES/ares.htm <br />Caltech Mars Society (1999) http://w</safety_wrapper <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>

 

[JonClarke]

Nuclear power is not necessary to the trip to Mars. The 20-40 kW needed for a Mars transfer vehicle can be easily supplied from solar panels. Of course if you have NTR propulsion, especially in an all-propulsive mission profile, it makes sense to use some of the waste heat to run a Brayton generator.<br /><br />Josh raised the argument that “Aerocapture is also impractical if you use solar panels for power. Using aerocapture results in nearly 50% fuel savings, so that must be counted against a solar option as well…” This won’t work. Solar cells can be folded up and stowed, as demonstrated on shuttle missions and on Mir. Another option is simply to dump them. An operating nuclear reactor needs cooling and thus a large radiator surface. You can’t dump this or fold it away. This is not incompatible with aerocapture but requires a different type of heat shield, typically a very large coolie hat rather than the bionic currently popular. Borowski’s BNTR study used all propulsive mode for the crewed mission – no aerocapture.<br /><br />Jon <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>

 

[JonClarke]

Nuclear power systems are often advertised as massing less than the solar counterparts. For example, in the statement by gunsandrockets that the “SAFE-400 reactor masses only 512 kg and produces 100 kWe.” <br /><br />The reality is much more complex. The core may well mass this amount, but the power system masses a good deal more. This includes the shielding, control systems, power conversion system and thermal control. For example a study of the power system for the NASA DRM 3.0 (http://gltrs.grc.nasa.gov/cgi-bin/GLTRS/browse.pl?1999/TM-1999-208894.html), came to the conclusion that a 160 kW reactor system would require 12 tonnes, including all systems. So I suggest that the SAFE-400 would mass between 4 and 8 tonnes all up.<br /><br />One difficulty is that the design of solar and nuclear power systems is often approached differently. For example 160 kW for DRM 3.0 study above was determined by adding all the estimated demands, regardless of whether they are ever likely to be concurrent. However the previously referred to Kerslake study (http://www.grc.nasa.gov/WWW/RT1999/6000/6920kerslake2.html) showed that by intelligent and efficient management of loads the same mission (could be met by a solar power system that supplies less than 50 kW continuously and masses 8 tonnes. <br /><br />The conclusion of this is that solar and nuclear power systems are roughly comparable, mass wise. Other studies, with different assumptions, come to similar results. For example Landis showed that it was possible to have a practical array on Mars to supply a 100 kW to run the ISRU plant of the Mars direct architecture http://powerweb.grc.nasa.gov/pvsee/publications/mars/marspower.html . It is very clear that many mission planners are not cognizant of these facts, and use completely unrealistic assumptions, for example version 1.0 of the DRM assumed that a fixed 120 kW array would mass <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>