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I notice again that you give no citation for your claims about surface insolation during severe dust storms on Mars.
This link has a dust opacity map (at the end of the article) which states: " The scale bar runs from a nearly clear opacity value of 0.05 to 0.40, which represents an approximately two-thirds reduction in the amount of sunlight reaching the ground." A 66% reduction, so 33% of the norm, not 0%, 5% or 10%.
http://www.planetary.org/blogs/emily-la … /1050.html
I think confusion arises because of statements like "only 1% of the insolation at the top of the atmosphere reachers the surface". This means directly, but the photons are bouncing around and a large proportion do reach the surface even in the severest dust storms. People are assuming because you can't make out details, there's little light around, but that's not the case. There's a lot of light around and a lot of dust. So, difficult to see things, but energy is getting to the surface.
This link (see Fig 2) also shows that insolation at the surface never goes down to 0% or anything near 10% of the norm.
https://link.springer.com/article/10.10 … 017-0360-x
Just trying to do my bit to calm this dust storm hysteria!
Louis:
Quoting your post in part: "I'm not a dust storm denier.
Opportunity died of old age not dust storms. It lived 55 times over its designed life time - surviving for over 14 years (living through about 6 major dust storms) without direct human intervention. That's the definition of the triumph of PV power.
Closer to the 5% to 10%? Where's a citation to back your claim? This academic paper gives a figure of 37% for a severe dust storm:"
Louis, that posting proves in fact that you ARE a severe dust storm denier! I need no citation beyond the publicly-released images when Mariner 9 reached Mars in 1969 to prove my point.
The Tharsis Mons volcanoes, including Olympus Mons, were invisible for nearly half a year. The general plain was not visible for some 9 months. The relief from the plain to the tip of Olympus Mons is about 50,000 feet (some 15 km). The depth of dust-filled air was clearly deeper than that.
It almost doesn't matter what the actual dust or air density is with dust-filled depths like that. So says the physics of particles suspended in gases relative to visible light transmission. The transmissibility of sunlight through dust-filled air over distances that long is essentially zero. Not 37%. Not 35%. 0%!!! Period!!! It's an exponential relation you use to figure it, with that distance in the exponent, for crying out loud!
THAT Mariner 9 data is a "worst case dust storm" on Mars! We've already seen it. Multiple times. Which makes your citation inapplicable to the worst case that we already know WILL occur! Roughly once a decade, but irregularly, with lesser storms more like your citation in between.
Sure, Opportunity lasted way far longer than its intended life. But that says ABSOLUTELY NOTHING about what sun-obscuring dust storms are like, except for the really bad (months long) one that killed it. It was still working before the storm, and never worked again after the storm. QED: the dust storm killed it.
I have never said that retropropulsive landing doesn't work. In point of fact, I championed that concept before you, or anyone else on these forums, advocated for it.
I have NOT championed multiple passes for aerodynamic braking at Mars, because Mars's upper atmosphere densities vary by factors of 2+, and that's way too much variability to cope with. Spacex does NOT propose that for "Starship" anyway. What they propose is to "aphelion-out" on the transfer trajectory and let Mars run over "Starship" from behind, for a (single-pass) direct entry. No different than any lander probe all the way back to Viking.
That sets your speed at entry interface to something near 7.5 km/s, when Mars escape speed is 5 km/s. You only have the actual encounter geometry as your one and only significant degree of freedom. You must control that geometry to a grazing entry between 1 and 2 degrees below local horizontal. Too steep, and it's a race whether you crush or incinerate first. Too shallow, and you bounce off into deep space forever. Tricky, that!
And, I HAVE NEVER SAID NOT TO USE SOLAR ON MARS! I ONLY said that solar should NOT be the ONLY power source you take! The other practical source in that cold, near-vacuum environment is NUCLEAR! No way around that.
GW
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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Louis,
If you cover a solar panel with sand, does it still produce power?
If you figure out how to solve that one, then there are several countries in the Middle East who'd be willing to pay big bucks.
The advantages of solar are their simplicity, redundancy, and flight heritage that have been proven with many successful missions. The challenges for solar missions to Mars have remained numerous regarding dust accumulation on solar panels, limited solar insolation from dust storms, and available sunlight at northern and southern latitudes. These very reasons supported decisions to move away from solar powered rovers such as Spirit and Opportunity and replace them with a nuclear powered Multi-Mission Radioisotope Thermoelectric Generators (MMRTG) as seen on the Mars Science Laboratory.
...
The independent studies cited herein have pointed out some of the advantages of nuclear surface power and how the Kilopower reactor can reduce several risks associated with the Martian environment that has been relentless to the solar powered missions. The study concluded that both the ISRU and crew phases of the early Mars missions were easily achieved with several 10kWe Kilopower reactors. The Kilopower based system won the mass and power trades for the crewed missions by a factor of two even at solar favorable sites, which provides additional support for nuclear systems when moving further from the equator.
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Louis:
You keep saying I cite nothing. Not true, I cited the photography from the Mariner 9 orbiter in 1969. That was an orbiter, not a lander, so there are no surface data from that mission. The dusty layer depth exceeded 15 km worth of volcanic mountain relief in Tharsis Mons, as observed from space. That's a lot of scattering that prevents photons from reaching the surface. Go look up transmissibilty-vs-scattering in any physics book. The relation is an exponential with particle size, particle volumetric density, and scattering path length in the exponent.
In your second citation, the Figure 2 data you reference actually say quite the opposite of what you claim. For "opacity" values in the 4 to 6 range, insolation at the surface falls by factors of 4 to 6. Factor 5 drop is to 20%, etc. The dust storms in the surface data since Viking show as opacity peaks in the 4-6 range, but no one has seen Olympus Mons obscured the way it was during Mariner 9 in 1969.
What that means is that we have already seen a dust storm more intense than anything seen in the surface observations since Viking in 1976. Per your Figure 2 in your cited report, the worst-case reduction I read from the scale was about 10% of what it was at "normal" conditions prior to the opacity peak. Which INEVITABLY implies the reduction in insolation during the 1969 event would have to fall in the 0-10% range of "normal" insolation. Just like I said.
You have to design for worst case, or else you will kill people. It's too hostile an environment not to do that.
There are dust storms and then there are occasional giant dust storms on Mars. They are not all the same. The ordinary ones do not cripple solar power too much, as we have already seen. But the occasional giant dust storms DO cripple solar power to one extent or another (that is what we have been arguing about).
