Nuclear Transportation?

C M Edwards · 2004-10-06 15:41:21

Gas Stations! An ingenious solution.

Euler · 2004-10-06 19:14:38

- As I said above, the reason I favor a chemical power store (such a Methanol and O2) is it's greater energy density. Despite the need to carry both the fuel and the oxygen, chemical energy is still signifigantly more energy dense than either fuel cells or simple batteries. But either is certianly do-able. Both would eliminate the need for a ISRP, but I'm not sure this would make up for the loss in efficency. Also, the hydrogen for the sabatier process would probably be stored as water (reclaimed from the engines after combustion), and broken into hydrogen only when necessary, to avoid having to carry hydrogen tankage.

Both fuel cells and batteries use chemical energy. While batteries generally use different types of chemical reactions than engines, many fuel cells can actually use some of the same fuels that engine use such as methanol. Fuel cells are typically a little more efficient than engines, but they are also much more expensive.

I favor batteries as the best way to store energy. The limited range is an issue, but I think the simplicity and increased efficiency of batteries is enough to offset that.

Most of the discussion on nuclear power has revolved around fission. However, there are at least 5 types of nuclear reactions that could be used: radioisotope, nuclear isomer, fission, fusion, and antimatter. The other sources could all become useful if technology for harnessing them becomes better.

GCNRevenger · 2004-10-06 21:26:30

Fuel cells operated by Methane (perhaps Methanol) with onboard Oxygen supplies, feeding electric motors in the wheels, I think are the best option to power a medium-to-long range rover:

As fuel cells don't use "combustion" persay, there is very little if any wasted oxygen due to incomplete reaction. They are also more efficent then combustion/thermal engines since they produce electricity directly, so much less energy is wasted as heat that your combustion engine just throws away.

There is also an energy savings with electric motors connected directly to the wheels rather than a transmission and shaft arrangement. They are also very reliable, and really have no moving parts other then gas valves and the motors in the wheels... no pistons, no crankshaft, no turbines, no gearbox, no nothing, just sealed electric motors and gas valves.

Yes, a fuel cell system will cost more. Yes it might be a little bit heavier then a piston or turbine engine. But, you still get much more useful energy from the same fuel mass as burning it, no transmission to lug along, and are much more reliable. You also need less oxygen since it uses it efficently and leaves less unreacted. Oh yes, and with separate electric motors in every wheel, you can maneuver much much more easily over rough terrain, even spin around 360 degrees by reversing opposing wheels and climb over awkward rocks better.

I don't know if its practical to make a turbine/electric combustion engine small enough, reliable enough, and efficent enough for a Mars rover like this, but I suppose that is possible too. I am pretty sure that a fuel cell system would be more efficent per pound of fuel and be more reliable though.

Anyway, gas stations and batteries...

Batteries for a medium-to-long range rover paired with the "gas station" aproach, I don't know if I like that that... the trouble with is, you have to spend considerable time at the power plant to recharge your batteries, time that you won't be driving around doing somthing useful. With a turboelectric or fuel cell system, you just drop off your waste water, fill up the Methane/LOX tanks, and head to the next site while the reactor cooks the water and CO2 to make more fuel & oxygen. Batteries also are pretty heavy for the amount of energy they store.

As far as "nuclear energy," fission is the ONLY one available and probobly the only one that will be available for the forseeable near future in a semi-portable, flight-weight and maintenance-free package with useful outputs. RTGs just don't put out much juice, the fuel for nuclear isomer reactors isn't readily available (microgram quantities) nor can the energy be converted easily (mostly gamma, little thermal), practical fusion reactors are 3-4 stories tall, and we can barely make any atoms of Anti-Hydrogen at all much less gram quantities. Fission is the only option for large amounts of power on Mars in a compact, reliable, and trouble-free package.

I wonder about having the "gas station" devided into two pieces, the reactor that could be ringed with a high berm of dirt, and the ISRU plant outside connected by a power cable. It would save having to mess with radiation shielding.

Austin Stanley · 2004-10-06 22:21:45

I appolgise for some of the confusion I may have caused, I have a tendicy to see "fuel cell" and think battery, when in fact the two are very diffrent things. While batteries are almost certianly unworkable, fuel cells may very well be usable, however I still favor a ICE for a number of reasons.

While it is true that fuel cells are more efficent than conventional combustion engines, you must have a fuel cell that is signifigantly larger than the internal combustion engine you could get away with. This allows for a more powerful engine, that is both smaller and lighter than it's fuel cell counterpart. A more powerful engine gives you both greater mission capacity, and a greater safety factor. I especialy belive that a high performance engine will be critical for a martian rover, to allow it to travers difficult terrain while carring heavy loads.

