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#1 2016-01-07 09:57:45

Tom Kalbfus
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Registered: 2006-08-16
Posts: 4,401

Nukemobiles on Mars

So what do you think? Is a nuclear reactor powered car on Mars a good idea? One of the main problems for rovers on Mars is range, if a nuclear powered car performs anything like a nuclear powered submarine, if could pretty much range over the entire planet, it would need to carry spare parts, such as extra tires and so forth. What do you think?

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#2 2016-01-07 19:55:08

SpaceNut
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Posts: 10,650

Re: Nukemobiles on Mars

How would you cool a reactor?

As the radiator would be huge and the air is to thin to wick the heat away otherwise from it.

My question is could we use an RTG power system safely?

What would we need to shield the crew?

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#3 2016-01-08 09:17:52

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

Re: Nukemobiles on Mars

Water is the most efficient shielding material for a nuclear reactor on a weight basis.  For gamma shielding, lead would allow a more compact shield, although mass would be similar.  But water is the shield par-excellence for screening out neutrons, because it is dense and hydrogen rich.  The required thickness depends upon the neutron flux at the inner surface of the shield.  Beyond a certain core size, this is roughly proportional to power density assuming the power profile of the core is reasonably flat.  The larger a reactor is, the more efficient it tends to be in terms of total power/weight, because because power is roughly proportional to core volume, wheras total neutron leakage (and shield mass) is proportional to surface area.  This ratio is complicated by cooling requirements for very large reactors, as the height of the core ends up being limited.  But generally, bigger is better.

The shielding problem is probably the biggest issue for a mobile reactor small enough to fit into a wheeled vehicle.  The waste heat problem could be dealt with by either running the reactor at very high temperatures (and radiating heat at high temperatures) or by using scooped up regolith as a heat sink.  Dumping excess heat during operation wouldn't neccesarily be a problem, as the reactor can use condensed atmospheric CO2 as working fluid.  This could be collected during Martian night-time at very little energy cost and stored as a pressurised liquid.  During day, the reactor would operate by boiling the liquid CO2 in a heat exchanger, passing it through a gas turbine and venting the expanded gas directly into the Martian atmosphere.  Decay heat could be vented through a roof mounted radiator.  At a temperature of 1000K, a radiator would dump 50kW/m2 into space by radiation alone.  Due to the need for high temperatures and high power density (to keep core and shielding mass down), a liquid metal cooled reactor would work best.  A small core would need a high enrichment to counteract neutron leakage.

Because of the shielding problem, this sort of vehicle would not work well as a car sized unit.  It would work much better as a travelling base.

An RTG using a low gamma emitting isotope would get around the shielding weight problem and could be constructed in small sizes.  The limitation here is the need to be able to safely dump the heat at all times.  If the cycle dumps heat at 1000K, then 2m2 radiator at the top of a vehicle would allow 100kW to be dumped.  If the engine cycle is 20% efficient, that would allow an engine power of 20kW (26.8HP).  Enough to move a vehicle at relatively slow speeds.  Strontium-90 would be an affordable radio-isotope for this application.  We would need about 187kg to generate 100kW of heat, so weight-wise it is doable.

But remember these isotopes must be launched from Earth.  A launch accident involving hundreds of kg of any isotope with decent heat generation would be a very messy event.  I have some doubts about the practicality of an RTG car for this very reason.  Why not a use a vehicle with lithium polymer batteries and a big roll-out PV panel?  Not quite as versatile as a nuclear vehicle, but a heck of a lot cheaper and easier to engineer.

