Nukemobiles on Mars

RobertDyck · 2016-01-22 23:39:53

kbd512 wrote:

The lithium ion batteries also pose operational issues and hazards to the crew, namely loss of capacity from cell deterioration and fire from cell explosion. There's no doubt in my mind that NASA would have the astronauts replace the batteries after every mission to prevent problems associated with cell deterioration. How many batteries do we want to send to Mars from Earth? Sending payloads all the way to the surface of Mars is expensive.

Batteries that caused problems on Boeing 787 Dreamliner are actually lithium polymer, not lithium ion. They have the same energy density (amp-hours per kg) but significantly cheaper. Lithium ion are more expensive, but much less susceptible to fire. I see Wikipedia reports batteries are lithium ion, but I read news articles that said they were lithium polymer.

I have a little experience with lithium polymer. When I worked at Micropilot, technicians would test autopilot software by flying a hobby RC airplane with their autopilot installed. Not every test succeeded, sometimes the plane would fly away until its fuel tank was empty. Micropilot was located in a rural area just outside the city specifically for these tests. Technicians had to enter farmer's fields, past startled cows more than once to retrieve their plane. I remember one day they tried a then-new lithium polymer battery. When the plane crashed, the battery ballooned. It didn't burn, but the technician looked up the manufacturer's instructions for disposal. It said to use a utility knife to cut open the battery, in a well ventilated area, where it could endure fire. He did so outdoors, in a brazier style BBQ. He wore a Lexan full face shield, leather welder's gloves, and lab coat. About half a second after cutting it open, the inside glowed like red hot charcoal, and it spewed thick black smoke. This was on a day with a breeze, the rest of us watching stood up wind so the lithium smoke blew away from us. It only burned a few seconds, but the technician was startled by the intensity. He said he'll never do that again.

In 2005, NASA invited members of the public to make presentations to them. The NASA administrator at the time hoped for new ideas from individuals who were not traditional aerospace engineers. I proposed replacing the battery of the EMU spacesuit. It had used silver nickel batteries that would last only 11 charge/discharge cycles. Since they charge the batteries fully before each EVA, that meant only 11 EVAs. I found 2 manufacturers, one produced a battery rated for 1,500 charge/discharge cycles, but the manufacturer emphasized temperature had to be very stable. The other manufacturer rated it for more robust temperature conditions, but only rated it for 500 charge/discharge cycles. That's more EVAs than the expected life of ISS, so replace the battery in each suit and you'll never have to replace them again. Or on a Mars Mission, every astronaut could go outside for a maximum duration 8-hour EVA every solar day (sol) on the surface. Obviously not going to happen, but a single battery module per suit could do it. Both batteries had the same number of amp-hours as the existing one, and 6 cells connected in series would provide the required voltage. Lockheed-Martin got the contract instead of me. A representative of Lockheed-Martin was in the audience when I gave my presentation. They bought the American subsidiary of the manufacturer of the robust batteries. How does a guy working out of his garage compete with a multi-billion dollar company that can buy my supplier?

Anyway, these are lithium-ion batteries. They have a metal case, not aluminized plastic film. They're a lot tougher than lithium-polymer.

kbd512 · 2016-01-22 23:51:14

JoshNH4H wrote:

Am-242 is produced from U-238 as follows:
U-238 absorbs a neutron and becomes U-239
U-239 decays to Np-239, which decays to Pu-239 (Half of the U-239 will become Pu-239 in about 60 hours)
Pu-239 absorbs 2 more neutrons and becomes Pu-241
Pu-241 decays to Am-241, with a half-life of 15 years
Am-241 absorbs a neutron and becomes Am-242
It's worth noting that the isotope you want is actually Am-242m, which is what's called a metastable isomer of Americium. According to Wikipedia this process produces 10% Am-242m and 90% Am-242, which has a half life of just 16 hours and thus would not be useful for a Martian reactor.
Obviously it's possible to produce, but it will require a big investment in Earthside nuclear infrastructure. Is it worth it?

If NASA requires reliable power for mobile surface exploration of Mars, then the investment is worth the reward. If NASA intends to create a base on Mars and then never move far from where the habitat touched down at, probably not. NASA says it wants to explore the surface of Mars, but the available solar and battery options are simply not amenable to that goal.

Uranium fuel technology presently available is the most advanced because the overwhelming majority of research funding was spent on it. Unfortunately, it's just not good enough for mobile space power applications. Every attempt to circumvent physics will lead us right back to the original problem.

There's a substantial mass and volume difference between a reactor the size of a 55 gallon drum, which is what's required if we're stuck with Uranium fuel and traditional radiation shielding materials, and a 100 pound propane tank, which is what we could have using Americium fuel and optimized radiation shielding materials.

kbd512 · 2016-01-23 00:18:37

JoshNH4H wrote:

kbd512-
The idea with distance was not to recharge the vehicles as a convoy, but rather to run power via wires from an appropriate distance. This may be more desirable than solid shielding both in terms of mass and in terms of modularity

You still need a properly shielded reactor, two or more vehicles with electric motors powerful enough to tow a vehicle disabled by electric motor failure, and batteries in the vehicle(s) powered by the reactor carrier.

If the power cable snaps because it is tensioned to keep it off the ground or is abraded from dragging it on the ground, then you need to carry replacement power cables. Every aspect of the power and transportation solution must be durable and redundant.

Explain what you mean by modular, with respect to using reactor carriers. Do you intend to use the reactor carrier to power a Mars base by landing the reactor away from the base and then driving it to the base?