If you size your panels to do their job at 35% of normal insolation, and insolation drops to 20% of normal or less (as it sometimes does per the Figure 2 in your citation), then you are crippled during those occasional events. You must have a backup, or else people die in the cold like Opportunity did.
What the rest of us have been saying is take a second source of power. You finally came around to that opinion, but you have been touting burning up some of your return propellant for that purpose. The rest of us see that as an unnecessary risk when there is nuclear, which does NOT care whether it is light or dark, dusty or clear; and it is not too dangerous to use.
Here on Earth there is a day-night cycle of electric power usage. Usage is lower (but not zero!) at night when folks are asleep and most of their equipment is off. Power usage peaks during the day. It is likely to be similar on Mars, with its similar day-night 24-ish hour cycle, except the numbers will be larger because of the extreme cold and the need for life support in that near-vacuum of an atmosphere.
The worst case is mere survival in the near-darkness of the maximal worst case dust storm (the 1969 event, worse than any since). That is the min sizing of your nuclear backup. At that min, you make no propellant, and you drive no rovers, until the dust thins, and your solar starts adding a little day power to the mix once again.
You are quite crippled in what your people can do during the height of the storm at min nuke backup size. How much more than the min nuke backup is the tradeoff that lets your people accomplish anything useful during the storm, such as still making at least a trickle of return propellant.
That's what engineers like myself have to think about. Kbd512 and Oldfart1939 have been saying the same things I have been saying about this. They are trained to look at things this way, just like me.
GW
GW Johnson
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So the only way to have solar is to up the panel count so as to make the system for the lowest possible with more batteries to handle the overcharging if we get more energy from them as the levels will not be equalized for the duration of the making of fuels for mars return home. Going with the estimate of expectation per day that is nearer to the max without solving for how often the dusts will drop the levels recieved means that we will not have the power for all uses.
So as to make fuel at a given average rate you will need to make sure we never drop below that level amount of energy.
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A claim without a link to a paper or article is an assertion not a citation.
Do you have a link to a science paper that backs your claims? Just because you can't see Olympus Mons doesn't mean you can't operate solar.
"However, there is still appreciable diffuse sunlight available even at high opacity, so that solar array operation is still possible."
https://www.sciencedirect.com/science/a … 6593901056
Note that: "even at high opacity".
I also read that the 2001 dust storm was comparable to the 1971 (not 1969) Mariner 9 dust storm, by the way. The 1971 photos were at 100 metres a pixel, so that might have been affecting the imaging. Personally I would exclude Mariner 9 from consideration because there simply isn't enough data there whereas the comparable 2001 storm is much better recorded.
This paper describes how dust accumulation can reduce the available insolation being captured by a PV Panel by 25-32% per cent in the summer season.
https://www.nature.com/articles/s41598- … 6-8#MOESM1
Obviously a human mission to Mars can do something about that through artificial dust reduction - not just relying on the passing of the seasons.
This paper states:
"Of major concern are dust storms, which have been observed to occur on local as well as on global scales, and their effect on solar array output. While atmospheric opacity may rise to values ranging from three to nine, depending on storm severity, there is still an appreciable large diffuse illumination, even at high opacities so that photovoltaic operation is still possible."
https://ieeexplore.ieee.org/document/111816
Note the "large" diffuse illumination.
I really don't think you should use an emotive word like "cripple" to describe the effect of a dust storm, even the most severe, on a PV system. No one's going to die of cold. I'd expect there to be something like 5000 KwHes of battery storage at the base when you take into account the 6 Starships' battery systems as well. That's 20 sols at a generous 10 Kwe constant. And then you have your meth-ox supply to fall back to power methane electricity generators. Beyond the initial arrival, that will provide enormous storage capacity. But even on arrival, on my proposal, you will have brought enough to power the base for 100 sols. Remember also that each of the Starships is bound to have some residual fuel on board - that could easily amount to a few more tons of storage. But of course, as my citations show, your PV system will still be producing energy even in the worst case dust storm scenario.
I reject the notion that I am proposing "buring up some of your return propellant" for survival purposes. I am saying design your propellant plant so it can produce an additional x amount to power electricity generation in dust storms and even possibly at night time on a regular basis (as that may well be more efficient than chemical battery storage overall). That x amount might be no more than 2% (20 tons of fuel-propellant, say). If so, a 5,000 kg propellant plant might become a 5,100 Kg plant...big deal!!! You will need more water and CO2 extraction for sure but again in terms of mass requirement it will probably be fairly trivial.
Meanwhile, don't forget, a supporter of nuclear power is arguing for transporting to Mars and deploying perhaps 100 x 10Kw radioactive Kilopower units. That's your logistical problem and good luck with it because I wouldn't want that!
I'll take a look at Fig 2 again - not sure our interpretations agree...
I've never been averse to taking along a few emergency RTGs - just in case.Pack them in the suitcase. Why not?
But for me failsafeness is like one of those Russian dolls. For me it goes solar, meth-ox, chemical batteries, and RTGs. RTGs would only ever be deployed in some nightmare emergency scenario that we can't even envisage - just to give the pioneers a chance. I would also keep some PV panelling inside Starships, undeployed to help mitigate the effects of a meteorite strike, should that (extremely unlikely) occur. It's the same idea as having those oxygen candles on board. You have to give your people a chance.
Louis:
You keep saying I cite nothing. Not true, I cited the photography from the Mariner 9 orbiter in 1969. That was an orbiter, not a lander, so there are no surface data from that mission. The dusty layer depth exceeded 15 km worth of volcanic mountain relief in Tharsis Mons, as observed from space. That's a lot of scattering that prevents photons from reaching the surface. Go look up transmissibilty-vs-scattering in any physics book. The relation is an exponential with particle size, particle volumetric density, and scattering path length in the exponent.
In your second citation, the Figure 2 data you reference actually say quite the opposite of what you claim. For "opacity" values in the 4 to 6 range, insolation at the surface falls by factors of 4 to 6. Factor 5 drop is to 20%, etc. The dust storms in the surface data since Viking show as opacity peaks in the 4-6 range, but no one has seen Olympus Mons obscured the way it was during Mariner 9 in 1969.
What that means is that we have already seen a dust storm more intense than anything seen in the surface observations since Viking in 1976. Per your Figure 2 in your cited report, the worst-case reduction I read from the scale was about 10% of what it was at "normal" conditions prior to the opacity peak. Which INEVITABLY implies the reduction in insolation during the 1969 event would have to fall in the 0-10% range of "normal" insolation. Just like I said.