Last edited by Antius (2016-01-08 09:18:33)

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#4 2016-01-08 10:48:48

Tom Kalbfus
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Re: Nukemobiles on Mars

Antius wrote:

Water is the most efficient shielding material for a nuclear reactor on a weight basis.  For gamma shielding, lead would allow a more compact shield, although mass would be similar.  But water is the shield par-excellence for screening out neutrons, because it is dense and hydrogen rich.  The required thickness depends upon the neutron flux at the inner surface of the shield.  Beyond a certain core size, this is roughly proportional to power density assuming the power profile of the core is reasonably flat.  The larger a reactor is, the more efficient it tends to be in terms of total power/weight, because because power is roughly proportional to core volume, wheras total neutron leakage (and shield mass) is proportional to surface area.  This ratio is complicated by cooling requirements for very large reactors, as the height of the core ends up being limited.  But generally, bigger is better.

The shielding problem is probably the biggest issue for a mobile reactor small enough to fit into a wheeled vehicle.  The waste heat problem could be dealt with by either running the reactor at very high temperatures (and radiating heat at high temperatures) or by using scooped up regolith as a heat sink.  Dumping excess heat during operation wouldn't neccesarily be a problem, as the reactor can use condensed atmospheric CO2 as working fluid.  This could be collected during Martian night-time at very little energy cost and stored as a pressurised liquid.  During day, the reactor would operate by boiling the liquid CO2 in a heat exchanger, passing it through a gas turbine and venting the expanded gas directly into the Martian atmosphere.  Decay heat could be vented through a roof mounted radiator.  At a temperature of 1000K, a radiator would dump 50kW/m2 into space by radiation alone.  Due to the need for high temperatures and high power density (to keep core and shielding mass down), a liquid metal cooled reactor would work best.  A small core would need a high enrichment to counteract neutron leakage.

Because of the shielding problem, this sort of vehicle would not work well as a car sized unit.  It would work much better as a travelling base.

An RTG using a low gamma emitting isotope would get around the shielding weight problem and could be constructed in small sizes.  The limitation here is the need to be able to safely dump the heat at all times.  If the cycle dumps heat at 1000K, then 2m2 radiator at the top of a vehicle would allow 100kW to be dumped.  If the engine cycle is 20% efficient, that would allow an engine power of 20kW (26.8HP).  Enough to move a vehicle at relatively slow speeds.  Strontium-90 would be an affordable radio-isotope for this application.  We would need about 187kg to generate 100kW of heat, so weight-wise it is doable.

But remember these isotopes must be launched from Earth.  A launch accident involving hundreds of kg of any isotope with decent heat generation would be a very messy event.  I have some doubts about the practicality of an RTG car for this very reason.  Why not a use a vehicle with lithium polymer batteries and a big roll-out PV panel?  Not quite as versatile as a nuclear vehicle, but a heck of a lot cheaper and easier to engineer.

What if you wanted to make those isotopes in space? Launch some nice and safe Uranium into space along with a cold reactor, then you turn on the reactor and have it make those isotopes in space, so they never have to be launched from Earth, and the reactor would generate some power while doing this, the power could be used to propel a spacecraft to Mars for instance.

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#5 2016-01-08 21:03:42

SpaceNut
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Re: Nukemobiles on Mars

Or even use the thorium that is on Mars such that all we bring is enough for a kick start of the system that is delivered....

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#6 2016-01-09 03:39:25

kbd512
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Re: Nukemobiles on Mars

SpaceNut wrote:

How would you cool a reactor?

On Mars in a M113 or MTVL, you could use a pump to actively cool the reactor, rather than heavy radiators.  Some of the waste heat is required to warm the vehicle and its rubber band tracks.

SpaceNut wrote:

As the radiator would be huge and the air is to thin to wick the heat away otherwise from it.

CO2 from Mars could be collected, pressurized, and used in a coolant loop.  Hot CO2 could be run through heat pipes to warm the vehicle and then dumped.

SpaceNut wrote:

My question is could we use an RTG power system safely?

Why bother?  RTG's are already heavy without shielding required for use in close proximity to humans and have much lower power output compared to fission reactors.

SpaceNut wrote:

What would we need to shield the crew?

