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#51 2016-01-16 22:24:43

kbd512
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Registered: 2015-01-02
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Re: Nukemobiles on Mars

SpaceNut,

Perhaps our definitions of micro-sized reactors vary, but a Toshiba 4S reactor is as large as a semi-trailer truck and undoubtedly weighs more.  The technology required to send a reactor that size to space, let alone land it on Mars, simply does not exist.  Various nuclear power generation architectures scale up well, but very few scale down to the size of something that could conceivably be carried by a manned vehicle.

The way I see it, the most important problem to solution is limiting gamma emissions from the core.  Gamma emissions require the most dense and expensive shielding, in terms of both cost and weight.  By way of comparison, the neutron emissions aren't a very serious problem from a cost and weight standpoint.  Supposedly, there is a polymer material called HGD that's more effective than lead and weighs substantially less.  There's not enough information on the material to know how effective it is or isn't at gamma ray shielding, but if so-called active gamma ray shielding materials like this can be engineered to completely absorb or deflect gamma emissions towards a relatively harmless exit point from the reactor (for example, towards the ground in front of the vehicle), then it's possible to design a small reactor with favorable weight and shielding characteristics for mobile applications.

Last edited by kbd512 (2016-01-16 23:45:28)

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#52 2016-01-17 10:50:10

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

Re: Nukemobiles on Mars

Which is a tow behind rover size and we need to be even small than that which was the point of finding the information for the topic which is well beyond me in many ways.....That said we are looking for reactors then that are sub micro or nano in size, RTG type power sources and Nuclear batteries in nature for use with a long term power source rather than solar and batteries for the large RV rover....

We know that fuel type does change the requirements of reaction, containment, radiation protection and in many ways mass of what is deliverable to Mars.

http://www.livescience.com/261-personal … years.html

050513_battery_pic_02.jpg?1296068635

The Wafer Test Fixture that the researchers used to test the new porous-silicon diode and its interactions with tritium gas. The diode is the dark wafer in the center of the top plate.

Not sure of the power levels from the device but its function is simular to a solar cell fuel cell and RTG with how it functions and using isotopic materials.......

A new type of battery based on the radioactive decay of nuclear material is 10 times more powerful than similar prototypes and should last a decade or more without a charge, scientists announced this week.

A 25-Year Battery Long-lived nuclear batteries powered by hydrogen isotopes are in testing for military applications

Widetronix’s batteries are powered by the decay of a hydrogen isotope called tritium into high-energy electrons. While solar cells use semiconductors such as silicon to capture energy from the photons in sunlight, betavoltaic cells use a semiconductor to capture the energy in electrons produced during the nuclear decay of isotopes. This type of nuclear decay is called “beta decay,” for the high-energy electrons, called beta particles, that it produces. The lifetimes of betavoltaic devices depend on the half-lives, ranging from a few years to 100 years, of the radioisotopes that power them. To make a battery that lasts 25 years from tritium, which has a half-life of 12.3 years, Widetronix loads the package with twice as much tritium as is initially required. These devices can withstand much harsher conditions than chemical batteries. This, and their long lifetimes, is what makes betavoltaics attractive as a power source for medical implants and for remote military sensing in extremely hot and cold environments.

https://medium.com/war-is-boring/poweri … 72975d7de8

Many of the isotopic rtg's fuels used have been around since apollo.....

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#53 2016-01-17 10:53:23

GW Johnson
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From: McGregor, Texas USA
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Re: Nukemobiles on Mars

I looked briefly at the provided links.  That's civilian industry finally picking up on what the US Navy did over 60 years ago.  I did notice these are intended to be buried,  so that hundreds of tons of dirt is your shielding.  Those USN designs,  especially the submarine systems,  are the most compact,  and the safest,  high-output reactor power systems that ever have been.

The latest USN designs are the 8th and 9th generation,  life-of-the-ship-without-refueling designs (30+ years,  perhaps 50+ years).  They fit in a reactor compartment space about 30 ft dia and 40-50 feet long,  and require about 100 tons of lead shielding to confine the radiation to the reactor compartment only.

