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#51 2017-05-01 17:03:57

Dook
Banned
From: USA
Registered: 2004-01-09
Posts: 1,409

Re: Air. Shelter. Water. Food.

The first dome picture could work but you're talking about moving a gigantic amount of regolith for a dome of any real size. 

It would be better just to bury a habitat and have plastic panels only on the sides to let some sun in.  If the base is in the northern hemisphere only the south facing side would need windows.  Still, if the pressure inside was only 5 psi you're talking about a pressure on each window of about 5,000 pounds.  Is it worth risking everyones life on just to get a little sunlight?

The second picture won't work because your foundation weight has to be equal to the uplifting force.  Even a very small dome, lets say 25 foot across, at 2 psi would have an uplifting pressure of about 275,000 pounds of force.  Mars gravity is .38 percent of the Earth's so you would need a foundation that is gigantic.

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#52 2017-05-01 17:29:18

SpaceNut
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Posts: 19,719

Re: Air. Shelter. Water. Food.

The ISS cupola windows and even the shuttles were able to sustain this level of pressure without breaking....So the framing and glass just needs to be made in the correct manner....

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#53 2017-05-01 17:48:30

Dook
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Registered: 2004-01-09
Posts: 1,409

Re: Air. Shelter. Water. Food.

The ISS and space shuttle don't have large windows like a greenhouse probably would but we could make all the panels on our greenhouse smaller, that would reduce the amount of total pressure on each window panel. 

Instead of 8 sq ft panels maybe reduce the panels by 1/4 to just 2 sq ft panels.  Then, at 5 psi, the total pressure on each panel would be 1,250 pounds.  Is it possible?  Yes, but you're betting your life on it and they have to last forever and you can't have any pressure changes or you would increase the chances of failure. 

I think it's too much risk just to get some natural sunlight.

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#54 2017-05-01 18:11:52

SpaceNut
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Posts: 19,719

Re: Air. Shelter. Water. Food.

As you noted only the windows facing towards the sun even need to be installed into the frame and with the ISS they have a shutter which can be moved to protect them from incoming rocks. The Windows: Fused silica and borosilicate glass are 12 cm thick glass is actually a composite of four laminated panes consisting of a thin exterior 'debris' pane that protects it from micrometiorites, two internal pressure panes, and an interior 'scratch' pant to absorb accidental marking from inside.

URUGw.jpg

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#55 2017-05-01 20:32:30

Dook
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Posts: 1,409

Re: Air. Shelter. Water. Food.

A greenhouse that has windows that are 5.6 inches of glass would be way too heavy to ship from the Earth. 

You could have a surface habitat that has it's roof and three sides covered in regolith and one side facing south using these windows.  That one side of windows probably wouldn't provide as much radiation protection as 8.2 feet of regolith though.

It's just not that important to get a little sunlight.  LED grow lamps inside would be fine.

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#56 2017-05-01 20:51:41

Oldfart1939
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Registered: 2016-11-26
Posts: 1,898

Re: Air. Shelter. Water. Food.

One way of achieving a dome structure of some degree of permanence could be achieved initially using the polymer film dome, but replacing it from the inside with a stronger structure, subsequently rendering the "bubble" superfluous. The damage to the dome from micrometeorites should be significantly less on the planetary surface, but a multi-thickness of different materials would seem prudent. A roughly 3" thick set of panels would probably suffice. Outer panels of Fused Silica, middle panel of high strength Polycarbonate, inner panel of borosilicate glass (Pyrex).  Using triangular panels also allows a curved structure to evolve more easily, and is also statically stronger.

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#57 2017-05-01 21:28:41

Dook
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Re: Air. Shelter. Water. Food.

The pressure needed to inflate a thin plastic dome would be very small, barely exceeding Mars atmospheric pressure, not enough to allow a crew to work without Mars suits. 

Building a greenhouse out of hard panels only gets you heat on your buried habitat since you can never pressurize the greenhouse.  And since you can never pressurize it there's no need for the panels to be built very strong, 1/4" plastic would be fine.