My M113 / MTVL design requires shielding for the driver's compartment and reactor compartment.  A Detroit Diesel 6V53T weighs 769kg.  I think that between the electric motors, redundant reactor cooling system, and shielding, the power pack weight is a wash.  The absence of the Allison X200-4 transmission saves 442kg, but that weight savings goes right back into crew compartment shielding.  The driver's compartment is the most heavily shielded part of the vehicle because it's where the electronics and communications equipment is located, it's nearest to the reactor, and it's where the crew will go to wait out a SPE.

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#7 2016-01-09 11:26:46

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

The smallest nuclear power reactor systems that I know of are the ones inside submarines.  Those are almost small enough to fit in a railroad locomotive,  if you delete a lot of the shielding.  Deleting all the shielding is what they had to do to fly a functioning core in the NB-36.  With even minimal shielding,  they are nowhere near compact enough to fit in a road vehicle of any kind.  You'd have seen an atomic truck or car by now,  if reactor designs existed that were small enough. 

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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#8 2016-01-09 14:45:55

RobertDyck
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Re: Nukemobiles on Mars

Many have said we have technology to build a working nuclear aircraft. One problem with the NB-36 was its design. They had a compressor at the intake of engines on the wings, air was ducted through the wings to a heat exchanger in the body of the aircraft, then hot air ducted back through the wings to the engines, and routed through a turbine. The compressor and turbine had to be immediately in-front/behind each other so a drive shaft from the turbine could drive the compressor. But they didn't put the reactor in the engines, the reactor was in the aircraft body. Requiring this complicated ducting. All that is heavy. To do it right, you have to put the reactor right in the engine. If you have multiple engines, you have multiple reactors. One reactor per engine.

Project Pluto used an unshielded plutonium reactor. It was designed as a cruise missile, no crew on board. It used radiator fins to directly transfer heat from the reactor to air flowing through the RAM jet. It didn't have any complicated liquid cooling system, or primary/secondary heat exchanger. A working aircraft requires that. Keep mass low by keeping it simple: KISS.

SAFE-400 was an actual reactor design for space. It was designed to use highly enriched uranium. SP-100 development was completed in 1992, produced 2000 kW thermal, 100 kW electric, mass 5422 kg. SAFE-400 was completed in 2007, produced 400 kW thermal, 100 kW electric, 512 kg. It was so much lighter primarily because the reactor was smaller. The power converter was more efficient, producing the same electricity from less heat. But realize, neither SP-100 nor SAFE-400 were shielded. Mars Direct would have a light truck carry the SP-100 reactor to a crater, with a power cable trailing back to the ERV. The unshielded reactor would be dropped at the bottom of a crater. Astronauts would just have to remember to never walk in the direction of that reactor. When I wrote the discussion threat on this form "updating Mars Direct", I suggested using SAFE-400. And instead of a truck to drop off the reactor, I suggesting directly attaching mobility pieces identical to the Curiosity rover, so the reactor would drive itself to a crater. But again, no shielding.

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#9 2016-01-09 17:44:16

kbd512
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Posts: 1,187

Re: Nukemobiles on Mars

GW Johnson wrote:

The smallest nuclear power reactor systems that I know of are the ones inside submarines.  Those are almost small enough to fit in a railroad locomotive,  if you delete a lot of the shielding.  Deleting all the shielding is what they had to do to fly a functioning core in the NB-36.  With even minimal shielding,  they are nowhere near compact enough to fit in a road vehicle of any kind.  You'd have seen an atomic truck or car by now,  if reactor designs existed that were small enough. 

GW

You wouldn't see reactors inside terrestrial vehicles because there's no power density advantage over internal combustion engines for terrestrial applications.  In other words, there's no business case for them.  There's no comparison between PWR's that produce many MW of power and SAFE-400, which produces 400kWt and 100kWe.  However, SAFE-400 is 20 inches by 12 inches, not including the thermal management solution.

I don't know how much shadow shielding would be required to reduce the dose to 1 mrem/hr, but it would be substantial.

The max core temperature was stated as 1020C, so we need gamma and neutron absorbing materials that can withstand 1500C.  I think Boron Carbide for neutron and Tungsten for gamma.