In a large boat,  these are 500 MW (thermal) pressurized water reactors with a primary steam loop and heat exchanger in the reactor compartment along with the reactor and its fuel rod control systems.  The secondary steam loop is in the machinery space compartment,  and presents no radiation hazard.  It feeds turbines,  some of which drive electric generators (typically near 160 MW electric capacity or thereabouts in a large boat),  the others geared directly to the ship's screw or screws.   

These are about half to a third the power of a big unit at a major power plant.  You can scale down the size quite a bit for something nearer only 200-300 KW thermal for some sort of a truck-and-trailer nuke Mars rover.  But I doubt you can scale down the shielding so much,  as it is simply thickness that is required to stop the various forms of radiation. 

This thing is going to be bigger and heavier than you want,  but yes,  it can be done.  The size of the thing will be about like a railroad locomotive set:   A and B units together.  If it could be scaled a whole lot smaller,  USN would have done so already.  They've had 6 decades doing it in smaller boats as well as the big ones. 

The USN did experiment with one sodium-cooled reactor design in SSN-575 Seawolf,  but had so many dangerous problems with it that they replaced it with the pressurized water design used in SSN-571 Nautilus.  Submarine reactors have been pressurized water ever since,  for 6 decades now.  I think there is an applicable lesson in that history.  The submarine application is not all that different from the space exploration application:  both require very compact solutions with humans in close proximity. 

GW


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#54 2016-01-18 06:57:59

Antius
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Posts: 974

Re: Nukemobiles on Mars

A naval reactor is volume constrained, but not really weight constrained.  In fact the weight can be useful providing ballast.  And a naval reactor can dump waste heat at low temperatures into the sea, so doesn't need to operate at high temperatures.  The navy have no incentive to build anything other than a LWR.  It is a cheap established technology with self-regulation built into it and excellent fuel temperature margins.  A liquid metal reactor wouldn't really offer them an advantage that would be worth the billions needed in development.  A space reactor is clearly very different in terms of design constraints - all waste heat must be radiated into space or transferred to atmosphere by dumping working fluid and weight is much more constraining.

Bad news for the idea of using a trailer reactor for charging batteries.  The energy density of lead-acid batteries is 0.17MJ/kg.  To store 200kWe x 8 hours would require 34 tonnes of batteries.  With the best lithium-ion batteries, that can be reduced to 6600kg.  So at best, a trailer reactor provides no advantage other carrying the reactor in the vehicle and using it to directly power the drive.

A micro-reactor is the way to go.  The 400kWth SAFE reactor appears to fit the job and the shielding mass appears to be workable at ~10 tonnes.  It won't win any races, driving it would be like the ultimate 1st gear.

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#55 2016-01-18 14:13:19

kbd512
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Re: Nukemobiles on Mars

Antius wrote:

A naval reactor is volume constrained, but not really weight constrained.  In fact the weight can be useful providing ballast.  And a naval reactor can dump waste heat at low temperatures into the sea, so doesn't need to operate at high temperatures.  The navy have no incentive to build anything other than a LWR.  It is a cheap established technology with self-regulation built into it and excellent fuel temperature margins.  A liquid metal reactor wouldn't really offer them an advantage that would be worth the billions needed in development.  A space reactor is clearly very different in terms of design constraints - all waste heat must be radiated into space or transferred to atmosphere by dumping working fluid and weight is much more constraining.

I didn't see how a LWR the size of a railcar was in anyway comparable to something like SAFE-400, either.

Antius wrote:

Bad news for the idea of using a trailer reactor for charging batteries.  The energy density of lead-acid batteries is 0.17MJ/kg.  To store 200kWe x 8 hours would require 34 tonnes of batteries.  With the best lithium-ion batteries, that can be reduced to 6600kg.  So at best, a trailer reactor provides no advantage other carrying the reactor in the vehicle and using it to directly power the drive.

The problem I have with the nuclear-powered battery charger solution is that more weight and more vehicles are required to implement it.  The idea behind the nuclear power solution was to eliminate some of the issues with the solar power solution, namely batteries.

Antius wrote:

A micro-reactor is the way to go.  The 400kWth SAFE reactor appears to fit the job and the shielding mass appears to be workable at ~10 tonnes.  It won't win any races, driving it would be like the ultimate 1st gear.

What is the shielding configuration you are using for this?