You have to design for worst case, or else you will kill people. It's too hostile an environment not to do that.
There are dust storms and then there are occasional giant dust storms on Mars. They are not all the same. The ordinary ones do not cripple solar power too much, as we have already seen. But the occasional giant dust storms DO cripple solar power to one extent or another (that is what we have been arguing about).
If you size your panels to do their job at 35% of normal insolation, and insolation drops to 20% of normal or less (as it sometimes does per the Figure 2 in your citation), then you are crippled during those occasional events. You must have a backup, or else people die in the cold like Opportunity did.
What the rest of us have been saying is take a second source of power. You finally came around to that opinion, but you have been touting burning up some of your return propellant for that purpose. The rest of us see that as an unnecessary risk when there is nuclear, which does NOT care whether it is light or dark, dusty or clear; and it is not too dangerous to use.
Here on Earth there is a day-night cycle of electric power usage. Usage is lower (but not zero!) at night when folks are asleep and most of their equipment is off. Power usage peaks during the day. It is likely to be similar on Mars, with its similar day-night 24-ish hour cycle, except the numbers will be larger because of the extreme cold and the need for life support in that near-vacuum of an atmosphere.
The worst case is mere survival in the near-darkness of the maximal worst case dust storm (the 1969 event, worse than any since). That is the min sizing of your nuclear backup. At that min, you make no propellant, and you drive no rovers, until the dust thins, and your solar starts adding a little day power to the mix once again.
You are quite crippled in what your people can do during the height of the storm at min nuke backup size. How much more than the min nuke backup is the tradeoff that lets your people accomplish anything useful during the storm, such as still making at least a trickle of return propellant.
That's what engineers like myself have to think about. Kbd512 and Oldfart1939 have been saying the same things I have been saying about this. They are trained to look at things this way, just like me.
GW
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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My proposal would essentially involve: 1. Excess PV (so whatever the seasonal norms expected in the absence of dust storms, the sytem will add on a percentage to compensate for averaged dust storm reduction in insolation at the surface during the mission period...so maybe something like 20%...someone will need to do the calculation). 2. Meth-ox storage/electricity generation. This has two aspects: taking a store with you from Earth that is available on arrival on Mars and a slight expansion of the propellant plant production facility to replenish that store if it has to be used (and also possibly to provide for night time energy demands) 3. Chemical battery storage. The six Starships will have extensive battery storage which can be tapped. In addition there will be a dedicated battery storage facility (or series of facilities) for the base.
Essentially 2 and 3 will allow you to deal with extreme dust storm conditions of the type referenced by GW, like the 71 storm.
Using these three elements it will be possible to ensure that energy input for the base (life support and transport etc) is secure and also that the energy input into the propellant plant does not fluctuate wildly, although it may not be an absolutely steady rate of supply as you can achieve with nuclear.
So the only way to have solar is to up the panel count so as to make the system for the lowest possible with more batteries to handle the overcharging if we get more energy from them as the levels will not be equalized for the duration of the making of fuels for mars return home. Going with the estimate of expectation per day that is nearer to the max without solving for how often the dusts will drop the levels recieved means that we will not have the power for all uses.
So as to make fuel at a given average rate you will need to make sure we never drop below that level amount of energy.
Last edited by louis (2019-10-06 17:33:49)
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MK1 prototype to get
https://www.teslarati.com/spacex-starsh … s-spotted/
https://www.nextbigfuture.com/2019/09/s … entry.htmlSame batteries that burst into flame and total your car to power starship is not such a good idea...
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Louis,
The logistical problems associated with transporting 100 Kilopower reactors pales in comparison to the problem of setting up thousands of solar arrays over an area that spans multiple football fields in size. The reactors are NOT appreciably radioactive until AFTER they've been turned on. In contrast, a Pu238 fueled RTG is many thousands of times more radioactive than a fission reactor that has never been turned on.
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https://ntrs.nasa.gov/archive/nasa/casi … 017750.pdf
Development of NASA’s Small Fission Power System for Science and Human Exploration
https://ntrs.nasa.gov/archive/nasa/casi … 005435.pdf
The Kilopower Reactor Using Stirling TechnologY (KRUSTY) Nuclear Ground Test Results and Lessons Learned
https://ntrs.nasa.gov/archive/nasa/casi … 012354.pdf
Nuclear Systems Kilopower Overview
https://ntrs.nasa.gov/archive/nasa/casi … 021391.pdf
Space Reactor Design Overview
https://sites.nationalacademies.org/cs/ … 059559.pdf
Small Fission Power System Feasibility Study
Reading real quickly the amount of radiation exposure when they are off is less than a chest xray....
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So what are the logistical problems associated with transporting 100 Kilopower units? Have you actually thought them through? I am a bit sceptical.
Solar arrays can be made part of dedicated units like ATK fan systems or zig-zagged panels or rolls of flexible PV. Those are all much easier to handle than what is a pretty delicate piece of equipment that may have to be offloaded and transported and even buried under regolith... x 100.
Louis,
The logistical problems associated with transporting 100 Kilopower reactors pales in comparison to the problem of setting up thousands of solar arrays over an area that spans multiple football fields in size. The reactors are NOT appreciably radioactive until AFTER they've been turned on. In contrast, a Pu238 fueled RTG is many thousands of times more radioactive than a fission reactor that has never been turned on.
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Louis,
Yes, I've thought about how virtually everything you've proposed, I've proposed, and everyone else here has proposed doing while I've been a member here might actually work, if it could actually work. Periodically, I re-run numbers using updated technology. I do this because I'm interested in knowing if any of these concepts, irrespective of who comes up with it or who actually implements it, can actually work passably well from a basic mathematics perspective.
I liked the idea of taking liquid fuels with us until I did some math on how much would actually be required and the fact that none of the stuff that people are proposing taking with them is even remotely storable without active refrigeration and attendant power requirements ranging into the 10's of kilowatts to keep those cryogens cold.
I liked large fission reactors until I considered the cost and complexity of operating them. They don't load-follow, so they require active control.
I liked thin film solar until I understood how dead soft the substrate material is.
I liked solar power satellites a whole lot better after some recent advancements were made in high power gyrotrons that were developed for nuclear fusion research.
