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#26 2017-05-13 19:27:18

SpaceNut
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Re: Mars Cart

kbd512 wrote:
Dook wrote:

Do I still want to use lead-acid batteries?  Yes.  Do you still want to risk the entire spacecraft and crew lives by shipping multiple lithium-ion batteries?

If the batteries the colonists depend on only have half the energy storage capacity required to make it through the night until the next morning comes along and the PV panels start delivering electricity again, are they going to hold their breath or learn to do without something else important like thermal control, food, or water because Rocket A is only capable of delivering Payload tonnage B?

It'd be nice to use the Marscat for more than a couple hours per day, too.

Using Lead-acid batteries, the battery tonnage you're looking at just to power WAVAR would be enough to ship a nuclear reactor, never mind the batteries required by the rovers and life support equipment.

Dook wrote:

ISS uses lots of lithium-ion batteries and there have been no fires?  There are video's on the internet of people's phones catching fire.  It's rare but it happens in lithium-ion batteries much more than lead-acid batteries.

Spacecraft batteries are better built than the batteries in Samsung's cell phones.

Dook wrote:

Properly designed lead-acid batteries won't short either?  Vibration from launch, aero-capture, parachute deployment, and landing could cause a battery to short.  You would have to have batteries powering the Mars Hab but the batteries in the rover or Marscat wouldn't have to be powered.  You could install the acid plates on Mars and add water.

Lead-acid batteries are vibration and G-force tested.  East Penn Manufacturing's batteries can pass a 2.2 million cycle +5G/-5G test on their Lead-acid batteries.  If you ran that same test on any spacecraft made to date, it'd be in a million pieces in a matter of seconds.

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#27 2017-05-13 19:30:33

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Re: Mars Cart

Terraformer wrote:

Wouldn't Nickel-Iron batteries make more sense for Mars? Lower density, but far more tolerant of abuse - and you can probably fabricate most, if not all, of the battery on Mars (still waiting for Open Source Ecology do develop plans for one...), so the mass is less of an issue.

Oldfart1939 wrote:

I use lots of battery powered lawn tools: lawnmower and weed eater. Also a chain saw. All are powered by Lithium ion batteries. And no, I haven't burned my housed down yet. Conversely, I can mow my entire lawn on a single charge of the E-Go 56 V battery.

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#28 2017-05-13 19:31:39

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Re: Mars Cart

kbd512 wrote:

The batteries required to store tens of kilowatts of power are very heavy, even if we use Lithium-ion, and more than twice as heavy if we use Lead-acid.  In real life, the Lead-acid batteries will require more frequent replacement than Lithium-ion as a function of duty cycle, so quadruple the weight of Lithium-ion batteries with equivalent amp-hour rating and that's pretty close to what a Lead-acid solution would weigh.

120,000W / 900W = 133.3 Lead-acid batteries

133 * 51.8 = 6,889lbs / 3,131kg / 3.131t

Lead-acid will last about a year on Mars with the kind of duty cycle and temperatures we'd subject them to:

6,889 * 2 = 13,778lbs

120,000W / 3,120W = 38.5 Lithium-ion batteries

38 * 80 = 3,040lbs / 1,382kg / 1.382t

Just a check to see how much 4 times as many Lithium-ion batteries would weigh:

3,040 * 4 = 12,160lbs

So, my previous supposition that you'd about quadruple the weight for Lead-acid is pretty accurate.

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#29 2017-05-13 19:34:36

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Re: Mars Cart

DOOK wrote:

I'm not concerned about minimizing batteries.  The Mars Hab will need a dependable battery bank.  I'm not sure exactly how many.  On Mars they would have constant power from the RTG and additional power from the solar array in daytime. 

Lead-acid batteries will last a year on Mars?  They should last the normal 8-10 years.  The batteries in the Mars Hab should be the same temperature as the inside of the hab.  The batteries in the Long Range Rover and Marscat would have small battery heaters under them.  No battery would be discharged for more than 1 or 2 hours except in an emergency.

kbd512 wrote:

The Lead-acid battery in my truck doesn't last 10 years or 8 years, it lasts 2 years and it's capacity is greatly diminished after a year or so.  The capacity of Lead-acid batteries is even worse than Lithium-ion in low temperatures.  Go read the manufacturer's specs or talk to an auto mechanic and ask them how many Lead-acid batteries they've seen that lasted 8 years, never mind 10 years.  The gel cells are a different technology.  There are multiple Lead-acid technologies in commercial use.

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#30 2017-05-13 19:40:18

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Re: Mars Cart

DOOK wrote:

How many times have I drained 50% to 80% of the capacity on my battery?  When I was young and working on my car a lot (I had a Vega with a grumpy carburetor) I would drain my battery sometimes.  Hasn't happened in the last 30 years. 

The Long Range Rover and the Marscat will have multiple deep cycle batteries and they will be used for about an hour a day then recharge.  The Long Range Rover has it's own solar array on top so it can recharge itself.  The Marscat would be plugged in to the base power.  So, they won't be discharged to 50% unless there is some emergency.