All that's required is a shadow shield.  Although beneficial, the entire reactor does not require shielding.

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#10 2016-01-09 18:19:53

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

Re: Nukemobiles on Mars

RobertDyck wrote:

Many have said we have technology to build a working nuclear aircraft. One problem with the NB-36 was its design. They had a compressor at the intake of engines on the wings, air was ducted through the wings to a heat exchanger in the body of the aircraft, then hot air ducted back through the wings to the engines, and routed through a turbine. The compressor and turbine had to be immediately in-front/behind each other so a drive shaft from the turbine could drive the compressor. But they didn't put the reactor in the engines, the reactor was in the aircraft body. Requiring this complicated ducting. All that is heavy. To do it right, you have to put the reactor right in the engine. If you have multiple engines, you have multiple reactors. One reactor per engine.

Even if we could develop a nuclear aircraft, to paraphrase one of the reactor designer engineers quotes on that task, it's impractical if not foolish.

RobertDyck wrote:

Project Pluto used an unshielded plutonium reactor. It was designed as a cruise missile, no crew on board. It used radiator fins to directly transfer heat from the reactor to air flowing through the RAM jet. It didn't have any complicated liquid cooling system, or primary/secondary heat exchanger. A working aircraft requires that. Keep mass low by keeping it simple: KISS.

GW said that the thermal flux melted the structural components.  Materials have improved since the 60's, but fast reactors that put out hundreds to thousands of MWt are... wait for it... really hot.  Hot enough to BBQ nearly all aerospace structural materials.

RobertDyck wrote:

SAFE-400 was an actual reactor design for space. It was designed to use highly enriched uranium. SP-100 development was completed in 1992, produced 2000 kW thermal, 100 kW electric, mass 5422 kg. SAFE-400 was completed in 2007, produced 400 kW thermal, 100 kW electric, 512 kg. It was so much lighter primarily because the reactor was smaller. The power converter was more efficient, producing the same electricity from less heat. But realize, neither SP-100 nor SAFE-400 were shielded. Mars Direct would have a light truck carry the SP-100 reactor to a crater, with a power cable trailing back to the ERV. The unshielded reactor would be dropped at the bottom of a crater. Astronauts would just have to remember to never walk in the direction of that reactor. When I wrote the discussion threat on this form "updating Mars Direct", I suggested using SAFE-400. And instead of a truck to drop off the reactor, I suggesting directly attaching mobility pieces identical to the Curiosity rover, so the reactor would drive itself to a crater. But again, no shielding.

The 512kg for SAFE-400 includes no radiation shielding or thermal management.  In other words, 512kg for the reactor core and Brayton cycle generator.

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#11 2016-01-10 07:00:18

Terraformer
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From: Lancashire
Registered: 2007-08-27
Posts: 2,508
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Re: Nukemobiles on Mars

Why not use a Strontium-90 battery, rather than a full reactor? It undergoes beta decay, so shielding should be much less of a concern. Though too expensive to  use in a Terran vehicle, the price required would be insignificant for a Mars mission. It's got a half life of ~29 years, so you shouldn't have to worry about power for quite a while.


"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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#12 2016-01-10 12:59:04

Tom Kalbfus
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Posts: 4,401

Re: Nukemobiles on Mars

GW Johnson wrote:

The smallest nuclear power reactor systems that I know of are the ones inside submarines.  Those are almost small enough to fit in a railroad locomotive,  if you delete a lot of the shielding.  Deleting all the shielding is what they had to do to fly a functioning core in the NB-36.  With even minimal shielding,  they are nowhere near compact enough to fit in a road vehicle of any kind.  You'd have seen an atomic truck or car by now,  if reactor designs existed that were small enough. 

GW

On Mars, you make your own roads, and the roads can be as wide as you need them to be, there are no forests or other terrain besides deserts on Mars. If you need a rover as big as a submarine, the terrain on Mars should prove no major obstacle.