I keep hearing that better batteries are just around the corner, in another 2 to 5 years, and just like fusion power, they're always 2 to 5 years away- much like driver-less cars and pilot-less aircraft carrying passengers.
I've settled on these small, stupidly simple, and horribly inefficient fission reactors as the most viable power option, mostly because serious money is being devoted to this project and serious effort to make something affordable and usable for multiple different types of missions is being considered, rather than some revolutionary new technology that will ultimately squander NASA's R&D money because it's only good for a single use case and nothing else. I'm not fixated on a Mars colony, which is just one small rock in a much larger solar system and infinitely larger universe. Unlike larger reactors or reactors with greater thermal power output to surface area, they load-follow and can't melt down without outside help. That's why there aren't any electronics included. They don't need any.
All that said, your skepticism is duly noted.
Now, do you have any technical arguments to make, maybe something based in math or engineering that could be supported or refuted through research and review?
ATK solar arrays are unprotected paper thin semi-conductors on flexible backings. They can be broken with two fingers. Thin film arrays are thinner than paper. As long as you don't abrade the thin films, they're better than semiconductor wafers for durability, but the dust on Mars and the moon may as well be loose sandpaper.
Kilopower is made from blocks of metals and the reactor core is designed to withstand reentry intact. There's nothing delicate about it. If a fission reactor was delicate in any way, it'd never survive operation. We've had fission reactors in operation longer than photovoltaic panels have existed.
I can guarantee that digging 100 post holes with a drill, even through concrete, will be faster than erecting something that comes in tiny pieces, but ultimately covers multiple football fields. A decent drill could bore the holes in a matter of minutes. You couldn't possibly get all those panels to where they need to be erected at, much less actually erect and connect them, in less time than it takes to dig that many holes.
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Somehow, transporting 100 Kilopower reactors and getting them set up beats trying to VERY CAREFULLY lay out 23 acres of thin film PV panels, which is what a 90,000 meters^2 grid covers. Most people have no concept of how large an area that encompasses. I sure as H*ll am not going out every morning and brush sand or dust off this thin film solar farm.
It's always easy to propose such ideas until one attempts implementing them; they are simply dreams.
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Point 1 -- ignoring the Mariner-9 dust storm just because it made no surface observations is stupid. It was an event. It was observed. It was the very worst ever seen. That is documented history, discounting it because I do not cite some idiotic report is also stupid.
Point 2 -- My knowledge of the detailed characteristics of solar photovoltaic cells is obsolete, I have not kept up with that. But when I took a course in solar during engineering school, the sense was solar thermal could make at least some use of diffuse radiation, solar PV could not. PV required direct radiation to do any good. Something about the energy of the photons triggering whatever reaction there is in the PV material.
Point 3 -- Kbd512 and Oldfart1939 are quite right in pointing out that it is far easier to set up 10+ Kilopower units for 100+ KWe power than it is to set up and maintain-clean multiple acres of solar panels. Laying thin-film solar on the dusty sand is NOT a viable option.
Point 4 -- any skepticism about Kilopower should take the form of wondering whether NASA will complete the testing, scale-up, and demonstration of this source in a timely fashion. Their track record with SLS does NOT inspire confidence.
-- corollary to point 4 -- has anyone considered the possibility that NASA may deliberately fail to bring the supporting Kilopower technology on-line in a timely fashion precisely so as to keep Spacex from beating them to Mars with men by a decade? We've seen bad face-saving behavior, and favoritism among contractors, out of government agencies, and especially politicians, before.
GW
Last edited by GW Johnson (2019-10-07 07:39:39)
GW Johnson
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"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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Well I note that in response to my query about the logistical challenge of deploying 100 x 10 Kw Kilopower Units you raise a lot of other issues not related to that.
As for their robustness, not much is robust when accidentally dropped from 100 feet, unless packaged in depth.
To reiterate some of the issues:
- If the KP units have to be off loaded, that is a huge undertaking. How long does it take to maybe winch down 100 separate units? 10 sols? 25 sols? Remember - there's plenty of other stuff needs doing as well.
- If the KP units have to be unpacked from complicated packaging, that could be a huge undertaking, given there are 100 and maybe only 6 crew. How long will it take to unpack one unit? 2 hours? 4 hours? If 4 hours that's 400 hours of unpacking taking up maybe 50 sols of work. And where does the unpacking take place? If the KP units are going outside, presumably the packaging stays on until you are at its final resting place...but then you are unpacking out in the open...and that sounds like a job for humans in EVA suits...the last thing you need I would say.
- If the KP units have to be transported away from the Starships, how long with that take - 200 there and back journeys? 10, 20 sols?
- If the KP units have to be buried in regolith, doing that 100 times is a huge undertaking. That could be another 25 sols of work.
- The more units, the more cabling. This could be an issue if your KP units have to be sited say 1 Km away from the base. 100 times x cable length is a lot of cable. Yes. I know similar issues apply to the solar array but a lot of the cabling can be integrated into the arrays.
- If the KP units stay on the Starship, that creates a huge risk. You have concentrated your energy system in one location. In the event of fire, explosion, subsidence or some other calamity, you have no energy system.
I don't think there will be any major issues on Mars with deploying a 30,000, 50,000 or even 70,000 sq metre array.
Firstly, ATK style PV fan arrays would be deployed on the surface to provide power for rovers and the hab. These could be deployed within hours. Then I think it would be a question of using robot rovers to lay down flexible PV arrays. This won't be complicated:
Take a look at this:
https://www.bbc.co.uk/news/uk-wales-sou … s-41443312
Quote from the article: "The Rapid Roll system allows flexible solar panels to be unrolled like a carpet from a trailer in two minutes."
Quick calculation: the system generates 11 Kw so maybe 40 Kwhs per day at a guess. Call that 20 Kwhs for Mars. So if you need 24.5 MwH on Mars, that means you need 1225 rolls. At 2 mins each and let's say add on another 2 minutes for taking on the rolls.
So 4 mins x 1225 rolls would equal 81 hours of work - maybe 10 sols' work at a minimum. Even if the rovers were only operating 4 hours a sol, that would be 20 sols' work. Certainly should be completed in under a month and of course it will be incremental, so after 15 sols you'd be getting half your power already.
Of course this is just a simple illustration. In reality, there are probably quicker ways of unrolling the arrays with dedicated machines.
The Starships will be contributing power as well.
