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Louis, you aren't going to deploy a large solar array covering acres of ground in a dust storm either. It's a good idea not to land in a dust storm for all sorts of reasons.
How about using interstellar ions as propellant? Two thin meshes one positive, one negative, will impart force on any charged ions between them, generating thrust. Of course, you still need to generate power to do it.
Why bother?
You could import 300 tons of PV equipment on Mission 2 if you wanted - enough to generate 300 MwHes per sol.
Alternatively you could import a highly automated PV manufacturing facility and begin manufacturing PV panels on Mars. More than that you can begin constructing a Mars ISRU PV manufacturing facilities using 3D printers after Mission 4, once various vital industrial machines have been imported. Some specialist materials and parts might still need to be imported but they would be low mass.
Any decision on power supply will be informed by cost benefit analysis. Maybe the analysis will opt in favour of solar panels. For early missions where power requirements are modest, it may indeed be cheaper to develop a space-solar solution than a nuclear reactor.
But the more energy you need, the less true that will be. An SP-100 reactor would generate 2MW of heat around the clock and the core weighs about 700kg. You would be hard pressed to match that sort of power density with solar panels on Mars. And the bigger the reactors are, the better their fuel utilisation and power density.
Living on the planet will require water in abundance. It will require large amounts of heat for surface greenhouses or large amounts of electricity for compact artificial lighting. And of course reducing metals, making concrete, making fuel, air, carrying out mining and manufacturing is power hungry. Living on Mars will require a lot more energy than living on Earth. That energy must be cheap if any serious colonisation effort is to stand a chance.
Is not steam extraction something I advocated some time ago?
It's a variant on something done in somewhat-depleted oil fields for decades now.
GW
I'm quite new here, so have missed a lot of valuable discussion. Apologies if I have duplicated things that were discussed before.
Generating steam at temperatures of 200-300C is something that could be accomplished using native graphite moderated reactors burning natural uranium. But these will be low power density monstrosities. I wonder if a better long-term approach would be to import fast reactor cores and reactor vessels and build the pipework and secondary systems on Mars
100 tons of shielding in a ship displacing 6000-7000 tons is not unreasonable at all!
What do you think the bulldozed berm around a kilopower unit will mass, maybe 100 tons?
Don't need to fly that 100 tons of shielding to Mars. Just use 100 tons of the local dirt. Fly a 2-5 ton electric (rechargeable) bulldozer to Mars instead!
What is so damned hard to understand about how to deal VERY EFFECTIVELY with the radiation from a kilopower unit?
GW
You don't need to move the 100 tonnes of dirt at all if you put the reactor in a pit, some 50cm wide and 3m deep. To do that, you only need to shift 1-2 tonnes of dirt and rock. Much easier than trying to push 100t around the core. Either way, its not a big deal I guess. The bigger the reactor is, the more it benefits from self shielding effects.
From the reference below:
'For the case of the Mars-Phobos system, the L1 point lies between Mars and Phobos, only 16.6 km from the center of the moon and only 3.1 km from the surface. The L2 point is on the far side of Phobos and only 20 meters further from the surface.'
https://trs.jpl.nasa.gov/bitstream/hand … sAllowed=y
A docking station could be built at the Phobos-Mars L1 point. This would seem to make it much easier to export things from the Mars system to Earth.
Phobos has an orbital speed of 2.14km/s (Mach 6.2). A cable extended from Phobos to the top of the Martian atmosphere, would be within reach of advanced air breathing ramjet aircraft, or a fully reusable rocket vehicle. Upon capture of the vehicle, the cable could be wound in to Phobos-Mars L1 point just a few km above the Phobos surface. Solar/Nuclear electric powered vehicles could then be used to transfer payload from Phobos to high Earth Orbit, using Phobos surface material as reaction mass.
GMO for a power gathering satellite would mean that the system could only be active for about half the time as the orbit passes over the night side every day. A polar orbit is in shadow for much less time over the year.
Several relay satellites in GMO should be used such that the polar orbiting power gathering satellites can send their microwave beams to them for onward transmission to ground stations. This may be subject to capacity reduction when there is a dust storm.