RobertDyck · 2016-01-23 00:20:43

Chart from Wikipedia:

This indicates the path is: 238U → 239Pu → 240Pu → 241Pu → 241Am → 242mAm
It also indicates a lot is lost. Of the 239Pu, 64% is lost to fission. Of 241Pu, 72% lost to fission, 25% becomes 242Pu, only 3% becomes 241Am. Of that, 10% lost to fission, and 79% becomes 242Am; only 10% becomes 242mAm. The remaining 1% must be rounding error.

kbd512 · 2016-01-23 01:59:16

RobertDyck wrote:

Batteries that caused problems on Boeing 787 Dreamliner are actually lithium polymer, not lithium ion. They have the same energy density (amp-hours per kg) but significantly cheaper. Lithium ion are more expensive, but much less susceptible to fire. I see Wikipedia reports batteries are lithium ion, but I read news articles that said they were lithium polymer.

In general, lithium batteries have operational issues in extreme environments. On Mars, the batteries would have to be warm enough to charge them, so they would need to be stored inside the vehicle's pressure hull.

RobertDyck wrote:

I have a little experience with lithium polymer. When I worked at Micropilot, technicians would test autopilot software by flying a hobby RC airplane with their autopilot installed. Not every test succeeded, sometimes the plane would fly away until its fuel tank was empty. Micropilot was located in a rural area just outside the city specifically for these tests. Technicians had to enter farmer's fields, past startled cows more than once to retrieve their plane. I remember one day they tried a then-new lithium polymer battery. When the plane crashed, the battery ballooned. It didn't burn, but the technician looked up the manufacturer's instructions for disposal. It said to use a utility knife to cut open the battery, in a well ventilated area, where it could endure fire. He did so outdoors, in a brazier style BBQ. He wore a Lexan full face shield, leather welder's gloves, and lab coat. About half a second after cutting it open, the inside glowed like red hot charcoal, and it spewed thick black smoke. This was on a day with a breeze, the rest of us watching stood up wind so the lithium smoke blew away from us. It only burned a few seconds, but the technician was startled by the intensity. He said he'll never do that again.

Any cell rupture in the confines of the vehicle would likely suffocate the crew.

RobertDyck wrote:

In 2005, NASA invited members of the public to make presentations to them. The NASA administrator at the time hoped for new ideas from individuals who were not traditional aerospace engineers. I proposed replacing the battery of the EMU spacesuit. It had used silver nickel batteries that would last only 11 charge/discharge cycles. Since they charge the batteries fully before each EVA, that meant only 11 EVAs. I found 2 manufacturers, one produced a battery rated for 1,500 charge/discharge cycles, but the manufacturer emphasized temperature had to be very stable. The other manufacturer rated it for more robust temperature conditions, but only rated it for 500 charge/discharge cycles. That's more EVAs than the expected life of ISS, so replace the battery in each suit and you'll never have to replace them again. Or on a Mars Mission, every astronaut could go outside for a maximum duration 8-hour EVA every solar day (sol) on the surface. Obviously not going to happen, but a single battery module per suit could do it. Both batteries had the same number of amp-hours as the existing one, and 6 cells connected in series would provide the required voltage. Lockheed-Martin got the contract instead of me. A representative of Lockheed-Martin was in the audience when I gave my presentation. They bought the American subsidiary of the manufacturer of the robust batteries. How does a guy working out of his garage compete with a multi-billion dollar company that can buy my supplier?

I want NASA and BAE to steal my idea. I just want humans to actually explore the surface of Mars and I don't care who gets credit. I'm not trying to make money off this. I have a day job writing software for statistical analysis programs and that pays my bills.

RobertDyck wrote:

Anyway, these are lithium-ion batteries. They have a metal case, not aluminized plastic film. They're a lot tougher than lithium-polymer.

Ok, but how much do these batteries weigh?

RobertDyck · 2016-01-23 09:11:49

My idea for the spacesuit had protections. Put the lithium ion batteries in a plastic bag in the PLSS backpack. That puts them outside the pressure envelope of the suit. Use a fluoropolymer for the plastic bag so it's immune to battery acid. I recommended PCTFE because it's immune to battery acid, can handle the extreme temperature swings in space, and the most gas impermeable. Aluminize the polymer film to further plug up pores in the polymer film, but put the side with the aluminum on the outside of the bag so battery acid can't get to it. Furthermore, instead of air, fill the bag with argon gas. Earth's atmosphere is 0.9% argon, so this is non-toxic, inert, and inexpensive. If a lithium battery of any sort ruptures, as soon as the lithium metal anode touches oxygen it bursts into flames. It doesn't need an ignition source, it *IS* an ignition source. But if the bag is filled with argon, and it's outside the suit pressure envelope so surrounded by vacuum, it can't ever get oxygen. That way it's absolutely impossible for the battery to burn. No fire, no smoke. So the thick black smoke won't happen either.

Batteries have to be temperature stable. The batteries I chose were relatively robust, but can't handle the extremes of space. To keep temperature relatively stable, press the plastic bag containing the batteries against the astronaut's back. Against the neoprene air bladder of the suit. So the plastic bag containing the batteries is outside the pressure envelope, but just pressed against it. That allows the astronaut's own body to regulate battery temperature. It's just a matter of arranging thermal insulation: put insulation around all sides of the battery except the side to the neoprene air bladder of the suit, the astronaut's back.

A vehicle can do the same thing. Put the batteries in a pressurized module pressed against the pressure hull of the vehicle, but outside the pressure hull. For safety. The habitat will also require batteries. Mars Direct used a nuclear reactor for the ERV, but solar for the hab. That's fine in space, during transit from Earth to Mars, but once on Mars there's night. Mars has day and night just like Earth. Solar panels don't generate electricity during night. You can't ask astronauts to stop breathing between sun-down and sun-up, so you will need batteries to run life support at night. Lithium-ion batteries have the highest energy density, so lowest mass for a given battery capacity. Yes, the battery module will be inside the pressure hull. Put the batteries in a sealed module, make the containing box or bag out of something impervious to battery acid (such as PCTFE or some other fluoropolymer), and fill the module with a noble gas. Argon is the cheapest noble gas. Helium is the lightest, but I don't think you need to fill the battery module with helium. Besides, you want batteries to be temperature stable, and they can generate heat when a lot of power is drawn from them. I'm not sure but I suspect argon transfers heat better than helium.

kbd512 wrote:

I want NASA and BAE to steal my idea. I just want humans to actually explore the surface of Mars and I don't care who gets credit. I'm not trying to make money off this. I have a day job writing software for statistical analysis programs and that pays my bills.