If temperature increases or decreases from 77 degrees F it cuts battery life?  The Long Range Rover and Marscat would be used at mid day when it's about 70 degrees.

kbd512 wrote:

How many times have I drained 50% to 80% of the capacity on my battery?  When I was young and working on my car a lot (I had a Vega with a grumpy carburetor) I would drain my battery sometimes.  Hasn't happened in the last 30 years. 

The Long Range Rover and the Marscat will have multiple deep cycle batteries and they will be used for about an hour a day then recharge.  The Long Range Rover has it's own solar array on top so it can recharge itself.  The Marscat would be plugged in to the base power.  So, they won't be discharged to 50% unless there is some emergency.

If temperature increases or decreases from 77 degrees F it cuts battery life?  The Long Range Rover and Marscat would be used at mid day when it's about 70 degrees. 

If you plug the vehicles in to base power, then they don't need very big batteries, but now you have an extension cord connected to a piece of earth moving equipment.  It would work if someone is handling the cord.

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#31 2017-05-13 19:44:52

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Re: Mars Cart

DOOK wrote:

The fission reactor would work?  I'm not against using a reactor but the electrical power requirements could be greatly reduced if we get rid of the rocket fuel manufacturing and other non-life support uses like trying to make steel.

Solar panels require batteries?  The Mars Hab will have built in batteries. 

Start draining 50% of my battery capacity and see how long it works?  Why would anyone design a Mars vehicle to do that? 

We could have Mars vehicles that operate around the base using extension cords instead of battery power and someone (you think a robot) could handle the cord?  We could.  I think having a Marscat that can be driven independant, without having to worry about running over it's cord or have another person move the cord around, outweighs the long electrical cord. 

Will the Mars Hab need power at night?  Of course.

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#32 2017-05-13 19:46:49

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Re: Mars Cart

DOOK wrote:

Then we would launch a big RTG or reactor module and another supply module, they dock and go to Mars. 

Six months later we would launch a Mars hab with a small RTG in it, and 4 crew (2 couples), into orbit and dock with a rover hanger (With a Long Range Rover, Mars cart, and Marscat) and both spacecraft would blast off to Mars.

On landing the crew sets up the thin solar array and the small RTG then 2 crew drive the Long Range Rover over to the pre-landed buried habitat and supply modules.  They bring everything back to the base on the Mars cart.  Then they go and get the big RTG or reactor and supplies and bring them back to the base.  They may have to make multiple trips. 

Then they use the Marscat to dig a 100 foot wide circular pit that is 17 feet deep and has tapered sides.  They would only use the Marscat for about 1-2 hours a day so it would take some time.

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#33 2017-05-13 19:48:47

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Re: Mars Cart

DOOK wrote:

How much regolith can the Marscat move in one hour a day?  Hmm, don't know.  The front bucket would be 4 feet wide, about 1 foot deep.  At first digging would be easy so maybe they could make 60 passes an hour for the first two weeks.  Then as they got down they would hit permafrost and would have to let it heat up and evaporate, and they might hit rock, and they would have to drive the regolith up the side so they would be down to 40 or 30 passes an hour. 

I'm not against using lithium-ion batteries in the Long Range Rover and Marscat if they don't have electrolyte in them during the space travel.  The water can be added when they land on Mars while they are still in the Rover Hanger.

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#34 2017-05-13 20:19:13

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Re: Mars Cart

kbd512 wrote:

Rough figures, but indicative of the minimal amount of work required...

If we had a 10 yard high HESCO barrier, then all we need is about 1,145 cubic yards of material to create a suitable barrier for permanent settlement (2.5 yards of material).  That would take about 64.5 days with Lead-acid batteries, assuming a fission reactor for recharging.  We need 75kWh of capacity with Lead-acid to get us to 30% depth-of-discharge, so the batteries give at least four years of reliable service.

75,000 * .3 = 22,500

75,000 / 900 = 83

83 * 51.8 = 4,299lbs of Lead-acid batteries

80% depth-of-discharge is no real issue for Lithium-ion and there's far less voltage drop, so...

28,000 * .8 = 22,400

28,000 / 3,120 = 9 * 80 = 270lbs of Lithium-ion batteries

Alternatively, we use fewer Lead-acid batteries and kill them immediately or kill them in about a year.

With a fission reactor, we can recharge the batteries twice per day using the Lead-acid or four times per day with the Lithium-ion.  So, 64.5 days with Lead-acid or 32.25 days with Lithium-ion.

This looks feasible, provided we use a HESCO barrier and there's a way to dump regolith into the barrier container.  I can't recall if I've ever seen HESCO that high... crap.  I need at least double the amount of material to stack HESCO that high.  So 129 days with Lead-acid or 64 days with Lithium-ion.  Even so, it's still doable.  If there's a way to get that thing closer to the ground, this would be a whole lot easier and faster and safer.

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#35 2017-05-13 20:19:23

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Re: Mars Cart

Dook wrote:

Marscat

My idea is to have seven 25' buried circular cylinder habitats each connected by a short 5' hallway with the living habitat at the center and six hydroponics and growing habitats around it. 

