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By the way, one way that SAFE-400 was so much better than SP-100 was use of a Brayton cycle electric generator.
Safe-400 is a very usefull devicie: it may be used even for propulsion, to feed a NEXT ion engine to bring the spaceship in L2 before the mission. Douring the trasnfers, it can be keep far from the habitat via an extensible truss and may contribute to counterweight the habitat for artificial gravity, while powering a mini-magnetosphere for radiation shielding.
I like it.
Last edited by Quaoar (2014-03-12 14:10:09)
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There's no reason why the crew have to be at 1g *all* the time. Can people tolerate going from 3rpm to 6rpm, so that we could stick a gym on the "lower" (outer, highest g) deck at 1g, and spend the rest of the time at half a g?
"I'm gonna die surrounded by the biggest idiots in the galaxy." - If this forum was a Mars Colony
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There's no reason why the crew have to be at 1g *all* the time. Can people tolerate going from 3rpm to 6rpm, so that we could stick a gym on the "lower" (outer, highest g) deck at 1g, and spend the rest of the time at half a g?
It may also be possible to use a short arm internal centrifugue for high gee cicles of exercise in a microgravity travel, but we have no data. So we need a space centrifugue for testing and exercise protocol optimization.
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You could use bottled compressed gas storage on the rover without swapping out welding gas bottles. Just build the gas tanks into the rover, and fill them at your gas plant when you drop by to fill up. That reduces manual labor in a spacesuit, at the expense of higher pressures and storage volume required of your stationary gas plant. This is true for both plain compressed gases, and for pressurized liquids like propane. Just different storage pressures.
GW
How much range of autonomy can have a fuel cell powered rover with internal gas tank of reasonable size?
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JoshNH4H wrote:RobertDyck wrote:Um, no. NASA found humans can tolerate a maximum of 6 RPM.
Actually, the research in this area is generally contradictory and it's not really possible to obtain a maximum value for acceptable RPM with any degree of certainty.
We have very few data and we absolutely need an artificial gravity module in space to test and verify the RMP limits, the ability to adapt at high rotation rate and the long term effects of a reduced gravity environment.
At this time, the only well prooved evidence is that microgravity is very unhealty and it has to be avoided in a more than two years Mars mission, because it would be very dangerous for an astronaut to break the thighbone during a surfare excursion. So, if you compare the risk of some drizzle douring trasfer for an high rotation rate to the risk of a femour fracture or a vertebral collapse douring exploration, the second is more and more mission critical.
Agreed, though it is my understanding that by giving astronauts arthritis medicine bone losses were reduced to very low levels
-Josh
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I have argued for the Centrifuge Accommodation Module. One engineer who worked for NASA before Shuttle was decommissioned told me they argued for one last flight of Shuttle, do deliver it. But Obama didn't allow it. Since end of Shuttle, I've argued for the Russian shuttle Ptichka to deliver it. Along with Russian solar panels to power it. Oh, wait! Ukraine. Damn!
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Agreed, though it is my understanding that by giving astronauts arthritis medicine bone losses were reduced to very low levels
Diphosphonates reduce bone mass loss, blocking osteoclast activity, but this "frozen bone", even if it has a good mass, may also be fragile because trabeculae may not be perfectly oriented to counteract the loads. This effect may be particulary critical in high stressed zones like the Ward triangle of the femour neck. So we have still very few data to realy on this kind of drugs in a long term space mission.
Considering also that bone mass loss is not the only negative effect of microgravity on human body: there are also retinal damage for fluid body shift ocular hypertension, anemy, cardiac hypotrophy...
Last edited by Quaoar (2014-03-13 10:36:05)
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There's little benefit to gravity when you're asleep, or bed rest studies would be no surrogate for microgravity at all. That means you should be experiencing close to 1 gee during daily work and exercise. Less is tolerable for other activities, less yet for sleep. Even zero gee would likely be OK for sleep.
In a design with different decks at different gee levels, I'd put the work stations and gym stuff on the deck with the highest gee, and the supply storage at the least gee, with the sleeping quarters up there near (or at) lowest gee. How much you get depends upon the nature of your design. But go for 1 gee on that lowest deck.