I'm unemployed. I lost my job when I tried to run for federal politics in Canada. Someone really didn't like me, screwed me over before I could get started. I've been struggling with a little home business doing computer repairs ever since.

kbd512 wrote:

RobertDyck wrote:
Anyway, these are lithium-ion batteries. They have a metal case, not aluminized plastic film. They're a lot tougher than lithium-polymer.
Ok, but how much do these batteries weigh?

Actually, lithium polymer batteries are not lighter than lithium-ion. They tried, but did not succeed. When they tried to make the batteries lighter, they reduced battery capacity too. So battery mass for a given capacity is actually the same. Lithiuim-ion are a little more expensive than lithium-polymer, but lithium-ion is what you have in cell phones, smart phones, and laptop computers. So you already use them.

If you want the details, I may as well post them now. The idea was already stolen.

nominal capacity at 0.2°C: 27 Ah (yes that's Amp-hour, not milliAmp-hour)
minimum capacity: 25 Ah
nominal voltage: 3.6 volts
case material: stainless steel
weight: 980g (that's grams, as in 0.980kg)
positive anode: lithium nickel cobalt oxide
negative anode: graphite
specific energy at 0.2°C: 99 Wh/kg
operating temperature: -30°C to +60°C
recommended charge temperature: 0°C to +40°C
storage and transport temperature: -40°C to +60°C
cycle life at 20°C and 100% DOD (depth of discharge): >1000 cycles to 80% nominal capacity
(0.5C charge; 0.5C discharge): > 2000 cycles to 60% nominal capacity

This is actually improved over what they published in 2005. They said 500 cycles. This is a German company, the other supplier was a French company that claimed cycle life of 1,500 charge/discharge cycles, but only if temperature was carefully controlled. This shows the German company has further improved their product.

And further note operating temperature. They had said operating temperature could not drop below -20°C, which is important considering these big batteries are intended for electric cars. Winter in my city drops below -20°C for weeks at a time, just yesterday was the last of a 2 week stretch this year. The coldest nights of the year can get below -30°C; only at night, and only the very coldest nights, but it does happen. When I grew up the absolute coldest nights got down to -40°C, although the last time that happened was one night in January 2005. Global warming is making winter more tolerable, this year the low was -27.2°C and it looks like we're past the worst.

Also notice they have larger batteries now. The 27 Ah batteries were the largest in 2005, now they have 45 and 55 Ah cells. But the Extravehicular Mobility Unit (EMU) spacesuit uses 27 Ah batteries.

Last edited by RobertDyck (2016-01-23 09:58:53)

RobertDyck · 2016-01-23 10:10:27

I may as well post the other supplier as well. The French battery supplier is Saft. Their space catalogue is here

Last edited by RobertDyck (2016-01-23 10:11:22)

SpaceNut · 2016-01-23 10:36:37

There are othertrade offs for Lithium ion batteries in what can they be drawn down to is 25% of a full charge as it can cause the batteries to internally short and or phase reverse upon recharging. The full charge storage value of them is 75% of a full charge that is why you initilly must charge the battery before use.

There was work being done on phase change materials for wicking the heat from the battery pack around 2005 but I have not been around the industry for a while now.

JoshNH4H · 2016-01-23 11:27:19

kbd512 wrote:

You still need a properly shielded reactor, two or more vehicles with electric motors powerful enough to tow a vehicle disabled by electric motor failure, and batteries in the vehicle(s) powered by the reactor carrier.
If the power cable snaps because it is tensioned to keep it off the ground or is abraded from dragging it on the ground, then you need to carry replacement power cables. Every aspect of the power and transportation solution must be durable and redundant.
Explain what you mean by modular, with respect to using reactor carriers. Do you intend to use the reactor carrier to power a Mars base by landing the reactor away from the base and then driving it to the base?

The logical solution in my eyes would be to have a tensioned steel (Or possibly Aluminum or whatever) cord hanging a couple meters off the ground with the power cable hanging, connected every meter or two. The rovers would have some onboard power storage capacity for life support and multiple replacement for both.

Nuclear reactors can produce variable output, so that you could chain a number of these vehicles to each other and drive them at the same time. I personally of course don't plan to do any of this, but if I were to be planning a Mars mission nuclear powered vehicles would almost certainly not be a part of it.

It occurs to me that I've never heard of a mobile land-based nuclear reactor. Do we know how the bumps and bangs of driving would affect the stability of the core in the long term?

kbd512 wrote:

If NASA requires reliable power for mobile surface exploration of Mars, then the investment is worth the reward. If NASA intends to create a base on Mars and then never move far from where the habitat touched down at, probably not. NASA says it wants to explore the surface of Mars, but the available solar and battery options are simply not amenable to that goal.
Uranium fuel technology presently available is the most advanced because the overwhelming majority of research funding was spent on it. Unfortunately, it's just not good enough for mobile space power applications. Every attempt to circumvent physics will lead us right back to the original problem.
There's a substantial mass and volume difference between a reactor the size of a 55 gallon drum, which is what's required if we're stuck with Uranium fuel and traditional radiation shielding materials, and a 100 pound propane tank, which is what we could have using Americium fuel and optimized radiation shielding materials.

