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#26 2017-02-01 09:05:22

Oldfart1939
Member
Registered: 2016-11-26
Posts: 826

Re: Long Range Rover

Actually, the buffer gas could be wasteful utilization of the CH4, although this may foul spark plugs. CO2 has a negative effect on combustion processes.

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#27 2017-02-01 11:08:01

elderflower
Member
Registered: 2016-06-19
Posts: 410

Re: Long Range Rover

CO2 is mostly negative due to cooling and displacement of Oxygen, which is why it is used in fire extinguishers. It will also absorb some heat and some of it will dissociate to CO and Oxygen. The amount of dissociation depends on the combustion temperature. If the Oxygen generated by dissociation then reacts with more carbon to generate more CO there will be a net heat release from this process.
Using excess CH4 will produce more CO as well as soot.
The best choices of heat engine would probably be Sterling or Brayton cycle machines. Bothe have heat supplied from an external source so combustion is carried out in a separate chamber which could be easily changed and /or cleaned. A fuel cell using LOX and methanol- same as used on earth for charging batteries on ocean crossing boats- might be a better bet, although they would need some development to get weight down and power up.

Last edited by elderflower (2017-02-01 12:04:14)

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#28 2017-02-01 13:43:03

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

Re: Long Range Rover

elderflower wrote:

Dook, Only about 70% of the nominal  displacement of a normal reciprocating engine is ingested. This is due to failure to totally clear the exhaust gases and to pressure losses in the inlet and exhaust systems. Of this 70% only about 23% is actually O2. The balance is N2 and a bit of Ar.
To replace the Nitrogen/Argon buffer gas an engine on Mars would probably use atmospheric CO2. You definitely need some buffer gas as the combustion temperature using LOX and CH4 is far too high for this type of engine.
Torpedoes have been driven by peroxide and fuel for many years. Most have given up on this mixture due to unscheduled disassembly events with disastrous consequences, but it might be possible to develop a safer system. This would allow use of a storable oxidiser which could be prepared on Mars.
Peroxide can be a monopropellant or can be used with fuel eg methanol, which can be made from synthesis gas quite easily and is also storable.
Peroxide needs stabiliser addition and that would have to be shipped from earth. Some tin compounds are generally used.

Your suggestion that a rover powered by an internal combustion engine did not depend on gaseous oxygen?  But it does.  Internal combustion engines can't burn liquid oxygen.  The oxygen has to be in a gaseous state.  Now, could the rover pull along a liquid oxygen tank that sends oxygen gas into the engine?  Yes, but it's very impractical. 

You say that only 23% of an engines intake is oxygen?  Okay, so that would be 7 cu in of oxygen gas taken in per round at it's operating rpm of 3,000 rpm.  That is 21,000 cu in of oxygen per minute, or 1.26 million cu in of oxygen gas per hour.  Your LOX tank would have to be huge and it would be so heavy that your rover couldn't pull it over Mars rocks and hills. 

And you still haven't provided the engine with a buffer gas unless you are going to use Mars CO2.  I don't know that an engine would run on 70% CO2.  And you still have to have a fuel tank. 

It's just not practical when compared to solar cells and batteries.

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#29 2017-02-01 13:43:57

Oldfart1939
Member
Registered: 2016-11-26
Posts: 826

Re: Long Range Rover

There really isn't much need for the inert sweeper gasses for an engine on mars, since the external pressure is only ~ 0.01 ATM. The burned gasses would exhaust themselves readily. It would take a bit of research, but an engine running on O2 and CH4 should be easily designed. The metals incorporated would probably need to be something similar to Inconel for maximum heat and oxidation resistance.

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#30 2017-02-01 14:07:31

RobertDyck
Member
From: Winnipeg, Canada
Registered: 2002-08-20
Posts: 4,872
Website

Re: Long Range Rover

VW's 'dung' Beetle: The car that leaves nothing to waste... thanks to its methane gas-powered engine

A car powered by methane gas has been created by a team of British engineers.

The vehicle named the 'Bio-Bug' is run reliably on biogas, which is produced from human waste at sewage works across the country.

Excrement flushed down the toilets of just 70 homes is enough to power the pioneering VW Beetle car for 10,000 miles - the equivalent of one average motoring year.
...
The Bio-Bug is a conventional 2 litre VW Beetle convertible, which has been modified to run on both conventional fuel and compressed methane gas.

The car, which has a top speed of 114mph, is started using unleaded petrol but automatically switches to methane when the engine is 'up to temperature'.

If the methane tank runs out the Bio-Bug reverts back to petrol.
...
The gas is generated through anaerobic digestion - where bugs which are starved of oxygen break down biodegradable material to produce methane.
...
The Bio-Bug does 5.3 miles per cubic metre of biogas...

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#31 2017-02-01 18:56:59

GW Johnson
Member
From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 2,543
Website

Re: Long Range Rover

Burning fuel plus oxygen in an internal combustion engine requires diluent gases to limit flame temperatures to values that real-world materials of construction can stand.  Nitrogen works in this role,  but is practically unavailable on Mars.  CO2 will not serve in this role,  because it is a combustion product gas that "poisons" the chemistry,  slowing it down to much less effective reaction rates. 

