How to cheaply and efficiently power a Moon Base

[Frank]

How to cheaply and efficiently power a Moon Base.

Powering a Lunar Moon Base isn't simple because the moon is in shadow for long periods of time.  Solar power must have weeks of backup which would require a lot of batteries or some sort of backup generator that runs on maybe hydrogen and oxygen.  Solar panels must be oriented to the sun to maximize efficiency, or more panels must be employed.

One idea is to place the solar panels in space, perhaps at EML-1 and beam the power back to the moon.  This still requires a very large array to capture the energy on the moon.

 

This idea is similar, it uses a satellite in a polar orbit such that it is always exposed to the sun (except for short eclipses with the Earth).  Power collected by the solar panels is transmitted down a rotating tether which has a battery charger and swapable battery at the end.  This arrangement (without the charger and power cable) is known as a Momentum Exchange Tether or rotovator/lunavator or skyhook.  There it is.  Use the mass of a solar panel array as an anchor for a rotovator, charge a battery, drop it and pick up another to be charged.  Batteries are convenient to move around and place where they are needed as opposed to fixed power lines and they are their own backup.

 

A single launch of a falcon heavy could put ~20t of solar panels, chemical rockets and tethers with power cables into lunar orbit providing around 30 kW of power. A single ~2h orbit would likely be enough to charge a 50 kWh battery, basically delivering continuous 25 kW power with battery reserves in units of 50 kWh. The battery swap doesn't change the momentum energy in the system.  A 50 kWh battery using 200 Wh/kg is 250 kg (but would weigh 42 kg on the moon) which is the maximum mass of this first stage tether.

Not only does this arrangement not require landing 20t of solar panels on the moon, it also allows transfer of payloads up to 250 kg to or from lunar elliptic orbit or EML-1. This saves rocket fuel launched from earth or mined from the moon to do the lifting.

Also anything that can be packaged into 250 kg chunks can be sent to the moon or from it. And if the transfers are bidirectional, no net energy is used!

 

The idea of a momentum exchange tether isn't new, but using battery swap to power a moon base is, as far as I know.

Here is a paper on a momentum exchange cislunar system which is meant to send cargo to and from LEO.

http://www.tethers.com/papers/CislunarAIAAPaper.pdf

It requires two rotovators.  I think a first step of just the lunar portion would allow efficient transfers between 400 km elliptic lunar orbit (or EML-1) and the lunar surface.  This could mean water at EML-1 by combining sent hydrogen with lunar oxygen or LOX from electrolysis of the water in a reusable cycle scenario.

Once power and transportation is solved, building a lunar moon base seems attainable.

 

[Moontanman]

Why not position the base at one of the Moon's Poles?  Supposed to be water ice there and there are relatively large places that are sunlit all the time... 

[Ken Fabian]

38 minutes ago, Moontanman said:

Why not position the base at one of the Moon's Poles?  Supposed to be water ice there and there are relatively large places that are sunlit all the time... 

I got pulled up before on another forum by suggesting this - because of axial tilt (1.5 degrees I think?) and the moon wobbling in it's orbit (nutation) the poles are not constantly sunlit. I have heard claims that  some polar mountain peaks may be constantly lit (named Peaks of Eternal Light) but I don't believe any have been identified. But having several solar arrays at different longitudes, close to the poles, could be linked by much shorter power transmission cables than doing that near the equator.

My scepticism that lunar bases or colonies can pass any kind of benefit vs cost analysis remains strong, but as thought experiments I still find these questions interesting.

[Frank]

Yes, this is possible, in fact likely the best place for it given a polar orbit, otherwise it might be ~29 days until the tether returns to the same location.  And ice should be easier to extract than oxygen from regolith, but some object to using the limited resource as fuel.  I don't know enough to make an informed argument.

Another nice thing about a system like this is it's easier to move than a solar field if we decided we wanted a base in a different location.  I guess it's not all roses if a base is anywhere but the poles, the equator orbit would have less sunlight and other places are only visited once per rotation...  I say we go with the polar moon base for this to work.