What will the diameter of the reactor be with shielding?

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#56 2016-01-18 17:39:04

SpaceNut
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Re: Nukemobiles on Mars

Thanks Antius, I was looking at the lead acid batteries for the shielding characturistics rather that charge available for the difference in using a dead mass of just lead...

I am now searching for the cooling design stuff of the Safe reactors which employ Heatpipe systems for cooling... It sort of sounds like a mass heat exchanger....

edit:

A bit of goggling/bing did yield the information for heat transfer from the reactors core to the disapation area Ie.  radiator.....

Power systems

image002.jpg

HeatPipe System for SAFE-400


Which got me thinking about magnetic bottling fields to contain the radiation as to whether this is a possibility....


Acronym: SAFE = Safe Affordable Fission Engine

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#57 2016-01-18 21:07:04

SpaceNut
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Re: Nukemobiles on Mars

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#58 2016-01-18 22:15:01

RobertDyck
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From: Winnipeg, Canada
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Re: Nukemobiles on Mars

SpaceNut posted a message in Light weight nuclear reactor, updating Mars Direct, implying he wants me to comment on this. Ok.

One source is Nuclear Reactors and Radioisotopes for Space, last updated November 2015. It has a good description of SAFE-400. It also says:

A smaller version of this kind of reactor is the HOMER-15 – the Heatpipe-Operated Mars Exploration Reactor. It is a 15 kW thermal unit similar to the larger SAFE model, and stands 2.4 metres tall including its heat exchanger and 3 kWe Stirling engine (see above). It operates at only 600°C and is therefore able to use stainless steel for fuel pins and heatpipes, which are 1.6 cm diameter. It has 19 sodium heatpipe modules with 102 fuel pins bonded to them, 4 or 6 per pipe, and holding a total of 72 kg of fuel. The heatpipes are 106 cm long and fuel height 36 cm. The core is hexagonal (18 cm across) with six BeO pins in the corners. Total mass of reactor system is 214 kg, and diameter is 41 cm.

I did a further search. Google for "HOMER-15" just gets Homer Simpson cartoon episodes. But if you look for the full name "Heatpipe-Operated Mars Exploration Reactor" you get useful information. Here's one paper:
THE HEATPIPE-OPERATED MARS EXPLORATION REACTOR (HOMER)
Department of Energy's (DOE) SciTech Connect - Office of Scientific and Technical Information (OSTI)
Published 1 Oct 2000

Is this what you're look for?

Last edited by RobertDyck (2016-01-18 22:16:35)

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#59 2016-01-18 22:20:02

JoshNH4H
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Re: Nukemobiles on Mars

Unless you're looking at a really, really massive transportation system travelling very long distances I don't think a nuclear reactor is going to make much sense.  It makes much more sense to keep your nuclear reactor stationary, and use it to produce chemical fuels to be burned for energy.  It's worth noting that the high power levels used in cars are mostly for high acceleration.  Actually travelling quickly doesn't use very much energy, especially if you're on a rail or at least a smooth road.  That's doubly true on Mars, with its thin atmosphere.  If a vehicle is travelling at 40 kph along a paved road without elevation change its power needs are very small.  Should there be an elevation change, the vehicle can slow down until its engine can handle it.

This article says that a car can have 9 lb of rolling friction per 1000 lb of curb weight (90 N drag per 10,000 N weight).  Considering Mars' lower gravity and lack of air drag, a 1-tonne vehicle travelling at 40 kph would only need to expend 0.5 HP to keep moving.

For scale, one of the really long (40 foot) intermodal containers can have a gross mass of 30 tonnes, and therefore could in principle be pulled along by a 15 HP motor if it could get sufficient friction.  The image that I hope you're all thinking of is four massive shipping containers being pulled by one tiny little smart car.  The acceleration would be terrible but when you're not driving, who cares?


-Josh

If you try to talk to me about cold fusion or propellantless drives I will ignore you.
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#60 2016-01-19 05:54:20

Terraformer
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Re: Nukemobiles on Mars

So, a 20 tonne vehicle might only need 7.5 kW in order to move at 40 km/h?

Storing 10 hours worth of energy for driving, then, would be 75 kWh. About 750 kg for NiMH batteries, half that for Lithium Ion.

