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Regarding the air to fuel ratio being about 12.5 to 14.7, that makes sense because 80% of the air is nitrogen and only 20% is oxygen. That gives you an oxygen to fuel ratio of (12.5/5 = 2.5, 14.7/5 = 2.94) between 2.5 and 3 to 1. Of course, oxygen gas and gasoline vapor are not the same density, so this does not tell us the ratio of mass of the two.
As I understand it, part of the antipollution device in a car involves re-injecting some of the exhaust back into the cylinders. Reburning the exhaust completes the combustion of some partial combustion products and heats the fuel/air mixture. Also consider this:
I just drove home in my Saturn and noted that it was going 60 miles per hour and the engine was turning at 2200 rpm (I have a tachometer). I took my foot off the pedal, drastically decreasing the gasoline flow to the engine. But the car and engine do not slow down quickly; the engine is still turning at least 2100 rpm, and won't slow to idle speed for a minute or two. So it is burning gasoline with way too much air at that point.
A vehicle on Mars has to accommodate this situation without wasting oxygen. The best way to do that is to have an "air reservoir" from which the air intake draws, and have two inputs into the air reservoir: one oxygen, the other exhaust. When the engine is going at full power only oxygen is admitted into the air reservoir. But if the driver takes her foot off the pedal, as the fuel input decreases, the oxygen input needs to decrease as well and the amount of exhaust being recycled into the air reservoir would increase.
Now, to repeat the paragraph at the end of my last message:
Assume 10 miles per gallon
10 miles is 16 kilometers
1 gallon is 3.75 liters
Thus 16/3.75 = 4.27 kilometers per liter
1 liter of water has a mass of 1 kilogram
Gasoline has a density just a little bit less than water; let's say 0.95
So we are getting 4.27 kilometers with 0.95 kilograms of gasoline.
That's just the fuel side; how much oxygen do we need?
2C8H18 + 25O2 = 16CO2 + 18H2O
This says that two gasoline molecules (I'm using the octane molecule to represent it) need 25 oxygen molecules to burn completely. You can verify this by adding up both sides of the equation: they both have 16 carbon atoms, 36 hydrogen atoms, and 50 oxygen atoms. BUT oxygen molecules and gasoline molecules do not have the same mass. What is the mass of 2 octanes and 25 oxygens? To determine this, we have to know what a MOLE is. It is a quantity of atoms that has a mass equal to the element's atomic mass:
1 mole of hydrogen atoms = 1 gram
1 mole of carbon atoms = 12 grams
1 mole of oxygen atoms = 16 grams
Note that 1 mole is a constant; 1 mole of hydrogen atoms is the same number of atoms as one mole of carbon or oxygen atoms.
One can quickly verify these numbers intuitively by noting that protons and neutrons have about the same mass and electrons, in contrast, have a mass less than one thousandth as much as a proton. Most hydrogens have one proton and no neutrons (1 particle per atom); carbons have six of each (12 particles per atom); oxygens, eight of each (16 particles per atom).
Now we can determine the mass of 1 mole of octane:
8 carbons = 8 x 12 = 96
18 hydrogens = 18 x 1 = 18
96 + 18 = 114 grams
Two moles of octane thus have a mass of 114 x 2 = 228 grams
And the mass of a mole of oxygen is:
2 oxygen atoms = 16 x 2 = 32 grams
The mass of 25 moles of oxygen is 32 x 25 = 800 grams
Therefore, to convert all this into a simple ratio of masses, if 228 grams of gasoline requires 800 grams of oxygen, then 1 gram of gasoline requires 800/228 = 3.51 grams of oxygen
In my previous post I said "3 or 4"; that was a fast estimate, but it is about right.
Now, let's go back to our previous calculation that 0.95 kilograms of gasoline gets us 4.25 kilometers. We can now determine how much oxygen we need as well:
0.95 x 3.51 = 3.333 kilograms
Thus we can travel 4.25 kilometers on 0.9 kg gasoline plus 3.333 kg oxygen = 4.233 kg of both:
4.25/4.233 = 1.004 km per 1 kg of fuel/oxygen mix
I said 1 kilometer per 1 kilogram. Not bad, for an estimate off the top of my head, without doing all these calculations.
And that assumes the lousy mileage of 10 miles per gallon. Methane is NOT gasoline, but its energy value is I think slightly more per kilogram, so this calculation based on gasoline is conservative. I believe it has more energy because a methane/oxygen rocket engine has a better specific impulse (exhaust velocity) than a kerosene/oxygen rocket engine.