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#58 2017-05-01 22:39:31

Dook
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Re: Air. Shelter. Water. Food.

Also, the idea that we can use a methane/oxygen powered bobcat is not practical for Mars.

A 100 cubic inch internal combustion engine at 2,500 rpm uses 250,000 cubic inches of oxygen per minute. 

A large compressed oxygen cylinder contains 150 cubic feet of oxygen, that's 259,200 cubic inches.  So one large oxygen cylinder will provide enough oxygen to run your bobcat for one minute.  Let's say the bobcat carries ten oxygen cylinders, that's ten minutes of work and then you have to refill all of your oxygen bottles.  And, I'm not even taking into account the number of methane cylinders you would need to run the bobcat. 

Any rover or vehicle on Mars has to be powered by solar/battery or RTG.

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#59 2017-05-01 22:44:42

Oldfart1939
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Registered: 2016-11-26
Posts: 1,898

Re: Air. Shelter. Water. Food.

No, the Bobcat would carry a Dewar flask containing LOX. Compressed gasses aren't practical, as you just pointed out. This is simply per Robert Zubrin in Entering Space.

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#60 2017-05-01 23:07:35

Dook
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Registered: 2004-01-09
Posts: 1,409

Re: Air. Shelter. Water. Food.

LOX has to be kept cold, -297 or it evaporates and you have to release it, you can't attempt to contain it inside a pressure vessel because it will explode the pressure vessel.  But, the expansion ratio is high, 861 to 1.  So, a hundred gallon LOX tank will get you 86,100 gallons of oxygen gas.

1 gallon equals .13 cubic feet, so, if my math is right, that 86,100 gallons of oxygen gas will give you 11,193 cubic feet, which converts to 19,341,504 cu inches.  Enough to run your 100 cubic inch bobcat engine at 2,500 rpm for 77 minutes. 

But you would have to find some way of keeping the LOX super cold or just let it evaporate.  And the 100 gallon LOX tank would be very heavy.  And you would have to get all of  your LOX shipped from the Earth and then transfer it to your bobcat LOX tank.  And your engine that uses LOX would have to have some kind of heating tube from the LOX tank to the engine so that the LOX converts to oxygen gas fast enough for the internal combustion engine to use. 

There's just no way this is at all practical for use on Mars.  Just because it's possible doesn't mean it's practical.

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#61 2017-05-01 23:14:56

Dook
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Posts: 1,409

Re: Air. Shelter. Water. Food.

LOX weighs 9.5 lbs per gallon.  So, the 100 gallon LOX tank would weigh 950 lbs on the Earth, actually much more than that because you would lose a lot in 6 months of boil off so the tank would have to be much larger. 

On Mars, the 100 gallon LOX tank on the bobcat would add an extra 361 lbs. 

And you would still need methane tanks.

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#62 2017-05-02 01:40:31

RobertDyck
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From: Winnipeg, Canada
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Posts: 6,162
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Re: Air. Shelter. Water. Food.

Dook wrote:

How are you going to get the hold down pegs through the permafrost?

I don't believe permafrost is right at the surface. Mars Phoenix landed at the north pole, at a location covered in tens of meters of dry ice during the winter, but sublimes during summer. They thought there might be water ice at that location, that's why they landed it there. Turns out ice was just below the surface, less than one scoop of their shovel. Don't expect the same at a location near the equator. I have said any human base would be set up between the tropics. Others have said more northern due to Mars elliptical orbit, the northern most limit may be slightly farther than the northern tropic, but not much. At these locations expect permafrost to be quite deep. That's another reason I have argued to land somewhere near a more substantial ice deposit: frozen lake or glacier. That means I expect Mars regolith to be soil with rocks for at least the depth of a tent peg. For a science mission, I envision pounding pegs with a sledge hammer, or possibly a light jack hammer.

Dook wrote:

Inflatables are not durable enough for a settlement to depend on.  Food is life support.  It can't fail every year or two, it has to be built to last.