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#13 2016-01-10 13:15:07

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

Re:  NB-36 -- there's some confusion here.  The NB-36 was a B-36 bomber modified to carry a partially-shielded operating reactor in flight for a test.  That reactor was not part of an operating nuclear aircraft propulsion system.  The nuclear aircraft propulsion system was a paper design never flown. 

The NB-36 flight was an early feasibility demo for even carrying a reactor in an airplane at all.  The most notable thing about that test flight was the crash containment vessel they built for the reactor core:  capable of withstanding a 500 mph crash without leaking.  It was tested on the rocket sled into a granite mountain at 500 mph before flying. 

Re:  Project Pluto in the late 1950's / early 1960's-- yes,  that reactor was entirely unshielded,  and further,  it had no containment at all.  It was a bare solid core through which the propulsive airstream passed at subsonic speed,  before nozzling down to become the exhaust jet.  The worst development test problems were with the structural supports for the core,  which reputedly operated about 10 F away from melting. 

As a project,  it had 3 fatal flaws (1) the new solid-propellant Minuteman ballistic missiles about to debut were both cheaper and safer,  (2) the exhaust stream was intensely radioactive from core and support erosion debris,  and would have killed friendly folks on the ground while on its way,  and (3) it was a low altitude Mach 3 cruiser,  whose trailing shock wave would also have killed friendly folks on the ground. 

One estimate/guesstimate I saw said the unintended casualties on the way would have outnumbered the folks killed at the target by its megaton-range warhead.  Definitely a doomsday weapon to be used only when committing MAD suicide. 

All that being said,  it should be possible to power a tank-like tracked rover on Mars with some sort of nuclear power.  It won't be as heavy as a tank,  but the core and shielding will make it heavier than you want.  It needs to travel at about the speed of an ordinary tank (maybe 20-30 mph) to be in the least practical.  That means a crude guess for drive power requirements might be at least 500-1000 HP,  even in Mars's lower gravity.  Probably more,  as you have to drive uphill a lot in rough terrain,  and soft surfaces make drag a whole lot worse.  Could easily be as high as 3000 HP. 

You'll need a compact,  shadow-shielded nuclear "something" capable of power levels like that.  HP or KW makes no difference:  it's low 1000's either way. 

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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#14 2016-01-10 18:08:36

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

Re: Nukemobiles on Mars

GW Johnson wrote:

All that being said,  it should be possible to power a tank-like tracked rover on Mars with some sort of nuclear power.  It won't be as heavy as a tank,  but the core and shielding will make it heavier than you want.  It needs to travel at about the speed of an ordinary tank (maybe 20-30 mph) to be in the least practical.  That means a crude guess for drive power requirements might be at least 500-1000 HP,  even in Mars's lower gravity.  Probably more,  as you have to drive uphill a lot in rough terrain,  and soft surfaces make drag a whole lot worse.  Could easily be as high as 3000 HP.

GW,

Here on Earth the M113 doesn't need 500 hp or even 1000 hp, let alone 3000 hp, to make it up a 60% gradient.  The M113 has a 275 hp diesel engine and the vehicle doesn't weigh 38% of its loaded weight.  The notion that you need that kind of power for driving around relatively flat terrain simply does not square with reality here on Earth.

Here on Earth, the rubber band tracks that certain variants of the M113 use substantially reduce rolling resistance, have substantially greater maintenance-free life in comparison to steel tracks, and greatly reduce vibration and thus improve ride quality.  Obviously you need heating elements in the tracks to warm them to keep the rubber above its glass transition temperature, but the one thing a nuclear reactor never run out of is waste heat.

For off-road use, any vehicle carrying a substantial payload simply has an easier time with the terrain when it's tracked.  Even on Mars, where gravity is substantially less, a tracked vehicle is going to have an easier time negotiating terrain.  Physics aren't suspended just because we're driving around a planet with 38% the gravity of Earth.