As long as most of the daily work and exercise takes place between 0.8 and 1 gee, I'd bet it's pretty effective. You'll only get zero gee at the center of rotation itself. Even a little bit of gee makes free-surface water possible: that makes conventional cooking and conventional water/wastewater handling feasible (including real toilets and real showers and baths).
Sure would be interesting to dock together a string of modules, maybe Bigelow inflatables, and spin it alongside the ISS as a free-flying "centrifuge annex". Sort of a habitat prototype for a Mars or asteroid ship. We could rather quickly establish answers to (1) how much gee is enough? and (2) how fast is too fast to spin? In a year or so.
Brute force / hard way / worst case cost: maybe 10 to 15 modules 10 m long, for a 100 to 150 m long baton, at longest. At one per launcher, and $80M per launch, that's 0.8B to $1.2B to launch them. If that's 20% of your program cost (rule-of-thumb), your program falls in the $4B to $6B range. That might be "cheap" at twice the price!
We do seem to have strayed far from rover propulsion. But it sure has been interesting!
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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10 to 15 modules 10 m long, for a 100 to 150 m long baton, at longest. At one per launcher
Now design it for one launch. One launcher. Period. Or worst case, a couple launchers. Total. For the entire mission.
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Awww, I was trying to be nice and let Atlas-5, Delta-4, and Falcon-9 do the job. So, use 2 or 3 Falcon-Heavies (which will fly very soon), or even 1 SLS (if it ever does fly). Probably around 10 ton per module, I'd hazard a guess.
I think you'll really spend more on the modules than on the launchers. And I was trying to upper-bound things, like I said in the post. (Only NASA would actually let it be that expensive.)
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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Hmmm. Too bad nitrogen is in short supply on Mars. Apparently. (There's a bunch on Titan, with a weak gravity well on that moon, although Saturn's gravity well is pretty strong.)
Because, nitric acid and NTO are both decent, easily-storable liquid oxidizers. Liquid methanol is an easily-storable liquid fuel. I'd hazard an educated guess that nitric acid and methanol are hypergolic. Run it fuel-rich, to reduce flame temperatures, at the expense of an increase in the smaller component, and I'd bet you can build piston, turbine, or external-combustion engines of several kinds. There's your rover propulsion.
So, where do we get nitrogen on Mars? Anyone?
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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A presentation I made at the 2005 Mars Society convention in Chicago. I still have the PowerPoint version, but this is converted to a Word document.
http://chapters.marssociety.org/winnipe … esting.doc
This requires more energy than a CO2 freezer because it uses pumps to compress Mars atmosphere. But it's the only way to get N2 and Ar. The Ar can be used to fill sealed window units.
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Hmmm. Too bad nitrogen is in short supply on Mars. Apparently. (There's a bunch on Titan, with a weak gravity well on that moon, although Saturn's gravity well is pretty strong.)
Because, nitric acid and NTO are both decent, easily-storable liquid oxidizers. Liquid methanol is an easily-storable liquid fuel. I'd hazard an educated guess that nitric acid and methanol are hypergolic. Run it fuel-rich, to reduce flame temperatures, at the expense of an increase in the smaller component, and I'd bet you can build piston, turbine, or external-combustion engines of several kinds. There's your rover propulsion.
So, where do we get nitrogen on Mars? Anyone?
GW
The atmosphere would be my primary suggestion. As you are surely aware, the composition of Mars' atmosphere can be broken down into three primary components: 95-96% CO2, 2-2.5% N2, with most of the balance being Argon. The numbers are different depending if you use the results of Viking or of Curiosity. I'm inclined to use the Curiosity results, since Viking's sensors were presumably inferior to those aboard Curiosity. In any case, there is Nitrogen in the air, and it is extractable. By compression and cooling, the CO2 can be chemically separated. Because CO2 will be the primary source of Carbon, which is useful in everything from Steel production and Iron extraction to polymers and composites and electrolysis. Nitrogen by comparison is a minority component of some polymers, as well as being a necessary filler gas in air at low concentrations. It is also useful in fertilizer.
My point is that even if we don't have extra nitrogen, obtaining Carbon should account for most of our nitrogenous needs. From the removal of CO2, production of useful compounds can proceed chemically, since Argon is unreactive in almost all respects.