I expect a science mission or construction team for a base to include inflatable(s). I have also said permanent settlement will require something more substantial, and something that can be made easily in-situ. An inflatable greenhouse will have to withstand Mars environment. I've argued for PCTFE, which is a fluoropolymer. On Earth the premium polymer film for a greenhouse is Tefzel, a co-polymer of ethylene (C2H4) with TFE (fluoromonomer C2F4). Tefzel lasts many years. PCTFE is all fluoropolymer (C2F3Cl), it's more expensive but stronger, highly resistant to UV, and able to withstand the cold of Mars. Expect PCTFE to last many years. But PCTFE is hard to make; don't expect an early settlement to be able to make it. And sand storms could limit it's life. That's why I said a permanent settlement would use glass, and based on hardness to avoid getting scratched during a major sand storm, I've argued for tempered glass.

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#63 2017-05-02 06:28:41

elderflower
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Re: Air. Shelter. Water. Food.

A dome, perhaps like the Eden Project, could be made with an outer layer or two of plastic pillows filled with water and allowed to freeze in situ. Inside this you would have transparent insulation. This would give mass to counter internal air pressure and provide radiation and meteoroid protection. You might need to repair or renew occasionally. It does require the location of a large amount of water to fill it. The ground side of the dome most be pressure tight and insulated. It would be more nearly a partly buried globe rather than a dome to avoid the need for a massive foundation.

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#64 2017-05-02 08:53:42

Oldfart1939
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Registered: 2016-11-26
Posts: 1,898

Re: Air. Shelter. Water. Food.

The Bobcat engine would be a completely new design. You may not know it, but lots of busses run on Methane these days, especially those operated by the government in the National Parks. Lots of POVs also have been converted to run on either Methane or Propane. When Oxygen is used instead of air the engines become a lot smaller since they don't need to compress the volume of air (23% Oxygen) in order to run. We don't need to consider pressurization in a Dewar flask (a big vacuum-thermos bottle), since the LOX will be used by a fuel injection system. The 100 gallon tank is far too big and exceeds the vehicle requirements by a factor of 4. A 100 kg tank of LOX should operate the Bobcat for at least an hour and that's ~ 25 gallons. Methane can be stored in a liquid state if kept under pressure, the way the busses do it. Most Earthside forklifts run on bottled propane since they don't generate Carbon Monoxide. Ditto, Methane.

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#65 2017-05-02 09:14:35

Dook
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Registered: 2004-01-09
Posts: 1,409

Re: Air. Shelter. Water. Food.

RobertDyck wrote:
Dook wrote:

How are you going to get the hold down pegs through the permafrost?

I don't believe permafrost is right at the surface. Mars Phoenix landed at the north pole, at a location covered in tens of meters of dry ice during the winter, but sublimes during summer. They thought there might be water ice at that location, that's why they landed it there. Turns out ice was just below the surface, less than one scoop of their shovel. Don't expect the same at a location near the equator. I have said any human base would be set up between the tropics. Others have said more northern due to Mars elliptical orbit, the northern most limit may be slightly farther than the northern tropic, but not much. At these locations expect permafrost to be quite deep. That's another reason I have argued to land somewhere near a more substantial ice deposit: frozen lake or glacier. That means I expect Mars regolith to be soil with rocks for at least the depth of a tent peg. For a science mission, I envision pounding pegs with a sledge hammer, or possibly a light jack hammer.

Dook wrote:

Inflatables are not durable enough for a settlement to depend on.  Food is life support.  It can't fail every year or two, it has to be built to last.

I expect a science mission or construction team for a base to include inflatable(s). I have also said permanent settlement will require something more substantial, and something that can be made easily in-situ. An inflatable greenhouse will have to withstand Mars environment. I've argued for PCTFE, which is a fluoropolymer. On Earth the premium polymer film for a greenhouse is Tefzel, a co-polymer of ethylene (C2H4) with TFE (fluoromonomer C2F4). Tefzel lasts many years. PCTFE is all fluoropolymer (C2F3Cl), it's more expensive but stronger, highly resistant to UV, and able to withstand the cold of Mars. Expect PCTFE to last many years. But PCTFE is hard to make; don't expect an early settlement to be able to make it. And sand storms could limit it's life. That's why I said a permanent settlement would use glass, and based on hardness to avoid getting scratched during a major sand storm, I've argued for tempered glass.

