From the following "Space" article LINK



What is the reason for the lower power requirements of Oppy?

I thought the two rovers were identical.

Maybe that Gusev zone requires more energy than Meridiani to advance due to the type of terrain. More slope to down and go up.

Rodolfo

The only thing I can think of is that Meridiani is a slightly more benign place than Gusev, specifically (as the article indicates) the temperatures are a few degees warmer so the heating requirements in mid winter will be slightly better.

I think it's because Oppy had that stuck heater switch and they shut the heater off. Spirit probably still has the intermittent heating going on.

Just a guess, going from memory.

Wasn't Spirit generating close to 900 watt-hours a month or so back?

Yes but that was in August\September. 956 on Aug 21, 885 on 12 September. The fall of in Insolation is quite rapid at the moment as summer ends, it would have dropped by about 75 watts just because of that. There probably has been some dust build up which would account for some of the additional loss but the largest component is caused by the fact that spirit is travelling down a fairly steep north facing slope while the Noon sun is still in the Southern Sky.

Spirit was facing north while climbing up Husband Hill. It can't be facing north again going down on the opposite side

Good point. I'm not sure of the orientation to be honest but I recall that SS did say they needed to get off slope quickly while the power situation was favourable. I took that to mean that the downward route was predominantly North facing. Looking at the route map it appears that the orientation is probably generally south easterly, tilted at an average of around ~10%. That shouldn't cause too much of a drop in power on it's own.

I'm still convinced that orientation is the main problem, it may be that the missing power is caused by the fact that she's in a valley now and the effective sunset is a lot earlier that it was when she was on top of the hill.

Can anyone estimate what her orientation and average tilt has been over the past week or so?

That would only affect it's ability to generate the required amount of power, but the article suggests that it is the power requirement that is different.

Likewise, stuck power heaters etc affect power requirements, but the team have the ability to switch off the heater on Spirit in the same way as they did on Oppy if power becomes tight, so those should equal each other out too.

That must be the main reason. The only other could be slight differences in elektronic efficiency....we have seen other differences (i remember threads before where we found out they aren't exactly identical twins.....). Slight differences in capacitors, coils, semi-conductors, intrinsic dielectrical properties of wiring, length of wiring, heat losses, etc. can add up to some % difference.

12/5/2005

Significant New Power Information from JPL is here:

PIA03607: Solar Power on Mars

http://photojournal.jpl.nasa.gov/catalog/PIA03607

Original Caption Released with Image:

This chart illustrates the variation in available solar power for each of NASA's twin Mars Exploration Rovers over the course of approximately two Mars years. Two factors affect the amount of available power: the tilt of Mars' axis and the eccentricity of the Mars' orbit about the sun.

The horizontal scale is the number of Martian days (sols) after the Jan. 4, 2004, (Universal Time) landing of Spirit at Mars' Gusev Crater. The vertical scale on the right indicates the amount of available solar power as a ratio of the amount available at the equator when Mars is closest to the sun (perihelion). The red line indicates power availability at Spirit's landing site (Gusev). The blue line indicates power availability at Opportunity's landing site (Meridiani).

The vertical scale on the right applies to the dotted line, indicating the latitude north or south of Mars' equator where the noon sun is overhead at different times of the Martian year.


and more here

PIA03608: Solar-Panel Dust Accumulation and Cleanings

http://photojournal.jpl.nasa.gov/catalog/PIA03608

Original Caption Released with Image:

Air-fall dust accumulates on the solar panels of NASA's Mars Exploration Rovers, reducing the amount of sunlight reaching the solar arrays. Pre-launch models predicted steady dust accumulation. However, the rovers have been blessed with occasional wind events that clear significant amounts of dust from the solar panels.

This graph shows the effects of those panel-cleaning events on the amount of electricity generated by Spirit's solar panels. The horizontal scale is the number of Martian days (sols) after Spirit's Jan. 4, 2005, (Universal Time) landing on Mars. The vertical scale indicates output from the rover's solar panels as a fraction of the amount produced when the clean panels first opened. Note that the gradual declines are interrupted by occasional sharp increases, such as a dust-cleaning event on sol 420.

Interesting graph. The worst time would be around the Sol 910, it is more than 7 months from now, it is around Junio-Julio.

Rodolfo

Thats around the next martian winter.