It's the manufacture of nitrogen oxides that is concerning to me, from a price perspective. N2O4 is an equilibrium reactant with NO2, which is produced by the oxidation of nitric oxide, NO. NO is produced from Ammonia by catalytic oxidation using a platinum catalyst. Obviously they are rather tough to handle, but IMO that's less of a concern because we do know how to do that. Assuming that the chemical energy driving the reaction comes from Hydrogen produced by water electrolysis, chemical to chemical efficiency of the reaction will be about 38%. The overall electrical to chemical efficiency will probably be about a third, perhaps less. This is ignoring the heat that is recoverable from the synthesis reactions.
Doable, I'd say, and probably the best option. We may want to consider Ammonia as a fuel, because it is more efficient from an energy perspective: chemical to chemical efficiency with Ammonia displacing Methanol is 57%, so electrical to chemical will probably be about 45%. Ammonia also has a lower flame temperature than Methanol; With NO2 used instead of pure Oxygen I would expect the flame temperatures to be far below your 2500 K (4000 F) limit.
-Josh
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Doable, I'd say, and probably the best option. We may want to consider Ammonia as a fuel, because it is more efficient from an energy perspective: chemical to chemical efficiency with Ammonia displacing Methanol is 57%, so electrical to chemical will probably be about 45%. Ammonia also has a lower flame temperature than Methanol; With NO2 used instead of pure Oxygen I would expect the flame temperatures to be far below your 2500 K (4000 F) limit.
Is N2O4-NH3 hypergolic?
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copied these three posts as they speak to the energy for motion.
Well... At least Alice can do some basic math, so maybe I'll read some more stuff on her website to see if she has any actual solutions, rather than complaints. Thus far, I've seen lots of complaints and scant mention of feasible solutions, apart from the usual "kill the poor people by mandating mass abortions" crapola that the anti-humanists are so fond of.
From her "Energy Skeptic" handbook:
A Sodium-Sulfur battery that could store 1 day of electricity for the US would weigh 450 million tons, cover 923 square miles, and cost 41 trillion dollars. That's clearly not practical, so solar and wind and batteries are not going to work without a miraculous new battery energy density improvement.Whereas, if that same battery was 10 times more energy dense for the same cost, then it'd only be 4.5 million tons, cover 92.3 square miles, and cost 4.1 trillion dollars. That's slightly less infeasible, yet probably still grossly unrealistic. Hopefully, we're beginning to appreciate the scale of the battery energy storage problem here. People who aren't being deliberately obtuse fully recognize that even this "1 day of energy storage battery" is an absurdity.
She indicates why she thinks nothing significantly better than Lithium-ion is on the horizon, yet says she doesn't understand how batteries actually work (a "sure sign" of an "expert"), along with why it is that increasing the cell voltage of a Lithium-ion batteries is not very practical. It's not completely infeasible as she seems to suggest, but it does require the introduction of some fairly toxic heavy metals (not a major problem for Lead-acid, apparently, because we recycle better than 90% of those).
Now that we all know that this "battery for everything" is a bunch of unmitigated stinky brown stuff, maybe we can stop pretending that battery powered trucks or electricity grids are the answer to anything, except how to strip even more resources, destroy more of Earth's environment, cause more pollution, and ultimately not solve a damn thing.
However, Alice seems to be missing something important, as it relates to trucks. They're not using all of the engine's power at all times, despite the fact that the engine is built to deliver it, if demanded by the driver. Increasing the displacement of an engine increases fuel burn because keeping a heavier rotating mass in motion requires greater fuel burn (both at idle and any other rpm range). In most cases, maximum thermodynamic efficiency is achieved at WOT (wide open throttle). However, fuel economy under varying loads is generally not. There's a "fuel island" that most intermittent combustion engines like to "sit on" for best fuel economy. Believe it or not, you can make a large displacement gasoline fueled aviation engine turn in some numbers very reminiscent of a diesel engine's brake-specific fuel consumption, but you also have to demand a lot less power from the engine than what you get at WOT and most pilots like "going fast".