Another issue with fuel cells is the issue of fuel purity. The high efficency fuel cells that most propose to use are fairly sensative to their fuel make-up, if stray chemicals (such as CO) were to get into the tanks, it could damage or even disable the fuel cell. ICE engines are pretty much immune to this problem, they will run on just about anything that is combustiable, and it is hard to imagine a chemical injecting that could damage the engine.

Another point in favor of ICE engines is relability and safety. While a ICE may have more moving parts that a solid state fuel cell, these are devices that have been around for about a 100 years, their operation is very well understood, and they can be designed very reliably. Fuel Cells, being more recent technology, cannot bost the same relability that an ICE can. Also, if the engine was to break, repair is at least conceviable, unlike with advanced fuel cells where repair is pretty much impossible.

All this is not to say there are not drawbacks to an ICE as opposed to fuel cells, but I belive they can be minimised. Fuel cells have the potential/necessity to be linked in series which allows continued power generation in the event one of the devices fails. However, due to ICE supperior power/weight ratio, you could simply carry a back-up engine, and still come out ahead of the fuel cells.

The increased inefficency of ICE as opposed to fuel cells could also be minimised by the proposal I made. With fuel eliminated as a cause of concurn by a towed reactor/ISRP the supperior power/weight ratio of ICE engines becomes more atractive.

But again, to me the best argument in favor of a ICE is the ability to have a more powerful engine. Mars is a rough and rugged planet with no roads and pleanty of obsticles. Rovers will be called upon to carry heavy loads over difficult terrain. If your rovers engine is not powerfull enough to cross a section of terrain or a steep gradiant there is nothing you can do about it. That part of the planet is simple inacessable. A ICE allows you to have a more powerful engine for much less mass than any other sort (well I guess a rocket engine would beat it, but...) and that to me is what is most crucial.

----------

As to other power sources, I'm all for their utilisation, but we have to face the facts. RTG are great, and have excelent energy density and fuel efficency, but would have to be much to heavy to generate the necessary power.

Batteries have good fuel efficency, but there power to weight ratio is weigh to low to make long range travel possible. And they suffer from many of the same problems I listed with Fuel Cells above.

Nuclear isomers hold much promise, but the technology is still in it's infancy, and not even close to being ready for a mars mission. In addition, gamma radiation would be a serious concurn.

Likewise with fusion. The technology is simply not there. In fact, they are further behind than nuclear isomers are. In any case, a fusion reactor is likely to be far to large to put on a mobile vehicle, at least for the forseable future.

Anti-matter does deserve some serious consideration I think, however. Seriously! The amount of energy stored here is obviously enourmous, and we can already create and store small quantities of it for signifigant lengths of time (days). Penning traps are not incredibly massive, and it is conceivable at least that a trap small enough to fit on a rover and store the anti-matter long enough for a rover mission could be designed. The anti-matter could then be slowly released into a tungston block, where it would react and heat the block up. The tungston would be sumerged in a coolant, which it would heat and power a turbine. The block and the coolent should take care of most of the radiation as well.

This is not to say that anti-matter doesn't still have serious problems. It's never going to be cheap to produce for one. I think CERN said that they might be able to produce it at the cost of 100,000W for every 1W of anti-matter, a HUGE improvment, but still incredibly expensive. But I don't think it should be dismissed out of hand. It certianly is a potetial power source in the future.

Austin Stanley · 2004-10-06 22:31:18

I don't normaly double post, but I didn't see your reply untill my was posted. Weird that we would make such similar and opposing positions in such a short period of time.

--

Anyways, about gas-stations, it occured to me that if you had a ready supply of water back at the gas-station there would be no need to haul the water back with you, which could signifigantly increase the rovers range. When you are carrying the water you are essentialy having to lug a full tank of gas both ways. The water at the station could be provided by a local source, or transported from another source in a logistic train sort of style.

Also while I don't favor batteries, a potential solution to having to hang around and wait for them to charge up is to have a ready set there waiting for you, which the reactor had already charged up. You get there, pop out your batteries (which should be simple), pop the new ones in, set the old ones to charge, and away you go.

Euler · 2004-10-07 00:22:18

While it is true that fuel cells are more efficent than conventional combustion engines, you must have a fuel cell that is signifigantly larger than the internal combustion engine you could get away with. This allows for a more powerful engine, that is both smaller and lighter than it's fuel cell counterpart. A more powerful engine gives you both greater mission capacity, and a greater safety factor. I especialy belive that a high performance engine will be critical for a martian rover, to allow it to travers difficult terrain while carring heavy loads.

I don't think that the rovers will need a huge amount of power. The weaker Martian gravity will reduce the amount of power that is needed, and the rovers will not be traveling very fast due to rough terrain.