Who say it's not feasible to do Mars with existing solar panels and batteries*? Is it you? Is it Robert Zubrin? With what claims is this supported? Will these claims still be valid in ten years, or if they were made in The Case For Mars, were they still valid ten years ago? Are they valid now, 20 years after publication?

The thrust of my argument boils down to this question, which is applicable to every economic decision: If we spend money doing something, will we see an equal or greater return?

In this case: Should we spend money on a Uranium-fueled nuclear reactor, an Americium-fueled nuclear reactor, or generate chemical fuels back at base/at some stationary location and refuel as needed? How much money is it worth spending on development to develop a system that won't really be used that many times when another system might be a bit heavier or a bit less capable, but cheaper up-front?

For example: If instead of carrying a nuclear reactor, you carry a methanol/LOX powered ICE** generated back at base. Your range falls from "unlimited" to 1000 km. Is that so bad? How much money is it worth to be able to spend months driving around the planet? How does this compare to your expected development cost of either nuclear system over a chemically fueled one?

Extra mass is not a mission killer, per-se. It's an additional mission cost, but only one of many different kinds. If the cost to LEO is $5,000/kg, let's say the cost to the surface of Mars is $50,000/kg. That's $50,000,000/tonne. And that's definitely a lot of money, probably more than I'll make in my entire life. But it still doesn't make sense to spend $500,000,000 to save $50,000,000 on eight missions. It's these tradeoffs, considered at every level, that makes SpaceX a cheaper launch provider than United Launch Alliance, not some inherent lack of talent or ability.

*To be clear, I am not per se claiming that I think Mars ought to be solar powered, so much as I'm using that scenario as a rhetorical device. I believe that Mars could be powered either by solar or nuclear, both having benefits and drawbacks, and which comes out ahead depends on the political and social environment, who's doing the mission, and the state of solar and nuclear technologies at the time. It's important to remember that even if nuclear comes out ahead of solar now, solar is progressing faster. Even if this is more about funding than physics, it's important to remember that the amount of money NASA (or any Mars mission planner) is able to dedicate to energy technology development is going to be smaller than the amount that the energy sector or government as a whole can.

**GW Johnson once pointed out that the Nitrogen in air actually serves an important function in internal combustion engines, namely that it keeps the combustion temperature from getting too high. For this reason, it actually makes sense to loop some fraction of the CO2 exhaust back into the intake to keep the flame temperature down. Cogeneration could be used for cryocoolers to keep the LOX liquid.

Antius · 2016-01-23 15:11:46

Why not use Plutonium? It has a similar critical mass to americium and is much more abundant.

JoshNH4H · 2016-01-23 15:14:27

Alternatively U-233 would also be just as good

RobertDyck · 2016-01-23 15:30:45

Antius wrote:

Why not use Plutonium? It has a similar critical mass to americium and is much more abundant.

Americium has a much smaller critical mass.
U-235 is the highest, then U-233, then Pu-239, and Am-232m the smallest.

Last edited by RobertDyck (2016-01-23 15:31:41)

kbd512 · 2016-01-23 15:36:33

JoshNH4H wrote:

The logical solution in my eyes would be to have a tensioned steel (Or possibly Aluminum or whatever) cord hanging a couple meters off the ground with the power cable hanging, connected every meter or two. The rovers would have some onboard power storage capacity for life support and multiple replacement for both.

That could certainly be done, but for a 100M cable I think the cable would have to be tied to a spring loaded mast atop each vehicle to prevent the cable from dragging on the ground when the rovers go over uneven terrain. It's a good idea that deserves a test to see how well it works.

JoshNH4H wrote:

Nuclear reactors can produce variable output, so that you could chain a number of these vehicles to each other and drive them at the same time. I personally of course don't plan to do any of this, but if I were to be planning a Mars mission nuclear powered vehicles would almost certainly not be a part of it.

While it's true that reactors can very output, there's typically a level of heat production where other aspects of power production, like the operation of the electric generators, work most efficiently. I think the best solution would be to match the power production capacity to the expected load and then size the reactor appropriately.

What types of power production technologies would you use?

JoshNH4H wrote:

It occurs to me that I've never heard of a mobile land-based nuclear reactor. Do we know how the bumps and bangs of driving would affect the stability of the core in the long term?

There are reactors installed in ships and submarines and there have been reactors installed in aircraft. Those vehicles all subject the reactor to dynamic loading. Reactors are not in use for vehicle power here on Earth because power-to-weight ratio is more important and there's no shortage of gas stations.

JoshNH4H wrote:

Who say it's not feasible to do Mars with existing solar panels and batteries*? Is it you? Is it Robert Zubrin? With what claims is this supported? Will these claims still be valid in ten years, or if they were made in The Case For Mars, were they still valid ten years ago? Are they valid now, 20 years after publication?

How far can you drive in a battery powered vehicle before you have to stop to recharge the batteries? Here on Earth, where we have very smooth roads and there's no requirement to carry everything you need to survive, including oxygen and water, how far do all-electric vehicles go before you have to recharge the batteries?

I agree with Dr. Zubrin's goal of getting humans to Mars, but disagree with some of the methods he wants to use. After 20 years, he's still fixated on a specific way of accomplishing a task that does not take advancements in technology into account. However, he's an aerospace engineer and I'm not.

For cargo, "lift, throw, and let it go" is decidedly expensive in terms of mass requirements for propulsion. For cargo, I think "lift, deploy the SEP tug's solar panels, and wait a year or two for it to arrive on Mars" is a better option that delivers more payload to the surface of Mars. Faster is only better in certain contexts.

JoshNH4H wrote:

The thrust of my argument boils down to this question, which is applicable to every economic decision: If we spend money doing something, will we see an equal or greater return?

Define what you consider "an equal or greater return."