GW Johnson wrote:

You'll need a compact,  shadow-shielded nuclear "something" capable of power levels like that.  HP or KW makes no difference:  it's low 1000's either way. 

GW

Yes, you do need a substantial shadow shield.  Even then, you're going to take a little bit more radiation than background levels.  However, it's not substantially more.  You also need a substantial amount of water.  You have a relatively heavy reactor up front and a relatively heavy water tank in the rear.

The benefit to using nuclear power is that you don't have to haul impossible-to-get fuel, low energy density batteries, or low output solar panels around the surface of a planet that's substantially further from the Sun than Earth is and doesn't have any gas stations.

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#15 2016-01-10 19:13:03

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

Re: Nukemobiles on Mars

So first problem after delivering the reactor powered truck to mars is providing the insitu CO2 liquidation plant to be able to fill the reactors cooling system.....

So another specialized payload must be launched to mars but also must be movable to the reactor in order to be able to fill it up so that it can be used.

Once it can start then it can maintain its own systems level of coolant( liquifided Co2) and created insitu atmospher within the truck for the crew as well as the power and heat to climate control there ride.....

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#16 2016-01-10 21:24:31

kbd512
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Posts: 1,187

Re: Nukemobiles on Mars

SpaceNut wrote:

So first problem after delivering the reactor powered truck to mars is providing the insitu CO2 liquidation plant to be able to fill the reactors cooling system.....

Why wouldn't you have CO2 in your coolant loop when you land on Mars?

SpaceNut wrote:

So another specialized payload must be launched to mars but also must be movable to the reactor in order to be able to fill it up so that it can be used.

No specialized payload is required.  Land the rover with everything required for it to function as a rover.  If you want to delete the mass of consumables, like food and medicine, and then stock the rover with landed supplies on Mars, that might make sense.

SpaceNut wrote:

Once it can start then it can maintain its own systems level of coolant( liquifided Co2) and created insitu atmospher within the truck for the crew as well as the power and heat to climate control there ride.....

Ship the rover with reactor coolant.

Regarding making oxygen from CO2, the reactor could potentially refill depleted LOX bottles by provide the heat required to do something like this:

Make Oxygen from CO2

Waste heat isn't a terrible problem on a frigid planet like Mars if you find ways to use it to your advantage.

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#17 2016-01-10 21:41:46

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

Re: Nukemobiles on Mars

I have been following another potential power source for Mars in one that produces heat and electrical power.

E-Cat

http://ecat.com/

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#18 2016-01-10 23:07:07

kbd512
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Posts: 1,187

Re: Nukemobiles on Mars

SpaceNut wrote:

I have been following another potential power source for Mars in one that produces heat and electrical power.

E-Cat

http://ecat.com/

SpaceNut,

I've been following that, too.  I have no idea large a 400kWt E-Cat would be.  If it's roughly the same size and weight as SAFE-400, great.  It requires no shielding, so that's a plus.

It clearly works and produces an enormous amount of heat, but I have no idea how much fuel is required by the reaction or if the fuel requires special storage.  My understanding is that some component of the fuel is pyrophoric.  That may not be a good thing to have in an aluminum vehicle filled with O2, but it's that or the problems associated with a fission reactor mere inches from where you live.

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#19 2016-01-11 05:35:03

Terraformer
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Registered: 2007-08-27
Posts: 2,508
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Re: Nukemobiles on Mars

I don't think 500 W/kg heat (pure Strontium-90) is "low density". Or if you like, an average of ~400 W/kg over 30 years. Using Strontium Titanate in an RTG, you might get 50 W/kg electric to start with, declining to 25 W/kg after 30 years.

I think a 100 kWe system that masses 4 tonnes should not be disregarded just because it hasn't been done before. We know Sr-90 can be used in RTGs, and most RTGs get low efficiency because they use thermocouples rather than turbines. But at this size, we can justify a turbine system, so efficiency should be much higher.

I think atomic batteries are underrated.