The lack of a cheap source of hydrocarbons, and the need for an additional large, cryogenic fuel tank certainly reduce the appeal of ICE, and also of fuel cells. If a battery powered rover has enough range to meet the requirement of Martian settlers, then it is a clearly superior choice.

GCNRevenger · 2004-10-07 08:52:23

When I say "energy density" i'm thinking the amount of energy you put to use from a given weight of fuel (or batteries) to the wheels or keeping the crew warm. As far as energy density goes, batteries are lousy and won't provide that much range per mass I don't think. The rover won't need loads of energy, but it will need it to go for a long time, so energy density is crucial.

Electric cars here on Earth don't have that great a range, and they don't have to keep the crew compartment warm or equipment running. The reduced Martian gravity will help when climbing, but alot of the energy used will go to horizontally pushing the rover's mass which won't be any easier then on Earth.

So that leaves us with chemical combusion and fuel cells... I am thinking that either option will use an electric final drive to increase maneuverability and eliminate mechanical componets like the transmission, the drive shafts, govenors, and universal joints. DARPA has such a vehicle, The Shadow, for the special forces powerd by a pair of Diesel generators and it is extremely agile.

I don't think you give the high efficency of fuel cells enough credit: a fuel cell for a car is actually smaller and not any heavier then a comperable piston engine. So much of the energy produced in a piston engine is wasted as heat, that you can get away with a pretty small fuel cell. A piston or turbine engine will also need to lug along a generator, whereas a fuel cell produces electricity directly. "However, due to ICE supperior power/weight ratio, you could simply carry a back-up engine, and still come out ahead of the fuel cells." Just isn't true, especially with the spare generator.

They really are way more efficent, so much that the small or negligible additional weight is more than made up for the improved fuel efficency even for similar energy output. Its not even a close contest... and since you need less fuel, you don't need as big of an engine because the vehicle has less mass. If you need a breif burst of extra power, then you have a battery or a bank of ultracapacitors just like Hybrid vehicles on Earth. These could be used to take advantage of regenerative braking too.

There is also the issue of incomplete combustion: how much of the oxygen actually injected into the engine is combusted? In a fuel cell, this is almost 100%, and as oxygen is pretty heavy (32g/mol) plus the insulation for a LOX tank, the reudced oxygen need is a huge improvement. You can't compare the energy density of the fuel alone, you have to take into account the oxidizer too, and I think that the lower oxidizer efficency of combustion engines will make a signifigant difference versus fuel cells.

"Another point in favor of ICE engines is relability and safety. While a ICE may have more moving parts that a solid state fuel cell, these are devices that have been around for about a 100 years, their operation is very well understood, and they can be designed very reliably. Fuel Cells, being more recent technology..."

Actually that isn't really that true... we've had fuel cells for a long time, almost as long as we have had practical ICE piston engines. The Apollo vehicles were powerd by fuel cells, and that was decades ago. Shuttle is also fuel cell powerd, and was updated to modern polymer exchange cells years ago, which are now flight tested. The simple lack of moving parts makes them far far more realible, as there simply isn't much to break.

And if you break your piston engine or turbine 200km from base, and you need a new piston rod or a compressor blade, your dead anyway unless you brought one, even if you can dismantle the engine and fix it before you freeze to death. Its a simple engineering concept, moving parts, particularly ones under high stress and temperature, each have a possibility of failure and this risk of failure is additive. More moving parts, less safety.

Hydrogen will be at a major premium for a long long time when we get to Mars even if water is relativly available, so I think that its worth it to capture the waste water from whatever reaction is used to recycle it. The gas stations need to be kept as light and simple as possbile so that they are maintenance free, so I think that drilling for water or whatnot should be avoided, nor is it really nessesarry if you bring a little extra H2 now and then to restock it... About antimatter, I don't think that will even be a remote option, given simply how hard it is to make the stuff. The conversion from thermal to mechanical energy isn't all that efficent either.

RobS · 2004-10-08 09:20:55

I quite agree with GCNRevenger. Here are some numbers about solar power that I think help to illustrate its problems and maybe some roles it can play.

The sun provides an average of 500 watts (1/2 kilowatt) of light energy at Mars's distance from the sun. When there are no dust storms (about two thirds of the year) that translates into 6 kilowatt-hours per sol of usable solar energy per square meter of a STEERABLE solar array. A flat NONSTEERABLE solar array would receive 6 / pi times 2 or about 4 kilowatt-hours per sol of sunlight. There is a publication on the web I've cited before and it repeats these numbers based on the Mars Pathfinder data. It says that when there is no dust storm, a nonsteerable surface receives between 3 and 4 kilowatt hours of sunlight per sol.