Space Daily has reprinted the chart and captions here:

http://www.spacedaily.com/news/mars-mers-05zzzzzj.html

Rovers Exceed Expectations

http://www.cornellsun.com/news/2006/01/20/...ml&mkey=2150912

There are many differences, but primary is the latitude location difference - secondary differences of considerable significance has been the increase power usage caused by Spirit's balky wheel, (drawing much more current, now remedied by alternate driving, but still a slight factor) and the stark differences in terrain, taking some degree more power to move 1000 meters uphill than it takes to drive 1000 meters on semi-level terrain. Drain on the MER-B batteries have probably been less deep and less frequent than for MER-A overall.

I would think too, having experienced years of working with batteries, that to some degree frequency of movement (driving frequently, versus staying in one local for extended periods of time) effects battery condition and the amount of energy that can be stored with a given potential. MER-A has driven over rougher terrain, causing this deeper and more frequent drain than that of MER-B, hence MER-A batteries would probably require more power supplied longer to reach the same potential that MER-B could achieve with less power supplied, simply due to differences in the condition of the batteries. (MER-B has been frequently parked (virtually) in several places over much of its time on Mars - while MER-A as been more frequently on the move in higher power consumption territory.)

Hence, it would not be unreasonable to expect that the condition of the MER-B battery would be in better overall condition than that of MER-A, and that the MER-A battery would be more difficult to charge to as high a potential as the MER-B battery. One thought to keep in mind - these batteries (like any other) are approaching the point where they will simply refuse to take a full charge, no matter how clean the panels are, or how much sun is available. They are the best that could be built, but all rechargeable batteries have a limit to the number of times they can be charged to the full potential they could achieve when new.

Don't know about average tilt numbers, but she has not been in ideal conditions for charging ever since she began going down into the inner basin, being tilted away from the most sun, being on a South facing slope and the Sun going lower in the North every day - hence the race to get to a North facing slope soon... and you're cetainly right about the 'valley' local, also limiting exposure a bit by early mountain-caused sunsets - and to some degree, sunrises, I would expect.

Note though, that long pauses (like at Comanche, and this latest 'bright spot') have helped build up power at each layover. During those stops, to do science observation, the batteries get a chance to catch up a bit.

Thanks for the info.

Steve Squyres and the team have said there is battery capacity for ca. 10,000 cycles for a theoretical lifetime of 6 to 7 years. This was one of the major upgrades based on the Sojouner experience. So perhaps recharging the batteries is not the issue so much as available sunlight /dust build-up hindrance to recharge the batteries in the first place. The heaters must be powered or the rovers will die, according to Steve and the team. Hence the importance of tilt angle as you described above.

Are all the drive motors on the wheels in operation?
( Spirit had a problem with a right front wheel drive motor ... )

This article has a very nice front page presentation of the well-known CGI representation of the MERs here:

http://www.cornellsun.com/media/paper866/d...ts/87irlrsv.pdf

Yes they are. Spirit indeed HAD a problem with its right front wheel, drawing more current due to increased friction, which disappeared after the new driving strategy (driving forward and backwards alternately) was introduced. The wheel problem was probably caused by the way the lubrication was distributed due to turning in the same direction all the time. Now it's fine.

TWIN MARS ROVERS STILL EXPLORING AFTER TWO YEARS
------------------------------------------------

With a longevity unthinkable even to the humans that built them, NASA's
remarkable Mars rovers remain hard at work after two years on the Red
Planet's surface. Both motorized robots are still going strong in their
continued scientific pursuits.

http://spaceflightnow.com/news/n0601/24marsrovers/

Thanks for the pointers to the articles !!

There is an interesting dichotomy emerging - highlighted by the differences between the first paragraphs of the articles from Cornell and Spaceflight Now.

“The Mars rovers Spirit and Opportunity have nearly tripled their expected lifetime on the Red Planet.” – Cornell Daily Sun

“Spirit and Opportunity reached the Red Planet on January 3 and 24, 2004, beginning planned 90-day missions...” – Spaceflight Now

We’ve been told that the life expectancy of the rovers was 90 days (that is all they were guaranteed for.)

Remember when Spirit got to 180 days it was said the mission had ‘doubled the expected life.’ *) hence, you would think that if they lasted 270 days, they would have ‘tripled’ their ‘expected lifetime’ – and that today we would be hearing that Spirit has been going over eight times it’s expected lifetime.

But, now we learn from Squires that, "All the components in the rover were designed to last the equivalent of 270 sols before we launched and we've now been on Mars more than 700 sols."