In space, where there is generally line-of-sight to the Sun, solar technology works quite well in the inner solar system. On rotating planetary bodies with atmospheres, it has limitations that nuclear power does not have. The ROI for nuclear power is the exploration capabilities that the technology permits.

JoshNH4H wrote:

In this case: Should we spend money on a Uranium-fueled nuclear reactor, an Americium-fueled nuclear reactor, or generate chemical fuels back at base/at some stationary location and refuel as needed? How much money is it worth spending on development to develop a system that won't really be used that many times when another system might be a bit heavier or a bit less capable, but cheaper up-front?

The only way you'll really get to explore Mars in a reasonable time frame is with functionally unlimited power. That's what nuclear reactors provide. If the goal is to actually explore Mars, then advanced nuclear power fulfills the requirement better than competing technologies at this time.

NASA has no issue spending billions of our tax dollars on things it has never once used, so I see no valid argument that explains why nuclear technologies continually receive a pittance in development funding while new rocket engines that are little improved over existing rocket engines receive so much development funding to produce such meager results.

JoshNH4H wrote:

For example: If instead of carrying a nuclear reactor, you carry a methanol/LOX powered ICE** generated back at base. Your range falls from "unlimited" to 1000 km. Is that so bad? How much money is it worth to be able to spend months driving around the planet? How does this compare to your expected development cost of either nuclear system over a chemically fueled one?

The ability to drive for months around the planet is equivalent to the ability to truly explore. There's no logical argument that passes muster for not taking a more balanced approach to allocation of funding for energy production that includes nuclear as well as solar, battery, and chemical technology.

JoshNH4H wrote:

Extra mass is not a mission killer, per-se. It's an additional mission cost, but only one of many different kinds. If the cost to LEO is $5,000/kg, let's say the cost to the surface of Mars is $50,000/kg. That's $50,000,000/tonne. And that's definitely a lot of money, probably more than I'll make in my entire life. But it still doesn't make sense to spend $500,000,000 to save $50,000,000 on eight missions. It's these tradeoffs, considered at every level, that makes SpaceX a cheaper launch provider than United Launch Alliance, not some inherent lack of talent or ability.

NASA clearly doesn't care about cost-per-mission. If it does, there's no plausible explanation for Saturn V or the Space Shuttle. Part of their mandate is to advance the state-of-the-art.

JoshNH4H wrote:

*To be clear, I am not per se claiming that I think Mars ought to be solar powered, so much as I'm using that scenario as a rhetorical device. I believe that Mars could be powered either by solar or nuclear, both having benefits and drawbacks, and which comes out ahead depends on the political and social environment, who's doing the mission, and the state of solar and nuclear technologies at the time. It's important to remember that even if nuclear comes out ahead of solar now, solar is progressing faster. Even if this is more about funding than physics, it's important to remember that the amount of money NASA (or any Mars mission planner) is able to dedicate to energy technology development is going to be smaller than the amount that the energy sector or government as a whole can.

Solar and battery technology are the first technologies I reviewed. I had no desire to stuff a nuclear reactor in a manned rover. I'm advocating for the use of the lightest and most performant power production technology that acknowledges reality - Mars is 50% further from the Sun than Earth is, has day/night cycles similar to Earth, and no roads or gas stations.

JoshNH4H wrote:

**GW Johnson once pointed out that the Nitrogen in air actually serves an important function in internal combustion engines, namely that it keeps the combustion temperature from getting too high. For this reason, it actually makes sense to loop some fraction of the CO2 exhaust back into the intake to keep the flame temperature down. Cogeneration could be used for cryocoolers to keep the LOX liquid.

Will internal combustion engine rovers and an ISPP plant be cheaper to develop and operate?

Here on Earth, how many hydrocarbon production facilities operate for a year or two without human involvement?

How are you going to power the ISPP plant?

Antius · 2016-01-23 15:53:35

If the purpose of the rover is exploration, then it should be possible to place small solar powered propellant factories along the intended exploration routes. If the factories are landed two years before and each is capable of storing 0.5 tonne of bipropellant, then the production rate would need to be about 0.8kg per day. A few m2 of solar panels should be enough at equatorial attitudes. Each factory would weigh no more a few hundred kg. A hundred or so of these factories would allow a methane/LOX powered rover complete coverage of the planet.

kbd512 · 2016-01-23 16:41:41

Antius wrote:

If the purpose of the rover is exploration, then it should be possible to place small solar powered propellant factories along the intended exploration routes. If the factories are landed two years before and each is capable of storing 0.5 tonne of bipropellant, then the production rate would need to be about 0.8kg per day. A few m2 of solar panels should be enough at equatorial attitudes. Each factory would weigh no more a few hundred kg. A hundred or so of these factories would allow a methane/LOX powered rover complete coverage of the planet.

Does the 300kg include EDL mass?

What would the physical volume of these plants be?

Is it possible to send all of them on one or two rockets and then deorbit and land them at evenly spaced intervals from each other?

Would we have to launch another set of fuel production plants for subsequent missions or do the plants contain enough hydrogen for at least one reuse?

Does that solution illustrate the lengths we'll go to to avoid using nuclear power?

We'd have to produce 50t of chemical fuel for a rover to ring the surface of the planet once using an internal combustion engine. A nuclear reactor would permit the rover to ring the surface of the planet multiple times in a single mission, if desired, and it would still be capable of that type of performance for another two missions.

Assuming reactor shielding and operations pose no insurmountable issues, which is an open question at this point due to simple lack of testing, does that adequately illustrate why nuclear power is so preferable to chemical power?

JoshNH4H · 2016-01-23 18:00:23

RobertDyck-

Per Wiki, Am-242, Pu-239, and U-233 are all on par, while U-235 is significantly higher.

kbd51-

If your primary focus is mass reduction, I think a power transfer system using distance (and the thin Martian atmosphere) as shielding is probably better. Operationally, I think shielding with matter might be better. Have you considered sandbagged regolith? It would be more massive but because most of the shielding mass would be imported it wouldn't increase the effective mass all that much.