"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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#20 2016-01-11 06:31:28

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

Re: Nukemobiles on Mars

kbd512 wrote:
GW Johnson wrote:

The smallest nuclear power reactor systems that I know of are the ones inside submarines.  Those are almost small enough to fit in a railroad locomotive,  if you delete a lot of the shielding.  Deleting all the shielding is what they had to do to fly a functioning core in the NB-36.  With even minimal shielding,  they are nowhere near compact enough to fit in a road vehicle of any kind.  You'd have seen an atomic truck or car by now,  if reactor designs existed that were small enough. 

GW

You wouldn't see reactors inside terrestrial vehicles because there's no power density advantage over internal combustion engines for terrestrial applications.  In other words, there's no business case for them.  There's no comparison between PWR's that produce many MW of power and SAFE-400, which produces 400kWt and 100kWe.  However, SAFE-400 is 20 inches by 12 inches, not including the thermal management solution.

I don't know how much shadow shielding would be required to reduce the dose to 1 mrem/hr, but it would be substantial.

The max core temperature was stated as 1020C, so we need gamma and neutron absorbing materials that can withstand 1500C.  I think Boron Carbide for neutron and Tungsten for gamma.

All that's required is a shadow shield.  Although beneficial, the entire reactor does not require shielding.

The shielding does not need to be inside the reactor vessel.  The reactor internals would need to meet these materials constraints, the shielding does not.  This is how AGR's are able to use concrete biological shields despite running at outlet temperatures of 600C.

You could probably estimate required shielding thickness and mass by drawing correlations with existing reactors.  Otherwise, its MCNP or Microshield.  In terms of mass, water is about the most efficient shield for mixed neutron/gamma attenuation.  For gamma rays you can use half-thickness values to get a rough approximattion os shield thickness.  In water, neutron flux will fall off more rapidly than gamma flux, so this might be an acceptable approximation.

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#21 2016-01-11 06:40:12

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

Re: Nukemobiles on Mars

Terraformer wrote:

I don't think 500 W/kg heat (pure Strontium-90) is "low density". Or if you like, an average of ~400 W/kg over 30 years. Using Strontium Titanate in an RTG, you might get 50 W/kg electric to start with, declining to 25 W/kg after 30 years.

I think a 100 kWe system that masses 4 tonnes should not be disregarded just because it hasn't been done before. We know Sr-90 can be used in RTGs, and most RTGs get low efficiency because they use thermocouples rather than turbines. But at this size, we can justify a turbine system, so efficiency should be much higher.

I think atomic batteries are underrated.

Imagine the radioactive consequences of spilling 1tonne of strontium-90 into Earth's atmosphere due to a launch accident.  Most large nuclear reactors produce about 1tonne of mixed fission products each year.  So a launch accident involving a 1te strontium battery would be like Chernobyl all over again.  A U-235 fission powered rover would be safer at launch but would mass a lot more.

Why not a lithium-polymer battery powered rover, with a big roll-out panel?  Stop at dusk, roll out the panel, charge in the morning, drive in the afternoon.  How does the weight work out compared to a nuclear reactor?  I bet they are about the same.

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#22 2016-01-11 07:12:48

Terraformer
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From: Lancashire
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Re: Nukemobiles on Mars

Could we minimise the risk of a launch accident by using strontium titanate in a strong containment vessel, and launching over land so that if it does split, it won't be dispersed by water currents? Alternatively, making it dense enough that if it lands in water it will rapidly sink to the bottom.

I think the important thing with Sr-90 is to keep it from dispersing. If we can do that, the safety concerns shouldn't be a problem, only the political ones.


"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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#23 2016-01-11 09:14:22

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

Re: Nukemobiles on Mars

Terraformer wrote:

I don't think 500 W/kg heat (pure Strontium-90) is "low density". Or if you like, an average of ~400 W/kg over 30 years. Using Strontium Titanate in an RTG, you might get 50 W/kg electric to start with, declining to 25 W/kg after 30 years.