I’m beginning to wonder if NASA/Squires/et al, are now beginning to worry that they may be accused of initially misleading the world as to expectations, i.e., it is less extraordinary that missions last only three times the expected than it is that they would surpass the expected by a great deal more than eight times!

It's a funding thing, you know...

* Two links that speak of doubling at 180+ days...
NASA says 'doubled its primary mission.' - see caption under photo 'Mars Rover A Big Hit'
Discover.com says 'doubled its expected lifetime'

(I hope fortune remains with these missions, and we see survival well into the future !!)

It's standard JPL practice to design and test things for 3x the expected mission life. So they planned for a 90-day mission but had test data that said they could last for 270 days if conditions were as expected.

Quite - if you're asking for a certain 90 sols, you dont design FOR 90 sols, you design for much more. So you want 90, design it so you know it'll last 270, and then you KNOW 90 will be fine.

Remember - it wasnt a component failure that was assumed to be the ultimate fate of the rovers, but dust deposition. We've just been lucky in that regard.

The design life of the hardware, and the expected life of the power source and not one and the same

I've heard Steve mention that if you'd sat him down and given him truth serum the day they landed, he'd have said perhaps 180 sols would be when they'd ACTUALLY die.

Doug

And remember that mechanical wear and tear is only one of the concerns. The power system's initial biggest challenge was surviving a late summer major dust storm which didn't arise but which had a 20% chance of happening, 90 days would have been difficult if the planet had been shrouded in dust and Tau had been at around 5. After that the rovers had to survive mid winter - the minimum sustained power level for operations is around 300 Watt haours and any extended period below ~275 watt hours is enough to actually kill one of the rovers (we believe). Spirit dropped to 288 watt hours briefly on Sol 204 when a driving constraint stopped her in an unfavourable position at almost precisely the wrong time.

Even though the power situation has generally been much healthier than that since then the best case margin for error at the Sol 200 mark was only aroun 25% and during that incident it was only around 0.5%. That was very close.

What exactly is expected to happen at that 275 watt-hour point that will kill the rovers? I have heard the arm-waving answer "the electronics will die" but I'm wondering if anyone can explain the chain of events that would happen at that 275 watt-hour point.

--Emily

I am an educated electronic engineer but I work as an I/T Specialist.

When the output watt is lower than the required limit: 275 watt-hour, it means that the electronic devices won't be able to work properly since the theyt aren't able to identify the binary signal: 0 or 1. For the motor electric won't have enough power to advance their wheels, joints, PMA, HGA, etc. However, the electronics won't die unless when the sun becomes higher and the solar power will start to charge up the battery and the electronic device will be resucited.

But, the other side, about the heating dispositives, I am not sure that they depends upon to the electrical energy source. If it is true, the electronic parts will be damaged during the Mar's night that might be lower than the lower range limit (I think it is between 45 to 40 centigrades below zero).

Rodolfo

As one of the folks who regularly quotes this number I've also got to say that I'm also very interested in finding out what that chain of events is.
The Rovers have been using "Deep Sleep" for a long time now and as I understand it that almost completely shuts down the rovers at night with some mechanism put in place that wakes them up when the power output from the solar arrays reaches a specified level. The only component that I'm aware of that is really sensitive is the Mini-TES - if the potassium bromide disc that it uses cools below -50 to -60C it will fail.
There probably are other components that have a temperature limit below which they will actually break but I've no idea what they are and it seems probable that anything with a truly critical low temperature limit would have an RHU to protect it.
My instinct on this (as an electrical engineer) is that power levels below 275 watts would simply reduce capability while power generation capability was low rather than trigger a rapid and complete system failure however Steve S. has repeatedly stated that the Rovers will die if the power level drops below around 300 watts.

Rupert Scammel's MER Technical data site lists the following (which came from a fairly early pre-flight presentation so these could be wildly inaccurate)
Engineering Housekeeping - 141 Watt hours/sol
Imaging - 62 watts/hr
Driving - 57 Watts/hr
Data Relay - 94 Watts/hr
RAT - 74 watts/hr
Deep Sleep mode presumably reduces the 141 watt hours per sol requirement fairly significantly.

I think basically - you have to have the rovers 'on' for enough time during the day to pack the WEB with heat to keep it reasonably warm thru the night. Even with deep sleep it's so well insulated that it can keep at sensible temps in there thru the night.