However, you still need to charge the thing. You *could* use solar panels, but if we're developing new technology anyway, why not beam the power from orbit and avoid the dust storms?


"I guarantee you that at some point, everything's going to go south on you, and you're going to say, 'This is it, this is how I end.' Now you can either accept that, or you can get to work." - Mark Watney

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#61 2016-01-19 06:13:17

JoshNH4H
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Re: Nukemobiles on Mars

Friction will probably be somewhat higher than rubber-on-road, and of course you'll want some extra capability, but ideally yes.  How does beamed power make dust storms less of a problem?


-Josh

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#62 2016-01-19 06:25:24

Antius
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From: Cumbria, UK
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Re: Nukemobiles on Mars

According to Wiki the Challenger 2 tank has a mass of 62.5 tonnes an engine power of 890kW.  So that’s 14.25W/kg, not dissimilar to what we calculated for the nuclear rover with LiH and tungsten shielding (10W/kg).  And the challenger can do 25mph over rough ground, 37mph on the road.

https://en.wikipedia.org/wiki/Challenger_2

Disappointingly, I cannot find any energy storage solution that would make a lightly shielded trailer reactor a more practical option.  I ran some numbers last night for a regenerative internal combustion engine, burning stored hydrogen and oxygen and then regenerating it in an electrolysis cell.  If the H2/O2 are stored as pressurised gases then the required pressure vessel weighs several tonnes.  If they are stored as cryogenic liquids then the heat pump, radiator and cryogenic tanks and insulation weigh tonnes.

I looked into the option of using stored thermal energy in phase change materials (i.e. liquid CO2 and super-heated material) as an energy storage mechanism and running a heat engine between them.  But the maximum practical energy density is about 1MJ/kg when the efficiency of the engine is factored in (~40%).  To store 200kW x 8 hours would require an energy storage system massing several tonnes.  An the heat engine would add to that.

The best option appears to be direct-drive nuclear-electric power cycle, with the densest shielding available.  If I had the time it would make an interesting post graduate project to examine the optimum shielding and spatial arrangement for such a vehicle – i.e. location of reactor and use of ancillary equipment as additional shielding.  But we can at least see broadly what the optimum solution looks like.

Last edited by Antius (2016-01-19 06:26:56)

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#63 2016-01-19 14:59:36

kbd512
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Re: Nukemobiles on Mars

JoshNH4H wrote:

Unless you're looking at a really, really massive transportation system travelling very long distances I don't think a nuclear reactor is going to make much sense.  It makes much more sense to keep your nuclear reactor stationary, and use it to produce chemical fuels to be burned for energy.  It's worth noting that the high power levels used in cars are mostly for high acceleration.  Actually travelling quickly doesn't use very much energy, especially if you're on a rail or at least a smooth road.  That's doubly true on Mars, with its thin atmosphere.  If a vehicle is travelling at 40 kph along a paved road without elevation change its power needs are very small.  Should there be an elevation change, the vehicle can slow down until its engine can handle it.

My design reference MTVL (a current production M113 variant with 6 road wheels to the original M113's 5 road wheels to increase cargo volume by 20%) would have a maximum mass of 20t.  On Mars, that means the vehicle weighs 7.6t.

The Earth-bound Canadian MTVL variant has top hatches and turret rings, upgraded armor and spall liner, a 400hp diesel power pack with, Soucy rubber band tracks, diesel storage tanks, a 38MM glacis plate and 12MM hull of welded 5083 aluminum, and a rear troop compartment with a hydraulically actuated rear ramp.  Combat weight is 20t (fully fueled, basic ammo load, two crew and ten equipped infantry dismounts).

We're basically gutting the base vehicle and refitting it for our intended use.  There will be no turret rings or top hatches.  The only entrance to the vehicle will be the rear cargo ramp.  The weight of the power train components is replaced with the weight of a small reactor, active cooling solution, and two 40kW electric motors.  The reactor, reactor shielding, Brayton cycle electric generator, and active CO2 cooling solution will be packaged into a removable power pack for servicing and disposal.  The electric motors will be bolted to the hull and separated from the rest of the power pack for servicing.