The habitats would be 8 feet tall, have a 6" ceiling panel, and be buried under 8.2 feet of regolith so we would need to dig down 17 feet and level the bottom.         

So, digging a circular pit 17' down and 85 feet wide at the bottom would require moving about 96,417 cubic feet of material.  Tapering the sides to a 45 degree angle would require moving about another 43,350 cubic feet of material.  So, that is a total of 139,767 cubic feet, if my math is correct. 

A Marscat that is the same size as an Earth bobcat has a 5.6 foot wide by 2 foot deep bucket.  So, that's about 11.2 cubic feet of material per load.  If the Marscat can make 60 passes an hour, one a minute, it will move 2,016 cubic feet in three hours.  Using three Marscats that would be 6,048 cubic feet moved a day. 

It would take 23 days to dig out the pit before you could start to build the seven buried cylinder habitats. 

The three Marscats (1,600 lbs each, total of 4,800 lbs) would be in the Rover Hanger along with a Long Range Rover (2,000 lbs) and a tracked Mars cart.

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#36 2017-05-13 20:20:22

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Re: Mars Cart

louis wrote:

I think your 8 feet of regolith shielding is excessive from everything I have read, especially as plants handle Mars level radiation v. well.

I agree with a high ceiling if we can manage it, because that enables more plant trays. Not sure why you want to build a complicated system why not build straight trenches out from a central habitat like a cross? 

My real concern though is health and safety,as the more you attach in a pressurised environment to your central habitat, the greater the scope for disaster, specifically fire or a catastrophic pressure event - but also if we are talking about a farm hab environment, spread of mould or other pathogens.

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#37 2017-05-13 20:22:02

SpaceNut
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Re: Mars Cart

DOOK wrote:

Using 8.2 feet of regolith for shielding is the number that other people came up with, not I.

Plants handle Mars radiation well?  The shielding is for the people, not the plants. 

Why build a complicated system instead of straight trenches?  I didn't think that circles were complicated but the reason is to keep everything close together so we could then build a 100 foot wide dome greenhouse over the top of it to provide heat and possibly be able to pressurized the dome to grow plants.

The more you attach to your central habitat the greater the chance for fire or pressure failure or mold or other pathogens?  The buried habitat will be pressurized, the pressure will equal the weight of the regolith on top but even if a pressure loss does happen the habitats will be strong enough to keep from collapsing.

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#38 2017-05-13 20:33:55

SpaceNut
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Re: Mars Cart

Dook wrote:

Marscat could work but not with 4,900 lbs of batteries?  Each Marscat would weigh 1,600 lbs and have 12 batteries.  So, the batteries would weigh about 600 lbs.  All of that is Earth weight.

kbd512 wrote:

I fat-fingered the Lithium-ion battery weight in Post #265.  It's 720lbs of Lithium-ion batteries, not 270lbs.  So 720lbs gives you 1 hour of usage at a consumption rate of 22kW/hr.  That assumes the use of two 30hp electric motors consuming an average of 22kW/hr, like you said you wanted in your Marscat.

Edit: Stupid mistake alert.  Disregard the next two paragraphs.  The Tesla EV battery packs are only 5.3kWh per pack.  660lbs of EV batteries is only 63.6kWh.  If it was 21.2kWh for 55lbs, nobody would be using gas powered cars anywhere.

Something is "off" with that number for Lithium-ion, though (maybe it's old tech), because a 21.2kWh Tesla EV battery pack only weighs 55lbs and produces double the voltage of that 12V 260AH SmartBattery.  If the Marscat had 4 Tesla Lithium-ion packs, it'd have 84.8kWh of electrical power storage onboard and the packs would weigh 220lbs, which does not include aluminum cooling plates.  That's enough power for 3 hours of continuous operation.  660lbs of 21.2kWh Tesla EV battery packs provide enough power for 9 hours of continuous operation.

If the Marscat works to fill HESCO for 9 hours per day, the regolith shield will be done in a matter of weeks.  We could even skip HESCO and just pile up regolith.  The 254.4kWh 660lb Lithium-ion pack has enough juice for us to use it just like a bulldozer and dispense with the weight of HESCO barriers.

The 600lb Lead-acid battery pack only provides 10.8kWh of power with 12 batteries, assuming you completely drain the battery, which would quickly kill the batteries.  I'm not sure what you're going to do with that little battery capacity in an earth mover.

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#39 2017-05-13 20:36:03

SpaceNut
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Re: Mars Cart

DOOK wrote:

I think I'm going to have to increase the number of batteries to 14 but lithium-ion is fine.  I still like the idea of adding the electrolyte when they get to Mars.  Even so, three hours of use would discharge each battery about 30%, which might be too much.

And if I only dig down 12 feet instead of 17 then I can cut the three Marscats down to only 2 and it would take 23 days, 3 hours of use a day, to dig out the area for the buried cylinder habitats.