However - once you get to a point when you spend most of your day just charging the batteries and not doing anything, then you're going to get a colder WEB before you go to sleep, and thus a much deeper thermal cycle in there over night. Once you get that deep cycling, then you're in the regieme that ends up with massive thermal cycling on batteries, on telecoms equip - you can't find lock with DTE comms, maybe even the UHF drifts so that Odyssey cant find it, then you get to a solar-power-only mode, and things just spiral from there.

I'm not sure how one might 'fix' that - but what I would do is just leave the rover 'sleeping' till abotu 3pm, and then keep the rover very very busy for an hour, then shut it down for deep sleep - that way you've packed as much energy in at the end of the day to get the WEB as warm as possible before night starts.

I'm guessing they're using that 275w figure as the point at which they cant do enough 'stuff' inside that WEB to keep it warm enough to remain healthy over night

BUT - that is all just my guessing. It might make sense to email an engineer about it

Doug

This is easy to reproduce at home:

1. Take your girl friend's fancy new Apple notebook computer, pull out the plug and run it until it indicates a 'low on battery' status.
2. Put the thing in your freezer (at -12C or something) and leave it there for a few hours.
3. Take it out and try to recharge the battery.

See what happens? Nothing. Will not recharge! Now, if you bring it back to room temperature it might just work again, but if you repeat his experiment 2 or 3 times you will kill the battery permanently. (Go ahead, try it... don't believe me for it)

Now, remember that the MERs have exactly the same type of Li-ion batteries, so my guess is that when you can't maintain the temperature of the WEB above freezing the battery will be dead within 2 or 3 sols.

There is nothing that beats a bit of science @HOME!

The MER battery's have a better thermal profile that your average laptop battery, but the point is fairly true all the same

Doug

As for freezing a battery to death, oddly enough it works the other way around too. At least for many battery types heating a flat battery will revive it temporarily. A number of wrecked sailors and airmen owe their lives to this old trick.

tty

I used to freeze tamagochi's for fun

Doug

Er... ...no girlfriend, and a freezer full of Apples?

I won't mock, my wife used to go nuts at film in the fridge!

Bopb Shaw

Perhaps you're talking about *literally* freezing the battery after a certain point, with bits inside cracking and electrodes etc being damaged at Martian night-time temperatures? I presume there's a gel in there somewhere, any battery specialists out there?

Bob Shaw

Actually not so difficult to grasp: a battery is nothing else than a chemical reaction in a casing. Chemical reactions are very sensitive to ambient temperature. When you freeze a battery it will grow crystals in its electrolyte that will increase its internal resitance.
Heating a Li-ion battery (>70C or so) will kill it instantly, it might even explode.
(Although this might work with other types)

Don't try that @HOME!

February 9, 2006 | If NASA's Mars Exploration Rover Opportunity were human, it would be more than 600 years old.

The rover has lasted almost eight times its life expectancy, logging more than 700 Martian days on the Red Planet's surface. There is little wonder that the robotic dune buggy has developed joint problems.

As the rover entered its third Earth year on the Martian surface (two Martian years), mission engineers developed their plan to cope with Opportunity's arthritis: the steering motor on one of the rover’s front steering wheels has died and a broken wire in the instrument arm’s shoulder joint motor has hampered the mission. The arm contains many of the mission's science instruments, including the Rock Abrasion Tool used for grinding soil and rock samples, and the Microscopic Imager. However, the rover can still drive with its other steerable wheels, and with extra current the arm is still usable. By developing a new way to stow the arm, engineers have the rover driving again after more than an Earth month of analysis-related delays.

http://skyandtelescope.com/news/article_1666_1.asp

I've overhauled my solar power estimate charts with the most recent power generation information. I've also properly reworked the calculated data in these charts to include an accurate model of the available insolation for each rover and also included an estimated value that factors in the effects of Tau using the values published in the MER Analyst Notebook.

Click to view attachment
Spirit Key Dates:
If the current dust deposition rate holds (~0.08% power loss per sol) Spirit's normal power generating capability will drop below 300whr per sol on Sol 865 (June 9th) and stay below that until Sol 997 (October 22). Those power levels effectively eliminate any significant mobility unless she has a very favourable location.
The worst period is from sol 919 to Sol 943 when power will be no more than 275whr per Sol unless she is oriented towards the Sun.

Click to view attachment
My reworked figures actually put Opportunity in a much more favourable power situation. Aggregate dust loss over the past 100 Sols has been very close to 0.04% per Sol. That means that Opportunity should be able to generate more than 457whr throughout the winter period and while that is a restricted power budget it is more than Spirit currently has available.