If I were designing roving science equipment for a martian exploration mission or an early colony, I would definitely look towards chemical fuels. Again, I'm fairly agnostic on the nuclear/solar issue. Nuclear is less politically feasible but probably lower mass. Solar would probably be higher mass when you include all the necessary power storage (Batteries for the night, probably, and significant meth/lox or methanol/lox reserves for long duration solar storms). It really depends what the mission goals are. I think it's worth mentioning that once the colony starts building its own generators they're probably going to have to be solar thermal, so neither PV nor nuclear will be particularly useful to them then.

Re: Dynamic loads, they're obviously something that a reactor could be subjected to, but they might be something that makes the reactor a bit more dangerous to operate. The acceleration of a car hitting something is higher than anything experienced in a ship, after all.

Teslas can go about 300 mi on a charge, I think. Really it depends how much battery you have. In any case, I'm talking chemical fuels. It still depends how big your tank is, but there's no reason not to have a big one if you need it. Geologists on Earth still have plenty of exploration to do even after doing more than any Mars mission possibly could. What I'm saying is that the return from focusing on one particular area and sites within a few hundred km of there seems like it would give a pretty good return on investment. A colony would be better equipped to do exploration anyway; I think an exploration mission's prime goal is to determine if a promising site is viable for a colony or not.

Solar Electric Propulsion has its benefits and drawbacks. You might not actually save that much (And remember, it's money and not launch mass that is the figure of merit). It could take quite a long time to get cargo to Mars with present SEP technologies because of low thrust. More importantly, some of the reduced fuel mass goes to increased engine mass.

Because NASA is not now going to Mars, going to Mars represents a paradigm change. Part of that paradigm change is doing things in a cost effective way. Some technologies may be better and some worse, but neither solar nor nuclear power makes exploration impossible, and so we have to look at what we gain and lose. Nuclear is more capable in some ways but requires a bigger development program and won't be very useful once it's colony time. I'm not arguing for solar so much as arguing that it's not a clear-cut decision, and that a lot of the arguments we make on this forum aren't really based in fact as much as they're based in our vision for Mars.

SpaceNut · 2016-01-23 18:01:16

I would say from the read of others that a concensious of a reactor powered rover will not be delivered as a single piece and that it would need to be partially assembled once on mars before use. If that is the case then sending it in a modular assembly makes sense and any mass that one would finally have is just a number of payloads to manage in terms of the finish product.

As for the solar power drop chemical factories these could be made as lage or as small as to what we intend for the distances between to act as the refueling stations for the rover. The rover when using an ICE would need to be almost run in a continous manner as the batteries are really the backup and not meant to be for real full powered moving motion.

I was also thinking of beamed power from a reactor design to the moving rv as a means to keep constant distance between it and the rv. Beamed power can come rf energy such as microwave or by thermal heating such as a laser to make a sterling engine run or some other power generation combination.

SpaceNut · 2016-01-23 18:57:49

The current space mission solar array designs seem to be the firmular circular fans of Nasa seen on Orion and others in the UltraFlex solar array.

http://nmp.jpl.nasa.gov/st8/tech/solar_array4.html
or start with the beginning page for the array
http://nmp.jpl.nasa.gov/st8/tech/solar_array1.html

Solar energy striking a square meters worth of panels is 1367 watts at earth orbit. The 5.5-m diameter UltraFlex 175 array will be capable of providing 7 kW of power with a deployed specific power of 175 W/kg. The solar cells used in the UltraFlex 175 solar array can attain 28% efficiency, meaning 28% of the energy that strikes them is converted to electricity. The on orbit Mars will be 715w to 492w for Perihelion to Aphelion with the surface being much lower.

For comparison the ISS arrays produce 30.8 kW from each array and have a specific power of 32 W/kg.

We do know that there are more efficient cells but by time we get to go to mars hopefully there will be even higher to chose from that will be of less mass per wattage of output.

http://www.atmos.washington.edu/2002Q4/ … house.html

http://zebu.uoregon.edu/disted/ph162/l4.html

http://tomatosphere.org/teachers/guide/ … griculture

At slightly more than 75°N, Devon Island has solar irradiance similar to the solar irradiation on the Martian Equator. You can verify this fact by locating the solar conditions on Earth, at 75°N on the graph above.

So a solar greenhouse is already a testable item at the Mars societies location....

kbd512 · 2016-01-23 21:05:09

SpaceNut,

How many of those panels would we need to charge a 200kWh battery on the surface of Mars?

SpaceNut · 2016-01-23 23:08:23

Just a baseline for panels the rover solar arrays generate about 140 watts of power for up to four hours per sol (a Martian day). Thats 560Wh of energy recieved which was for moving and keeping the rovers electronics plus batteries from freezing during the martian night. So that resource must power the manned rover for another 20 plus hours at minimal current draw on the storage batteries. That said its not just about the panels when looking at the rover design....

The radiation on Earth is at 1400 W/m2 while on Mars is only 600 W/m2. Solar energy can be harvest using Solar cell at 40% of what we would harvest on earth. We typically say that Earth recieves 1000 W/m2 at the surface that said we can go with Mars as being 400 W/m2.

Giving the power as kWh implies a 24 hr clock for a multiplier of use (discharge) or in charging but a charge rate is not always the discharge or use rating value.

Just a sudo array number for earth orbit would be 200kW / 7 kW = 28.5 approximate panels with a panel mass of 40 kg for a total mass of 1128 kg. With earth surface recieving only 5 kW for the same array which would cause an increaseinn the total number of panels for the wattage. But for mars the panel would be recieving and outputting just 2000 W of electricity. So if we do want 200 kW of power we need 100 of our best panels to create it and thats only going to be for a 4 hour period around high noon....