The "basic physics" she refers to provides a solution to her conundrum:
Presuming well-designed and properly inflated tires and a good level paved road surface, tire drag is 1.6% to 2% of vehicle weight at 70mph. Every part of the tire has to flex as it passes under the axle. A railcar produces 1/9th or 1/10th of that drag force at equivalent speed because its wheels don't flex. Aerodynamic drag force at 70mph is 23.5lbf/ft^2 for every square foot of frontal area of the vehicle under power. For any vehicle, but especially a lightweight personal transport vehicle, aerodynamic drag force is considerable and typically more than rolling resistance. Now you know why Tesla's engineers spent so much engineering effort on reducing the drag coefficient. The Cd of a Tesla Model 3 is 0.23 and he frontal area is 23.9ft^2. Those are impressive numbers, BTW. Many bullets don't turn in a Cd that good.
23.9(frontal area in square feet) * 0.23 (Cd) * 23.5 (pounds of constant aerodynamic drag force at 70mph per square foot; doesn't change for different vehicles; only changes with elevation and temperature- atmosphere is thinner the higher you go) = 129.1795lbf (pounds of drag force that the motor must overcome to keep the vehicle at 70mph)
Now we add to that the rolling resistance / drag force produced by the vehicle's tires (this increases with increasing speed, but increasing tire pressure has a drastic effect on rolling resistance; the engineering toolbox website has a good graphic that shows the curves for tire inflation pressure and speed)
0.02 (rolling resistance coefficient at 70mph) * 4,100lbs (Model 3 mass with the 75kWh battery) = 82 pounds of force from rolling resistance
129.1795 + 82 = 211lbf
That's the drag force that the electric motor and battery have to overcome to keep the Tesla Model 3 at 70mph. More power will be required going up a hill and less power will be required going down a hill.
The equation for horsepower (1 brake horsepower is 550ft-lbs/sec) required would be as follows:
211(pounds of force to overcome) * 102.667(fps at 70mph) / 550 = 39.386hp = 29.37kW
In other words, a Tesla Model 3 with the 75kWh battery pack could maintain that speed for 2.55 hours or 153 minutes. Please note that for maximum range, according to mileage claims, you obviously can't travel at 70mph, because aerodynamic drag force goes up dramatically as speed increases.
So, kbd, what's "the solution":
1. Drive at 55mph. The double nickel saved lives and it drastically reduced aerodynamic drag, which is typically the dominant factor affecting the power requirement for a highway vehicle. Power to overcome drag goes up drastically with increasing velocity. You can only reduce Cd and frontal area to a certain point, so going slower is the most practical option for saving fuel. That's what the physics says.
2. Decrease rolling resistance of the tires. There's clearly a limit to this and if the tire doesn't grip the road sufficiently well (provide adequate traction), then you have a drifting machine more than any type of usable grocery getter (Model 3) or grocery hauler (Class 8 heavy duty truck).
3. Use a form of power that does not require keeping a heavy rotating mass in motion, such as a high temperature diesel fuel cell, which can be 70% to 80% efficient. This single change is the greatest possible improvement to fuel economy of the heavy trucks that keep a technologically advanced society supplied with goods and services. It's hard to underestimate how this affects fuel consumption, since rolling resistance force is tied to weight.
If a diesel engine converts 35% of the fuel into mechanical power, then 16.1MJ/kg. If a diesel fuel cell converts 70% of the fuel into mechanical power, then 32.2MJ/kg. The fuel cell also produces half of the emissions for the same amount of mechanical work. If the highly aerodynamic diesel engine powered DOE "Super Truck" was getting 12mpg, then a high temperature diesel fuel cell truck of the same design will be getting 24mpg with the same load and a the same speed.
Apart from drastic expansion of the railways, there isn't any other practical way. Reduce the drag force (to a point), increase tire inflation pressure (to a point), and use a more fuel efficient "engine" to provide power. That's it. That's all there is to it. No "magic" can or will happen here.