Thanks for the long range forcast Helvick - fantastic work.

Here's hoping for a few windy days on the slopes of McCool hill.

James

Very impressive work!

What is responsible for the occasional severe drops in power output? For example, on the Spirit chart there's one at around Sol 440 and another at around 580. Did she have a bad orientation the sun on those occasions?

I apologize if this has been discussed previously; it's hard to keep up with all the threads here.

TTT

Thanks Helvick! The report is probably one of the most wanted article to know among UMSF members. Now, we are more concerned about the power level survival for Spirit. So, we start to look for any signal from Spirit rover driver to leave "YA" from HP. The good probably ones is that Oppy might be traveling as forever all round year toward Victoria.

Rodolfo

The x-axis labeling on my charts is a bit whacky, I need to fix that. The labels actually refer to the data point where a line underneath the text would intersect the x-axis. The points you refer to actually happen at Sol 418 and sol 557 respectively.

The biggest problem though is that I only have around 30 reliable data points for Spirit and 40 for Opportunity and they tend to be somewhat clustered so there are some odd artificacts in the chart as a result. The Spirit Sol 440 dip actually happens at around Sol 418 and was caused by an increase in atmospheric Opacity to 1.5 on sol 418. However that was immediately followed by a cleaning event so there was a fairly significant bounce back. I don't have detailed data for the period leading up to Sol 418 but I suspect it would have varied between 550 and 600 whr. By Sol 422 power was up to 800whr because of the cleaning event.
The Sol 557 event is, I think, more of a general pointer to the fact that power generation wasn't a huge priority in the drive up the side of Husband hill. By the time the summit was in view the orientation had improved significantly leading to the 956 whr number at the summit on Sol 581.

There are two main threats to the rovers Sol to Sol solar power generating capability:

• Orientation
  If the rover spends any significant amount of time in an unfavourable position at the wrong time then the amount of power generated on that Sol can be affected quite dramatically. The rover planners and drivers are very very good so this happens very rarely but there is one well documented example for Spirit on Sol 203 as she was approaching Clovis on the West Spur. The local slopes were up to 25deg and during the Clovis approach the rover ended up in a south facing hollow after a short autonomous driving sequence ended up ~1m further than planned. Power dropped from 370whr on Sol 202 to 288whr on Sol 203 as a result.
  
• Atmospheric Opacity
  The average atmospheric opacity (Tau) in spring\summer tends to be around the 0.8-0.9 range and that tends to drop down to 0.3-0.5 as they move through winter. However local\global dust storms cause that value to rise significantly. A large dust storm in the Margaritifer region affected Opportunity around Sol 627 when Tau rose from the summer average of around 0.9 to more than 1.6. The numbers in the Daily Reports for Sol 627 to 630 show the effect:
  Sol 625 675 whr Tau =~0.9
  Sol 627 593 whr Tau =1.6
  Sol 628 479 whr Tau = ~1.9
  Sol 629 470 whr Tau = ~2.0
  Sol 630 496 whr Tau = ~1.8
  The relationship between power generated and Tau is quite complex. Direct (ie beam) sunlight drops off according to the formula Beam=G0*exp(-tau). Where G0 is the amount of solar radiation that would be seen if there was no atmosphere. Indirect (diffuse) radiation increases with Tau but there isn't a nice simple formula for it. For the most part Tau changes between 0.5 and 0.9 don't have a severe overall effect. When Tau increases to 1.5 the power drops by about 25% and 2.0 causes a drop of about 42%. The Opportunity Sol 627-629 period seems to have been the one of the worst. There was a similarly significant storm around Sol 489-~520 which led to power dropping from around 600whr to 410whr.

To further complicate matters these two things interact. When Tau is very low then orientation effects are much more significant because direct sunlight predominates. In the equatorial and southern hemisphere regions of Mars Tau tends to be higher in Summer and lower in winter due to the overall weather patterns of Mars which tends to loft dust into the atmosphere during the SH Spring and Summer. The net effect for the rovers is generally beneficial but it means that orientation becomes critical in wintertime as mistakes can be punished by a power loss of 30% or more.
[Edited to Add]
There are one or two points where power generated exceeds the theoretical "Zero dust Loss" case on my charts. The most significant of those are at Sols 310-312 just inside the rim of Endurance where she was generating 10-17% more than she would have if she had been on a perfectly flat plain because she was optimally tilted towards the sun while still being high enough not to have early morning\late evening sunlight blocked by the crater rim. It happened again on Sol 339 when she was beside the heat shield but I don't have any explanation for the power surplus at that point.