Solar arrays do not convert all of that light into electricity. The old panels converted about 10-12% of it. The current ones can convert 20%. Well developed materials in the labs can do better than 30% and highly experimental materials are now pushing toward 40%. I will assume that in 20 or 25 years Mars explorers will have solar arrays able to convert 1/3 of the sunlight to electricity (that assumes arrays closer to the 40% conversion number, since 40% is the conversion in a lab and cells tend to degrade over time). If one assumes that we can make 1/3 efficient solar cells, then an array can make for a martian explorer about 1.3 kilowatt hour per square meter per sol. This will be lower much of the time, but if we assume very simple mechanisms to provide partial steering--such as arms able to raise a side of the array to face the rising or setting sun--then 1.3 kilowatt hours per sol per square meter is achievable.

How much power do we need? Life support needs about 1 continuous kilowatt per person per sol, or 24.6 kilowatt-hours per sol (let's round it off and call it 25). This is my rough estimate. It means a vehicle with four people needs about 100 kilowatt hours per sol and therefore would need an array 100/1.3 = 77 square meters. That's the size of a very large room, or about 8.5 meters by 8.5 meters (about 28 feet square). There is no easy way a vehicle could have an array like that deployed AND move around.

On the other hand, IF the life support system could be made much more efficient, the array MIGHT shrink down enough in size so that it could fit on the roof of the vehicle, plus very wide overhangs on both sides, the front, and back. But I don't know if that is possible.

One must also consider the situation in dust storms. At the peak of a global dust storm, insolation drops from 4 kilowatt hours per square meter per sol to one tenth of that, or 0.4. Clearly, a vehicle could weather a storm of that sort only using stored power; the array otherwise would have to cover 770 square meters and would be 28 meters or 92 feet across!

So far we haven't even considered moving the vehicle. Let us assume the vehicle needs 30 kilowatts of power for six hours every sol. That's 180 kilowatt-hours more of power, requiring 180/1.3 = 140 square meters of array (12 meters or 40 feet square).

Conclusion: a vehicle could not move around AND make its power at the same time. However, a vehicle (or more likely at least a pair of vehicles) could set up a base camp (a portable "gas station") and operate from the solar arrays there, then move on to another base camp. Let us say we have an expedition that uses 300 kilowatt-hours per sol per vehicle and has two vehicles plus a "gas station truck." The truck would mass 1,000 kilograms including methane and oxygen tanks, sabatier reactor, electrolysis equipment, and cryogenic refrigerator. The truck would have capacity to store 1,000 square meters of solar array, massing 1,000 kilograms (including the power conversion equipment). The expedition would select a site and deploy the 1,000 square meters of solar array by unrolling or unfolding them. They would produce 1,300 kilowatt-hours of per per sol, enough to produce about 500 kilograms of methane and oxygen per day. The vehicles would drive out from camp for up to several days, explore an area, possibly clear a trail to the next site, then return to the base camp, fill up, transfer waste water to the gas station truck, then pack up the arrays and move them to the next base camp.

It would be inconvenient and labor-intensive compared to a nuclear reactor, and it STILL could not go out during dust storm season.

-- RobS

SpaceNut · 2004-10-08 10:09:24

Great numbers on the solar cell energy and such for a rover use.

Hour to get between sites is in the probable 4 to 6 hours before you would need to turn around and go back to base camp for maximum use of daylight hours. Distance is much harder to resolve to a number since you would be stopping and starting in order to carry out experiments and observations.

Not sure about the weight and cryrogenic refrigeration, I think this makes the vehicle to large and very heavy. Not to mention that you now must shield the crew from the cold of the tanks but also from any explosion of them.

I not sure about the amount of life support number needed to keep the crew warm from electric heaters, versus giving them air to breath from the electrolosys process.

The need to keep the first vehicles simple and light weight comes to mind from the lunar buggy.

RobS · 2004-10-08 10:31:04

I am assuming the "gas station truck" is separate from the vehicles used by the crew. The crew's vehicle would need cryogenic methane and oxygen tanks, whether the fuel is made at the landing site or at a base camp. Everyone assumes that.

A day of exploration doesn't involve contant movement. One goes a while, stops, performs an EVA, moves a while again, etc. That's where the six-hour per daytime figure comes from. Note the gas station I designed is able to power two vehicles for two people each plus the gas station truck (which I assume also needs the same amount of power) with a surplus of 33%, which makes up for dust on the panels and in the air, emergency needs for power, etc.

As for heating the vehicle, the fuel cells generate waste heat, even when they're 75% efficient, so there's spare heat to keep the vehicle with the crew warm. The gas station truck could also have storage for supplies and they could be maintained at a certain temperature from the waste heat of the sabatiers and electrolysis.