It might be cheaper to install PV on the surface and provide power storage or round the planet transmission, in combination with differential pricing when it becomes commercial, to allow for night use. This is also subject to interference by dust storms but maybe to a greater extent as it affects energy capture. Round the planet transmission might be by microwave but dust storms would interfere with that too.
About 11% of the time. Mars has an equatorial radius of 3396km and Areostationary orbit lies some 17,032km above the surface. So Mars will cast a shadow some 6800km wide on an orbit some 64,200km in circumference.
All the same, a solar power satellite will be in shadow for 2.7hours per Martian day. To deal with that problem there would need to be either multiple satellites staggered along the orbit; energy storage; or deliberate shutdown of equipment at specific times of day.
Most likely a combination of the above. On the plus side, the timing and duration of the power loss is entirely predictable down to the microsecond. That makes it much easier to deal with. There are some loads that we could simply switch off for 2.7 hours per day. If people are asleep, then residential loads are lower anyway. Heating is a function that can take place when power is abundant and switch off when it is not; because heat can be stored very efficiently and at very high energy density in bulk materials (think storage heater).
There's little point to sleeping in 1g. Particularly in space, since it makes more sense to use the central, best shielded part of the spaceship for sleeping.
Indeed. The effects of 0g on the body are very similar to the effects of bed rest, to the extent that bed rest is used here on Earth as a reasonably good proxy for modelling the effects of zero g. This suggests that a fully grown adult could sleep in zero g with little ill effect, provided that their waking hours were mostly spent in gravity.
But there is another less well known reason for zero g deterioration of bones. Without the force of gravity, new bones cells cannot align themselves properly when attaching to the bone matrix. This is likely to be especially problematic for anyone still growing, especially babies and small children. Everyone sent into space so far has been fully grown. Whether 0.38g is sufficient gravity to prevent issues of this type, I do not know. I am tempted to say 'probably yes' given that 0.38g is still substantial gravity. We won't know for sure until we carry out partial gravity experiments on developing animals in orbit.
Living in 1g on Mars would mean building big centrifuges with thrust bearings and electric motors to keep them spinning. And they would need a big rotation radius in order to avoid Coriolis sickness. Possible, but not cheap.
For Elderflower re #14
Nice! Thanks!
The 100 10 Kw power plants could be fitted with conveyor belt passages between the reactor and the surface.
The excess heat that would normally radiate to space could be directed to the incoming ice/regolith mixture which could be monitored by sensors to confirm that melt has occurred before advancing the conveyor.
If Calliban's concept for a larger reactor wins the competition for funding, it too could be fitted with a moving passageway for material to be heated in this manner.
(th)
I would suggest a solution mining approach. Use reactor waste heat to heat water to 100 degrees centigrade in some sort of heat exchanger and pump it underground using a centrifugal pump. Next, inject cold liquid CO2 into the well in controlled quantities. The hot water will melt the ice and evaporate the CO2 creating a layer of melt water covered in pressurised CO2 gas. The pressure will push the melt water out of extract wells drilled along the glacier. Heating and melting the ice will take a lot of energy, about 500KJ/kg. So you need to inject about 1kg of hot water for every 2kg you extract. That's ignoring heat losses. Using steam would be far more efficient, because the latent heat of boiling/condensation of steam is about 2MJ/kg. But that implies higher temperatures.
I accept that a Mars Mission will rely on sound engineering and you can't have multiple levels of failsafeness. But nuclear power is intrinsically unsafe, just like rocket fuel or wires carrying electricity. I think there is certainly a question mark over adding intrinsic unsafeness to a Mars Mission, when a perfectly viable - and in the view of many, including Space X, it would seem so far, better - alternative is there for the picking.
Since no amount of logic will sway you on something that you clearly believe in passionately, I give up trying. Believe what you want to believe.
Mars has the advantage of being outside of Earth's gravity well and having a much shallower gravity well. If you want to build things in Earth orbit or for Earth orbit, it may be cheaper to ship materials or finished products from Mars than from Earth surface. Mars has a complete selection of industrial elements and a relatively benign surface environment. Maybe it is a useful staging post for colonisation of the wider solar system. It's distance is a problem.
"And if nuclear is "safe" why do nuclear reactors on submarines have a 100 tons of shielding."