The power pack and electric motors are mounted forward of the first pressure bulkhead, which is the front of the driver's compartment.  The second pressure bulkhead and hatch separates the driver's compartment from the cargo area of the vehicle.  The driver's compartment is intended to be occupied only briefly during EVA so that an astronaut performing an EVA can depressurize the rear cargo compartment without the requirement for the other astronaut to suit up and for precise maneuvering requiring a better visual than externally mounted cameras would provide.  The rover is intended to be tele-operated by iPhone or iPad or similar device from mission control, orbit, and from the surface- inside or outside the vehicle.

JoshNH4H wrote:

This article says that a car can have 9 lb of rolling friction per 1000 lb of curb weight (90 N drag per 10,000 N weight).  Considering Mars' lower gravity and lack of air drag, a 1-tonne vehicle travelling at 40 kph would only need to expend 0.5 HP to keep moving.

The rolling resistance for tracked vehicles is typically higher than for wheeled vehicles, but tracks also reduce ground pressure.  The use of rubber band tracks greatly reduce rolling resistance and vibration compared to steel tracks and are half the weight.  The tracks and road wheels must use heat from the reactor to keep the rubber above the glass transition temperature, but there's no shortage of thermal waste to warm the vehicle.  Governing top speed to 40 kph and avoiding abrupt maneuvers will reduce the possibility of throwing a track.

JoshNH4H wrote:

For scale, one of the really long (40 foot) intermodal containers can have a gross mass of 30 tonnes, and therefore could in principle be pulled along by a 15 HP motor if it could get sufficient friction.  The image that I hope you're all thinking of is four massive shipping containers being pulled by one tiny little smart car.  The acceleration would be terrible but when you're not driving, who cares?

The reference vehicle must have sufficient power-to-weight ratio to tow a disabled MTVL over most terrain and that's why we have more powerful motors than what's minimally required for adequate mobility.  That's also why we use a four vehicle convoy.  My DRM has a crew of four astronauts and splits them between two vehicles with two fully functional spare vehicles for storage of the other half of their consumables (food and water), spare rover parts (rover wheels, tracks, bolts, and tools for track/wheel replacement), and scientific instruments.

My intent was for mission control to plan routes using satellite imagery, feed data into the portable vehicle control devices that the astronauts have, and then the astronauts would have final say in navigating their vehicles.  Ideally, the vehicles would incorporate software for obstacle and vehicle avoidance so as to permit semi-autonomous navigation between exploration sites.  That way, the astronauts could concentrate on vehicle maintenance, EVA planning, and samples analysis.

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#64 2016-01-19 16:21:32

kbd512
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Re: Nukemobiles on Mars

Antius wrote:

Disappointingly, I cannot find any energy storage solution that would make a lightly shielded trailer reactor a more practical option.  I ran some numbers last night for a regenerative internal combustion engine, burning stored hydrogen and oxygen and then regenerating it in an electrolysis cell.  If the H2/O2 are stored as pressurised gases then the required pressure vessel weighs several tonnes.  If they are stored as cryogenic liquids then the heat pump, radiator and cryogenic tanks and insulation weigh tonnes.

My first thought about how to solve the power problem was a combination of lithium air, super capacitors, and a rollup solar array stored on top of the vehicle.  I traded various battery, super capacitor, and solar technologies like roof mounted trackable arrays in an attempt to devise a solar solution with similar or better weight and mobility to the nuclear solution.  There were no solutions I could devise that substantially decreased vehicle weight while retaining vehicle mobility.  Even the latest generation of batteries and super capacitors are heavy, given the vehicle's power requirements.

Antius wrote:

I looked into the option of using stored thermal energy in phase change materials (i.e. liquid CO2 and super-heated material) as an energy storage mechanism and running a heat engine between them.  But the maximum practical energy density is about 1MJ/kg when the efficiency of the engine is factored in (~40%).  To store 200kW x 8 hours would require an energy storage system massing several tonnes.  An the heat engine would add to that.

Apart from exotic or developmental battery/capacitor/solar technology, it would seem that any energy solution approximating the capabilities of the nuclear solution still masses multiple metric tons and involves operational use complexities and potentially dangerous vehicle configurations to keep the batteries warm.