The secret to the telsa battery is in how its constructed to keep from catching fire...by small cells that have cooling built in to wick the heat away....

tesla battery pack images

The low level discharge is for storage and not use levels....when you discharge li-ion batteries that far you risk damaging the cell structure causing a barrier to charging to form which can cause the battery to short and not charge to full capacity. Recomended discharge in operation is only to 70% of its value before recharging....

http://www.techrepublic.com/blog/five-a … tery-life/

Characteristics of Rechargeable Batteries

Safe Lithium-ion Battery Designs for Use, Transportation and Second Use

Guidelines on Lithium-ion Battery Use in Space Applications

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#40 2017-05-13 20:37:52

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Re: Mars Cart

DOOK wrote:

I wasn't clear enough on the lithium-ion battery discharge, three hours of use (two 30 hp motors in each Marscat) would use 30% of the batteries (14 lithium-ion batteries).  So, the batteries would be drained to 70% of their capacity. 

Now, I have to figure out how long it would take to fully recharge the two Marscats from base power so they can be used the next day for 3 hours again.

Dook wrote:
Oldfart1939 wrote:

One way to minimize the regolith moving would be to excavate a trough just deep enough to have half the cylinder sunk in place, then use the excavated "spoil" to recover it. That would cut the battery usage a lot. If we need more regolith, just dig up the surface and keep heaping it on until "enough" has been moved. That's how I was envisioning a "passageway" from the habitat units exteriors into the greenhouses would be built.

That's why I changed the depth from 17 feet to 12. 

I could raise it further but the seven buried cylinders need 8 feet on top of them for radiation protection and I would like to put a 100 foot domed greenhouse over them to provide heat and I didn't want there to be a big hill inside the greenhouse. 

A hill a few feet high would be fine but not too much.

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#41 2017-05-13 20:40:53

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Re: Mars Cart

kbd512 wrote:

Edit: Stupid mistake alert.  12 packs are 63.6kWh, which only equates to 2 hours 20 minutes of run time at 80% DoD for the Marscat.

The depth-of-discharge issue applies to every battery, but Lithium-ion batteries are designed to discharge 80% of their rated capacity and that's what all manufacturers test against.  12 Lithium-ions (12 Tesla packs) gives 9 hours of power per Marscat.  The batteries should last for at least a thousand of cycles if you discharge them that way.  If the cats are stored outside, and for all practical purposes they have to be, then RHU's (Radioisotope Heater Units) must keep the batteries warm.  I like RHU's because there's no reliance on batteries or base power to keep the packs above their minimum charging temperature at night.  The RHU's provide a gradually declining output over a decade or so.  RHU's are small (26mm / 1in by 32mm / 1.3in) and light (40 grams / 1.4oz), but expensive because they contain Pu238.


Dook,

Again, apologizes for the mistake.  I can't seem to get my numbers straight for these stupid EV batteries.  The Tesla battery packs are only 5.3kWh and weigh 55lbs.  Something in the back of my head kept telling me that couldn't be right, or nobody would be using gasoline.  That means it's 220lbs for 21.2kWh, so 660lbs only nets 50.88kWh at a 80% DoD, which means 2 hours 20 minutes or run time per charge.

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#42 2017-05-13 20:46:49

SpaceNut
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Re: Mars Cart

DOOK wrote:

I don't know where I want to store the Marscat's yet.  They will probably be stored outside and connected to the base power to recharge every night.  I would like to bring them inside the rover hangar but I don't like the idea of having to pressurize and depressurize the rover hangar each day to drive the Marscats out to do work.  Also, the rover hangar would need a large door and if we pressurized the rover hangar to even 2 psi the force on the door would be incredible.  I could just not pressurize the rover hangar at all and use it as a warmer place to store the Marscats while they recharge.

And I still need to figure out how long it takes to recharge the batteries over night and how much energy it will need.  At night the RTG will be the only power source. 

Those radiation heaters should be installed under all the batteries in the Marscats and the Long Range Rover.

Maybe pre-deliver the rover instead of having a rover hangar?  The crew that arrives has to have it's own Long Range Rover because they will need a way to go and get stuff.  You guys think that everything is going to land near the main base, they're not.  Things are going to be landing all over the place.  You're going to need a good sized rover that can go far out and pull a Mars Cart loaded down with heavy things (buried habitat components, greenhouses, food, water, supplies) back to the base.

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#43 2017-05-13 20:48:27

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Re: Mars Cart

Dook wrote:

One Marscat uses 134,280 watts in 3 hours of use.  Two Marscats would use a total of 268,560 watts in 3 hours of use. 

So, if you use the Marscats for 3 hours a day and recharge them for 21 hours a day they both need a combined 12,788 watts per hour to recharge and be ready the next day.  That's a problem. 

I think a big RTG would produce 600 watts an hour, two 10'x50' thin solar arrays would produce about 450 watts each (900 total), a thin solar array on the top of the Mars Hab covering the regolith would produce another 635 watts an hour, and a solar array on the entire outside of the Mars Hab would produce about 360 watts an hour.  So, that's a total of 2,095 watts produced every hour in the day, and 600 watts an hour at night.  That's not going to be enough.   

