Lunar Rover Power

Powering a rover through the two week lunar night is receiving a bit of attention. Most of the attention seems to be on better batteries and thermal control with some suggesting that nuclear is the only reasonable option for such a long time period of extreme cold and darkness. It may be possible to power a lunar installation through the night without using extreme storage or the dreaded nuclear. It may be time to think about the advantages of a Lunar SPS to bridge the gap between the periods of abundant sunlight energy on the ground.

Lunar SPSWhile Lunar stationary would be restricted to orbits so far out as to make the required rectennas too large for rovers and other small early prospecting missions, a lower Lunar orbiting SPS would be able to service much smaller installations. Required rectenna size is inversely proportionate to distance from the power source. Half the distance being half the required diameter and a quarter of the area required. A rectenna on Earth receiving from GEO needs to be a mile or more in diameter with the beaming distance of 22,300 miles or more if not directly under the SPS . A Lunar SPS at 250 miles or so altitude could efficiently hit a rectenna 1% of that or around 50 feet in diameter. Fifty feet diameter is possibly  reasonable for a small rover on a world with  light gravity and no wind. It is certainly reasonable for a stationary prospector of small size.

The drawbacks are both the intermittent nature of the available power and the variable angle with distance of the rover from the SPS. The intermittent nature of the available power will probably be solved for powerless periods of two to three hours followed by intense charging periods with relatively small battery systems.  Rectennas can be designed to accept power from multiple angles with relatively minor design work. The variable distance means that much of the low angle power will be wasted unless a much larger rectenna is built into the surface station. That waste could easily be more acceptable than nuclear depending on the political environment during a particular mission or exploration era. Even wasting 90% of power could be more acceptable than ground systems that must shut down for half of their working lives.

Once a Lunar SPS is operating in this manner it can be used for a multiple of ground stations accross a period of decades.

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I do construction for a living and aerospace as an occasional hobby. I am an inventor and a bit of an entrepreneur. I've been self employed since the 1980s and working in concrete since the 1970s. When I grow up, I want to work with rockets and spacecraft. I did a stupid rocket trick a few decades back and decided not to try another hot fire without adult supervision. Haven't located much of that as we are all big kids when working with our passions.

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About johnhare

I do construction for a living and aerospace as an occasional hobby. I am an inventor and a bit of an entrepreneur. I've been self employed since the 1980s and working in concrete since the 1970s. When I grow up, I want to work with rockets and spacecraft. I did a stupid rocket trick a few decades back and decided not to try another hot fire without adult supervision. Haven't located much of that as we are all big kids when working with our passions.
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14 Responses to Lunar Rover Power

  1. George Turner says:

    For just hunkering down, how about deploying a large insulated mylar umbrella over the rover, with its edges touching the ground, to reduce heat loss from the regolith underneath the rover? This would act like adding insulation on top of the soil around the edges of an underground house or basement, forcing the heat retained by the soil’s thermal mass to travel a longer sideways distance to get out from under the blanket, maintaining much more of the residual daytime heat. It would be much like a dome tent and the rover might even benefit from the soil’s IR energy being focused on it by the mylar.

    Then you make the umbrella do double duty as a thin-film planar rectenna.

  2. Andrew Swallow says:

    A rover can be powered by batteries. The space power satellite (SPS) could power the recharge point during the lunar night. The rover ‘refuels’ by changing batteries.

  3. ken anthony says:

    We could continuously provide power to earth vehicles but we only do that with some trains and buses. Most vehicles used stored energy (gasoline.) Googling tells me pumped water (and gravity) is the most efficient method of storage (I would have thought flywheels with magnetic bearings.)

    To get over our nuclear problems I just wish these guys would give us our cigarette pack sized 250 hp thorium nukes which would be ideal on any rock.

  4. ken anthony says:

    Correction: 250 KW or 335 hp.

  5. ken anthony says:

    Of course, bananas have potassium which is radioactive so…

    One banana, two banana, three banana, four
    Four bananas make a bunch and so do many more.
    Over hill and highway the banana buggies go
    Coming on to bring you the Banana Splits show

  6. gbaikie says:

    Why not land a depot for rover.
    Have rover run on Hydrogen fuel cell, and it refills at depot.

    So, rover lander is depot which holds a few gallons of liquid oxygen
    and enough Hydrogen or methane to go with the oxygen {and could just extra rocket fuel which isn’t used up in landing}.
    Rover starts with full tanks and when runs low returns to lander for refill.

    More complicated could be saving the water and converting back into rocket fuel. So lander would have solar panels have the power to do this. Making it operational as long one can split the water.
    And there endless complication, one then mine lunar water and get surplus of rocket fuel.