This article indicates flexible solar can be as light as 70 ounces per sq. feet = 0.31 Kg. I am happy to accept that we might double that for Mars for various reasons to 0.6kg. For a 50,000 sq.metre system that would be 30 tons - much, much lower than the minium 150 tons for the Kilopower units.
https://www.solarpowerworldonline.com/2 … r-modules/
Louis,
Yes, I've thought about how virtually everything you've proposed, I've proposed, and everyone else here has proposed doing while I've been a member here might actually work, if it could actually work. Periodically, I re-run numbers using updated technology. I do this because I'm interested in knowing if any of these concepts, irrespective of who comes up with it or who actually implements it, can actually work passably well from a basic mathematics perspective.
I liked the idea of taking liquid fuels with us until I did some math on how much would actually be required and the fact that none of the stuff that people are proposing taking with them is even remotely storable without active refrigeration and attendant power requirements ranging into the 10's of kilowatts to keep those cryogens cold.
I liked large fission reactors until I considered the cost and complexity of operating them. They don't load-follow, so they require active control.
I liked thin film solar until I understood how dead soft the substrate material is.
I liked solar power satellites a whole lot better after some recent advancements were made in high power gyrotrons that were developed for nuclear fusion research.
I keep hearing that better batteries are just around the corner, in another 2 to 5 years, and just like fusion power, they're always 2 to 5 years away- much like driver-less cars and pilot-less aircraft carrying passengers.
I've settled on these small, stupidly simple, and horribly inefficient fission reactors as the most viable power option, mostly because serious money is being devoted to this project and serious effort to make something affordable and usable for multiple different types of missions is being considered, rather than some revolutionary new technology that will ultimately squander NASA's R&D money because it's only good for a single use case and nothing else. I'm not fixated on a Mars colony, which is just one small rock in a much larger solar system and infinitely larger universe. Unlike larger reactors or reactors with greater thermal power output to surface area, they load-follow and can't melt down without outside help. That's why there aren't any electronics included. They don't need any.
All that said, your skepticism is duly noted.
Now, do you have any technical arguments to make, maybe something based in math or engineering that could be supported or refuted through research and review?
ATK solar arrays are unprotected paper thin semi-conductors on flexible backings. They can be broken with two fingers. Thin film arrays are thinner than paper. As long as you don't abrade the thin films, they're better than semiconductor wafers for durability, but the dust on Mars and the moon may as well be loose sandpaper.
Kilopower is made from blocks of metals and the reactor core is designed to withstand reentry intact. There's nothing delicate about it. If a fission reactor was delicate in any way, it'd never survive operation. We've had fission reactors in operation longer than photovoltaic panels have existed.
I can guarantee that digging 100 post holes with a drill, even through concrete, will be faster than erecting something that comes in tiny pieces, but ultimately covers multiple football fields. A decent drill could bore the holes in a matter of minutes. You couldn't possibly get all those panels to where they need to be erected at, much less actually erect and connect them, in less time than it takes to dig that many holes.
Last edited by louis (2019-10-07 07:55:32)
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Starships will NOT be contributing power; they will be CONSUMING power.
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Eh? They each (each of six) will have 400 Kwhes of battery storage for fin actuation. Once on the surface they don't need fin actuation and apart from one, they are not returning to Earth. They may be consuming some power for temperature control and so on. But some of the Starships will be completely emptied of cargo and propellant-fuel...so what power will they be "consuming"?
Starships will NOT be contributing power; they will be CONSUMING power.
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The batteries for the fins will have been drained of all power as they will be below the 70% safe discharge point for the type of battery being used. The reason they are drained is that we can not charge them with solar panels while making any of the entry to mars aero breaking or aerocapture which can last days to months depending on the path we take. A direct aerobraking means we are coming in hot and will expend all store charges on the cargo ships to effectively land.
The current incantation of starship does not have any solar wings to recharge the batteries from and while they might work with a methane gas generator for a short span we still will be taking a long period of time setting up the solar array on the ground to recharge them before they freeze. Once they freeze we are a goner. The fact that the electronics will also be damaged from the extreme mars cold since the batteries can not recover if that happens.
Yes a methane generator for the entry path will work using the boiloffs from traveling to act as a recharging system of which this is what boeing and altlas make use of to keep the ship operational after the stages are in flight Kbd512 has talked about this in other topics. But the use rate is the issue for burning fuels that you migh need for landing so that still is a problem for the inflight charging of batteries if the Starship does not have any panels for the journey to mars for either cargo or crewed.
They 400kwhr is just a number that can be used for a rate of useage since voltage of a battery will go down until we reach that 70% point and they are not designed to up its current output to compensate for the whrs as that generates internal heat that would kill the cells of the battery. That is why batteries have an ampere hour rating.
Now onto the kilowatt reactor use they will work just as good in the ship as they would on the ground for the cargo vehicles all that is needed is to give a radiator for the excess heat which could be built into the ship. We do not need all of them in the ship but it gets you power at the flip of a switch to start them. Once the ship is on the ground it still will need some power internally to keep everything working and not freezing. This level might be aas low as the 1kwhr unit for our needs which means we can get into the ship with minimal shielding due to its low energy output and radiation as well.
Remember that the reactors are not heavy like a three mile island plant its rather small so they will move easily and since these have not been activated prior to off loading them from the cargo area then if you drop them we will not have a huge radioactive element to contend with at that time as its the equivalent of a chest xray.
As far as units being close to humans burying them in a hole and covering them will take care of the radiation levels so the cables will not need to be long.
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SpaceNut - I was responding to OF's claim that the Starships would be consuming power which in context seemed to imply they would be net consumers of energy rather than net producers. I wasn't making any claims for how much electricity would be left in the batteries on arrival but I do claim that they will recharge the batteries.
I don't believe they will be net consumers because they will have pretty large solar panels that can be extended out from the body of the rocket.
You state: "The current incantation of starship does not have any solar wings to recharge the batteries from"
Well that then begs the question of how it will get to Mars because it will need power for fin actuation, for maintaining correct internal temperature, for communications and (on the human passenger model) for life support.
So either Musk has undergone a conversion to nuclear power or - much more likely - they aren't required on the Mk 1 and 2 version.
The batteries for the fins will have been drained of all power as they will be below the 70% safe discharge point for the type of battery being used. The reason they are drained is that we can not charge them with solar panels while making any of the entry to mars aero breaking or aerocapture which can last days to months depending on the path we take. A direct aerobraking means we are coming in hot and will expend all store charges on the cargo ships to effectively land.