How much power would a small nuclear reactor generate? 

I think some time ago Spacenut posted a link to a nuclear reactor that was 30 feet wide.

I found it, the Sandia Report 50-100 kwe gas cooled reactor.  I just don't like all the moving parts compared to an RTG.       

The two Marscat would use 268,560 watts total working 3 hours a day.  So, they need 12,788 watts total recharge an hour to be fully recharged.  So, we need to produce about 15,000 watts an hour of power all night long. 

The Buk RTG makes 3,000 watts and weighs 2,200 lbs.  If we increased it 5 times it would produce 15,000 watts an hour and weigh 11,000 lbs.

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#44 2017-05-13 20:55:38

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Re: Mars Cart

kbd512 wrote:

The Kilopower reactors NASA and DOE are developing are 10kWe and total system mass (radiators, shielding, reactor core, electric generators) is  1800kg.  These reactors use commercial LEU (Low Enriched Uranium), so you need a lot of Uranium (larger core, larger reflector, more shielding), but it's much cheaper than HEU (Highly Enriched Uranium), which is what SAFE-400 uses.  Apart from the generators, it has one moving part, a single control rod inserted into the middle of a Uranium cylinder.  That's pretty simple.

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#45 2017-05-13 20:56:22

SpaceNut
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Re: Mars Cart

kbd512 wrote:
Dook wrote:

The thing about needing all this power is it's only needed to recharge the Marscats for 21 days of digging, then they would build the habitats, then it might take them 10 days or so to bury the habitats with the Marscats.

I don't think it's a good idea to try to predict exactly how long a vehicle will need to operate.  If it's there, you'll use it for something.

Dook wrote:

Is there any way to reduce the size of the SAFE-400 to a smaller version that puts out about 15,000 to 20,000 watts?  Or is it as small as it can get?

There's a lower limit on how light you can make a viable fission reactor.  The reactor can be really small, but output does not scale linearly with size.  A 400kWe (not 400kWt, which is what SAFE-400 is) version of SAFE-400 is almost the exact same size.  You're probably familiar with the term "critical mass".  There's a lower limit to how much fissile material you can use to sustain a controlled chain reaction and there's also a limit to how thermally "hot" the core can get before it melts.

Awhile back, I had an argument here in the nuclear powered manned rover thread (Planetary Transportation subsection of this forum) with a moderator about what the smallest fissionable mass of material was for a controlled (nuclear reactor) versus uncontrolled (nuclear weapon) chain reaction and how core diameter relates to shielding if the objective is to minimize the total mass of the reactor system as much as possible and still provide adequate shielding for humans working in very close proximity to an operating fission reactor.  His contention was that critical mass was the determining factor in how small the core could be.  That's true to the extent that a critical mass of whatever isotope is required to reach criticality to begin with.  Certain isotopes require much less neutron moderation and reflection mass as a function of their fission cross-section, a measure of how likely fission is to occur when the nucleus of an atom of the fissile isotope is struck by a neutron.  Common fissile isotopes like U235 and Pu239 require neutron moderators (to "slow" neutrons to speeds likely to induce fission) and neutron reflectors (like the walls of a pin ball machine to bounce neutrons off of, except reflectors fling neutrons in every direction).  That said, the diameter of the core dramatically affects shielding mass as a function of the square-cube law.  Using Uranium or Plutonium requires rather thick neutron moderators and reflectors (Beryllium is about the lowest mass practical neutron reflector we have) and then the shielding has to surround the neutron reflector if humans are to be anywhere near the reactor during operation.

I wanted our nuclear scientists to research mass manufacture and use Am242m (a metastable isotope of Americium) to absolutely minimize core diameter.  Am242m, like Pu238, is incredibly expensive to produce as a function of the energy and materials expenditure required to produce it.  The reason I wanted to produce Am242m is that that isotope has a fission cross-section of thousands of barns, compared to hundreds of barns for U235 and Pu239, so it needs little to no neutron reflector surrounding the core to reflect neutrons to sustain a chain reaction.  A film of the Am242m isotope less than one thousandth of a millimeter in thickness can sustain fission on its own without a neutron moderator or reflector of any kind.

For any practical purposes, an isotope as "hot" (in the radioactivity sense of the word) as Am242m will require some sort of alloying metal or must be produced in ceramic form to make the isotope usable for thermal power output without melting the isotope in the reactor or fissioning before we ever insert it into the reactor.  Simply obtaining isotopically pure Am242m might work for making an insanely expensive and small nuclear weapon, but it'd be impractical for electrical power production.  Pu238 (the isotope we use in our RTG's) is manufactured as Plutonium Dioxide, a ceramic metal, to increase the temperature at which it melts.  Pure Plutonium metal melts at 639C, whereas Plutonium Dioxide melts at 2390C.  Am242 metal melts at 1176C, so much like Pu238, we'd either alloy it with another metal or turn it into a ceramic to withstand the temperatures produced in a fission reactor and to somewhat "moderate" the radioactivity so we didn't have Am242m films many times thinner than a sheet of printer paper spontaneously fissioning while we were trying to manufacture it.