The same reason that power cables from solar plants will be insulated and even safe cars are equipped with seat belts and brakes. Safety comes from sound engineering. It is all about the relative risk that people face from using the thing, whatever it is. It doesn't mean that accidents are impossible. It means that with the engineering in place, your chances of dying that way are small compared to other risks that you face. Without a certain amount of good engineering, nothing that humans build would ever be safe. Your house is only safe because the man that designed it followed building regulations. A shack built out of sand glued together by sugar would not be safe and poorly designed buildings have killed countless people over the years. That doesn't mean that using buildings is inherently unsafe. I'm guessing that your house is safe because it was designed to be fault tolerant. It is possible that it could collapse in an Earthquake tomorrow crushing you to death. But it is unlikely and hence, your house is relatively safe. I bet you don't lay awake at night worrying about your house collapsing. Likewise, I don't sit around worrying about reactors without shielding, because I know they are designed with shielding and that no magic wand is going to make it disappear. I could rattle on some more, but hopefully you get the point.
For Calliban re #67
First, thank you for the concise summary of reasons to bury reactors in pits, instead of building berms around them.
I'll memorialize your post with a search term:
SearchTerm:BuryReactor
Second, (and not for the first time) I find myself in disagreement with your argument for large nuclear reactors.
I'll open this counter argument with acknowledgment that your points are ALL valid, as I understand them, if the premises upon which (I presume) you are making them apply to a given situation.
However, I recommend redundancy in a life-or-death situation such as will apply on Mars (or anywhere away from Earth).
Efficiency in design which leads to vulnerability is (in my opinion) the tail wagging the dog. If I have a chance to sway public opinion (unlikely, I'll admit) I'd vote for many small reliable reactors instead of one big one. To my mind, arguing about 30% efficiency (assumed for NASA 10Kw Sterling engine design), vs 31% efficiency (or more) with another design is missing the point. I'd far rather have 100 small reactors in a field of them, than one big reactor that becomes the single point of failure for the enterprise.
All that said, thank you for your firm support of the principle that the human race MUST master atomic power (in all forms) in order to expand confidently out into the Universe.
(th)
Tahanson, mission designers will use fault trees to model the overall probability of failure of the mission and will have required reliability for each of its components, power supply included. Based on the required reliability, the power system designers will make the decision as to how much redundancy is needed in their power system design and how best to achieve it. I would expect to see a lot of optioneering and cost-benefit analysis. It will not be an arbitrary decision whether we use 1 reactor, 10, 100, or none at all. They will calculate exactly how much redundancy is needed to meet minimum requirements, choose the cheapest practical means and then balance further improvements against cost and mass penalty. These are not the sorts of decisions that people will make a whim or intuition.
In my opinion, it is quite unlikely that mission designers using risk management tools to deign it necessary to bring along 100x 10kWe reactors, instead of 1x 1000kWe reactors. Most likely, the single reactor will have redundancy built into it, like multiple pumps and cooling loops; but only one core, as duplicating this doubles the mass. I make that judgement because a duplicate pump and cooling loop is much cheaper and lighter than a duplicate reactor. I could be wrong about this. Designers may decide that dividing power production between two reactors is a justified way of achieving the required power system reliability. I am quite confident that they will not choose 100 units instead of just 1 or 2, for the same reasons as I am quite confident that the rocket engines in Musk's future vehicles will not have 100 redundant propellant pumps.
As for small reactors being less efficient in terms of power per unit weight and fuel utilisation, this is a fact established by the laws of physics. Critical assemblies leak neutrons at their periphery. A smaller assembly has a greater ratio of surface area to volume. Therefore, the smaller the assembly is, the more enrichment and fuel volume fraction needed just to keep it critical. You eventually reach the point where a 1kW reactor needs the same fuel volume as a 10kW reactor and is merely operated at lower power, as it would be impossible to maintain a critical assembly with a much smaller core volume.
What this means is that very small reactors, like those that we would engineer for a mission to Mars, tend to require highly enriched uranium or plutonium as fuel. A single 1MWe reactor will have lower enrichment requirements than 100x 10kWe reactors. It will have 1 or 2 sets of power generating subsystems, instead of 100-200. Large thermodynamic systems tend to be more efficient than smaller systems, because of lower thermal losses and more reversibility. It will weigh a lot less as well.
