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MSL's Power Source
Guest_exobioquest_*
post Nov 27 2005, 04:46 PM
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Hi, new here.

I'm wondering if any news has come down about finalizing what MSL will run on?

Will it be 2 Boeing's MMRTG (at ~100 watts?) or Lockheed Martinís SRG (again ~100watts?), have they decided yet? Willl MSL use the RPS to trickle charge a battery or will MSL run on the RPS only? God I hope solar is not a option is anyone pushing for it?
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helvick
post Nov 27 2005, 06:30 PM
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QUOTE (exobioquest @ Nov 27 2005, 05:46 PM)
God I hope solar is not a option is anyone pushing for it?
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I seriously doubt that anyone is pushing for it. Even with significantly improved efficiencies Solar just has too many drawbacks for long term missions. The MER's have been lucky and no-one should fool themselves into thinking that the circumstances that have allowed them to survive can be relied upon.

With technology as it is right now any roving surface mission with a long term primary mission needs an RTG. To replace a ~100W RTG with a solar solution that could survive a major dust storm you would need approximately 16x the surface area of the panels on the MER's. Solar Cell technology has improved quite a bit since the MER's were put together but even so the best case I can see right now would be panels 10-12x the area of the MER's. And that is just for a near equatorial rover.
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Guest_exobioquest_*
post Nov 27 2005, 06:53 PM
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QUOTE (helvick @ Nov 27 2005, 12:30 PM)
The MER's have been lucky and no-one should fool themselves into thinking that the circumstances that have allowed them to survive can be relied upon.


Thatís what I'm worried about, I'm sure the anti-nuclear people (remember the ones that went apesh!t over cassini?) are going to be piss off about MSL and are going to use MER's two amazingly hard to survive years at the equator as proof that solar can be used for every mars mission no matter the latitude or conditions.

Iím also worried that MSL is going to use batteries, sure it could survive for a Martian year with Li-ion batteries but how about 5-10 years of over time, not likely, if its RTG only it could potentially run around for decades! Everything else should easily survive decades and hundreds of km of operation if maintain at a relatively constant temperature of course, as is MER has push a lot of components to and beyond there temp ranges and its still running! The power source should be the only significant limiter in life-time performance, and the use of chemical batteries will definitely shorten it.

By the way what advances in solar cells have happened since MERís launch that could improve solar cells performance so much.
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helvick
post Nov 27 2005, 08:05 PM
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QUOTE (exobioquest @ Nov 27 2005, 07:53 PM)
Thatís what I'm worried about, I'm sure the anti-nuclear people (remember the ones that went apesh!t over cassini?)
*

They are still there but the work done in pushing Cassini through seems to have led to a lot of folks realising that the threat was overhyped. NH seems to have avoided the drama so I don't think MSL is at risk from that sort of campaign.
QUOTE (exobioquest @ Nov 27 2005, 07:53 PM)
Iím also worried that MSL is going to use batteries, sure it could survive for a Martian year with Li-ion batteries but how about 5-10 years of over time, not likely.
*

I haven't seen any detailed outline of MSL's power subsystem but I'd be amazed if it was likely to be a problem. If batteries are required then there are battery systems that survive far longer than the type used on the MER's, think about the batteries in a host of satellites that survive for 5-10 years and multiple discharge\charge cycles per day. The MER batteries were chosen because they provided the best power density for a short mission, they are not the only option.
QUOTE (exobioquest @ Nov 27 2005, 07:53 PM)
By the way what advances in solar cells have happened since MERís launch that could improve solar cells performance so much.
*

Spectrolab (the manufacturers of the triple junction cells on the MER's) now have cells commercially available that are 28.3% efficient vs the 23.6% cells used on the MER's. They say they expect to have 35% eficiency cells on the market in 2006 according to their PR blurb here: Spectrolab. That may be PR but I'd say it's right - I can't find the link at the moment but I have seen some test results of sample cells of theirs that hit 33\34%.
35% conversion efficiency would (probably) increase the power output of the MER arrays by 50% so these are not trivial increases in efficiency.
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Guest_exobioquest_*
post Nov 27 2005, 08:38 PM
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Well maybe your right about anti-nuclear nuts, I haven't heard them complaining about New Horizon, but then again there is no other option for New Horizon, but if people think solar is all that is needed on mars their going to question MSL. The PR problem could get worse depending on how long the phoenix lander lasts, if it last all the way up until it's covered in dry ice its not going to look good for the high latitude argument.