 

5 minutes ago, Ken Fabian said:

I got pulled up before on another forum by suggesting this - because of axial tilt (1.5 degrees I think?) and the moon wobbling in it's orbit (nutation) the poles are not constantly sunlit. I have heard claims that  some polar mountain peaks may be constantly lit (named Peaks of Eternal Light) but I don't believe any have been identified. But having several solar arrays at different longitudes, close to the poles, could be linked by much shorter power transmission cables than doing that near the equator.

My scepticism that lunar bases or colonies can pass any kind of benefit vs cost analysis remains strong, but as thought experiments I still find these questions interesting.

Hi Ken, oh, I see I should quote when responding...

I missed the part about constant sunlight in Moontanman's post.  Axial tilt should be OK for a tether system but wobbling might mean a larger landing target.  The mechanism to do a battery swap isn't obvious either.

When it comes to money, I'm also mystified.  Bigelow thinks he can set up space tourism to a space station https://en.wikipedia.org/wiki/Bigelow_Commercial_Space_Station.  Blue Origin wants a polar lunar colony https://en.wikipedia.org/wiki/Blue_Origin_Blue_Moon.

 

 

[Frank]

According to wikipedia,  "No peaks of eternal light have been positively identified on the moon, but many peaks have been detected that, via simulations based on imaging and laser and radar topography, appear to be illuminated for greater than 80% of a lunar year."

And "Further data from the SELENE spaceprobe determined that one peak at Peary Crater receives sunlight for 89% of a lunar year, the highest level of illumination predicted for any peak of eternal light on the moon.^[3]"

https://en.wikipedia.org/wiki/Peak_of_eternal_light

 

A battery swap from a tether seems easier, faster, cheaper to me.  Add to that the ability to ferry stuff to and from the moon efficiently and it seem worth looking into.

 

[Ken Fabian]

Frank, I think bringing along a nuclear power plant would be a serious contender for powering any Lunar base. For one thing it means the base can be sited where it's best suited for it's purpose rather than sited for utilising solar power. Solar could still be a useful additional power source, with high energy projects scheduled for Lunar day periods.

I'm not sure what to think of the tethered payload delivery system. Wouldn't the capture of moving mass result in significant instabilities? Can the payloads be delivered in the right direction with the right amount of momentum? I suspect there will be more straightforward ways to deliver payloads.

[Frank]

2 hours ago, Ken Fabian said:

Frank, I think bringing along a nuclear power plant would be a serious contender for powering any Lunar base. For one thing it means the base can be sited where it's best suited for it's purpose rather than sited for utilising solar power. Solar could still be a useful additional power source, with high energy projects scheduled for Lunar day periods.

I'm not sure what to think of the tethered payload delivery system. Wouldn't the capture of moving mass result in significant instabilities? Can the payloads be delivered in the right direction with the right amount of momentum? I suspect there will be more straightforward ways to deliver payloads.

Certainly, the upsides of NP are significant, but so are the downsides.  If we stop using nuclear on Earth because of the downsides, it's hard to understand why we would want to do it where there are limited resources to clean up a mess.  In any case, offering  an alternative isn't going to hurt anyone.

The tether end, from the lunar perspective, just drops down on a particular spot and pops back up.  There is an animation on wikipedia:

https://en.wikipedia.org/wiki/Momentum_exchange_tether#Rotovator

I started a new thread for ideas on how to make the swap here (I don't know exactly what would work):

http://www.scienceforums.net/topic/110975-fast-battery-swap-for-a-lunavator/

 

 

 

[Moontanman]

14 hours ago, Frank said:

Certainly, the upsides of NP are significant, but so are the downsides.  If we stop using nuclear on Earth because of the downsides, it's hard to understand why we would want to do it where there are limited resources to clean up a mess.  In any case, offering  an alternative isn't going to hurt anyone.