Permafrost is not at the surface.  Almost all the surface ice has been melted and evaporated away except for the possibility of salt water in places.

For a science mission, you envision pounding pegs with a sledge hammer to hold down a greenhouse?  If the greenhouse is an inflatable you just have to hold it down from the wind and the regolith moved inside to grow plants would be enough.

A settlement would require something that would be easily made in-situ?  Nothing can be easily made in-situ on Mars except oxygen and water and those two things will be made by machines brought from the Earth.     

I don't think there are sand storms on Mars.  There are dust storms but the dust is so fine that I don't think it could scratch plastic. 

I still think we can't pressurize a dome greenhouse.  It could be built over a buried habitat to provide heat but that's all.

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#66 2017-05-02 09:19:16

Dook
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Registered: 2004-01-09
Posts: 1,409

Re: Air. Shelter. Water. Food.

elderflower wrote:

A dome, perhaps like the Eden Project, could be made with an outer layer or two of plastic pillows filled with water and allowed to freeze in situ. Inside this you would have transparent insulation. This would give mass to counter internal air pressure and provide radiation and meteoroid protection. You might need to repair or renew occasionally. It does require the location of a large amount of water to fill it. The ground side of the dome most be pressure tight and insulated. It would be more nearly a partly buried globe rather than a dome to avoid the need for a massive foundation.

The greenhouse dome could be held down with bags of water placed on top of the greenhouse?  It would require 130,000 gallons of water to equal the uplifting force on a 2 psi greenhouse that is a 100 foot dome. 

If there is a pressure loss your entire greenhouse comes crashing down.

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#67 2017-05-02 09:42:56

Dook
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Registered: 2004-01-09
Posts: 1,409

Re: Air. Shelter. Water. Food.

Oldfart1939 wrote:

The Bobcat engine would be a completely new design. You may not know it, but lots of busses run on Methane these days, especially those operated by the government in the National Parks. Lots of POVs also have been converted to run on either Methane or Propane. When Oxygen is used instead of air the engines become a lot smaller since they don't need to compress the volume of air (23% Oxygen) in order to run. We don't need to consider pressurization in a Dewar flask (a big vacuum-thermos bottle), since the LOX will be used by a fuel injection system. The 100 gallon tank is far too big and exceeds the vehicle requirements by a factor of 4. A 100 kg tank of LOX should operate the Bobcat for at least an hour and that's ~ 25 gallons. Methane can be stored in a liquid state if kept under pressure, the way the busses do it. Most Earthside forklifts run on bottled propane since they don't generate Carbon Monoxide. Ditto, Methane.

The bobcat engine used on Mars would be a completely new design?  So it wouldn't use as much oxygen?  How? 

I may not know it but busses run on methane these days?  You can operate internal combustion engines on methane or propane on the earth but that's because the earth has an atmosphere with oxygen.  Internal combustion engines are not practical for use on Mars. 

When oxygen is used instead of air the engines become a lot smaller because they don't need to compress the air in order to run?  Internal combustion engines use pistons that suck in air and fuel, compress it, then a spark ignites the mixture to create a power stroke.  They are designed to compress the mixture.  If you are saying that a new type of engine will be used on Mars that doesn't compress the gas mixture, please give details on it. 

LOX would be used by a fuel injection system?  Inject liquid oxygen directly into a cylinder instead of changing it to a gas first?  Hmm.  When you move LOX it evaporates quickly because the lines are not frozen yet so all you get is oxygen gas.  Then, after a bit the liquid will start coming out once the lines have frozen.  The LOX lines going to your injectors would be very small because of the expansion ratio of liquid oxygen to gas is so high so they would not be sized for oxygen gas.  You would have to vent oxygen from the engine until you got LOX to it.  I don't know, it might be possible but I certainly don't think it compares well to solar powered electric motors.