Antius wrote:

The best option appears to be direct-drive nuclear-electric power cycle, with the densest shielding available.  If I had the time it would make an interesting post graduate project to examine the optimum shielding and spatial arrangement for such a vehicle – i.e. location of reactor and use of ancillary equipment as additional shielding.  But we can at least see broadly what the optimum solution looks like.

Ideally, we'd use an all-electric solution.  The all-electric solution is what I would prefer, but that doesn't seem feasible at this time.

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#66 2016-01-19 21:20:43

SpaceNut
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Re: Nukemobiles on Mars

Thanks for the lithium battery totals which you indicate is

Antius  wrote:

To store 200kWe x 8 hours would require with the best lithium-ion batteries, mass would be 6600kg.

Which mean that the rover radius can only travel just under 4 hours befor turning around and the waiting untill the totally dead batteries are fully recharged. So if we are using solar to charge that would mean we would be able to use the rover every other day or longer between cycles depending on what we deliver to mars surface at a stationary site for solar mass...

Part 1 is the interface charging conditioning units for connection to the batteries.

Battery Charging and Management Solutions

http://www.electronicproducts.com/Power … eries.aspx

High voltage 100 Amp Solar Battery Chargers and PV System Controllers

http://batteryuniversity.com/learn/arti … _batteries

Optimum Charging Profile for Lithium-ion Batteries to Maximize Energy Storage and Utilization

part 2 is the solar array selection of cell type and whether it is stationary or tracking....

These are what effects the charging time and conditions for the rovers use....

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#67 2016-01-20 05:43:24

Antius
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From: Cumbria, UK
Registered: 2007-05-22
Posts: 974

Re: Nukemobiles on Mars

I looked into alternative shielding materials yesterday as part of the effort to reduce shielding mass.  It turns out that hafnium hydride has a density of 11,400kg/m3 and has 2.7 times the hydrogen density of LiH (300kg/m3 vs 111kg/m3).  Since it is areal density of hydrogen atoms that counts the most in stopping fast neutrons, the use of HfH2 should reduce the inner shield radius by ~2/3rds.  What’s more, it is a much more efficient gamma shield than LiH, so there is potential that it could reduce required tungsten thickness as well.  My initial estimate suggests that the HfH2 inner shield should weigh only 70% as much as the LiH inner shield.  Due to the reduced radius, the tungsten shield should weigh only 20% of what it would for a LiH shield, assuming its thickness stays the same.

This reduces required shielding mass from 10 tonnes to about 3 tonnes.  Assuming that shield contact dose fluxes are similar, the reduced shield radius also makes distance a much more effective defence.  For the previous core, a 5m distance reduced dose by a factor of 25 over surface contact dose rate.  For a core shield with only half the radius, a distance of 5m reduces dose by a factor of 100 over contact doserate.

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#68 2016-01-22 13:59:20

kbd512
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Re: Nukemobiles on Mars

Antius wrote:

I looked into alternative shielding materials yesterday as part of the effort to reduce shielding mass.  It turns out that hafnium hydride has a density of 11,400kg/m3 and has 2.7 times the hydrogen density of LiH (300kg/m3 vs 111kg/m3).  Since it is areal density of hydrogen atoms that counts the most in stopping fast neutrons, the use of HfH2 should reduce the inner shield radius by ~2/3rds.  What’s more, it is a much more efficient gamma shield than LiH, so there is potential that it could reduce required tungsten thickness as well.  My initial estimate suggests that the HfH2 inner shield should weigh only 70% as much as the LiH inner shield.  Due to the reduced radius, the tungsten shield should weigh only 20% of what it would for a LiH shield, assuming its thickness stays the same.

The reactor's active cooling system would have to be very effective to prevent HfH2 from decomposing.  HfH2 starts to melt at 300C.

Antius wrote:

This reduces required shielding mass from 10 tonnes to about 3 tonnes.  Assuming that shield contact dose fluxes are similar, the reduced shield radius also makes distance a much more effective defence.  For the previous core, a 5m distance reduced dose by a factor of 25 over surface contact dose rate.  For a core shield with only half the radius, a distance of 5m reduces dose by a factor of 100 over contact doserate.