Conclusion: a vehicle with 1 tonne of propellant (222.2 kg methane and 777.8 kg oxygen), getting the equivalent of 10 miles per gallon, should be able to travel 1,000 kilometers. That assumes it doesn't have to run the engine to power the life support, which it will, so this number is at the high end.
Let me add that I think your way of calculating mileage is extremely clever and I congratulate you for figuring it out. But I think it assumes too much oxygen consumption to burn the fuel.
Now, here's yet another way to figure mileage. It takes about 5 kilowatt-hours to make 1 kg of methane plus oxygen (222 gms of methane, 778 gm oxygen, just to be clear what I mean!). I am not sure where I got the figure; I think it's in *The Case for Mars* somewhere. Let us assume that the methane and oxygen, burned in an engine, create mechanical energy at a 25% efficiency. This means 1 kg of propellant gives us 5 x .25 = 1.25 kilowatt-hours of mechanical energy. Diesel engines do better than this.
Let us say a 25 kw engine (thats 25 x 1.3 = 33 horsepower, if I remember the conversion right) can move our vehicle 20 kilometers in one hour (just 20 km/hour or 12 miles per hour; quite slow). To get 25 kilowatt-hours, we will need to consume 25/1.25 = 20 kilograms of methane/oxygen mix. So we come up with 20 kilograms of methane/oxygen to go 20 km = 1 km per kg again.
-- RobS
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http://www.teknett.com/pwp/drmayf/hpvsspd.htm]Formula for Horsepower and Speed
Takes less Horsepower than expected.
Click http://www.teknett.com/pwp/drmayf/HPVSSPD2.JPG]here for graph under Earth conditions.
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Four pages we've been arguing this! Sigh... I really do not understand what it is about the combustion engine that people absolutely must have it to power a vehicle on mars no matter what. Is it just the idea of driving a Humvee on mars? Is that it? Leather interior? Maybe a vehicle with 1 tonne of propellant should travel 1,000 kilometers but what happens if the engine fails? What if you get a LOX leak? The crew dies. How do you squeeze an oxygen/pressurization system into a Humvee? The crew can't stand up in it so are they going to have to travel sitting the whole 1,000 miles? No room for a double pressure bulkhead. No room for putting on space suits. Is the crew going to have to wear them the whole time? So you need to design all of this stuff, this 'air reservoir' thing, and keep it all under 2.2 tonnes while somehow giving it more redundancy.
My idea has multiple redundancy for mission critical and life support systems. I believe it is lighter than any combustion engine Humvee that you could come up with that does the same thing. And it's range is only limited by the amount of food the crew carries and the 135 day internal air supply. Oh, and the crew can stand up inside.
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Upright stance incide in something like this:
http://www.fiat.co.nz/showroom/ducato/d … ducato.htm
Every drive could fail. Every oxigen supply might leak.
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My idea has multiple redundancy for mission critical and life support systems.
The Oxygen in 1.48 m^3 Methane example could enable a person breathe for
44,673/150=298 days. A person needs 150 ft^3 per day.
In case of vehicle breakdown, Dook should be designing for sufficient solar energy to make the 150 ft^3 per person per day, and produce also CO2 removing chemicals..
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I very much agree with you, Dook, about two things:
1. It seems unwise to rely on a single internal combustion engine for a Mars vehicle, because it could fail catastrophically. This worries me a lot as well.
2. It seems very strange to rely on "old fashioned" technology on Mars. It feels like an emotional throwback.
When Zubrin advocated internal combustion in *The Case for Mars,* my reaction was the same as number 2. Personally, I would prefer a group of light-weight, reliable, high-efficiency, cheap, oxygen-methane fuel cells. These are under development right now, and they would use the same fuel as the ERV, and therefore would not require a separate fuel manufacturing system. Failing that, a hydrogen-oxygen fuel cell system would work. Detroit is working on building a chassis that would have a series of hydrogen fuel cells built into it, plus four independently powered wheels (each with their own electric motors and regenerative braking systems). This strikes me as extremely robust; six or eight small fuel cells, so if one fails there are plenty of others; four independent wheels, any two of which could power the vehicle forward and stop it.
BUT this technology is still very experimental. It could very well be that six fuel cells would all fail in a 5,000 kilometer trip while a single internal combustion engine engine almost certainly would not; the internal combustion engine is now extremely reliable, compact, and cheap, while fuel cells still use exotic materials, can require very narrow operating conditions, can be very difficult to repair in the field, require much more weight, and may simply be more bother than they are worth.