-- RobS

Dook · 2004-10-09 13:02:17

If it's too heavy for early missions then it's too heavy for any mission is correct. The whole point of mars direct is to go there and explore the planet from the very beginning.

Maybe it's enough for you to just go there, spend 30 days walking around, then leave, but why should we when we can do much more.

I've been working on an idea for a pressurized mars vehicle that uses solar and fuel cells and has almost unlimited range while providing double and sometimes triple redundancy for mission critical and life support systems. This vehicle is 400 pounds over the weight limit to be included in mars direct at the moment but it is a much better idea than some nuclear powered humvee that's thousands of pounds too heavy for mars direct not including life support. You say solar and fuel cells won't work but haven't even looked at the idea.

Also, you can't use a combustion engine on mars for more than a few miles. It consumes too much oxygen! We've already had this discussion. Review the previous posts!

GCNRevenger · 2004-10-09 23:50:36

I was thinking a scheme with a wider range, that the rover would spend upwards of 1-2 months between refueling to permit medium-to-long range trips to sites of interest without having to drag the power source with you, not just a few days on short hops from the fuel station. The fuel station wouldn't even be mobile on its own, just dropped off at the limits of the first tank of fuel from the rover (minus whats needed extra to carry the plant). I also think that the rover will spend a little more time moving per Sol then six hours, and will probobly be on the move for most of the Martian day time, like 7-8hrs a day. For an exploration missions, the crew will vary, but the power requirements for LSS will be largely determined by vehicle capacity and not how many people are in it at the time.

One of the issues with a solar array as power source that I think is worth pointing out, Rob and Dook, is that the Sabatier cycle or any energy storage mechanism is not 100% efficent. That is, for every watt produced by the array or reactor doesn't become a watt available for rover power. Even if this figure is pretty good, like above 50-66%, now you are starting to talk about needed a really huge solar array. As for Cryogenic tankage being hazardous, I think that just has to be lived with and the boiloff could be shunted directly to the power plant to eliminate its waste.

"If it's too heavy for early missions then it's too heavy for any mission..." Doesn't make sense, I don't understand. There will probobly not be enough mass budget in the first mission or three to provide for the semi-portable fuel plant(s) but the rover ought to be accomodated okay. Its really a means of extending the operating radius of medium/long range rovers when we have a base set up. Me least of all think that a little one-month trip on the surface is enough, and you must be confused or somthing to believe that I would think that. In any event, the trajectory for the mission (conjunction class I think its called?) requires a 500-600 day stay on Mars.

"I've been working on an idea for a pressurized mars vehicle that uses solar and fuel cells and has almost unlimited range... but it is a much better idea than some nuclear powered humvee that's thousands of pounds too heavy for mars direct not including life support. You say solar and fuel cells won't work but haven't even looked at the idea... Also, you can't use a combustion engine on mars for more than a few miles. It consumes too much oxygen! We've already had this discussion."

Well, even if you come up with some magic solar cells with >50% efficency, there is still no way you are going to pack a solar array of reasonable size onto a rover, especially with the inefficency of storage systems. With the Sun on Mars giving you about 3-4kw/m2, thats just not going to be enough, your design will fail. Is that sufficent consideration for you?

Why wouldn't a combustion engine work on Mars? It would have somewhat worse fuel economy then a fuel cell because of thermal-mechanical-electrical conversion and lower oxygen efficency hence cutting alot off the range, but it would work, it just isn't the best solution.

And how about you try "reviewing previous posts" for a change Dook... The whole idea of gas station with a small nuclear reactor is so that you don't -have- to lug along a big a reactor with you to take advantage of its nearly unlimited supply of power. You drop the thing off one time, then come back to it for fuel as needed.

It both separates the power generation from the vehicle so you can use a nuclear power source without having to lug it and its shielding, you can use a smaller reactor since it will operate at full power 24/7 while the rover only operates at full power about 1/3rd of the day, and a nuclear source is easier to deploy than a massive solar array. Oh, and its dust-storm proof too.

Dook · 2004-10-10 08:05:25

In my design the solar panels supplement and provide an emergency power supply only. My vehicle is not limited to six hours of driving per day.

Also whats the point of staying on mars for 500-600 days if they only have a rover (yours) that can't go very far because they couldn't bring the fuel stations along on the first trip?

Sigh...SOLAR IS SUPPLEMENTAL! EMERGENCY BACKUP! NOT THE SOLE SOURCE OF POWER! Look at the design before you comment on it!

My design will NOT fail. It has double and triple redundancy for movement and life support. How much redundancy does yours have? How heavy is yours? It's likely so heavy that it can't be included in mars direct so you are wasting your time.