Deployment is going to be a problem whether or not you are using nuclear or solar power on Mars. The more power you need, the bigger the problem gets. Moving the reactor isn't a big issue. You just mount it on a chassis that is strong enough. It doesn't need to move quickly and electric motors have plenty of torque at slow speeds. Same with solar panels.
For a nuclear reactor, you need to dig a pit at least a few metres deep and put the core at the bottom. Trying to bulldoze mounds of soil around the reactor is not an optimum shielding solution. The problem is that fast neutrons interact with high-Z materials in the soil producing a lot of secondary gamma. You need to shift a lot of soil to provide effective shielding. If you dig a pit, your shielding comes from soil already in place and your pit only needs to be wide enough to accommodate your reactor core. Maybe a metre wide and 3-4 metres deep. So the amount of soil you need to shift is minimised if you dig a pit. A top shield, power generation and waste heat panels sit above it. This is another activity that becomes more efficient the larger a reactor gets. The mass balance between solar and nuclear is a function of how much power you generate. The Kilopower units are not an efficient use of nuclear power for a manned Mars mission in my opinion. They are designed with a different purpose in mind.
For a 1MWe solar based solution, you need to deploy several acres of solar panels over a rubble strewn surface. A certain amount of assembly will be required. Given that discharge voltage for thin-film is low, you may need to connect local inverters and transformers to avoid excessive voltage drop and losses between different areas of your solar plant. You have acres of solar panels, electrical contacts and power electronics, that will be susceptible to damage and dust contamination. And all assembly must be carried out in space suits. There are reliability issues here that you won't have with a nuclear system. On the plus side, from Musk's point of view, it is much easier to develop a solar based system than a nuclear based system. Developing a space nuclear reactor would stretch his budget, even if it is a superior system from a mass and reliability perspective. And there is the complication that small nuclear systems tend to require the use of highly enriched uranium, which has security issues. In a sane world, the BS and bureaucracy that surround development of a space nuclear reactor wouldn't be an issue that stands in our way. But we don't live in a sane world. The people that stand in the way of space nuclear power (and all nuclear power) create the very problems that they then point to as reasons not to do it.
Extract from New Scientist: 'It would also need to be big – some 200 metres long and 12 metres in diameter – and powerful, requiring 165 megawatts of power to generate just 1 newton of thrust, which is about the same force you use to type on a keyboard'
That's a lot of power for not much thrust. If you are generating that much power, the photon pressure from the waste heat radiators will rival what this drive is able to produce.
Bad news regarding Apophis. It appears to be an LL Condrite. This is a breccia type material with no more than a few percent free iron.
https://en.m.wikipedia.org/wiki/99942_A … cteristics
The problem is that free iron is what platinum group metals, cobalt and nickel are dissolved within. And they are the big paydirt when it comes to asteroid mining. Really we need to be able to sell those things back on Earth, to at least subsidize the mission.
"Certainly on the basis of this detailed analysis, it would seem that a PV solution is far superior to a Kilopower solution which would amount to a minimum of 160 tons plus 40 tons for the non-power elements of propellant production."
Shipping dozens of small kw-level nuclear reactors to Mars makes no sense at all. It balloons the mass requirements for a system that is already one of the most reliable mission components. The original SP-100 concept from the 1980-90s had a mass budget of 4.58tonnes for a 100kWe system, using thermoelectric generators with a 2MWth heat source at an efficiency of 5%. With Sterling or Brayton cycle engines, power density would be greater still. Nuclear systems producing ten times as much power need not be ten times heavier. But very small fission systems are not very efficient because there are minimum effective sizes for critical assemblies.
https://ntrs.nasa.gov/archive/nasa/casi … 003294.pdf
Kilopower appears to have been designed as replacement for RTGs on deep space missions, where you need 100s-1000s of watts to power transmitters and heat components. Mass efficiency is not the driving concern, as the driver is to produce small amounts of power reliably for decades. The RTGs that they replace have even poorer power density, but huge energy density. Not necessarily a suitable system if power of a MWe or more is needed for 2-3 years. The fact that MW grade space reactors are not being developed, tells us that NASA probably aren't taking manned Mars missions very seriously.