If indeed we only want 200 kWh then that will be just 25 panels recieving full power during the 4 hour window.....

SpaceNut · 2016-01-23 23:45:45

Lithium ion battery Charging basics

The chemistry is basically the same for the two types of batteries, so charging methods for lithium polymer batteries can be used for lithium-ion batteries.
Charging lithium iron phosphate 3.2 volt cells is identical, but the constant voltage phase is limited to 3.65 volts.
The lithium ion battery is easy to charge. Charging safely is a more difficult. The basic algorithm is to charge at constant current (0.2 C to 0.7 C depending on manufacturer) until the battery reaches 4.2 Vpc (volts per cell), and hold the voltage at 4.2 volts until the charge current has dropped to 10% of the initial charge rate. The termination condition is the drop in charge current to 10%. The capacity reached at 4.2 Volts per cell is only 40 to 70% of full capacity unless charged very slowly.
•Charge temperature--must not be charged when temperature is lower than 0° C or above 45° C.
•Charge current must not be too high, typically below 0.7 C.
•Overdischarge protection--stops discharge when battery voltage falls below 2.3 volts per cell (varies with manufacturer).
•A fuse opens if the battery is ever exposed to temperatures above 100° C.

Here is the value for C in battery charging
http://www.microchip.com/stellent/group … 028061.pdf

LI-ION CHARGING
The rate of charge or discharge is often expressed in relation to the capacity of the battery. This rate is known as the C-Rate. The C-Rate equates to a charge or discharge current and is defined as:
I = M x Cn
where:
I = charge or discharge current, A
M = multiple or fraction of C
C = numerical value of rated capacity, Ah
n = time in hours at which C is declared.

kbd512 · 2016-01-24 07:13:05

JoshNH4H wrote:

Per Wiki, Am-242, Pu-239, and U-233 are all on par, while U-235 is significantly higher.

I'm no expert, but I'm aware of no other isotopes on par with Am242m, with respect to neutron cross section. A reactor using Am242m simply requires far less fissile material to obtain the same specific heat. It should follow that core diameter can be substantially reduced using fissile materials with very high neutron cross sections. This is important for mobile applications of nuclear power for a variety of reasons, but the most pertinent reason that I can think of for manned space applications of nuclear power is reduction of the required reactor shielding mass.

JoshNH4H wrote:

If your primary focus is mass reduction, I think a power transfer system using distance (and the thin Martian atmosphere) as shielding is probably better. Operationally, I think shielding with matter might be better. Have you considered sandbagged regolith? It would be more massive but because most of the shielding mass would be imported it wouldn't increase the effective mass all that much.

My focus on mass reduction is about keeping the landed mass of the payload to 20t or less. However, durability is also very important. Skimping on mass for the sake of saving an extra pound or two isn't helpful when you have no means to replace damaged equipment. If the mass requirement is so substantial that additional launches are required, then you have a problem.

There are a variety of reason why I selected the MTVL variant of the M113 as a suitable vehicle for Mars surface exploration, but the most important reasons were general durability, off-road mobility, and sufficient internal volume for consumables storage. There are lots of other vehicle designs that weigh less, but few have equivalent durability and mobility.

JoshNH4H wrote:

If I were designing roving science equipment for a martian exploration mission or an early colony, I would definitely look towards chemical fuels. Again, I'm fairly agnostic on the nuclear/solar issue. Nuclear is less politically feasible but probably lower mass. Solar would probably be higher mass when you include all the necessary power storage (Batteries for the night, probably, and significant meth/lox or methanol/lox reserves for long duration solar storms). It really depends what the mission goals are. I think it's worth mentioning that once the colony starts building its own generators they're probably going to have to be solar thermal, so neither PV nor nuclear will be particularly useful to them then.

Where is your colony going to obtain source materials and power required to construct its solar thermal power station?

JoshNH4H wrote:

Re: Dynamic loads, they're obviously something that a reactor could be subjected to, but they might be something that makes the reactor a bit more dangerous to operate. The acceleration of a car hitting something is higher than anything experienced in a ship, after all.

The vibration and dynamic loading aspect of reactor design requires testing, but vibration testing has been done before on various reactors here on Earth and no showstoppers have been noted.

JoshNH4H wrote:

Teslas can go about 300 mi on a charge, I think. Really it depends how much battery you have. In any case, I'm talking chemical fuels. It still depends how big your tank is, but there's no reason not to have a big one if you need it. Geologists on Earth still have plenty of exploration to do even after doing more than any Mars mission possibly could. What I'm saying is that the return from focusing on one particular area and sites within a few hundred km of there seems like it would give a pretty good return on investment. A colony would be better equipped to do exploration anyway; I think an exploration mission's prime goal is to determine if a promising site is viable for a colony or not.

If a solar powered rover could simply drive for all or most of the day, every other day, I'd consider that an unqualified success.

JoshNH4H wrote:

Solar Electric Propulsion has its benefits and drawbacks. You might not actually save that much (And remember, it's money and not launch mass that is the figure of merit). It could take quite a long time to get cargo to Mars with present SEP technologies because of low thrust. More importantly, some of the reduced fuel mass goes to increased engine mass.

If the SEP tug is less than $100M plus whatever the chemical TMI stage costs, then one launch is less expensive than two launches.

JoshNH4H wrote:

Because NASA is not now going to Mars, going to Mars represents a paradigm change. Part of that paradigm change is doing things in a cost effective way. Some technologies may be better and some worse, but neither solar nor nuclear power makes exploration impossible, and so we have to look at what we gain and lose. Nuclear is more capable in some ways but requires a bigger development program and won't be very useful once it's colony time. I'm not arguing for solar so much as arguing that it's not a clear-cut decision, and that a lot of the arguments we make on this forum aren't really based in fact as much as they're based in our vision for Mars.