The current incantation of starship does not have any solar wings to recharge the batteries from and while they might work with a methane gas generator for a short span we still will be taking a long period of time setting up the solar array on the ground to recharge them before they freeze. Once they freeze we are a goner. The fact that the electronics will also be damaged from the extreme mars cold since the batteries can not recover if that happens.
Yes a methane generator for the entry path will work using the boiloffs from traveling to act as a recharging system of which this is what boeing and altlas make use of to keep the ship operational after the stages are in flight Kbd512 has talked about this in other topics. But the use rate is the issue for burning fuels that you migh need for landing so that still is a problem for the inflight charging of batteries if the Starship does not have any panels for the journey to mars for either cargo or crewed.
They 400kwhr is just a number that can be used for a rate of useage since voltage of a battery will go down until we reach that 70% point and they are not designed to up its current output to compensate for the whrs as that generates internal heat that would kill the cells of the battery. That is why batteries have an ampere hour rating.
Now onto the kilowatt reactor use they will work just as good in the ship as they would on the ground for the cargo vehicles all that is needed is to give a radiator for the excess heat which could be built into the ship. We do not need all of them in the ship but it gets you power at the flip of a switch to start them. Once the ship is on the ground it still will need some power internally to keep everything working and not freezing. This level might be aas low as the 1kwhr unit for our needs which means we can get into the ship with minimal shielding due to its low energy output and radiation as well.
Remember that the reactors are not heavy like a three mile island plant its rather small so they will move easily and since these have not been activated prior to off loading them from the cargo area then if you drop them we will not have a huge radioactive element to contend with at that time as its the equivalent of a chest xray.
As far as units being close to humans burying them in a hole and covering them will take care of the radiation levels so the cables will not need to be long.
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Let's assume a cargo "Starship" can deliver 100 metric tons of cargo to Mars. I have chain hoists in my farm shop that can handle 2 tons, and any crane system built into "Starship" should be able to handle a lot more than that. Call it 5 tons for a realistic guessed number.
To offload 100 tons 5 tons at a time requires the crane's freight elevator platform to make 20 round trips. As a guess, assume 200 feet one way. Most of the commercially available power hoist equipment operates at around 1 foot/second. That's 200 seconds down, and 200 seconds back up, for each load. Add another 200 seconds as a guess to roll the load off the platform onto the ground. And another 200 seconds to roll the cargo onto the hoist platform, up at the cargo hatch. That's 800 seconds (less than 15 minutes) per trip.
15 minutes per trip times 20 trips is 300 minutes. 5 hours. To unload 100 tons! You could certainly do it in a single working day. Hardly a huge endeavor. I unloaded and stored 6 tons of steel in my shop, several hundred pounds to about half a ton at a time, off the big trailer, with chain hoists on trolleys running on overhead beams, in about half a day. I did it single-handed, meaning totally alone. And my hoists are manually chain operated, not power hoists at all. Plus, I'm an old man: 69 years old.
A kilopower unit weighs less than a ton, as near as I can tell. Those things can just be a part of that 100 tons you offload. The electric bulldozer you need is heavier, by far. And it can transport the not-yet-activated kilopower unit to the intended site quite safely.
As I said, you set up your kilopower unit and connect it up to the cabling, then you bulldoze a berm about it. You do not need to fully bury it, until its end-of-life decommissioning. Even so, you just bulldoze the berm over it when that time comes.
How many do you need? Well, that depends upon how many people you send how quickly, and how fast you make return propellant. At 10 KWe each, you need but 10 (under 10 tons) to have 100 KWe available. 100 of them (under 100 tons) gives you 1 MWe. What could you do with a MWe available?
As for "Starship" itself, it had better have some source of power. Not a storage device, a source. Batteries are storage devices, not sources.
When humans first get there, there is no place for them to go. They will be living inside those "Starships" on the surface, until actual habitation spaces can be erected. That living requires a lot of power, same as is required during the transit of the manned "Starship". You cannot do that transit job on batteries either (it's a 6 to 9 month transit).
These "Starships" had better have solar power, and even then, you are betting the solar works until you get the nukes set up. Because of the risk of one of those so-inconvenient pesky major dust storms.
Just trying to put some realistic perspective on this.
It's no big deal to unload 100 tons out of a landed "Starship", with stuff you could buy at Harbor Freight, if you don't mind the hardware weight. It is easily possible to do better than that.
Before you start the reactor, a Kilopower unit is quite safe to treat as ordinary cargo, and it weighs under a ton. You could easily ship a bunch of them to Mars in each "Starship" as part of its 100 ton load.
Nuke is base load power, which I think is night-time life support plus steady propellant production (chemical machinery usually likes to run steadily and continuously, which solar is most certainly not.
Power demands are far higher in daylight, which is when the crew is constructing stuff or going on exploration rides. That large uptick in power during the day is what you use the solar for. Just when you actually have it.
Bear in mind that to send 1 "Starship" home each opposition will require something like 1000-1500 tons of propellant. You must produce something like 1.4 to 2.1 tons per day, figured at 24 hours around-the-clock production. Which means 24 hour around-the-clock electrical power, which solar is most definitely not. And you ain't gonna transport enough battery there to make it work around the clock. Too heavy.
Now, for the rovers and bulldozers, I would like to see hydrogen-oxygen fuel cells, based off Martian water, so I can ship the hardware there empty and therefore lighter. Electrolyze the water during the day when you have the solar. You'll have to send water purification equipment, but you need that anyway.
GW
Last edited by GW Johnson (2019-10-07 18:16:08)
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The additional cargo tanks would also be used for all other gasses we do intake for processing and would serve for water as well to aid in the manufacturing of methane. They also could be used for waste removal from the one that is occupied. We can process it from there with bacterium and funnel off the methane that they would create.
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Louis,
If offloading 100 reactors is a major undertaking, then offloading 1,225 rolls of photovoltaic arrays is an even more daunting challenge. You're not going to set these things up right next to the ship you plan to use to leave Mars in, so any power array will be at least several kilometers away from the rocket or propellant plant that it's powering because most people would want to use the power plant for more than a single refueling. As a result, not having the power plant blown into the next ZIP code by Starship when it takes off is mandatory. Any such solar array will require far more assembly work than a fission reactor. There will be electrical connections and electronics galore, just as there is in real solar power plants here on Earth. If these rolls of solar panels are big enough to cover a football field, they'll be every bit as heavy as a nuclear reactor. If they're fabricated into smaller rolls, then there will quite literally be thousands of them. No solar arrays will be dropped in the dirt, either. No commercial solar power plant anywhere on Earth has ever done that and there's probably a good reason.