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#68 2017-05-02 10:03:25

GW Johnson
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Posts: 4,078
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Re: Air. Shelter. Water. Food.

I think you will find that to run an IC-type engine on a fuel plus oxygen you will bump into these two rather critical considerations: 

(1) however you input the fuel and oxygen,  the burning must take place at elevated pressure,  for either piston or turbine of any kind at all to work (which means some sort of compression is inevitable),  and

(2) fuel plus oxygen at stoichiometric typically burns near 5500-6000 F whereas fuel plus air at stoichiometric typically burns at about 4000 F.  The hardware cooling problem is far harder to deal with when using straight oxygen without a diluent gas.  Consider that iron alloys are a white-hot puddle of liquid if the iron reaches 2935 F.  And heat transfer is typically the slowest process of them all. 

That's not to say it cannot be done,  because it can.  I'd recommend 23-25% oxygen diluted with Martian-atmospheric CO2 as the oxidant stream fed to the engine along with the fuel.  They can be kept in 3 different tanks as liquid fuel,  liquid oxygen,  and liquid CO2.  Until the experiments have been run that say otherwise is OK,  I'd recommend mixing the CO2 and oxygen before adding the fuel and trying to burn. 

Doing the phase change to gaseous oxidant in the combustion zone is going to be a real problem,  one that makes your engine complex and short life,  because its combustion chamber becomes a liquid rocket engine.  To avoid that,  you need to feed your oxygen-CO2 stream to the combustion chamber as a gas,  so that only the minority fuel component need be vaporized during the combustion process.  That way your engine can look like a more-or-less conventional piston or turbine engine.  If you do not do it that way,  it cannot be a conventional design,  it will be something we have never before attempted. 

Doing something unconventional usually does not turn out very well.  For example,  operating a diesel engine with hydrogen peroxide instead of air actually cost more than one submarine crew their lives. 

Just food for thought.

GW

Last edited by GW Johnson (2017-05-02 10:05:49)


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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#69 2017-05-02 10:16:57

Oldfart1939
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Registered: 2016-11-26
Posts: 1,898

Re: Air. Shelter. Water. Food.

As I stated in my previous post--it would need to be a new design. Something that might be done at NASA?

As an afterthought GW, you recall the operating temperature curves for fuel-air mixtures. The highest temperature as you stated is exactly at the stoichiometric ratio. Run either fuel rich (as in my Lycoming O-540) or air rich (LOP operation). I'd recommend running the engine pretty fuel-rich, since we should be making Methane from ISRU. That plus a dose of atmospheric CO2 should get the operating temperature down and allow some compression, to boot.

Last edited by Oldfart1939 (2017-05-02 10:24:42)

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#70 2017-05-02 11:00:11

Dook
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Re: Air. Shelter. Water. Food.

Battery powered vehicles will work for an hour or two a day and then they would have to recharge.  The only real limit for a battery powered vehicle is on mechanical failure and battery life.  The vehicles have to be built somewhat lightweight and the batteries would have to be rebuilt every 8-10 years or so but the crew would just have to replace the acid plates and not the entire battery.  They would have a supply of them on hand.

Dust storms might not even stop a battery vehicle from being used unless it becomes too difficult to see because the vehicles can be recharged at night by the bases big RTG.

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#71 2017-05-02 14:51:37

kbd512
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Re: Air. Shelter. Water. Food.

I think Dook has the right idea, as far as vehicle power is concerned.

Electric motors don't require the support infrastructure that internal combustion engines require.  There are no lubricants or fuels to manufacture, store, or transport.  Current Lithium-ion battery technology is rapidly being eclipsed by graphene-based super capacitor technology.  Super capacitors recharge in seconds, can operate at temperatures far outside the acceptable range for Lithium-ion batteries, and have cycle lives measured in the low millions of cycles before capacity is substantially reduced.  The fact that the energy density is not as great as it is in Lithium-ion batteries is not that a major problem if the vehicle also has an onboard RTG and solar panels to recharge the capacitors.