As for methane-oxygen fuel cells, they are even more experimental. Who knows when they will be reliable and light enough for use on Mars. I use them in my novel, but that's a novel.
Solar power simply cannot produce enough power to move a vehicle. To power life support, maybe yes; if a vehicle needs three kilowatts at any particular time under emergency conditions, that's 72 kilowatt-hours per day, which would require about 60 square meters of panels (1/2 kilowatt per square meter of sunlight on a panel for half a day; 30% conversion; that's 150 watts per square meter, so one needs 7 square meters to produce a kilowatt-hour of power per hour, and the 7 square meters would produce 12 kilowatt hours during the day). Such an array would have to be about 8 meters long and 8 meters wide. One could fit that on the top of a vehicle as long as it is elevated off the ground enough not to hit boulders.
I think Zubrin went with an internal combustion engine eimply because it is the technology that's most proved. The others simply may not be as reliable. And Zubrin does not advocate traveling vast distances in a pressurized rover; no more than dozens of miles from base. The second crew, however, would have two such vehicles and then could travel much farther. In *Mars on Earth* Zubrin talks about the need to have something like an all-terrain vehicle, preferrably one per astronaut, so that if one breaks the other one can town the vehicle back and both astronauts can ride back.
-- RobS
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Every drive could fail. Every oxigen supply might leak.
Sure every drive could fail and every oxygen supply might leak but my idea has 2 separate gearboxes, each with 3 motors. My idea doesn't use LOX but oxygen gas in 4 separate bottles each opened one at a time. That is what I mean by redundancy. Sure something could fail, you have to plan for it. That is why every mission critical and life support system has a backup.
MarsDog: CO2 removal is a life support system. My idea has two oxygen bottles that together provide 135 days of oxygen for a crew of two. Also, in an emergency the 4 bottles that supply oxygen to the fuel cells can instead send oxygen inside the cabin. My idea has two C02 removal systems and the crew will have a repair kit with replacement components to fix things that may break. Also there will be 15 spare lithium hydroxide canisters for emergency CO2 removal. Again, multiple redundancy.
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It could very well be that six fuel cells would all fail in a 5,000 kilometer trip while a single internal combustion engine engine almost certainly would not.
Alkaline fuel cells are not experimental. They have been flying on the space shuttle since the beginning and a fuel cell was even onboard the Apollo capsules. In all of the space shuttles trips only one fuel cell has failed. In my design there are two fuel cells, each can easily charge the batteries. If both fuel cells happen to fail the solar panels can charge the batteries during the morning to noon hours, the vehicle can drive for a while, then it would have to stop and recharge the batteries again before night comes. It would take much longer but they would get home.
My idea has a flat mounted, fixed, 13.861 square meter solar panel array on top of the vehicle with a mylar mirror (heliostat) that focuses sunlight onto the array. This is expected to provide somewhere between 2-3 kw to assist the batteries in the daytime. Also a rolled up solar collector can be spread out on the mars surface and then connected to my vehicle to provide additional emergency power. This provides a triple redundant power source.
I think any mars vehicle would be tested thoroughly before it is sent to mars. Also with redundant systems it is unlikely that multiple systems will fail, and if they do, the crew may very well be able to fix them onboard. Also at a minimum my design provides 135 days of oxygen but with 4 more external oxygen bottles (for fuel cells) that is another 270 days worth of oxygen on board at all times.
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http://members.shaw.ca/tfrisen/how_much … ]According to NASA the average person needs 0.84 kg of O2 per day
One cubic meter of liquid O2 = 1141 kg
For 2 people 1141/(2*0.84) = 679 days.
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In order to fine tune the designs, a spreadsheet setup is needed.
The tradeoffs are complex, especially with weight constraints.
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For long trips, a trailer with extra fuel, food and parts would be convenient.
A large solar panel could be unfolded, when stopped, during peak sunlight hours.
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2023 Taklimakan Rally to resume in Xinjiang
https://www.bignewsnetwork.com/news/273 … n-xinjiang
maybe Mars will one day have its 'Sports' it might host Desert Racing and Olympics
RobS has posted about regenerative braking in this thread, we have already talked of landing on the Mountains like Olympus Mons and such downhill recharge system are now used in Musk's cars. I wonder if a system could be used to transport material down from a higher location on Mars, the Regenerative Braking a more simple process used by self-charging hybrids on Earth.
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![http://www.teknett.com/pwp/drmayf/HPVSSPD2.JPG]here](http://www.teknett.com/pwp/drmayf/HPVSSPD2.JPG]here){kind=link}