Whereas a 600hp Class 8 diesel engine is in the 4,000 pound range, a high temperature fuel cell would likely weigh less than 660 pounds. Since it consumes half as much fuel, a single 50 gallon fuel tank can take you 1,200 miles. The national average 7mpg for a class 8 truck requires 172 gallons of fuel to go the same distance, which is another 830 pounds of weight add to the tractor (plus another 100 pounds or so for the tankage), so the fuel cell powered vehicle is 3,000 pounds lighter in terms of "engine weight" alone. If each wheel has its own electric motor, then we're looking at saving at least another 1,000 pounds of transmission and drive train weight, so we either gain payload or reduce fuel consumption by decreasing rolling resistance. All in all, we're looking at knocking 4,000+ pounds of weight off the tractor with the fuel cell. This doesn't require using lighter materials, either, it's just the mass differential between a large high duty cycle ICE and a high temperature solid oxide fuel cell. The truck chassis will still be steel with Aluminum wheels, same as nearly all new big rigs. For on-road use in spring or summer months or places where it doesn't snow or ice up frequently, super singles do contribute meaningfully to improved gas mileage. CNT fabric sidewalls would nearly prevent blowouts. We can play endlessly with aerodynamics, but the most practical solution is to drive a little slower.
The final solution is to build more railways anywhere there isn't water, because physics simply won't accommodate any magical thinking about the relative efficiency of flexible vs non-flexible wheels. US Army studies showed (every single time they re-do "the test", looking for a different result that physics won't accommodate) that wheeled vehicles up to the 10t weight limit can generally get better fuel economy off-road than tracked vehicles, but after that the use of tracks (especially rubber band tracks) saves fuel because the tracked vehicle design ends up exerting lower ground pressure for a given payload, therefore reduced rolling resistance. That little "discovery" and "re-discovey" and "re-re-discovery" was solely due to rolling resistance between wheels vs rubber band tracks in an off-road environment. Again, physics doesn't care about what toys the boys want to play with. It says what it says, does what it does, and it's not changing to accommodate anyone's ideology or feelings or burning desires.
Want to know why we still make and use diesel combustion engine powered trucks?
Nothing that changes the math affecting the power requirement to move a given load a given distance has appeared, with respect to battery powered vehicles and the current battery technology is woefully insufficient to move Load X to distance Y at Rate Z.
We, as in all of humanity, live in a country called "Derpistan". In Derpistan, the locals don't like math. They much prefer their religion of choice. One religious group thinks diesel ICE's are the answer to all of life's energy problems and the other thinks that batteries and solar panels solve their energy problems. Both groups wave their magic wands, recite their religious dogma, and... NOTHING CHANGES AND NOTHING FUNDAMENTALLY "BETTER" APPEARS! Well, derpity-doo-dah all the live long day. Maybe one day we'll collectively realize how much precious time has been wasted chasing down non-solutions. You go up the power-to-weight and efficiency ladder whenever you start demanding more ton-miles per unit of fuel (of whatever type) expended, not down.
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Kbd, that is impressive. There are a few limitations on solid oxide fuel cells that might require you to modify your concept, but SOFC trucks deliver enough of an improvement in fuel economy that it would appear (to me at least) that they should allow us to stay ahead of the surplus energy cliff for a long time to come. Just a couple of things:
1. Realistic efficiency for a SOFC is closer to 50%. Still a big improvement on compression ignition engine, especially when reduced engine and drive train weight are accounted for.
2. SOFC is very vulnerable to damage from sulphur in fuel. The fuel needs very low sulphur content.
3. As a high temperature device, there will be warm up time. The truck should maximise distance travelled between rest stops.
4. Hydraulic recovery of braking energy could improve fuel efficiency even more. For big trucks operating in urban conditions, it could allow a 40% reduction in fuel consumption. Probably a lot less for long haul.SOFC sounds like a good option for powering a Mars vehicle. That would be especially true if could use hydrogen and CO2 in the fuel cell. I seem to remember that hydrogen will reacte exothermically with CO2.
I don't think grid battery energy storage is taken seriously by anyone as a potential solution for long-term lulls in renewable energy production. What it can actually achieve is to provide enough energy storage to allow CCGTs to be brought on load, if the windfarm drops off of load. That is where battery storage is useful. You don't need days or even hours of grid storage for that.