If you look at the expected operating lives of RTG's and super capacitors, they're more closely matched than they are when paired with current Lithium-ion batteries which have cycle lives measured in the low thousands of cycles.

It only takes about 62We of power to drive a homopolar generator that produces 1.5v * 1000a.  Ordinarily, we can't do anything with that kind of electrical power.  The voltage needs to go way up and the amperage needs to go way down.  However, super capacitors can recharge quite rapidly with that kind of amperage input with very minor creativity in the charging circuitry (parallel vs series recharging).  A 300We RTG can easily produce enough electrical output to drive a small electric motor to turn two or three homopolar generators.

Let's say we have a 339kg (let's just call it 350kg after we add the electrical connectors and charge controllers) super capacitor bank that stores 2.5kWh worth of electricity.  This is current commercially available super capacitor technology from Maxwell, model BCAP3400.  If the super capacitor is divided into two banks of 120v or 240v modules, then while one bank recharges the other can provide motive power.  The generator can recharge the other bank in seconds.  It only takes very little power, perhaps 5hp to 10hp, to continue rolling on a road using wheels at a speed of 20kph to 30kph, more like double that using tracks in an off-road environment that's relatively flat and level, so let's say 20hp or 15kW.

Every 60 seconds of continuous power with a 15kW draw from the super capacitor bank is .25kWh worth of electricity, so each capacitor bank stores approximately 5 minutes worth of run time (obviously it will be somewhat less than that if higher output is required and unless we increase the number of super capacitors to contend with current leakage and whatnot).  Presuming an improbable 24/7 use and consuming all of rated capacity, that's 288 cycles per day from a device rated for 1,000,000 cycles or 3,472 days (9.5 years, pretty close to the RTG's rated life time of 10 years).

After 10 years, we swap 450kg (RTG / super capacitors / electric motors) or so worth of parts and the vehicle's power train is as good as new.  At 20kph to 30kph, we're unlikely to throw any tracks but extra track segments are required, so let's say 1500kg worth of replacement parts every 10 years.  The track pads and road wheels can use the tough, very low Tg, and radiation resistant metallic rubber compound NASA's contractors developed for cryogen tank seals, called Thoraeus (sp?) rubber.

I seriously doubt any sizable LOX / LCH4 plant would weigh less than 1500kg, never mind all the storage and transport equipment required for the fuels and lubricants, never mind replacement engine blocks for piston engines or hot sections for gas turbines, never mind the tools and test equipment required to ensure the ICE's are functioning as intended.  I can swap electric motors, super capacitor banks, and a RTG with a torque wrench and CarbonX clothing to handle the hot RTG.  No individual component of this system would require a crane or winch, either.

Well, there it is.  No requirement for liquid hydrocarbon fuels or the massive logistics tail that follows them.  Small tracked vehicles, in the Japanese Type 60 class (a small armored personnel carrier; obviously we'd not need the additional armor of an APC) using RTG and super capacitor power, would require about 1500kg worth of replacement parts every decade or so.  That seems doable.  I could be off by a factor of 2 and we're still talking about far less total tonnage for initial or ongoing operations.

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#72 2017-05-02 15:44:37

RobertDyck
Moderator
From: Winnipeg, Canada
Registered: 2002-08-20
Posts: 6,162
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Re: Air. Shelter. Water. Food.

Dook wrote:

For a science mission, you envision pounding pegs with a sledge hammer to hold down a greenhouse?  If the greenhouse is an inflatable you just have to hold it down from the wind and the regolith moved inside to grow plants would be enough.

No, you keep ignoring what's in Robert Zubrin's book "The Case for Mars" as well as what I've posted on this forum. Since the "Case for Mars" conferences before the founding of the Mars Society, it was always intended to flatten the greenhouse with hold-down straps. An inflated structure will tend to be a sphere; if longer than wide it will be a cylinder. But making the greenhouse higher than a crew member can reach is useless. In order to make a greenhouse 10' high x 20' wide, you have to squish it.
greenhouse.jpg

Dook wrote:

Nothing can be easily made in-situ on Mars except oxygen and water and those two things will be made by machines brought from the Earth.