A combustion engine won't work on mars. GO BACK AND READ THE POSTS! Why didn't you join the discussion when it was ongoing? Combustion engines are not efficient for mars. You have to have a GIANT LOX tank and even then you can go only 80 miles at most while my vehicle goes over 1,000 miles. There are pages of math for you to review.

Find a few references for RTG power plants and you will see why nuclear power is not the smart way to go for a rover. They weigh 2,000 lbs minimum.

Rxke · 2004-10-10 12:30:23

Dook, instead of trying to burst a jugular, I suppose you read Cindy's initial post...

What about nuclear materials to heat and run the engines of trains, other land vehicles, etc.?

So she is *not* talking about the very first rover on Mars, but later stuff...

Weight-issues are less of a concern then.

GCNRevenger · 2004-10-10 13:03:38

You should try pointing out things your vehicle before caps-locking anything you know Dook, you did not say that your vehicle wasn't solar powerd like Helios in this thread...

AndI have read about your vehicle and I don't think its a good idea nor could it meet the hopelessly optimistic mass estimate and have left over payload mass. Where is the mass budget for food, lab equipment, rock samples, and other items to make the rover worthwhile? It is also going to get terribly cramped with all the gear, like LSS gas tankage and propulsion hardware, you try to cram inside the pressure hull along with an airlock. I have a feeling that it will be badly under-powerd too with its puny electric motors.

Overall both your threads on the Simple Mars Vehicle and Combustion Engines demonstrates several deep flaws in understanding and engineering... Carbon composit drive shafts? Using fuel cells to charge batteries? Lugging along pressurized Oxygen tanks? Heat lamps for internal LSS heating?

You also are overlooking several facts about ICE engines, and your hypothetical engine figures are not credible. Not all the gasses injected into the cylinder have to be fuel or Oxygen, and since you need about double the mass of Oxygen as Methane, then why don't natural gas powerd vehicles on Earth run out of fuel in minutes? They seem to get along pretty well to me... excess heat could go into LSS needs (waste water distilation, cabin heating) too. A typical natural gas powerd vehicle gets similar milage from liquid gas as it does gasoline, so you are only looking at needing about double or tripple that of oxygen by mass, maybe a little more. A 500km range is not unreasonable to consider.

http://www.army-technology.com/projects … ts/shadow/
Able to travel almost 800km with 95L (~70kg) of diesel fuel, so if it were operated on Methane/Oxygen/inert instead of diesel/air, you are talking in the region of 400-500kg of fuel & Oxygen. Obviously the rover will be a little bigger and perhaps heavier then this vehicle, but this illustrates that an ICE powerd Mars rover is not outside the realm of reason.

Also note how the vehicle uses much larger 50kW electric motors in every wheel, a little overkill for Mars but probobly about right considering extra vehicle weight, and this is a much better solution then anything dealing with drive shafts for reliability (no shafts, no govenors, no Universal joints, no gearbox), agility (it can turn 360deg in place or add power to only one wheel to clear rocks), and provides motor redundancy.

You are also doing an exceptional job in not reading the very posts in this thread. I am talking about a medium or long range rover to begin with, able to operate for at least a month unrefueled, which would tow the "gas station" a considerable distance before dropping it off, then going an even greater distance from the fuel station then it is from the landing site.

The whole idea is to NOT carry your power source OR the ISRU plant with you... a small solar array for supplimentary power is fine... so all that mass having to do with pressurized gas tankage, water electrolyzers, associated gas handling, all that can stay with the gas station.

OH, and you explicity overlooked my statement that RTG power generators don't have the nessesarry output and that a small nuclear reactor (not an RTG) is indeed possible. NASA's SAFE-300 reactor ought to only weigh about half a tonne and still produce about 100kWe or power or about double that of chemical energy if used as a Sabatier heat source.

Euler · 2004-10-10 13:49:26

Not all the gasses injected into the cylinder have to be fuel or Oxygen, and since you need about double the mass of Oxygen as Methane,

Don't you need 4 times as much Oxygen as Methane?
CH4+2O2=>CO2+2H2O

Anyway, it seems to me that it would be very difficult to get 1-2 month range out of the vehicle even with the best chemical fuels. I think it would be more resonable to design a vehicle to make shorter trips lasting only a couple of days.

GCNRevenger · 2004-10-10 15:24:42

Bleh yeah, it is closer to four moles with Methane, i'm thinking how much you would need to mostly combust Diesel fuel...

Two months would probobly be enough with a centrally located fuel station.

Dook · 2004-10-10 15:39:37

Ryke: I wasn't replying to Cindy's original post.