I still think core diameter and reflector diameter reduction should be scrutinized to reduce the required shielding mass.  If an Am fueled reactor could be developed, then a substantial reduction in core diameter and reflector diameter could be achieved with an attendant reduction in shielding mass and diameter.

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#69 2016-01-22 18:39:54

SpaceNut
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From: New Hampshire
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Re: Nukemobiles on Mars

https://en.wikipedia.org/wiki/Americium_nuclear_fuel

http://www.livescience.com/39874-facts- … icium.html

Americium currently has 19 known isotopes and 8 nuclear isomers. The half-lives for most isotopes and isomers range from 0.64 microseconds for 245Am to 50.8 hours for 240Am. 241Am and 243Am are the longest living with half-lives of 432.2 and 7,370 years, respectively. They are alpha-emitters and are now available in high-purity kilogram quantities. The nuclear isomer 242Am has a half-life of 141 years. Like most other actinides, the isotopes of americium with odd numbers of neutrons have relatively high rate of nuclear fission and low critical mass.

Ok so we need to pick which isotope to design with... seems there are 3 to work with.

Americium 242m and its potential use in space applications

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#70 2016-01-22 20:18:45

JoshNH4H
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Re: Nukemobiles on Mars

Does anyone have a link to a paper describing the development of the SAFE-400?  I can't seem to find one on google or ntrs, and it was my understanding that the SAFE-400 reactor core was basically just a core, and did not include any equipment to convert the nuclear heat into usable mechanical or electrical energy.  It is being described differently here in this thread, specifically with the 493 kg including brayton cycle converters, and I would like to see confirmation one way or another.

Edit:  Has anyone looked into using distance instead of mass for shielding?  I see no reason why the reactor couldn't drive far ahead or behind the main car of the rover with any human habitation being on the far side.


-Josh

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#71 2016-01-22 21:43:24

kbd512
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Re: Nukemobiles on Mars

SpaceNut,

Am242m is the isotope I had in mind.  If core diameter can be reduced by 50% over SAFE-400, then field replaceable power packs are feasible.  To put some perspective on how small the BAE MTVL variant of the M113 really is, it's external dimensions of 567cm L x 254cm W x 183cm H are quite close to the external dimensions of the 2016 Chevrolet Suburban 567cm L x 205cm W x 188cm H.  So, with that in mind, reactor size is everything.  Even though the reactor is literally as far forward as it could be without hanging off the front end of the vehicle, we also want as much distance as possible between our radioactive power source and astronauts.

Ideally, power packs should be small enough to remove from the vehicles with a manually operated hoist.  The entire unit would be adequately shielded for humans to be in very close proximity for brief periods of time, since they're obviously in close proximity while they're inside the vehicles that the reactors power, and the reactors would obviously be shut down for power pack removal and disposal.

My thinking on this is that at the end of three missions, the crew dig a pit for reactor core disposal, remove the reactor from the vehicle, bury the core, and then post a marker at the disposal site.  Each vehicle would have external power and oxygen connectors beside the rear ramp so the crew could work on the vehicle.  For reactor disposal, another vehicle would provide the required oxygen and power along with additional crew to assist with reactor disposal.

Following reactor disposal, the electric motors would also be discarded and replaced.  A fresh power pack would then be installed and taken critical.  The final maintenance task would be removal and replacement of the rubber band tracks.  The assisting vehicle and crew would also use their external electrical power connectors to keep the tracks warm during installation.

All electronics and batteries would be discarded and replaced after every mission.  Each arriving crew would have vehicle preventative maintenance tasks to perform and each mission would assign a rotating Reactor Duty Officer (RDO) responsibility.  Reactor vitals would be transmitted to the iPads in all four vehicles and with a crew of four, RDO is a six hour watch.  The remaining astronauts may perform EVA's for field geology as interesting finds dictate, rest, exercise, and perform vehicle maintenance.  However, RDO is a never-ending responsibility.  Prior to astronaut arrival and after astronaut departure, mission control provides the RDO.

Apart from GPS aided navigation, a satellite constellation around Mars is also required for constant communication to receive updates on reactor status.  Once manned exploration starts, I see this as a required capability irrespective of whether or not nuclear reactors are used.  The astronauts should always be able to phone home for help, even if exchanges take a half hour or so.  With nuclear powered rovers that can literally circle the equator in less than a month, it only makes sense to have communications satellites ringing the planet.