    You could rocket and roll. Have lander and mini lander. Have rover on mini lander. Lander lands. Then mini lander blasts of from it, taking rover to some location, then rover rolls off of mini lander.
    Rover travels about, and then returns to mini lander, and it takes them back to lander to refuel.
    This would allow landing on peak and getting rover to bottom of crater. Also allows a tall lander. Assume mini lander never tries to gets back on lander, rather mini lander when returns lands near the lander.

    It seems just starting with just depot is easiest. So just bring enough rocket fuel and not try the mining and splitting water, and have later variants attempt recycling the water and/or mining lunar water.
    So have cryogenic oxygen and fuel in depot and have the boil off saved and pressurized, and use pressurized oxygen and fuel for rover power.

  7. guthrie says:

    I understand setting limits to a thought experiment to explore other viable options, but is there a reason nuclear power is described here as ‘dreaded’?

  8. fred k says:

    I propose using solar cells on the rover. During the lunar day the solar cells gather energy from the sun.

    A lunar SPS could deliver its energy via an optical laser aimed toward the rover.
    L1 (near side) and L2 (far side) would be possible locations for the SPS

  9. johnhare johnhare says:

    Dreaded by people that freak out at the word as if every powerplant was a bomb waiting to go off. The most technically viable option in my opinion has serious PR issues with many loud and low information people. This affects government regulations on others as well as government launches directly.

    Fred K,
    L1 and L2 would be possible locations only if you could get a very accurate and very tight beam at such long distances. If you can, then you have a weapon that will make some people nervous.

  10. fredk says:

    John: Granted, 50K kilometers implies a lot of beam dispersion; I imagine if you have arbitrary amounts of power you can overwhelm the problem that way. I realize this only makes your “weapon” objection greater.

    However, we are only talking about delivering roughly 1300 watts/meter2, which shouldn’t scare anyone. Not to mention the only available target would be in close proximity of the moon, not the Earth.

    Although I suppose those with a military imagination could scale things up, relocate the SPS, and replace cheap drones with expensive Star Wars lasers. … OK, so we know who to pitch this to.

  11. Robert Clark says:

    Studies have shown some locations on the Moon at high altitude near the poles receive nearly continuous sunlight:

    These could be sites to locate lunar colonies.

    The lunar RESOLVE mission to produce propellant from water at a shadowed crater on the Moon finally got the go ahead:

    NASA Plans to Make Water on the Moon.
    APR 12, 2013 07:50 PM ET // BY IRENE KLOTZ

    NASA is developing a lunar rover to find and analyze water and other materials trapped in deep freezes at the moon’s poles and to demonstrate how water can be made on site.
    Slated to fly in November 2017, the mission, called Regolith and Environment Science and Oxygen and Lunar Volatile Extraction (RESOLVE), will have a week to accomplish its goals.
    To stay within a tight $250 million budget cap — including the rocket ride to the moon — project managers are planning to use solar energy to power the rover’s systems and science instruments. However, sunlight on the places where water and other volatiles may be trapped only occurs for a few days at a time.

    The reduced sunlight at such locations will limit the length of time the mission can operate. However, some of the long illumination sites are located near shadowed craters. So we might be able to place mirrors at the illuminated site to provide solar power to the RESOLVE rover in the shadowed craters.
    This would necessitate a more complex mission involving two landing sites however.

    Bob Clark

  12. Paul451 says:

    Nukes are going to be too heavy for a rover (at least a rover capable of being landed in one piece.) RTG’s are small enough, but might not provide enough power to keep the important bits warm over the lunar (fort)night.

    That means you either have a nuke power-plant on a lander, with the rover distance-limited by its ability to return to the lander at night. Or you are relying on something else to provide power at night… hence the OP.

    Would a microwave transmitter really be any more efficient than a laser? (Or even just a few stonking big dumb reflectors. I wonder if you could have a solar sail statite over the lunar pole. Minimum focus for the sun at 1AU is 8.7 units focal width per 1000 units of distance.)

  13. john hare says:

    I’m not really up on efficiencies of beamed power and depend on various tidbits I’ve read that seem to favor microwaves for efficiency, and lasers for tighter beams.

    I wish I knew enough about statites to have an informed opinion. My uninformed opinion is that it would take vastly more statite area to get the same results, which may or may not be offset by cheaper unit area for the space based portion.

  14. Luigi says:

    I am wondering whether a simple orbiting mirror to reflect some sun light on the PV cells of the rover wouldn’t be simpler. Sure, there is a limit to how much can we manipulate non-coherent radiation, but it is worth doing the maths. Also, for beamed power lasers are a better idea than microwaves, because the beam divergence is inversely dependent on the wavelength, so the shorter the wavelength, the better. In general, I think that once we have >1 MW CW power laser, everything will be easier, since we could build then beam powered ion drives, which would greatly benefit from not having to haul around their heavy power sources.

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