GCN: One of the very first things I said was "Go back and look at the posts!" If you had then you would know that my idea was not dependant on solar. You simply assumed. I don't wish to repeat pages and pages of discussion and information.

The weight of food and lab equipment is not included because they can be brought from the mars hab to the vehicle. I don't imagine rock samples will be taken from the earth so their weight is not included. Launch weight is the concern, not weight once the vehicle is on mars.

It will not be cramped at all inside. The inside is eight feet wide with a folding bed along one wall. There is a 2.5' wide walkway on the left side that runs from the front to the rear. Rock samples and food storage are in underfloor compartments. All the propulsion hardware fits easily under the floors. Six 8 hp motors produce 48 hp through reduction gearboxes. Torque and gearing are what matter, not top speed. It won't set speed records but then it's not intended to. I can easily add 4 more motors (2 front gearbox, 2 rear gearbox) at an increase in weight of 200 lbs to increase the horsepower to 80 hp. Also I can't believe you are calling my 8' wide by 17' vehicle cramped when you post a humvee! You can stand up in mine and you don't have to constantly wear a pressure suit.

Could you be more specific on how the threads on my vehicle and combustion engines demonstrate deep flaws in engineering?

I overlooked nothing about internal combustion engines lack of efficiency for use on mars. The figures are absolutely credible and they include the ratio of oxygen to fuel. Do the math yourself and see what you come up with. When you do you will see that the amount of oxygen required is huge. How are you going to provide this much oxygen on mars? The reason natural gas vehicles and all other ICE vehicles run fine on the earth is because they don't have to provide the oxygen.

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Copied from the Internal Combustion Engines Post:
Okay, the 152 cu in engine at it's maximum torque of 1,950 rpm uses 148,200 cu in of fuel/oxygen/CO2 per minute (4 cycle engine). Now if 20% of that is oxygen that is 29,640 cu in used per minute.

A 50 gallon LOX tank provides 11,948,624 cu in of oxygen gas which would last you 403.1 minutes or 6.7 hours. You could go just over 100 miles then turn around and head back.
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Having electric motors in the wheels is not better. Do you know what a slip ring is? How are you going to get the electricity to the rotating wheel and keep mars fines from clogging it or rocks from breaking off the contacts? This also exposes the wheels to shock and vibration. The Spirit rover has motors in it's wheels and look what happened to one of them. Also you don't get any torque multiplication that you would get from a gearbox. Gearboxes = less hp motors needed = less weight.

Okay, a one ton power plant. Now come up with a total vehicle weight including everything else needed: shielding, oxygen supply, CO2 removal, water supply, heating, food, vehicle structure and suspension, chairs, beds...

Rxke · 2004-10-10 16:03:17

Ryke: I wasn't replying to Cindy's original post.

Exactly.

But this thread is about nuclear powered vehicules. So of course, there's talk about what's better/worse etc. but you're all but hijacking this subject, IMHO.

Don't get me wrong: I've read your rover thread, and it has merit, but there are other ways, like ICE, nuclear, (maybe even the compressed air) which all have their specific merits, and bashing anything that isn't like *your* design will get us nowhere.
To prove I've read your stuff: you suggested yourself to maybe use an RTG as backup/refill 'trailing rover'...

Back to the subject.

I used to be quite anti-nukes, but a nuke-powered 'train' or monster-truck that could go on virtually forever would make a lot of sense when you don't have a lot of infrastructure. Hauling stuff from outopst A to B on the other sid of the planet, but... once you have a way to build automated (or man tended) 'refill-spots' for a relatively small investment, they'd probably become superfluous, no?

Cheaper to have several vehicules with a relative short/medium operation radius (like Dook's rover) than on expensive, heavy and 'when it breaks-we-have-real-problems' monster-truck...

repairing an ICE/solar/fuelcell driven rover can be straightforward, repairing a nuclear powered engine? *shudder*

Rxke · 2004-10-10 16:07:10

I mean... Where do you draw the line cq efficiency of chosen hardware?

A nuketruck in a land replete with fillingstations is a white elephant. But you start without fillingstations, so at what point (number of stations) would the nuketruck become obsolete? Or could it be used for something else, maybe? (Answering my own question: yes: mobile powerstation on sites where building has just started, no infrastructure yet etc...)

Dook · 2004-10-10 18:14:18

Hijacking the thread? Of the 39 replies to this thread I have posted 5 of them. So how am I hijacking the thread? In my very first reply I said "A nuclear powered exploration vehicle is possible but not efficient when compared to a combined fuel cell and solar powered one." Exactly on topic. Just because it is possible does not make it the best way especially if it is going to be included in mars direct.