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#72 2016-01-22 22:49:18

JoshNH4H
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Re: Nukemobiles on Mars

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?


-Josh

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#73 2016-01-22 22:57:49

kbd512
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Re: Nukemobiles on Mars

JoshNH4H wrote:

Does anyone have a link to a paper describing the development of the SAFE-400?  I can't seem to find one on google or ntrs, and it was my understanding that the SAFE-400 reactor core was basically just a core, and did not include any equipment to convert the nuclear heat into usable mechanical or electrical energy.  It is being described differently here in this thread, specifically with the 493 kg including brayton cycle converters, and I would like to see confirmation one way or another.

It's just a core.  The thermal management scheme was obviously tested.

JoshNH4H wrote:

Edit:  Has anyone looked into using distance instead of mass for shielding?  I see no reason why the reactor couldn't drive far ahead or behind the main car of the rover with any human habitation being on the far side.

First, you need another vehicle in your convoy to transport the reactor.  The reactor has to produce substantially more power to simultaneously recharge the batteries of multiple vehicles for convoy operations.  That means more waste heat rejection capability is required and more shielding is required.  To drive at 20kph continuously for eight hours, the mass requirement for lithium ion batteries capable of meeting that energy storage requirement would likely be heavier than a properly shielded reactor.

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.

Assuming the reactor never has a problem, if the vehicle it's mounted in breaks down and can't be repaired in the field, loss of mission and loss of crew is the most probable outcome.  If any of the manned vehicles in the convoy break down, you have to tow that vehicle.  Each manned vehicle only contains enough consumables for half of a 500 day surface stay.  If there's a reactor in every vehicle in the convoy, then one of the functional vehicles can tow the disabled vehicle at reduced speed.

Mechanical breakdown is why each vehicle has 2 40kW electric motors.  With Mars gravity, the power-to-weight ratio is very similar to that achieved by Earth-bound M113's.  If required, an Earth-bound M113 can tow another M113 at reduced speed.

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#74 2016-01-22 23:07:15

RobertDyck
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Re: Nukemobiles on Mars

Josh:

I keep quoting this article: Nuclear Reactors and Radioisotopes for Space, last updated November 2015.

The SAFE-400 space fission reactor (Safe Affordable Fission Engine) is a 400 kWt HPS producing 100 kWe to power a space vehicle using two Brayton power systems – gas turbines driven directly by the hot gas from the reactor. Heat exchanger outlet temperature is 880°C. The reactor has 127 identical heatpipe modules made of molybdenum, or niobium with 1% zirconium. Each has three fuel pins 1 cm diameter, nesting together into a compact hexagonal core 25 cm across. The fuel pins are 70 cm long (fuelled length 56 cm), the total heatpipe length is 145 cm, extending 75 cm above the core, where they are coupled with the heat exchangers. The core with reflector has a 51 cm diameter. The mass of the core is about 512 kg and each heat exchanger is 72 kg.

I found this paper, but it isn't free. Design and analysis of the SAFE-400 space fission reactor
There are a few papers on SAFE-100, it's smaller and less efficient predecessor.

Distance: Robert Zubrin's mission plan "Mars Direct" would use a robotic truck to place an SP-100 reactor with no shielding at the bottom of a Mars crater some distance from the Earth Return Vehicle. A power cable would trail back to the ERV. A crater would be chosen deep enough that the reactor would not be visible from a safe distance. That means a significant amount of regolith would separate astronauts from the reactor, so regolith of the ground itself would be shielding.

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#75 2016-01-22 23:24:04

JoshNH4H
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Re: Nukemobiles on Mars

RobertDyck,

The first article actually has a slight error.  In its table it says that the SAFE-400 is thermoelectric, when generating 100 kWe from 400 kWt requires either stirling or brayton cycle heat engines.  The second paper is behind a paywall, unfortunately. 

It's important that the mass given is only for the core, as the heat engine and radiators and (as you've been discussing, the shielding) will likely be quite massive, if possibly less than the SP-100 (5.4 t)

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


-Josh

If you try to talk to me about cold fusion or propellantless drives I will ignore you.
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