Thermal energy storage could also be valuable as it has generally low capital cost compared to other options. It means storing hot water in insulated tanks. In power generation concepts, heat is usually stored in concrete or rock masses. This is then used to generate power using a steam cycle (S-CO2?). Works best in combined heat and power mode. Typical energy storage density is 0.5-1MJ/kg, which is about the same as batteries. The difference is that rock and concrete are cheap as chips, it is the steam generating plant that costs money in this case.
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Calliban,
1. I'm reasonably sure that the US Army's SOFC program manages 70% to 80% efficiency using CHP and they're designing these things for off-road trucks and generators for base power. They don't baby their equipment, either. Anything incapable of withstanding a reasonable level of abuse won't see a field trial. In any event, there are a number of ways to improve electrical efficiency and cell life, which includes recirculation of exhaust gas through the cell to extract more power from the un-reacted Hydrogen gas in the exhaust stream.
More Efficient, Longer-Lasting Solid Oxide Fuel Cells
2. The US Army has also come up with materials that are tolerant of high Sulfur content in poor quality JP8 fuel. You do get more NOx emissions with SOFC, but less PM2.5. In my opinion, higher quality fuels are worth the extra refining cost because they make design of the rest of an internal combustion or fuel cell solution so much easier.
3. True, but there's a solution to this problem and the one actual unresolved issue with fatigue from thermal cycling as well. Surrounding the fuel cell with appropriate insulation or a molten salt heat source to prevent rapid temperature fluctuations will greatly increase service life.
NASA may have solved the thermal cycling issue with a mechanical solution:
High Power Density Solid Oxide Fuel Cell
2.5kW/kg and 7.5kW/L
A 600hp / 448kW class 8 truck fuel cell "engine" would therefore have a cell volume of 2.1 cubic feet and weigh 179.2kg / 395 pounds. That would easily fit between the frame rails of the tractor, even with insulation and everything else included. I used my mass estimate based upon "standard" 2kW/kg SOFC technology and increased that by about 50% for sake of durability. There's little reason to think that we couldn't make the solution "bomb proof" by generously beefing up the design, on account of how much lighter it is than a diesel engine.
The fact of the matter is, these things are tiny and maintain better power-to-weight ratios than gas turbines up to 1.5MW or so. That accounts for nearly all land vehicles by numbers, many diesels for boats and backup generators for ships, all light aircraft (defined as 10,000lbs or less here in America). I'm guessing that the fuel economy improvement would cover the mass differential for virtually all land vehicles outside of some very specialized one-of-a-kind industrial equipment for mining and construction (where slightly increased power system mass would likely be negligible anyway).
The pilots I know couldn't care less if the power going into the prop is provided by a gas-powered four-banger, a fuel cell, or miniature purple ponies. They'd welcome electric motors for "smoother than a gas turbine" operation with open arms. The electric motors for aircraft have already been perfected and they work beautifully, as long as you can supply the kilowatts. The pilots who have flown them have nothing but good things to say about them. Their only complaint is that the batteries can't supply the power for very long. Fuel cells could change that. For this to happen, fundamentally different thinking is required and fuel efficiency needs to become a national priority. The technology needs to be industrialized, plain and simple.
For larger ships and power plants, there's still a possibility of using co-generation techniques to improve fuel efficiency:
FUEL UTILIZATION EFFECTS ON SYSTEM EFFICIENCY IN SOLID OXIDE FUEL CELL GAS TURBINE HYBRID SYSTEMS
4. Since electric motors are already incorporated as a part of any fuel cell solution, it would also make sense to use the motors as generators to assist with braking. Four 112kW motors are the ultimate "Jake brake", but without the obnoxiously loud noise. Vehicle accessories like power steering, AC and heating, hydraulics or pneumatics to assist with braking or acceleration, could also be pressurized using waste heat from the fuel cell, as you already noted.