The tether end, from the lunar perspective, just drops down on a particular spot and pops back up.  There is an animation on wikipedia:

https://en.wikipedia.org/wiki/Momentum_exchange_tether#Rotovator

I started a new thread for ideas on how to make the swap here (I don't know exactly what would work):

http://www.scienceforums.net/topic/110975-fast-battery-swap-for-a-lunavator/

 

 

 

Who says we are stopping the use of nuclear on Earth and what does that have to do with the moon? Besides the fact that we use 1940s nuclear tech and there are other more advanced ways to use nuclear that cannot explode or melt down? 

 

A Moon base near the poles makes sense due to the availability of ices and solar power. If nothing else near the poles you could have more than one solar power station and at least one would always be in sunlight all the time and the transmission cables would be short.  

[Frank]

1 hour ago, Moontanman said:

Who says we are stopping the use of nuclear on Earth and what does that have to do with the moon? Besides the fact that we use 1940s nuclear tech and there are other more advanced ways to use nuclear that cannot explode or melt down? 

 

A Moon base near the poles makes sense due to the availability of ices and solar power. If nothing else near the poles you could have more than one solar power station and at least one would always be in sunlight all the time and the transmission cables would be short.  

Well, Germany and other countries a phasing nuclear out.  https://en.wikipedia.org/wiki/Nuclear_power_phase-out  My personal opinion is that no new nuclear sites should be developed and that reprocessed uranium should be used up in the existing sites.  Regardless, no harm in exploring solar as a main power source.

The thing is that putting 2x (more or less) the solar panels on the moon than would be needed in space means more launch mass per delivered kW.  Not only are more solar panels needed, but they need to be soft-landed on the moon which isn't free.  I suppose if a working lunavator/tether were working, that last penalty would be reduced.

With some slight corrections of the lunavator over the course of 29 days, a single spot close to the pole could be supplied with charged batteries ready to be inserted into rovers and other lunar robots and plants. 

 

[Moontanman]

40 minutes ago, Frank said:

Well, Germany and other countries a phasing nuclear out.  https://en.wikipedia.org/wiki/Nuclear_power_phase-out  My personal opinion is that no new nuclear sites should be developed and that reprocessed uranium should be used up in the existing sites. 

 

If indeed you are talking about pressurized water reactors I would agree, but the are other safe options.  

The main problem with nuclear power on the moon would be building a reactor on the moon, materials would be difficult to get there, and radiating away waste heat. The waste heat problem could be a deal breaker actually.. 

[Frank]

33 minutes ago, Moontanman said:

The main problem with nuclear power on the moon would be building a reactor on the moon, materials would be difficult to get there, and radiating away waste heat. The waste heat problem could be a deal breaker actually.. 

Yes, and the economic argument on Earth is getting weaker as well as solar PV cost drops and efficiency rises.  https://en.wikipedia.org/wiki/Cost_of_electricity_by_source#NREL_OpenEI_.282015.29

Plus, the lunar tether power is stable and predictable.  (I like V2G to fix that on Earth... someday)

 

[Ken Fabian]

2 hours ago, Moontanman said:

If indeed you are talking about pressurized water reactors I would agree, but the are other safe options.  

The main problem with nuclear power on the moon would be building a reactor on the moon, materials would be difficult to get there, and radiating away waste heat. The waste heat problem could be a deal breaker actually.. 

I have reservations about massive global expansion of use of nuclear around the world as a primary emissions reductions strategy but that's for another thread; enough to say I think the dominant environmental, security and weapons proliferation concerns won't apply to use at a moon base. Or the same level of concern over cost. Safe and reliable launch would be important. I believe the USSR put 40 or so small fission reactors into orbit over the years. There are probably some still up there, but dumping them back into Earth's oceans when done isn't something I'd like to see emulated but that wouldn't apply to this. It would also not be up to me!