So on a world like Mars,  you are far better off with fuel cells that make electricity,  coupled with electric drive.  You can augment this with solar,  but don't forget that solar is half as effective there as here,  and lost to you during sandstorms.  Fuel cells work with gaseous oxygen and gaseous hydrogen,  not the liquids.  That means you are looking at heavy,  high-pressure,  voluminous reactant storage.  There is no way around this.  Non-hydrogen fuel designs are heavier and lower-powered.  No way around that,  either. 

Nuclear as we know it requires a very large scale to be practical,  excepting the very power- and speed-limited schemes,  such as on Curiosity.  Many dozens of tons of vehicle are required for long range and higher speeds.  About the size of a railroad locomotive or larger.  Until you are willing to ship many dozens to a few hundreds of tons at a time in a single package,  I think you can safely forget that option. 

All in all,  I'm not at all sure a long-range rover that can safely carry humans at decent speeds is even feasible on Mars.  You might just be better off planning to land from orbit at different sites,  or to fly there in rocket vehicles. 

GW

Last edited by GW Johnson (2017-02-01 19:01:31)


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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#32 2017-02-01 20:44:55

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

Re: Long Range Rover

GW Johnson wrote:

Burning fuel plus oxygen in an internal combustion engine requires diluent gases to limit flame temperatures to values that real-world materials of construction can stand.  Nitrogen works in this role,  but is practically unavailable on Mars.  CO2 will not serve in this role,  because it is a combustion product gas that "poisons" the chemistry,  slowing it down to much less effective reaction rates. 

So on a world like Mars,  you are far better off with fuel cells that make electricity,  coupled with electric drive.  You can augment this with solar,  but don't forget that solar is half as effective there as here,  and lost to you during sandstorms.  Fuel cells work with gaseous oxygen and gaseous hydrogen,  not the liquids.  That means you are looking at heavy,  high-pressure,  voluminous reactant storage.  There is no way around this.  Non-hydrogen fuel designs are heavier and lower-powered.  No way around that,  either. 

Nuclear as we know it requires a very large scale to be practical,  excepting the very power- and speed-limited schemes,  such as on Curiosity.  Many dozens of tons of vehicle are required for long range and higher speeds.  About the size of a railroad locomotive or larger.  Until you are willing to ship many dozens to a few hundreds of tons at a time in a single package,  I think you can safely forget that option. 

All in all,  I'm not at all sure a long-range rover that can safely carry humans at decent speeds is even feasible on Mars.  You might just be better off planning to land from orbit at different sites,  or to fly there in rocket vehicles. 

GW

It depends on what your expectations are.  If you are expecting a long range rover that can travel 40 mph over rock fields, through deep sand, and go thousands of miles on Mars and back again, well, that's just not going to happen. 

You have to have a long range rover with the Mars Lander.  The first ISRU vehicle will land somewhere on Mars.  The Mars Lander with a crew will land somewhere else.  The two vehicles won't be close together.  Even if NASA adopts SpaceX's maneuvering rocket technology the distance of the two is going to be 100 miles or more.  There are just too many variables with aero braking into an atmosphere. 

My Long Range Rover (LRR) will go about 20 miles a day if used during the mid-day and it will carry about 45 days of oxygen for two crewman.  It has fully reclining seats for two and two beds so it can carry four crew if necessary.  This LRR is only 16 lbs over Zubrin's weight limit for a rover and it will fit underneath a Mars Lander without needing a built in hanger.

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#33 2017-02-02 06:31:12

elderflower
Member
Registered: 2016-06-19
Posts: 410

Re: Long Range Rover

It occurs to me that a conventional internal combustion engine using Mars atmosphere is probably not going to be at all practical. Given the extremely low pressure outside, the exhaust flow would be sonic. The flow will therefore be limited by the diameter of the exhaust duct and the upstream pressure will be too high to clear the cylinders on the exhaust stroke. The exhaust gas turbine would be multistage to reduce the pressure and extract power, and its low pressure stage would be very big. It would have to drive a similarly huge supercharger to get the required mass of gas into the combustion chamber.

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#34 2017-02-02 20:53:38

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 10,075

Re: Long Range Rover

A gallon of gas 6 lbs on earth burns in an engine to produce exhaust output of 19lbs of carbon dioxide....
http://siei.org/usefulequations.html
http://combustiontechnology.co.za/train … ustion.htm
Approximately 1,600 to 2,000 cubic feet of air is required to burn one gallon of #2 fuel oil at 80% efficiency (at sea level).  About 15 cubic feet of air is required to burn one cubic foot of natural gas at 75% efficiency (at sea level).

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#35 2017-02-04 20:48:08

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 10,075

Re: Long Range Rover

The information on this article is lacking the shielding mass or radiator size for this style of reactor but for a mars use it would seem to be plausable for use as its Small.

Molten Salt Reactor about 50 times the power density of current submarine nuclear reactors

The 650 MWth IMSR (Integrated Molten Salt) reactor is about the same size as the smAHTR (125 MWth) reactor.

The smAHTR reactor is 9 meters tall (30 feet) by 3.5 meters (12 feet) in diameter.

The 220 MWth S8G reactor for the Ohio submarines is 42 feet in diameter, 55 feet long; 2,750 tons

So the IMSR with supercritical CO2 turbines would have almost 3 times as much power in an area about 16 times less area. In the range of 150-200 cubic meters and about 200-400 tons.

http://www.uxc.com/smr/Library/Design%2 … SmAHTR.pdf

http://www.bnrc.berkeley.edu/documents/ … L_Pres.pdf

http://info.ornl.gov/sites/publications … b48861.pdf

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