Dook wrote:

Also can you insert the control rod to activate it for a month or so, then pull out the control rod to deactivate it when it's no longer needed?  Isn't that how nuclear power reactors work?

Yes.  After the neutron "poison" (substance that absorbs so many neutrons that the fission chain reaction can't continue) is inserted into the reactor, the chain reaction stops in seconds to minutes.  It'll be very close to throwing a switch in a very small reactor.  Radiation levels produced fall dramatically in seconds to minutes.  However, after a reactor is first taken critical for the first time, thereafter the components are radioactive because the byproducts from fission are radioactive.

Most people think that half lives of millions of years means "highly radioactive", but it's actually the opposite.  If something has a half life of seconds or minutes, it's so "highly radioactive" that you won't be able to handle it.  If it has a half life of millions of years, it's barely radioactive at all.  These highly radioactive but short-lived isotopes (Cesium, Strontium, an Iodine isotopes, for example) are what we call "fission products" and are the reason you can't immediately work on a reactor after shutdown.  The quantities produced in these very small reactors will also be small, but the isotopes will still be there and are highly radioactive for seconds to days after shutdown.

If you shut down a reactor of the sort that we're proposing to use on Mars, then humans can work on the reactor about two weeks later with minimal precautions taken.  At that point, the natural environment of Mars will be more radioactive as a result of GCR's than the site where the reactor operated.  The fission products captured in the Uranium, not the Uranium itself or the reactor itself, are what you have to concern yourself with.  In any event, these reactors are designed in such a way that the Uranium fuel is never removed from the core for the entire operational life of the reactor.

Dook wrote:

The crew could activate the smaller SAFE-400 for a month to recharge the Marscats.  Dig out the habitat area, build the habitats, bury them, recharge the Marscats and deactivate the reactor.

If you spend the money to ship a reactor to Mars, then you'll be using the reactor thereafter to generate power for a few decades.  It's entirely feasible to vary output to extend the life of the core.  If you're not using 100kWe, then turn the control drums inward or partially insert the rod to vary reactivity and thus output.

Dook wrote:

Then, a year or so later, they would get a second shipment of buried habitat components and reactivate the SAFE-400 for a month to recharge the Marscats again.

It's feasible, but in reality you want the power.  Even if you're only using it to operate WAVAR or grow food or dig more holes for habitat modules, you'll always find uses for the power.

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#46 2017-05-13 21:02:18

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 10,650

Re: Mars Cart

kbd512 I apologize if that was me as I think that was another...

Nukemobiles on Mars

I do hope that this captures the main points DOOK was trying to make about the MARSCAT....

To comment on the buring the tuna can habitat structures as Oldfart noted covering a ditch of just over half of the units diameters is all that one should need to cover them with and that would cut the enrgy draw from the batteries considerably.

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#47 2017-05-14 16:48:20

kbd512
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Registered: 2015-01-02
Posts: 1,187

Re: Mars Cart

SpaceNut,

No apologies are necessary.  I should be able to explain the technical merits and feasibility of the concepts I propose and I expect counter-arguments containing the same types of information.  If someone says "I think we should make airplanes with steel because steel is cheap and strong", then they have to be able to explain the technical merits and feasibility of constructing aircraft with steel.  If someone else says, "Normally we try to make aircraft as light as possible because heavy aircraft don't fly very well and steel is awfully heavy compared to aluminum", then that's a valid technical point that anyone contemplating building an aircraft with steel should take into consideration.

The point I made about fission reactors for mobile applications was a difference in valuation between what Antius and I were proposing to accomplish with the design of such a reactor and the points Josh brought up about the practical aspects of living in a nuclear-powered vehicle.  Basically, we determined that it was technically feasible to do it.  Whether we should or shouldn't put an operating nuclear reactor mere feet away from where people live is a different question entirely.

The Soviets apparently tested a prototype land vehicle consisting of four tracked vehicles (1 for reactor, 1 for steam generator, 1 for turbogenerator, 1 for control console and auxiliary power) with a 8.8MWT (1.5MWe) boiling water reactor using 74 HEU fuel bundles, but it never entered service.  This vehicle was intended to supply power to remote arctic air defense installations.  The vehicle chassis were based on the Soviet T-10 heavy tank.  The weight of this setup was 210t.  After its initial test run, the Soviet government shut down the project.  In the early Cold War era, between Russian and the US, we attempted to put a fission reactor in everything from a satellite to a tank.  The US Army also operated a small fission reactor in the arctic for a short period of time at a place in Greenland named Camp Century.

I agree that the Marscat is useful to have.  Some sort of combination utility vehicle and earth mover is requirement for surface mobility and maybe survival for permanent inhabitation of Mars.  On Mars, the Japanese Type 60 vehicle I proposed for the Marscat concept is a few hundreds pounds lighter than the curb weight of my Silverado and it has much lower ground pressure because the weight is distributed over a much larger surface area and much better traction because there is so much more surface area in contact with the ground.