35% efficiency?! Thatís got to be more then just 3 junctions! Talk about expensive but that should still be pennies for a space probe. What if solar panels are placed on a motorized stand and aimed at the sun, I'm sure that argument could be made, I donít know how much that would weigh in comparison to a RTG but it sure would be big and cumbersome in volume.

Nickel-Metal hydrides can survive several thousand recharge cycles and donít spontaneously decay, but could it last for a decade? Itís just after MER lasting for 2 years I would like to see MSL put MER in its place and outlast its optimum mission length by several times. By the way which one do you prefer the MMRTG or the SRG?
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DEChengst
post Nov 27 2005, 09:14 PM
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I'm quite sure MSL would survive after the batteries die. The main reason for adding batteries to a RTG powered probe is that RTGs supply the same amount of power 24/7. You don't design the RTG to give the peak power you need on the probe but you design it to deliver the average power you need. If you need peak power you run the probe from both the RTG and the batteries. If you're using less power than the RTG produces you recharge the batteries. Having dead batteries would limit the ammount of things you can do at once with the probe, but I don't think it would kill it.


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Guest_exobioquest_*
post Nov 27 2005, 09:30 PM
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Ya, but I don't think 100watts is enough to do much; MER supposedly needs 100 watts just to move around. Maybe if the RPS was at 200watts you could get most things done without the batteries, as is the laser chemcam thing is going to needs some big capacitors if its going to have enough power to burn through the layer of dust on rocks.
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helvick
post Nov 27 2005, 09:45 PM
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QUOTE (exobioquest @ Nov 27 2005, 09:38 PM)
35% efficiency?! Thatís got to be more then just 3 junctions! Talk about expensive but that should still be pennies for a space probe. What if solar panels are placed on a motorized stand and aimed at the sun, I'm sure that argument could be made, I donít know how much that would weigh in comparison to a RTG but it sure would be big and cumbersome in volume.
*

AFAIK that's still a triple junction design but I don't know for sure. The 28.3% units are triple junction.
Motorizing the panels doesn't have the payoffs that folks generally think they do. The complexity of a sun tracking mount for a ~1m^2 panel adds a lot of risk of mechanical failure. In most normal Martian SH summer conditions (tau~0.9) you gain 10-15% power but you won't need it then. In SH Winter at high latitudes when tau drops to 0.3 on average it might be worth as much as 50% at a time when it actually is useful as overall insolation will be down by almost that much. However for a long mission you have to design for when conditions are going to be at their worst, and that means you design for the high dust loading in the atmosphere with Tau >3. Under those conditions your aiming mount gains you nothing because >95% of insolation is diffuse and effective insolation is down to 20-25% so that is your limiting characteristic. For a long lived lander\rover you have to fly with a panel big enough to generate enough power when a major dust storm hits and the chances of that are ~20% per martian year. Since you have to have the panel area for that situation there is no need for an aiming mount and adding one is pointless.

Phoenix could well last quite a long time after it's primary mission but it will die as winter sets in because there are about 110 Sols of virtual total darkness at its latitude. I've been searching for data on the exact efficiency of the cells and the area of the panels but don't have anything at all yet. However assuming (for the moment) that it ships with panels like the MER's it would generate 850Watt hours on the day it lands (10 May 2008 I think), that will drop to 400 Watt hours by Sol 160, 185 by Sol 190, 200 by sol 210, 100 by sol 245 and zero around sol 300. That's assuming the dust deposition rates are the same as those experienced by MER. Anyway whatever the actual power output at the start is it will drop by >50% after 160 sols and fall rapidly after that. Those are my own calculations so I might be a bit off but they are based on Phoenix's landing site\date and the direct\diffuse Insolation model published by Applebaum and Landis that you can find online if you search for Mars Solar Power.