To be blunt, that means you don't have the right stuff for Mars.

Dook wrote:

I don't think there are sand storms on Mars.  There are dust storms but the dust is so fine that I don't think it could scratch plastic.

National Geographic clip about Mars dust storms:
hqdefault.jpg?custom=true&w=336&h=188&stc=true&jpg444=true&jpgq=90&sp=67&sigh=RJ06DeEcWl6r2Lm18AQONa1_8w0
Analysis by Mars rovers shows the dust is just sand pulverized to small particle size. That means it has the same hardness, and same edges just as capable of scratching. And based on moving dunes, I still assert that sand does move, just not very high off the ground. "Not very high" hasn't been defined yet, no direct measurement. But in either case, even that dust will scratch plastic.

Dook wrote:

I still think we can't pressurize a dome greenhouse.  It could be built over a buried habitat to provide heat but that's all.

My conclusion is a dome greenhouse needs a floor. You could continue to argue for a foundation, but a very large dome will require a massive foundation. One option is to drill into bedrock, secure it directly to bedrock. But rock has even worse tensile strength than concrete; it's great in compression but really bad in tension. A heavy building on Earth sitting on bedrock provides compression, but a holding down a pressurized dome is all tension. Rock could fracture, pulling up to release pressure. But a floor means the dome is just a building sitting on the ground.

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#73 2017-05-02 16:52:14

Dook
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From: USA
Registered: 2004-01-09
Posts: 1,409

Re: Air. Shelter. Water. Food.

RobertDyck wrote:
Dook wrote:

For a science mission, you envision pounding pegs with a sledge hammer to hold down a greenhouse?  If the greenhouse is an inflatable you just have to hold it down from the wind and the regolith moved inside to grow plants would be enough.

No, you keep ignoring what's in Robert Zubrin's book "The Case for Mars" as well as what I've posted on this forum. Since the "Case for Mars" conferences before the founding of the Mars Society, it was always intended to flatten the greenhouse with hold-down straps. An inflated structure will tend to be a sphere; if longer than wide it will be a cylinder. But making the greenhouse higher than a crew member can reach is useless. In order to make a greenhouse 10' high x 20' wide, you have to squish it.
http://canada.marssociety.org/winnipeg/greenhouse.jpg

Dook wrote:

Nothing can be easily made in-situ on Mars except oxygen and water and those two things will be made by machines brought from the Earth.

To be blunt, that means you don't have the right stuff for Mars.

Dook wrote:

I don't think there are sand storms on Mars.  There are dust storms but the dust is so fine that I don't think it could scratch plastic.

National Geographic clip about Mars dust storms:
https://i.ytimg.com/vi/JKBk_Kfucs4/hqdefault.jpg?custom=true&w=336&h=188&stc=true&jpg444=true&jpgq=90&sp=67&sigh=RJ06DeEcWl6r2Lm18AQONa1_8w0
Analysis by Mars rovers shows the dust is just sand pulverized to small particle size. That means it has the same hardness, and same edges just as capable of scratching. And based on moving dunes, I still assert that sand does move, just not very high off the ground. "Not very high" hasn't been defined yet, no direct measurement. But in either case, even that dust will scratch plastic.

Dook wrote:

I still think we can't pressurize a dome greenhouse.  It could be built over a buried habitat to provide heat but that's all.

My conclusion is a dome greenhouse needs a floor. You could continue to argue for a foundation, but a very large dome will require a massive foundation. One option is to drill into bedrock, secure it directly to bedrock. But rock has even worse tensile strength than concrete; it's great in compression but really bad in tension. A heavy building on Earth sitting on bedrock provides compression, but a holding down a pressurized dome is all tension. Rock could fracture, pulling up to release pressure. But a floor means the dome is just a building sitting on the ground.

You want to flatten the greenhouse with hold down straps?  If the greenhouse material is made in an oval shape it would not need to be flattened with hold down straps.  There are balloons of many shapes, they don't form a sphere when they are inflated.  Still, you're talking about a very temporary greenhouse, something that would be tested by the exploration teams, not depended on for life support.  Food is life support.  It can't be designed to last just two years.