If you drop an ATK fan, a roll of thin film, or a KiloPower reactor from 100 feet in the air, all three will be destroyed. The Uranium core of the reactor will be just fine. The cells in the ATK fan are so brittle that finger pressure will shatter them, so a 100 foot drop followed by a sudden stop is instant destruction. The thin film arrays are thinner than pantyhose material, so it tears easily and has a minimum bend radius that deformation during a fall would easily exceed, and everything but the core of a Kilopower reactor would be bent or broken. Either way, the point about not damaging the equipment you're dependent on for your survival really shouldn't have to be made to astronauts with multiple PhD's in science, engineering, and medicine.
The only part of a reactor that should be assembled at the emplacement site is the reflector, because the reactor can't go critical without that. The notion that no EVA's would ever be required to assemble a multi-MW power plant, even a gas turbine power plant, is completely absurd. I hope you know that much.
As far as total number of parts is concerned and total mass, a solar power solution wins that category in spades. For output equivalent to a nuclear reactor, there's no such thing as a solar power plant with fewer total parts requiring manual labor assembly. There will be thousands of inverters containing microchips and other solid state electronics that control panel and array output, thousands of wiring runs that have to be fabricated, thousands of supports that have to be assembled or fabricated. Even if the wiring can be pre-fabricated, it'll still have to be connected. The low atmospheric pressure and dust on Mars dictates a Paschen breakdown voltage somewhere between 300V and 350V, so lower voltages will have to be used to prevent damaging arcing and sparking on the photovoltaic arrays themselves. This sets an upper limit on achievable array efficiency through increased operating voltages. A hard vacuum is a comparatively fantastic electrical insulator, which is why higher voltages can be achieved by arrays in space. Once again, the dust is the problem.
I don't see any major problems with transport and assembly of pieces of cargo roughly the weight of as a small diesel truck engine and roughly the same shape as a small palm tree. Kilopower will weigh 570kg on Mars. A Cummins 6BT diesel engine weighs 500kg here on Earth. A pair of mechanics routinely take these things in and out of 1 ton trucks using hand cranes and stands. We're dropping these in holes in the ground that are roughly the same diameter and depth as a fence post hole, so we'll need a post hole digger and a hand crane, but that's about it.
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Each Kilopower unit will mass at least 1.5 tons - so a minimum of 7.5 tons but probably a lot more. However on Mars that equates to 2.8 tons in Earth equivalent, so I suppose that's some help.
I still don't see 5 units being lifted off in 15 minutes. They aren't all going to be waiting by the cargo door. Somehow you have to get them to the cargo door and then manouvre them on the hoist platform. All this being overseen by guys in space suits presumably.
This image suggests they will be pretty big. I'm not even sure whether this is a 1 kw or 10 kw model:
https://www.nasa.gov/directorates/space … wer/images
The I kw unit was 1.9 metres tall!
The Wikipedia page says the "The space rated 10 kWe Kilopower for Mars is expected to mass 1500 kg in total" - not under one ton.
Either way, with packaging, could you really cram 5 on to one hoist platform? I very much doubt it. If you can, then time will be taken up balancing them one on top of another and securing them.
A 100 Kwe output will never produce enough propellant for a return Starship. The estimate given by one analysis of 1 Mwe may be an overestimate but I think it's going to be somewhere between 400 Kwe and 1 Mwe (with other demands) - so between 40 and 100 units.
So you are recommending that you fire up the Kilopower units on the Starship on arrival? You do realise that has implications in terms of crew health. They will need to be protected from the radiation.
Propellant production does not require 24/7 production. There's no necessity about that. It can be a stop-start process if you wish. But with a solar based solution I think you'll see more like a natural rythm of high and low production over the sol, with production reducing overnight. Battery storage could be in the range of 10 MwHs. I would envisage production maybe being over the average for 4 hours, around average for 8 hours and then below average for 12 hours.
Let's assume a cargo "Starship" can deliver 100 metric tons of cargo to Mars. I have chain hoists in my farm shop that can handle 2 tons, and any crane system built into "Starship" should be able to handle a lot more than that. Call it 5 tons for a realistic guessed number.
To offload 100 tons 5 tons at a time requires the crane's freight elevator platform to make 20 round trips. As a guess, assume 200 feet one way. Most of the commercially available power hoist equipment operates at around 1 foot/second. That's 200 seconds down, and 200 seconds back up, for each load. Add another 200 seconds as a guess to roll the load off the platform onto the ground. And another 200 seconds to roll the cargo onto the hoist platform, up at the cargo hatch. That's 800 seconds (less than 15 minutes) per trip.
15 minutes per trip times 20 trips is 300 minutes. 5 hours. To unload 100 tons! You could certainly do it in a single working day. Hardly a huge endeavor. I unloaded and stored 6 tons of steel in my shop, several hundred pounds to about half a ton at a time, off the big trailer, with chain hoists on trolleys running on overhead beams, in about half a day. I did it single-handed, meaning totally alone. And my hoists are manually chain operated, not power hoists at all. Plus, I'm an old man: 69 years old.
A kilopower unit weighs less than a ton, as near as I can tell. Those things can just be a part of that 100 tons you offload. The electric bulldozer you need is heavier, by far. And it can transport the not-yet-activated kilopower unit to the intended site quite safely.
As I said, you set up your kilopower unit and connect it up to the cabling, then you bulldoze a berm about it. You do not need to fully bury it, until its end-of-life decommissioning. Even so, you just bulldoze the berm over it when that time comes.
How many do you need? Well, that depends upon how many people you send how quickly, and how fast you make return propellant. At 10 KWe each, you need but 10 (under 10 tons) to have 100 KWe available. 100 of them (under 100 tons) gives you 1 MWe. What could you do with a MWe available?
As for "Starship" itself, it had better have some source of power. Not a storage device, a source. Batteries are storage devices, not sources.
When humans first get there, there is no place for them to go. They will be living inside those "Starships" on the surface, until actual habitation spaces can be erected. That living requires a lot of power, same as is required during the transit of the manned "Starship". You cannot do that transit job on batteries either (it's a 6 to 9 month transit).