No explanation for the excess power when Oppy was at the heatshield?. I bet being parked next to a mirror had an effect.

I'd considered that and although it must be a factor it doesn't add up to the whole story. The reported power was 836 whr. Tau on that day was 0.9. My zero dust loss calculation puts power for the horizontal case at 713whr. The difference is 17% and the actual improvement is probably closer to 30% since there was a non trivial amount of dust on the panels at the time.
From the Navcam pictures on Sol 339 the heatshield occupied around 1/4 of a Navcam frame or about 1/48th of the the total sphere around Opportunity. If you look at the pictures you'll see that most of the mirror sides are pointed away from Oppy at the time. My gut feeling for this is that it couldn't add more than 1% to the available power at the time.
Local albedo affects power especially when albedo is high and Tau is near or above 1. This is a non trivial effect but even though the area around Heat Shield seems quite bright in the raw images the reported data indicates that albedo was fairly constant on the plains (0.14 +_0.1). I used 0.15 for my Opportunity calculations which would probably be erring marginally on the high side in any case.

I suspect it might be a reporting error. ~600-650whr on that Sol would tie in much more closely with the nearest datapoints before (Sol 333) and after (Sol 401) that point in time.

Clearly I've spent far too much time thinking about this.

A slight update. Browsing back through this I realised I'd forgotton about the charts mars_loon had found that showed insolation charts for both sites and more importantly Spirit's actual solar panel relative efficiency between Sols 1 and 700. I extracted the data from the image and used it to calculate what should be a much more accurate estimate of solar panel power output per sol for Spirit. I applied that value to the Tau adjusted power value to see how it matches up with my estimated power curve which is based mostly on the occassional value in the Daily Reports. The new line is labelled "Efficiency Adjusted".
Click to view attachment
It makes the whole thing quite a bit messier but I think it highlights times where power has been gained or lost due to orientation.

I've done a calculation that I hope some of you might be interested in. The question I posed myself was how much petrol contains the amount of energy that the rovers have collected on average in the last 700+ sols

If we assume average power collected at 600W.h/sol
this is equal to
=600x60x60J/sol
=2,160kJ/sol

Pentane, a major fuel component, has a heat of combustion
=3,509kJ/n
or=3,509kJ/72g

Therefore solar energy collected
=2,160/3509g of petrol/sol
density = 0.62g/cm*3

This is equal to 72mL of petrol/sol. It would run for 5 days on a single beer can full or 63 sols on 1 gallon. Over the 750 odd sols that spirit has been active it has managed on less than 55L of fuel. This is about the amount in one small car tank

Ahh - you're assuming 100% efficiency in the use of petrol.

Typically, an efficient Petrol engine is 30%, Diesel perhaps as much as 40% or more

So you'd need not 72 ml/sol, but more like 220 ml/sol

Doug

Abalone:

1950s Moon and Mars exploration plans assumed the use of diesel-engined pressurised tanks, complete with an oxygen source!

I wonder if one of the chemists out there could calculate the mass of the petrol plus the required oxygen for MER operations to date. It makes me wonder whether a MSL-class 'hybrid' solar/IC engine option might not be a goer - solar energy for trickle feed, step on the gas when required, and with a clear EOM strategy built in from the start (drive while you can, then become a fixed solar-powered station). Possibly an alternative to those nasty RTGs?

And imagine if the US ended up sending petrol to a far away desert for a change!

Bob Shaw

It was only intended as a comparison. If you use the petrol in a fuel cell you could get 70% eficiency. Maybe petrol was a poor choice, have worked it out for liquid hydrogen and its about 10.5g/day or about 8kg for the entire mission if we assume a fuel cell efficency of 80%. And before you ask, hey it not my fault there is no oxygen on Mars.

I only did the calculations to illustrate the small amount of energy that we are dealing with here

Abalone:

A fuel cell is certainly more efficient, but remains - today - a more difficult technology than IC. Can you turn a fuel cell off on Mars for weeks at a time, then restart it in a flash? I bet that IC engines would work just fine, petrol, diesel or H2/LO. Hell, they might even contribute to some global warming on Mars!

Bob Shaw