Wikipedia has some proximal examples pertaining to the difference in ground pressure exerted by a wheeled ATV (2psi) versus a tracked ATV (.75psi).  The combat loaded weight of the Japanese Type 60 is 8,000kg, 7,600kg empty.  The ground pressure of the loaded vehicle is .67kg/cm^2 or 9.52psi.  The modified vehicle I proposed weighs 23.75% of what the Earth-bound Japanese Type 60 weighs when it's on Mars, so its ground pressure is 2.26psi on Mars.  There are equations and models for military vehicles that indicate how vehicles perform in off-road environments and interested parties can Google "Bekker's equations" for more information.

Tires typically provide the best performance on relatively hard and flat surfaces, as a function of the ground pressure exerted by the wheels and traction achievable over soft ground.  That does not mean that tires can't work for off-road use, it just means that tracked vehicles of equivalent weight exert lower ground pressure and have better traction.  Bekker's equations were developed to model mobility trade-offs for military units using tracked versus wheeled vehicles to provide off-road mobility.  The US Army has extensively studied and tested the off-road mobility characteristics of its vehicles, but typically ignores what they know from both mathematics and testing in their purchasing decisions.

The vehicle I proposed has a dual purpose and the US military would also benefit from having small, light, ultra-low silhouette vehicles.  The V-22 or CH-53 and C-130 can easily transport vehicles in this weight class.  These vehicles can carry soldiers or cargo and mount machine guns, grenade launchers, 81mm mortars, 25mm to 40mm automatic cannons, or missiles systems to include FIM-92 (attack drones and transport helicopters), AIM-9 (attack helicopters and jets), AIM-120 (tactical fighters), FGM-148 (armored personnel carriers, artillery pieces, and heavy machine gun, automatic cannon, or mortar crews), or AGM-114 (tanks).

All current wheeled, "tactical" vehicles like the Stryker, LAV-25, M-ATV, and MRAP variants have some of the most absurd requirements of any military vehicles in existence.  The M-ATV must travel 1 kilometer after it takes a round of 7.62mm ammunition to its radiator or travel 30 miles at 30mph if two tires loose air pressure from hits from 7.62mm machine guns.  You could fire any 7.62mm machine gun at a Japanese Type 60 and only succeed in scratching the paint.  The curb weight of the new M-ATV is 12,500kg, it has a height of 8 feet 9 inches, and a CG height that starts roughly where the roof of the Type 60 begins.

The CH-53K can technically sling either a loaded M-ATV or stripped LAV-25.  The LAV-25 and Stryker series of vehicles are so large and heavy that only our C-130's can transport them and only after removal of add-on armor.  It is not possible to air drop a LAV-25 or Stryker from the C-130, so these vehicles are delivered by rail or ship.  Any off-road excursions through soft terrain will quickly illustrate why these vehicles are road bound.  Restrictions to roads or hard ground have made ambushes and IED emplacement favored tactics of our adversaries.  Restrictions to speeds and tire pressurization levels, even on roads, when add-on armor is fitted mean these vehicles are no faster in practice than the M113's they replaced.

It's possible to make a vehicle light or completely protected, but not both.  The "medium" category of protected vehicles is a euphemism for too heavy to air drop or air transport and too light to provide the protection of tank-based heavy APC's like the Israeli Namer.  Light tracked vehicles like the Type 60 can still be better protected than medium wheeled tactical vehicles.  The M113 MTVL is also a "medium" vehicle, but actually fits in a C-130 without disassembly, weighs less than the Stryker with add-on armor, makes rated speed on-roads, can travel over soft ground, costs less to produce, carries more soldiers, and is still better protected than the Stryker.  Even so, 3D warfare with "medium" vehicles is impossible using today's vertical lift assets.  This is not an argument for more M113's since M113's will never provide true air mobility without a new and obscenely expensive vertical lift asset development program, just letting everyone know what their tax money didn't purchase.

The M113 MTVL exerts a ground pressure of .67kg/cm^2, compared to the Stryker's 2.05kg/cm^2 for the base configuration, but is still substantially smaller (smaller target and air transportable without disassembly) and is just as fast as the Stryker on-road if add-on armor is fitted to the Stryker to provide equivalent protection (no real-world difference in speed).  The two selling points of the Stryker were more speed on-road and air transportability via C-130, neither of which actually happened.

No army should be tied to roads, never mind the most well-funded army in the world.  Ambushes can occur most frequently when travel is predictable.  True off-road capable light tracked vehicles make ambushes more difficult.  All light tactical vehicles should be air transportable and air droppable, make reasonable speed both on-road and off-road, limit terrain and weight induced mechanical failures, and provide good terrain traversal to make travel unpredictable.