These "Starships" had better have solar power, and even then, you are betting the solar works until you get the nukes set up. Because of the risk of one of those so-inconvenient pesky major dust storms.
Just trying to put some realistic perspective on this.
It's no big deal to unload 100 tons out of a landed "Starship", with stuff you could buy at Harbor Freight, if you don't mind the hardware weight. It is easily possible to do better than that.
Before you start the reactor, a Kilopower unit is quite safe to treat as ordinary cargo, and it weighs under a ton. You could easily ship a bunch of them to Mars in each "Starship" as part of its 100 ton load.
Nuke is base load power, which I think is night-time life support plus steady propellant production (chemical machinery usually likes to run steadily and continuously, which solar is most certainly not.
Power demands are far higher in daylight, which is when the crew is constructing stuff or going on exploration rides. That large uptick in power during the day is what you use the solar for. Just when you actually have it.
Bear in mind that to send 1 "Starship" home each opposition will require something like 1000-1500 tons of propellant. You must produce something like 1.4 to 2.1 tons per day, figured at 24 hours around-the-clock production. Which means 24 hour around-the-clock electrical power, which solar is most definitely not. And you ain't gonna transport enough battery there to make it work around the clock. Too heavy.
Now, for the rovers and bulldozers, I would like to see hydrogen-oxygen fuel cells, based off Martian water, so I can ship the hardware there empty and therefore lighter. Electrolyze the water during the day when you have the solar. You'll have to send water purification equipment, but you need that anyway.
GW
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Louis:
It is pathetically obvious that you have never gotten your hands dirty loading and unloading anything bigger than a sack of groceries.
I can (and I do) move a ton of steel across the floor of my shop all by myself, with a simple pallet jack. I could do that wearing an idiotically clumsy spacesuit. These things are designed to make such activities easy.
The stuff you ship to Mars must be banded to pallets (which in turn must be secured to the cargo deck during the flight). When you lower your crane elevator the first time, the biggest items on it are a roll of landing mat material, a big roll of sheet metal, and a pallet jack. You roll out the landing mat onto the dirt as a firm substrate, unroll the sheet metal on top of it to provide a smooth surface. Then you can use the pallet jack to unload onto the surface from the crane elevator platform.
Your next trip down is the small electric bulldozer/front end loader vehicle, which you drive onto the crane platform, and you drive it off when at the surface. It will pick up and move the cargo packages offloaded onto the landing mat structure.
The rest of it is just packages that are palleted. Use a pallet jack in the cargo bay to roll the pallets onto the crane platform. Then lower it. And use the other pallet jack to roll that pallet onto the landing mat, where the bulldozer/loader picks it up and moves it where it is needed. You can accumulate palletized packages on the landing mat surface while the bulldozer/loader is taking them to wherever they go, one by one.
You need one guy to operate the crane, who can double (or not) as the pallet jack operator in the cargo bay. You need another guy operating the pallet jack on the landing mat. You need a driver for the dozer/loader vehicle.
That's how a crew of 3 to 4 can unload 100 tons from a "Starship" in a day, while wearing those idiotic balloon spacesuits. I find it rather pathetic that I have to explain this operation to you. If you but look around, you can watch people doing this sort of thing at construction sites and freight ports every single day!
Why aren't you worried about getting better, more supple spacesuits, instead of worrying about this unload operation? That's my question to you.
And no I DID NOT say to fire up Kilopower units to supply electricity aboard "Starship" during the transits. I said they will have to do what was pictured in the 2017 and 2018 presentations: big solar panel wings. There is no place in the "Starship" concept to safely operate a nuclear reactor, even a compact one like Kilopower. I've said that multiple times on these forums. You simply refuse to read and understand it.
The fact that the so-called "Starship Mark 1" vehicles do not have this feature is meaningless. Flight test experimental vehicles rarely resemble the finished products they finally generate. I've written this before on these forums. You simply refuse to read and understand that, too.
And by the way, the six-leg landing leg design, of diameter only 9-ish meters, may double landing pad-foot area, but it makes the topple-over risk far worse. The footprint is just way too narrow for the cg height. That Mark 1 experimental flight test vehicle will NEVER successfully land off of a thick, steel-reinforced concrete pad! It looks NOTHING like whatever might successfully make a rough field landing on Mars or the moon!
GW
Last edited by GW Johnson (2019-10-08 08:33:23)
GW Johnson
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For Louis re this topic and the forum in general ...
Thank you Thank you Thank you for inspiring GW Johnson and kbd512 (in particular but not alone by any means) to create such outstanding, memorable, instructive and thoroughly entertaining (to the prepared mind) posts!
This forum would be much less valuable without your constant prodding. Keep it up, please!
I would only ask the authors who respond to your sallies to try to remember to put tags into the posts to make them easier to find.
In a recent post (probably in another topic) kbd512 made reference to an earlier post, to save time in a current post.
For everyone ... the forum archive could be even more valuable as an archive of knowledge if more care were taken in tagging posts.
This doesn't have to be done at the time of post, and indeed, it might be better if tags were added after feedback arrives.
(th)
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Tahanson43206:
You're talking "computer tags" to someone who can just barely do basic email. I designed my first airplane and my first half dozen supersonic missile engines with a slide rule. Not a calculator, a slide rule. I can put some keyword or words in my typed text, yes. But, what tags would you suggest?
We've been arguing about the safety (or lack) of nuclear, whether there are chemical backups to nuclear or solar, and whether or not solar works in the darkness of a giant dust storm (no, it does not, as the death of Opportunity proved). Then we got sidetracked into how easy or difficult it would be to unload one of Musk's "Starships" on Mars, assuming it can land in rough-field conditions safely, which I still very strongly doubt. And then we sidetracked further into the electricity supply on a "Starship" while in transit between planets.
And in other threads, we have argued about how closely Musk's prototypes actually resemble whatever "Starship" will have to become (and I say the resemblance is fleeting at best, this early in the flight test program). We have argued about how much return propellant must be made, and how fast, and how much energy is required, and whether it is wise to tie the machinery output to a variable solar power supply (usually chemical process equipment likes to run continuously at constant power levels).
Those are all different things, there are a great many of these topics, yet all are related, in that all require solutions, if Musk is ever to send any of these craft to Mars. If there's any keywords in there, maybe you can figure out what to use.
GW
Last edited by GW Johnson (2019-10-08 11:59:14)
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