I actually have a specific tractor design in mind that integrates the fuel tank with the chassis, a tubular design that utilizes otherwise wasted space and provides a smoother ride using independent suspension. It would have 4 super singles instead of 4 super singles and a pair of steering wheels. The drastic weight reduction of the tractor using the fuel cell makes this practical since there's not a 4,000 pound motor out front and another 1,000 pounds of drive train components. I think the Ford F-Vision design is pretty close to the right idea, but the batteries are not suitable for highway use because they simply don't have the energy density required. The F-Vision has a 300 mile range using its onboard battery. Note that the Nikola Motors Nikola 1 design uses a PEMFC with 100kg of onboard hydrogen storage and that provides a 500 to 750 mile range, and 1,000hp or 2,000hp for acceleration using an onboard 250kWh to 1MWh battery, and it has a dry weight of 18,000 to 20,000 pound range (or much more than that with the 1MWh battery). Nikola Motors said that if they were to add a 3MWh battery pack to provide equivalent range to the Hydrogen fuel cell powered truck, it would add approximately 30,000 pounds of weight to the vehicle, which basically makes it unusable as a heavy duty truck for carrying typical loads on most existing highways.
Batteries may be all the rage for small cars and there's great utility in battery powered micro cars for daily driving, but for heavy duty highway trucks the need for fuel cells is pretty obvious. While drivers are driving, they're getting paid, thus time and therefore money is at play. Most people want their product delivered yesterday as well.
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The Simple rover on the first page is also talking fuel cells.
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SpaceNut,
I don't think fuel cells or internal combustion engines of any kind will be practical for mobile applications. External combustion engines using novel fuels, such as the Silane-based fuels Robert and Josh mentioned, might be feasible. However, those fuels would have to be manufactured / stored / transported, at significant energy cost. The development work and equipment required to make such a novel combustion engine work is considerable, yet little concerted effort has been devoted to that end.
You'll not fling molten silica particles into the blades of any gas turbine engine's hot section and do anything except destroy the blades in very short order. I'm not aware of any gas turbine blade material, "stick" or "non-stick", that can withstand glowing hot silica particles being propelled into them by the mass flow through the engine. The entire reason the blades don't melt or erode to begin with is that there's a very thin layer of cool gas that insulates the blades from the highest combustion temperatures. When you coat them with molten sand, that process no longer functions as intended. Whenever gas turbine engines used by commercial airliners fly through clouds of volcanic ash, they flame out, may or may not restart for a brief period of time and function at significantly reduced power levels, and are subsequently written off if the jet manages to land in one piece.
Incidentally, ULA made a pure O2/H2 piston engine "work in space" by massively de-rating the engine. The engine in question made more than 600hp in the race car it was taken from, but the variant for the Centaur upper stage produces less than 1/10th of that power level because the waste heat from the engine has to go somewhere and the only options available are to radiate it away into space or to use the LH2 in the fuel tank as coolant. It's still the same size / weight as the race car variant, it's just far lighter than the combination of Lithium-ion batteries and GHe2 tanks it replaces. The control electronics and injector hardware are also markedly different since it's no longer using gasoline. Anyway, that I6 is an Aluminum block / head / pistons design with a special pressurized oiling system for lubrication. So yes, you can make a piston engine work in a vacuum, in micro-G, using pure O2/H2 boil-off, but it won't perform like the gas or diesel piston engines in a modern day motor vehicle. A good analog would be the early combustion engines found in cars like the inter-war Ford Models. They did work, but their power-to-weight ratio was pretty low. If you have to carry the oxidizer with you, then the power-to-weight ratio of the total solution is very low. It should be noted that ULA's engine only has to function for a period of minutes to days in normal operations.
The external combustion engine is probably the best "traditional combustion engine" idea I've seen thus far, but both the burner and working fluid tubes in that type of engine would likely still get coated with molten silica and fail at some point. If both the fuel and oxidizer could be stored in ambient Mars surface conditions, then there may still be an application for this type of technology.
The RTG idea has the benefit of producing continuous heat for a period of decades without the need to manufacture / store / transport fuels of any kind. If the radiated thermal power is directly used to heat CO2 working fluid to power an industrial SCO2 gas turbine, then the seals on the gas turbine or radiator and the gearing / power transmission are the life-limiting design factors. It's not an actual "RTG" because it's not generating thermoelectric power, rather a nuclear heat source driving a working fluid. There's definitely a minimum vehicle size that the RTG makes the most sense to use it in. If the weight and range requirements are lower, then Lithium-ion batteries make the most sense.
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