I don't know about the waste heat issue - it would be a serious consideration during  340 hours of continuous sunlight, where temperatures can get over 120 degrees C, with another 340 hours of night with temps down to minus 150 C. But I'm not sure it would be insurmountable. If radiative cooling is directed to open space and it is shaded and insulated from radiant heat from sun and surrounding sun heated lunar surface that might be sufficient.

Building them on site seems problematic - these are complex technologies with exacting standards. It's unlikely they even can be built entirely or even mostly with locally obtained lunar materials - I would expect modular reactors, probably purpose built to handle the changes from Earth gravity to high G acceleration to zero gee to, finally, low lunar gravity. But we are discussing a base or outpost, not a colony expected to be able to make it's own equipment.

 

Edited October 20, 2017 by Ken Fabian
improve clarity

[Moontanman]

1 hour ago, Ken Fabian said:

I have reservations about massive global expansion of use of nuclear around the world as a primary emissions reductions strategy but that's for another thread; enough to say I think the dominant environmental, security and weapons proliferation concerns won't apply to use at a moon base. Or the same level of concern over cost. Safe and reliable launch would be important. I believe the USSR put 40 or so small fission reactors into orbit over the years. There are probably some still up there, but dumping them back into Earth's oceans when done isn't something I'd like to see emulated but that wouldn't apply to this. It would also not be up to me!

I don't know about the waste heat issue - it would be a serious consideration during  340 hours of continuous sunlight, where temperatures can get over 120 degrees C, with another 340 hours of night with temps down to minus 150 C. But I'm not sure it would be insurmountable. If radiative cooling is directed to open space and it is shaded and insulated from radiant heat from sun and surrounding sun heated lunar surface that might be sufficient.

Building them on site seems problematic - these are complex technologies with exacting standards. It's unlikely they even can be built entirely or even mostly with locally obtained lunar materials - I would expect modular reactors, probably purpose built to handle the changes from Earth gravity to high G acceleration to zero gee to, finally, low lunar gravity. But we are discussing a base or outpost, not a colony expected to be able to make it's own equipment.

 

Check out molten salt reactors. https://en.wikipedia.org/wiki/Molten_salt_reactor but the real problem on the moon is there are no rivers or seas to absorb vented waste heat. Even on Earth it takes gigantic water cooling towers to do the job...  

[Frank]

How embarrassing, given a ~20t system mass, a payload of 1/16.7 is possible, so  up to 1.25 t (1250 kg) instead of 250 kg.   Of course, 250 kg of batteries can still be used and recharge would still be 2h.

This version seems a bit clearer:

http://www.tethers.com/papers/InterplanetaryTetherTrnsprt.pdf

 

[Frank]

Given a 40t lunar momentum exchange tether with solar power (powervator?) we get 2.5 t of payload that can be either powered or transferred to EML-1.

A hydrolysis module can be powered directly instead of charging batteries.  So we have ~60 kW to play with.

I found a 1991 paper for a concept "Conceptual Study of on Orbit Production of Cryogenic Propellants by Water Electrolysis"  https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19910010004.pdf

Scaling their findings to 20%, I get a module mass of 2122 kg and power use of 58.2 kW.  But I removed the cryogenic hydrogen generation because it is to be reused in the production of LOX only.  So we get ~70 t of LOX per year of operation.

If a lunar base is thought of as a Deep Space Gateway, there may be an economic argument for producing LOX and placing it at EML-1.  Basically, each 20 t of LOX is equivalent to a launch's worth of payload on its way to Mars for example.

Of course the cost of maintaining the base, collecting the water, putting the LOX into orbit, etc... may ruin the economics.  If nothing else, it may end up being cheaper than maintaining the ISS.

The mining part can produce water (directly or via reaction lunar oxygen with earth hydrogen) which can also be exported to EML-1 and potentially other materials (silicon for solar panels, aluminium and iron).

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110006938.pdf

I may look into the Solar Carbothermal Regolith Processing and Gathering costs later.  The solar thermal part is linked to solar thermal propulsion.