A reactor system with more than one power generation loop is no less reliable than a solar power plant backed up by a methalox gas turbine. Both have moving parts. The reactor achieves redundancy by having multiple pumps and power generation loops. If everything goes wrong, you freeze to death. Then again, if the propellant pump fails on the descent stage engine you are going to die. Given that there are so many ways of dying on a mission like this, the priority is to make sure no single component dominates risk; there will never be any perfect solutions.
Consider that nuclear submarines here on Earth need propulsion to get to the surface. If reactor power fails unexpectedly, the entire submarine could be lost along with its crew. The Earth's navies do not generally mitigate this problem by bringing backup reactors along. The systems are reliable enough and have enough stored energy to mitigate the hazard.
"He comes up with a combined figure of only 74 tons for both a 1 Mwe Propellant Production Facility and the mass of the PV facility generating the power for the process."
74 tonnes is a great deal of mass budget for a Mars-direct style mission. Is that what we are discussing here? Even for Musk's much more ambitious starship concept it sounds like rather a lot.
For Calliban ... this topic has languished for more than a month.
Please give it a boost with an insight about your chosen focus, or something related if you want to expand a bit.
(th)
Belter Lowda! I will rejoin this discussion soon. Have been busy of late.
"The fact you might require 100 KP Units to power propellant production on Mars is a game changer I think."
Using 100 small reactors, each with their own subsystems, is not an efficient way of exploiting nuclear power. If you want 1MWe say, them you build a 1MWe reactor. You don't build a hundred smaller ones.
Good replies all. For those interested, here is a link to a MIT study on the relative mass of thin film solar, RTG and nuclear fission based systems in support of Mars missions.
http://systemarchitect.mit.edu/docs/cooper10.pdf
Solar PV in broadly competitive with the baseline fission solution at latitudes between the equator and 30 degrees north. Outside of this area, system mass will be greater.
There are other complications that need to be addressed. The PV panels need assembling and weighting down, which is estimated to be about 30 man-hours work. The assessment does not seem overly concerned with the potential for damage to the panels assembled over a rubble strewn surface, or the inefficiency that this would impose by bending the panels into random orientation. Dust would need to be periodically removed, maybe using compressed CO2. Obviously, the array needs to be positioned on a flat or slightly southerly facing surface outside of shadow. Undulating of the surface due to rubble and other surface imperfections could be problematic. Overall the system can be made to work. It is more complicated and generally less mass competitive than a 100KWe fission reactor would be.
The reason we see so little innovation and development in small fission technologies is the difficulty imposed by licensing and regulations. And this all gets even more difficult if designs use highly enriched uranium. For this reason, it is very expensive to develop fission reactors. The plants themselves probably aren't that expensive if we produce them in series production. A lot of cost and BS that make nuclear power development so expensive on Earth, can presumably be dropped by an independent Mars base. Once a base reaches a population level of tens of thousands, it could begin building small, carbon moderated natural uranium reactors, without interference from Earth.
5. For Mission One on Mars, cost is hardly a consideration. Getting it right is all that matters. Even if it costs 100 times the cost of PV on Earth, if it works, the price will be right. Even if it costs $1 billion per annum to produce that 1 Mwe it really doesn't matter, if it works.
Are you sure about that? Cost is really what all this is about. There is no doubt that a Mars mission can be mounted using either Kilopower reactors or lightweight solar arrays. But cost per mission will be different. This is an important metric and so are the risk and capability limitations imposed on the mission. We could nullify the risk and capability limitations by devoting even more mass to even larger solar arrays and more mass to various means of energy storage. But cost would increase even more.
Do you seriously believe that cost does not matter? That would make it different from practically every space mission that we have carried out to date. If something costs a billion dollars more than it should do, then 'as long as it works' won't be a good enough justification for not using something that also works at a fraction of the price. The more it costs, the more likely the mission will be scrubbed and the fewer missions you will ultimately afford. Value for money always matters.
Louis, you keep trying to deny the consequences of the second law of thermodynamics! Part of me admires you for trying, frustrating as it is to watch. Essentially, you are trying to argue the case that a system running on low density and intermittent ambient energy; can outperform a system running on concentrated and controllable stored energy with much lower entropy. And you seem determined not to take no for an answer. People keep trying to do the same thing here on Earth and it is already starting to cost us a lot of prosperity. And expensive projects like space flight require a lot of prosperity, which is really a function of surplus energy.