Last edited by kbd512 (2017-05-14 16:53:11)

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#48 2017-05-14 19:08:51

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 10,650

Re: Mars Cart

I do believe we have a couple of concepts going from the pull behind cart to something thats more than a light weight rover to a complete mobile home of tracks each with there desired configurations of power and use.
The tracks versus tire to surface pressure is kind of a no brainer as that a simple surface area equation of contact. The question I would have is the wear issue that we are seeing on the rovers metal wheels at the linkage areas for each track plate as to if we would need to look at it....
Other than that its a matter of creating the list of mass to place in each version and build prototypes of each to try at the analog mars society sites. The more experience we gain from such items the better off we will be on mars.

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#49 2017-05-15 09:40:15

kbd512
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Registered: 2015-01-02
Posts: 1,187

Re: Mars Cart

SpaceNut,

The alternative is the steel wire wheels from Goodyear.  The new versions are definitely better than the old versions from the Apollo rover program.  The maintenance schedule issues that apply to stainless steel linked tracks do not apply to rubber band tracks.  Compared to steel linked tracks, rubber band tracks reduce weight by 50%, vibration by 70%, rolling resistance by approximately 25% (varies by design), and remove daily maintenance tasks associated with pin tensioning and torque connector tightening.  Soucy Defense Canada lists the operating temperature range as -73C to 80C.  If there is any feasible way to heat the tracks at night, then this is clearly the way to go.

The UHMWPE (Ultra-High Molecular Weight PolyEthylene) material that Soucy uses for the sprockets and idlers already performs quite well at deeply cryogenic temperatures, far colder than what Mars ever sees at its poles in winter, and is definitely the way to go for the running gear, as a function of its radiation resistance and resistance to perchlorate salts.  Soucy UHMWPE running gear and rubber band tracks have already been demonstrated in heavier commercial off-road vehicles made by the Russian Terranica Corporation's line of off-road vehicles with the same engine power output as the Japanese Type 60 Model C (most powerful version of the vehicle).

Soucy Defense

Terranica - English Version

Any general purpose utility vehicle will be heavy and off-road use typically creates maintenance issues for wheeled vehicles faster than tracked vehicles.  There are no roads on Mars, so 100% of usage is off-road.  Maybe the ground will be flat and hard where the lander lands, but maybe not.  Aluminum alloys are clearly unsuitable, but heated rubber or stainless steel should perform better.

For wheeled vehicles, rolling resistance on roads is approximately 2% of their weight.  For tracked vehicles, rolling resistance on roads is approximately 4% of their weight.  As a result, fuel economy for wheeled vehicles is better on roads.  However, the same is not typically the case in an off-road environment, where fuel consumption of wheeled vehicles (power required) can be every bit as high as for tracked vehicles of the same weight.  If rubber band tracks are installed in place of the linked steel tracks, then rolling resistance is less, but still higher than for wheeled vehicles.  If you intend to travel by road, then use wheels.  If you intend to travel off-road, then calculate the percentage of off-road usage.  If it's higher than 50%, then use tracks or expect pre-mature drive train mechanical issues.  This is not universal, it's a rough rule of thumb calculated from actual US military vehicles in actual use cases.

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#50 2017-05-16 20:22:13

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 10,650

Re: Mars Cart

kbd512 wrote:

There is an emerging technology that makes battery power competitive with radioisotope power, but further development is still required.

Silicon-Graphite crystal power cells last for many years with no appreciable output drop.  24V at 18A (432We) continuous for a module that weighs approximately two thirds of what a MMRTG weighs.  Some of these devices have been tested for five years or more with no discernible drop in voltage or amperage or cell deterioration from being left outside on the ground.  Voltage increases slightly, when heat is applied, to about 1.8V per cell (about 2V max at operating temperatures) from 1.5V per cell which has been tested to freezing temperatures.  These cells have the benefit of being highly modular, light, extraordinarily energy-dense (but not power dense), extremely cheap to manufacture compared to Pu238 isotope, and there is no radiation of any kind produced.

If electrical energy density for a given mass is of utmost importance, then there's no contest between NASA's decades-old Seebeck technology and crystal power cells.  These cells have never been tested in a high GCR environment, but the basic materials used in their construction have been used in fission reactors, so I expect no real issues there.

I don't know how to get around the thermal regulation requirements.  Insulation and electrical heating are required.  Some of that excess electrical output would be lost to electrical heating to increase the voltage.  Even so, it's definitely a better deal if useful electrical output, mass, radiation concerns, and fabrication costs are considered.  I estimate a cost of about $50K for a cell of the sort I described (24V at 18A; voltage will go up as heat is increased but I expect to sacrifice some of the total wattage for heating the cell).  Keeping the temp at 100F would produce about 1.8V, so it's the same 28V system as the MMRTG.

The good news is that these things would cost tens of thousands of dollars vs tens of millions of dollars for the Plutonium alone.  Nobody cares about launching more carbon and silicon into space since every spacecraft already has some in it.  The overall manufacture process is not simple and laborious.  Oldfart1939 might be able to figure out the best way to manufacture these cells.

This technology is a game changer.... link provided by Lious
http://gizmodo.com/this-graphene-coated … 1452245250

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