QUOTE (exobioquest @ Nov 27 2005, 09:38 PM)
By the way which one do you prefer the MMRTG or the SRG?
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I didn't think the SRG had been proven in any significant way yet. The MMRTG is an evolution of the existing RTG's, proven and known to be reliable. Much as I like the SRG idea I'd hate to see MSL used as it's first flight test.
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Guest_exobioquest_*
post Nov 27 2005, 10:18 PM
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From what I have read SRG has survived several years of ground testing without any appreciable wear, of course thatís what they claim and is only from ground tests, but still the performance advantages of the SRG over the MMRTG seem good enough to warrant its first use on MSL, the fact it only needs 1/4 as much fuel is a big one, considering the price per gram of Pu238 and the production difficulties of lately. Also it weighs less and is smaller, and vibration should not be a problem for a rover.
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mcaplinger
post Nov 28 2005, 03:59 PM
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QUOTE (exobioquest @ Nov 27 2005, 08:46 AM)
Will it be 2 Boeing's MMRTG (at ~100 watts?) or Lockheed Martinís SRG (again ~100watts?), have they decided yet?
*


According to
http://trs-new.jpl.nasa.gov/dspace/bitstre...6/1/05-0708.pdf
MSL will use an MMRTG. I would think that the use of batteries is inevitable, as there's no way the avionics can run directly from RTG output, much less the payload.

I'm sure they've done a detailed trade study of alternative RTGs, but I haven't seen those.


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Guest_exobioquest_*
post Nov 28 2005, 04:57 PM
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QUOTE (mcaplinger @ Nov 28 2005, 09:59 AM)
According to
http://trs-new.jpl.nasa.gov/dspace/bitstre...6/1/05-0708.pdf
MSL will use an MMRTG.  I would think that the use of batteries is inevitable, as there's no way the avionics can run directly from RTG output, much less the payload.

I'm sure they've done a detailed trade study of alternative RTGs, but I haven't seen those.
*


Well thats sad, hope they change their mind. By the way who put those notes on the pdf? I did not consider this but because the MMRTG puts out 2000w of heat they need a rather large radiator for the cruise stage, the SRG only puts out 500w.
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mcaplinger
post Nov 28 2005, 05:30 PM
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QUOTE (exobioquest @ Nov 28 2005, 08:57 AM)
Well thats sad, hope they change their mind.
*


The report I referenced is probably just a baseline; I don't know if the final RTG configuration has been picked. However, I'm not sure that the Stirling RTG is a clear-cut winner. It's more efficient and lighter for a given power output, true, but it's far more mechanically complex and has its own set of problems (vibration, electrical noise, etc.)


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Guest_BruceMoomaw_*
post Nov 29 2005, 01:18 AM
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Apparently the Stirling version simply won't be ready by then.
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Guest_exobioquest_*
post Nov 29 2005, 03:35 AM
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QUOTE (BruceMoomaw @ Nov 28 2005, 07:18 PM)
Apparently the Stirling version simply won't be ready by then.
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Is that inside info or can you reference it?
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mcaplinger
post Nov 29 2005, 05:22 AM
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QUOTE (mcaplinger @ Nov 28 2005, 09:30 AM)
However, I'm not sure that the Stirling RTG is a clear-cut winner.  It's more efficient and lighter for a given power output, true...
*


Actually, doing a little more research, the SRTG is even less obviously a win. It saves Pu mass, but the additional mass of the more complex conversion hardware partially kills that advantage. Depending on exactly what mass the MMRTG finally comes in at, it can have a higher energy density than the SRTG. Getting rid of the additional waste heat, while an issue, is a fairly easy problem to solve.

Quoted from "Advances in Planetary Aerobots" by Erik Laan et al:

"The MMRTG will be designed to generate
110 Watts of electric energy over a minimum lifetime
of 14 years (3 years on the Martian Surface)
weighing 24-34 kg (218.8 - 309.1 kg/kW) including 4
kg Plutonium-238 (Boeing)
Another development is aimed at increasing the
efficiency of the conversion process. Current RTGs
are capable of converting heat into electricity with an
efficiency of ~8%. A number of new conversion
processes were proposed recently. The Stirling
Radioisotope Power Source is one of them. This
SRPS is using a Stirling heat engine, which produces
an acoustic pressure wave to drive a piston in a linear
alternator producing the electricity. The SRG
(Stirling RTG) delivers 93-114 Watt of electric
energy, weighing 27 kg (236.8 - 290.3 kg/kW) and
contains 1 kg of Plutonium-238 (LMM)."


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