I think it's fair to say that both ISPP and portable nuclear power generation require further development. The affordability aspect is just laughable when put into context. NASA has thrown billions left (Boeing), right (SpaceX), and center (Sierra Nevada) to develop multiple space capsules that all do the exact same thing or don't do what they were intended to do (Orion).

Apart from one reactor and a smattering of RTG's, absolutely every attempt to utilize nuclear power in space has had the rug pulled out from underneath it for non-technical reasons. Even here on this forum, we're coming up with every possible argument against using nuclear power when it's pretty clear that all the alternatives have greater mass for less capability and other operational drawbacks that nuclear power does not have.

At some point in the very near future, a NASA administrator is going to have to tell Congress and the President to either educate themselves about what space exploration involves, with respect to power requirements for deep space exploration or to simply shut down the program. The most ignorant people in the room can't continually dictate what NASA develops for space exploration.

RobertDyck · 2016-01-24 10:14:16

kbd512 wrote:

I think it's fair to say that both ISPP and portable nuclear power generation require further development. The affordability aspect is just laughable when put into context. NASA has thrown billions left (Boeing), right (SpaceX), and center (Sierra Nevada) to develop multiple space capsules that all do the exact same thing or don't do what they were intended to do (Orion).

Some people want Apollo back. NASA and Congress are throwing billions at resurrecting Saturn V and Apollo.

And some people who grew up in the 1980s & 1990s want Shuttle back. Actually, in 1968 NASA wanted a small shuttle to deliver 7 crew members and a little luggage to an International Space Station. They would use Saturn 1B or its successor for cargo and to construct the station. Nixon slashed NASA's budget, forced them to use one vehicle for everything. And forced the military to share too. So Shuttle became a Frankenstein's monster that did everything, but nothing well. After the Challenger accident, it looked like Congress might not let NASA fly Shuttle again, so NASA started development of a mini-Shuttle to replace it, for crew only. They developed HL-20. Saturn 1B cost less than a single launch of Shuttle. So...

JoshNH4H · 2016-01-24 14:41:33

Critical mass of various isotopes in kg:

U-233: 15.8
U-235: 46.7
Pu-239: 10.0
Am-242[sic]: 8.8

Other isotopes are listed too, if anyone is interested. Critical mass is determined by nuclear cross section and by the number of neurons produced per fission event, so even if the cross section is high that doesn't necessarily mean that the critical mass will be low. For what it's worth, there are isotopes on this table with lower critical masses than Am-242m, but they're even harder to produce. While it's true that lower critical mass implies lower shielding mass, it's also true that any non-nuclear power source requires no mass to shield from radiation.

What drove you to 20 tonnes of landed mass? Let's say a Falcon Heavy can put 10 tonnes on the surface of mars. Each FH launch costs ~125 mn. That's not so much compared to the mission cost, so I don't think that it makes sense to put a hard limit anywhere. Instead, once the mission's purpose has been decided, we need to decide whether spending money on something makes sense compared to the value of the stated purpose of the mission.

Relative to a chemical rover, a nuclear one will cost more and take more time to develop. On the other hand, it gives you unlimited range (or rather, range limited by your provisions). But how much good does unlimited range do you when you only have 550 days of surface time and a handful of crew? A circle 1000 km in diameter is huge, after all. It's bigger than Texas or France and roughly comparable to the entire northeast United States.

I would have initial missions be focused on proving resource availability and finding a spot for an initial colony location. Once the colony is there it would increasingly seek to produce everything locally, with the seed of machines sent from Earth to help them do so.

I'm not opposed to nuclear power in general. I agree that vibrations from a rolling vehicle are a concern but aren't prohibitive. I agree that a solar electric vehicle would probably have lackluster performance. I'm unsure on whether a solar electric tug or chemical propulsion is better, although I may try to run some numbers on it.

My challenge to you is to consider everything as a cost/benefit optimization and make decisions accordingly, recognizing that we have yet to lock ourselves into any design choice, and also recognizing that NASA has virtually no pull over Congress. When you say this:

At some point in the very near future, a NASA administrator is going to have to tell Congress and the President to either educate themselves about what space exploration involves, with respect to power requirements for deep space exploration or to simply shut down the program. The most ignorant people in the room can't continually dictate what NASA develops for space exploration.

What you're really saying is that there will be no Mars mission in the foreseeable future. We can do better than that.

kbd512 · 2016-01-24 15:34:21

Rob,

You can't run a space exploration program off of nostalgia. NASA needs to decide what it wants to do and then focus all effort and funding on that goal.

If the goal is to get humans to Mars, then it must be accepted that there are certain aspects of the Martian surface environment that don't work in favor of solar and battery technology, namely the extreme temperatures and distance from the Sun. As SpaceNut pointed out, for solar panels to work as efficiently as they do on Earth on Mars, a 40% power density improvement would be required. The mass of the solar panels is only part of the problem. Apart from the mass and volume of solar arrays that would provide sufficient power for manned applications, the most significant problem is the weight of the batteries required to store that energy. The array mass and volume are trivial problems compared to the battery mass and volume problems.

Solar panels and batteries were insufficient to provide Curiosity's power requirement of just 125 watts. Up the power requirement a couple orders of magnitude and your energy production and storage problems are significantly exacerbated. The RTG aboard Curiosity provides 2kWt and 125We at start of mission. It should be pretty clear that simple RTG's won't work for a manned rover, either.

If Mars had gas stations, I would be all in favor of using chemical fuels. I think any ISPP effort should focus on fueling a MAV and possibly a TEI stage. If we can make the propellant required to return to Earth, mass requirements for going to Mars are substantially reduced.

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