The economy is an energy system. Fossil fuel EROI continues to decline and nothing with comparable surplus energy is presently available to replace fossil fuels, thanks to all attempts to block it by idealistically minded 'green' people. Ironically, these people are doing nothing to help humanity reach a more sustainable way of living. The sort of energy solutions they have in mind require enormously greater inputs of refined materials, like steel, concrete and silicon, that must all be extracted from the environment and processed – all at high environmental cost. The end result of all this will be much lower energy use, which will result in a proportional decline in human wealth and (quite probably) human numbers as well. My concern is that these idealists may end up permanently costing humanity its rightful place as an interplanetary species and they will certainly cost a lot of people their quality of life and probably life expectancy, as this folly continues to unfold. Becoming a multiplanet species is the most ambitious, expensive and energy intensive endeavour that humanity has ever undertaken. It is very difficult to see how it can happen in an environment where the surplus energy available to the human economy is contracting.
Regarding early Mars missions, we certainly can make them work at a functional level with solar power alone. But that comes at a price in terms of mission mass, reliability and flexibility; as GW, Oldfart1939, Kbd and many others here keep telling you. Basically, it means a more costly mission with lower capabilities. We can talk around various options and discuss the relative pros and cons, but ultimately there is no point deluding ourselves into imagining things are different to what they are. If we end up relying on solar power for bulk power production on Mars, it will be because politicians, lawyers and deluded idealists, have removed the option for nuclear power or made it so impractical that we cannot use it.
Entropy gets no respect :-)
Nuclear power might come into its own when it comes to growing food, which has a huge energy demand. I don't think we know yet whether nuclear powered multi-level (or underground) farm facilities would make more sense than PV powered or natural light farms (perhaps with reflectors to boost light levels). It will probably be partly down to population size as to what makes sense. If the population gets to one million, that's a lot of area to be covered by PV. Not impossible, but might not make a lot of sense. By that stage, however, solar power satellites capable of beaming energy down to the surface may be operational which will give us another option.
Solar power satellites may or may not be practicable as bulk power sources for Earth and Mars. It depends entirely on our success in harnessing space based resources that are already at the top of planetary gravity wells. It makes little sense trying to launch these things from Earth. The jury is still out on whether the same is true for Mars. One thing that is certain and was worked out quite early in development of this concept, is that solar power satellites are multi-GW powerplants. This is determined by the fact that rectenas have a minimum practical diameter of about 10km, assuming a satellite in GEO. Microwaves don't have enough beam coherence for rectenas to be reduced to much less than that. The original SPS concept theorized by NASA was a 5-10GW powerplant. I can't remember if that was at the powerplant transmitter or on the ground. But this isn't practical on less than GW scales either way.
The weakness of ground based solar power in meeting any large scale equipment load is that even under clear skies at the equator, its power output follows a half sinusoidal pattern. That means that it can only produce more than 50% of its peak capacity at that location for about 30% of the time. That imposes quite a huge limitation on the productivity of whatever the solar array is powering, whether it be a propellant plant or an underground food factory. A nuclear reactor that is rated for these loads will provide steady baseload power all of the time. Whatever its other benefits and disbenefits happen to be, this is a huge economic advantage if that equipment happens to be capital intensive and expensive to ship in from Earth. You really dont want to see it underutilized. It is for exactly this reason that all utility grade solar on Earth is backed up 100% by fossil fuel power plants. Essentially, it is the FF plant that meets most of the load burden and the solar plant reduces the fuel bill somewhat when it is producing. Obviously, that option isn't available on Mars. So we either take the productivity hit and only run our plant a third of the time; or we do what sensible people do on Earth and build a baseload powerplant. On Mars, that means nuclear reactors.
For a long time to come, it will make sense to import nuclear reactor pressure vessels, core, instrumentation and control systems, from Earth. However, it is likely to be far more practical to build things like heat exchangers and Rankin cycle power generation equipment on Mars, using ISRU carbon steels. This accounts for most of the mass of the power plant.