 

 

[Enthalpy]

On 10/19/2017 at 0:45 AM, Frank said:

... a satellite in a polar orbit such that it is always exposed to the sun...

You might want to re-think this. But not the main difficulty anyway.

The low-orbit speed is 1680m/s around the Moon and rotating a part with that tip speed is extremely difficult. I emphasize that we have no carbon nanotubes is significant amounts, nor do they have interesting properties as a rope up to now. The best materials is graphite fibre composite, which would require a thick base to attain such a tip velocity. That means: tons of rope for kg at the tip.

Then, I want to see convincing details about the residual height and speed, and how the payload survives the landing.

Such detailed descriptions could make science. Until then, all these rotavators remain science-fiction.

==========

The other aspect is that a lunar base must be buried or shielded in some way, and then it keeps naturally a good temperature during the two night weeks. Electricity for other uses can come from batteries: a few tons suffice.

[Frank]

Why rethink the polar orbit?

 

I don't think you've read the paper I linked.  The tether material is Spectra 2000 (and is readily available) and a Hoyt tether (sp?)  has multiple strands that can withstand a break.  That part is known.  Everything in space seems "extremely difficult", the question is always "can it be done with reasonable resources?".

This is the paper: http://www.tethers.com/papers/InterplanetaryTetherTrnsprt.pdf

I've been looking for (papers on the web, written by smart people) that explain why it can't work and haven't found any.  I have found other versions of it like the MXER.  Here's one from NASA: https://www.nasa.gov/centers/marshall/pdf/115871main_MXER_TS.pdf

 

Also, the Carbothermal process uses the Sabatier reaction which is similar to what would be used on Mars.

Edited October 25, 2017 by Frank

[Frank]

I may drop the whole momentum exchange tether idea in light of new (to me) information.  It seems the solar energy per mass fraction has potentially shot up 10x recently by removing the substrate from space solar panels or using concentrated photovoltaics.  Here is the 2011 paper:  http://www.asertti.org/events/fall/2011/presentations-workshop/Landis.pdf  promising 1,000 W/kg and 3,000 W/kg, but reality always bites us in the ass and Ultraflex actually delivered 150 W/kg (at 1AU)     https://www.orbitalatk.com/space-systems/space-components/solar-arrays/docs/UltraFlex_Factsheet.pdf 

So 3x better than ISS panels I used as metric, but they still need to be pointed at the sun with a solar tracker, adding mass and don't operate ~20% of a year.  The tether and catching apparatus etc...  have significant mass as well.  But the trackers and panels are known technology, whereas the momentum exchange tether is still conceptual. Tether mass to panel mass seems adversely disproportional. 

This means a tether would need less launch mass if some lunar regolith were used as ballast or it could deliver up to 3x the energy to a payload (battery or powered module).  So maybe 180 kW for a 40 t system with 2.5 t payload.  Still save the soft-landing cost.  Some of that power can go to orbit restoration via the new X3 hall thruster (5.4 N at 102 kW, ~3,000 s Isp) http://ns.umich.edu/new/releases/25192-thruster-for-mars-mission-breaks-records.  Two thrusters could be used if power from a previously charged battery payload were added to the solar panel output.

Lunar solar panels would mean some holidays ~20% of the time where battery operated machinery would stop and those batteries would be used to power essential systems like stereos and TVs for downtime leisure...

Hmmm.

 

[Frank]

For those interested in tether analysis "Three Dimensional Dynamics of a Flexible Motorised Momentum Exchange Tether", includes many references (2016):

https://pure.strath.ac.uk/portal/files/54060596/Ismail_Cartmell_AA2016_Flexible_motorised_momentum_exchange_tether.pdf

 

[Frank]

After reading all this info about space solar panels and ion drives, It occurred to me that the 100 kW megaflex https://www.orbitalatk.com/space-systems/space-components/solar-arrays/docs/MegaFlex_Solar_Array.pdf solar array being developed might power that X3 thruster making a tug for small loads, for example 2.5 t which happens to be the design load for the lunavator.