I don't have the right stuff for Mars? Okay Macgyver.  Please explain how you are going to make things easily on Mars. 

The sand on Mars will scratch a greenhouse?  Sounds like more evidence that we need to bury a series of habitats and grow hydroponics and vegetables and fruit trees in plastic regolith tubs instead of attempting to build greenhouses that you can't pressurize. 

You think the greenhouse needs a floor?  Connecting the floor to the dome would make the greenhouse a pressure vessel and evenly distribute the force all the way around so it would not lift.  And if you made all the panels small, maybe 2 sq ft, they would only have about 570 lbs of force on them. 

Adding a floor would about double the number of panels you would need, essentially reducing your greenhouse size to about half. 

LED lighting in a buried habitat is a better option for growing plants.

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#74 2017-05-02 17:15:12

Oldfart1939
Member
Registered: 2016-11-26
Posts: 1,898

Re: Air. Shelter. Water. Food.

If we can utilize this new supercapacitor technology in place of Lithium Ion batteries, that would make an enormous difference in overall logistics required by the Mars base. My experience with Lithium Ion batteries is in my lawnmower and weed eater, which is certainly better than the old IC driven stuff. I'm wondering whether there could be enough funding to build an electric powered small Bobcat, big enough to move enough regolith into the greenhouse to be worthwhile?
My previous comments were based on another Zubrin hypothesis, but the concept is now being overtaken by technological developments.

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#75 2017-05-02 19:24:46

kbd512
Administrator
Registered: 2015-01-02
Posts: 3,723

Re: Air. Shelter. Water. Food.

BCAP3400 costs $57.74 for 100 modules from Mouser Electronics, so that's $37,646.48 for 652 capacitors required by the example cited above, not including shipping.  Let's triple that cost since our government is involved.  They always over-pay, no matter what they're buying.

The Pu238 isotope costs roughly $8M per kg to produce in our breeder reactor, so roughly $62.4M for the 7.8kg inventory in a 300We RTG.  Efforts are underway to reduce that cost and increase output, but it should be apparent that Pu238 is not viable for any sort of utility electrical power production for a Mars base.  Even at half that price, it's still far too expensive to use in multi-kW output systems.  NASA projects a 40kWe continuous electrical output requirement for a Mars base.  I have no clue what they need that much continuous power for, but that's the stated output requirement for a starter base and they're obviously not paying for a SLS launch in terms of Pu238 so they're developing Uranium-fueled Kilopower fission reactors instead.

To produce 7.8kg of Pu238 for a vehicle power pack that lasts about a decade on Mars, the cost associated with the Pu238 production is reasonable when current $50,000/kg delivery costs are considered in the value equation.  An internal combustion engine for Mars is a major development project for reasons GW already noted.

The graphene-based super capacitors in Maxwell's labs are substantially lighter and have substantially higher capacitance, but you still need a nuclear power source and a homopolar generator to provide 24/7 power.

I think it's time we dusted off that Strontium-based nuclear battery design Dr. Paul Brown developed in the late 1980's.  We can't figure out what to do with all the Sr90 from our fission reactors.  Any reactor operator would pay NASA to take the stuff off their hands.  The technology works like a PV cell by directly converting radioactive decay into electricity by surrounding the source with a LC tank circuit.  Sr90 is a pure beta emitter, so there is no low energy gamma from spontaneous fission like there is in Pu238.  Unlike Am241 and Pu238 fueled RTG's, the Sr90 batteries operate at room temperature.  The output is in the low kW range for a unit in the same weight class as the GPHS-RTG.  We could either dramatically reduce the weight of the RTG or provide direct power to the motors and eliminate the super capacitors.  I still think keeping the super capacitors is desirable to provide instant high-output for low-speed torque and life support equipment like CAMRAS and MOXIE that have higher peak startup power requirements to operate pumps.  I'm just throwing this out there.  NASA never pursued this because of NIH.

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