I'm not bashing anyone, even GCNRevenger. It pisses me off when he makes incorrect assumptions about my design before he even looked at it and when he says "your threads demonstrate deep flaws in understanding and engineering" and he can't even do the simple math it takes to figure out an internal combustion engines oxygen/fuel use per minute when the math was already done for him. Then he proposes "A 500km is not unreasonable to consider" when it's completely unreasonable because you can't carry enough oxygen to supply an internal combustion engine for that long of a trip on mars.

I did look at a towed nuclear power source but it proved to be too heavy. At 1,000 lbs it can't go on mars direct. My current idea is within a few hundred pounds of being included on mars direct.

Another problem with the nuclear filling stations is that all exploration goes to places you've already been to. You have to go in a straight line and look at things you've already seen.

The debate is this: Can you make an exploration vehicle powered by nuclear fit within mars directs 2.2 tonnes weight limit? I don't think you can but if someone has it all figured out I would certainly like to hear it. My argument is that my design is better.

Euler · 2004-10-10 18:36:22

Bleh yeah, it is closer to four moles with Methane, i'm thinking how much you would need to mostly combust Diesel fuel...
Two months would probobly be enough with a centrally located fuel station.

With perfect combustion, there would be about 3.5 kg of Oxygen per kg of Diesel. However, Diesel engines always use up more oxygen than they need to combust the diesel fuel, so it is actually much worse than that. There has been a lot of research into submarines using diesel fuel and stored oxygen while under water, but they still use up so much Oxygen that the best energy density they can get is about 125 Whr/kg. That is less than the energy density of some types of batteries. A normal ICE would only get about 75 Whr/kg, fuel cells might get 1200Whr/kg.

You would also have to think about where you would get the diesel. It would probably be expensive to grow biodiesel in your greenhouses, but is might be even more expensive to try and synthesize it or import it.

A nuketruck in a land replete with fillingstations is a white elephant.

I think that if you can get a small enough nuclear reactor and solve the problems with radiation, then the filling stations would become a white elephant due to the nuketruck.

RobS · 2004-10-11 00:27:23

According to *The Case for Mars,* you can't burn methane in pure oxygen because the flame temperature is too high for an internal combustion engine. On Earth, the nitrogen lowers the overall temperature; on Mars, you'd use atmospheric carbon dioxide. That solves Dook's problem of consuming too much oxygen.

I wonder whether the diesel submarines than ran on stored oxygen had modern oxygen sensors and other electronics. I suspect nowadays one can mix diesel fuel, oxygen, and exhaust gasses very precisely to burn the fuel and oxygen efficiently and get the power level desired.

But even so, I agree with everyone else that fuel cells are more efficient than internal combustion engines. *The Case for Mars* prefers internal combustin engines, but I suspect that's because the book is over ten years old and is using outdated technology. Several of the rover models proposed to the Mars Society since by engineering teams at universities have assumed fuel cells. The new Detroit design that puts the fuel cells, electric motors, and batteries in the chassis and allows different car or truck designs to be bolted on and plugged into the chassis is a superior design, I think.

-- RobS

Euler · 2004-10-11 01:18:37

I wonder whether the diesel submarines than ran on stored oxygen had modern oxygen sensors and other electronics. I suspect nowadays one can mix diesel fuel, oxygen, and exhaust gasses very precisely to burn the fuel and oxygen efficiently and get the power level desired.

The figures I was quoting were from completely modern submarines that are currently being developed (most navies still use diesel subs). However, the diesel subs have to mix some Argon in with the O2, so you might be able to do a little better on Mars by mixing in some CO2.

But even so, I agree with everyone else that fuel cells are more efficient than internal combustion engines. *The Case for Mars* prefers internal combustin engines, but I suspect that's because the book is over ten years old and is using outdated technology. Several of the rover models proposed to the Mars Society since by engineering teams at universities have assumed fuel cells. The new Detroit design that puts the fuel cells, electric motors, and batteries in the chassis and allows different car or truck designs to be bolted on and plugged into the chassis is a superior design, I think.

Yes, I think we can be fairly confident that fuel cells>engines for use in a Mars rover.

I still think that a more energy efficient method of storing energy might prove more practical than either. Batteries, flywheels, capacitors, superconductors, etc. have all made significant improvements in recent years, and each deserves some consideration.

Rxke · 2004-10-11 02:00:00

The debate is this: Can you make an exploration vehicle powered by nuclear fit within mars directs 2.2 tonnes weight limit?

No it isn't. This thread was not about MD... But you're absolutely right, I was too harsh with my hijacking claim, my excuses. Sigh. Everyone goes offtopic after five posts.
IMO, the boards are a mess, I often wonder how guests find their way around here...

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