So if the mass of the thruster and solar arrays is 500 lbs plus 150 W/kg, we get

(500 pounds) + ((100 kW) / (150 (W / kg))) = 893.462852 kilograms

and doubling for structure and other equipment = 1,786.9257 kilograms

plus payload 2.5 t is about 4.3 t

pushing 4.3 t from LEO to EML-1 needs 3.8 km/s delta-v

So we're looking at somewhere around 3.8 km/s /(5.4 N / 4.3 tonne) = 35 days

But if we build a fleet of them, taking advantage of mass production, we can haul large payloads or fuel for pretty cheap (relatively).

 

Note, I didn't include ion drive propellant, so likely more than a month...

mass of propellant m0 = 4300 * Math.exp(3800 / (9.8 * 3000))

m0 = 4893.2990256160165

So propellant is about 593 kg

Is there an easy way to figure out time given the non-linear propellant/mass rate?

So more than 35 days and less than 40 days to reach EML-1 from LEO, maybe more because we can't take advantage of the Oberth effect?

 

[Frank]

I'm calling the lunar rotavator LunaMET (lunar momentum exchange tether).  This is a simplified version of the above given the possibility of SEP tugs (Solar Electric Propulsion tugs) which would slowly lift cargo from LEO to EML-1, can stow their solar panels for aerobraking return to LEO and can be used as power source for modules when not hauling cargo.  To simplify, use the lunaMET only to move cargo to and from the moon and EML-1 (no power lines down the tether).  At EML-1 would be the recharge infrastructure for batteries.  Also at EML-1 would be the electrolysis unit to convert water to LOX and either LH2 or LCH4.  EML-1 would also have a propellant depot which may simply(tm) be rocket upper stages.  So the SEP tugs would plug their solar panels (somehow) into the EML-1 station to run the production machines, notably battery chargers and water electrolysis.

 

It seems the lunar rover proposed by NASA would have a payload mass between 3 to 6 t (on the moon), so it might make sense to use this as a maximum cargo mass for the LunaMET.  Let's call this object (3 to 6 t) a cargo_unit.  https://www.space.com/5098-nasa-chariot-father-lunar-rover.html

Essentially, the counterweight matches tether mass and the hub has solar panels that are effectively two SEP tugs, one on each side, with deployed solar arrays at 90 degree offset so they don't obscure one another.  The counterweight mass would be lunar regolith.  Somehow brought up using the tether.  Maybe use the fat end of the tapered lunaMET cable as a counterweight and lift smaller mass to bootstrap the system.

The SEP tugs would be scaled to use a number of X3 ion drives (5.4 N, 102 kW) pushing a cargo_unit mass.  The number depends on acceptable LEO to EML-1 trip duration.

SEP tug: http://www.spaceflightinsider.com/missions/human-spaceflight/solar-electric-propulsion-nasas-engine-mars-beyond/

 

If cargo_unit is 5 t, a falcon 9 could put two of them into LEO.  To keep trip duration to about a month, two drives and ~200 kW of solar panels would be required.  LunaMET mass would climb to 80 t!   Some amortization calculations need to be done to figure out the best value for cargo_unit.  Launches needed VS expected savings in mass lifted/landed.
 

 

Given that the same rovers would likely be used on Mars, and 2.5 t would be the max. cargo there, 2.5 t might be the best value for cargo_unit.  It also happens to be exactly what the lunavator paper uses meaning in a 40 t LunaMET mass.

 

Of course, on the moon, the rover could carry two 2.5 t cargo_units.

 

Going with two Ion engines on the SEP tugs gives a bit of extra redundancy and speed.  Efficiency can be increased at the expense of trip duration by pushing multiple cargo_units.  Falcon 9 lifts 10 t in one go, so there is some flexibility as to the number of tugs to use.