I think a 100 kWe system that masses 4 tonnes should not be disregarded just because it hasn't been done before. We know Sr-90 can be used in RTGs, and most RTGs get low efficiency because they use thermocouples rather than turbines. But at this size, we can justify a turbine system, so efficiency should be much higher.

I think atomic batteries are underrated.

You explained why it hasn't been used for space-based RTG's in your first sentence.  Strontium-90's power density is .7W/cm3 to .9W/cm3.

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#24 2016-01-11 10:07:16

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

Re: Nukemobiles on Mars

Antius wrote:

The shielding does not need to be inside the reactor vessel.  The reactor internals would need to meet these materials constraints, the shielding does not.  This is how AGR's are able to use concrete biological shields despite running at outlet temperatures of 600C.

The shielding is not inside the reactor.  The reactor itself, which includes the walls of the vessel, is quite hot.  Anything in contact with the reactor vessel has to be capable of withstanding the heat generated by the reactor.  Boron Carbide is used to capture fast neutrons.  Tungsten is used to absorb gamma.

DU would be preferable for absorbing gamma, but you still need something to absorb neutrons.  I think uranium carbide or uranium boride should be considered to increase gamma absorption.  Both melt at temperatures far above what's required for this application.  This solution won't weigh less, but it will be a far more effective High-Z / Low-Z combination than boron carbide and tungsten.

Antius wrote:

You could probably estimate required shielding thickness and mass by drawing correlations with existing reactors.  Otherwise, its MCNP or Microshield.  In terms of mass, water is about the most efficient shield for mixed neutron/gamma attenuation.  For gamma rays you can use half-thickness values to get a rough approximattion os shield thickness.  In water, neutron flux will fall off more rapidly than gamma flux, so this might be an acceptable approximation.

The crew are so close to the reactor that a very effective shield is required.  This is a M113.  Space is at a premium.  Water won't cut it.

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#25 2016-01-11 10:56:14

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

Re: Nukemobiles on Mars

Antius wrote:

Why not a lithium-polymer battery powered rover, with a big roll-out panel?  Stop at dusk, roll out the panel, charge in the morning, drive in the afternoon.  How does the weight work out compared to a nuclear reactor?  I bet they are about the same.

The rover should ideally be capable of 24.65/7 operation.  The rover shouldn't be limited to driving at night.  If you have a dust storm and your solar panel effectiveness drops off, then what?  You still need power for life support.  The rover is also intended for robotic and tele-robotic operation, so the solar panels have to be attached to the vehicle.  I suppose you could mount a drum atop the vehicle to spool off a large flexible array and then slowly backup and retract the panel to prevent tearing a delicate flexible panel.  That would work and you could have a fairly large flexible panel.

No matter what your energy solution is, you have to carry the weight of your energy solution with you everywhere.  That rules out hydrocarbon fuels and limits the practicality of solar.  You also have to expend energy to heat the vehicle and its tracks.

Since I plan on using a convoy of 4 vehicles, two occupied and two robotic (my weight and volume calculations were initially for solar powered vehicles with consumables required for a nominal 500 day surface stay), the two robotic vehicles could share power with the manned vehicles.  I plan on having external power and air ports for the astronauts to use to explore near the vehicles and perform track replacement, if required.

The 4 vehicle convoy is necessitated by consumables storage volume, redundancy to contend with vehicle failures, spares storage, and scientific instrument storage.

Even if the reactor and shadow shield weigh 3t-4t, I think you still come out ahead with respect to the overall efficiency of the solution with nuclear and have 24.65/7 power and plenty of waste heat to use to heat the vehicle.

Unlike the solar panels and li-poly battery, the reactor and shielding combination won't degrade appreciably over the course of three missions.  After one mission, you have to replace the panels and batteries.  Worth it?  Do a trade study.

I'm not married to nuclear or solar, but I want to know what the weight of each solution would be, which solution requires the lightest replacement parts and therefore least expensive spares shipment for subsequent missions, and which solution is most technically feasible.

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