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Background: The moon has been selected as the base for human space exploration. Rather than ship tonnes of material out of earth's gravity well, interplanetary spacecraft and space stations will be manufactured on the surface of the moon and launched from there.

A level of industrial capacity allowing the mining and processing of ores has been developed on the moon, as well as manufacturing, and reasonably good material science using moon resources. As a rough guideline, if a material could have been manufactured on earth in the 1970s a reasonable substitute in terms of material properties can now be manufactured on the moon. If you can justify why a material exceeding that standard can be manufactured on the moon, you can use it.

Note: To clarify, the tech level does not need to be restricted to the 1970s (the target is in the future but with minimal additional scientific advances). I have placed this restriction on the level of material science because developing simple structural materials (e.g. steel) on the moon would pose significant challenges, but the details are beyond the scope of the question.

The question: How will these spacecraft manufactured on the moon be powered?

Criteria:

  • The more realistic the power source is based on current science the better: Currently implemented > Prototyped > In development > theoretical > hypothetical > impossible
  • The raw materials should be found on the moon, with as little mass as possible imported from earth. The more abundant and easily processed the raw materials are the better.
  • The answer should explain how the power source is able to power both the craft's propulsion and its other power requirements.
  • The end result must be a portable power source for the spacecraft, with both the drive for the spacecraft and any fuels required manufactured and produced on the moon. The same levels of credibility apply to the drive as to the power source.

Note: I am looking for the craft's principal power source. Because some power sources lend themselves more easily to providing propulsion I feel an explanation of the propulsion system used in conjunction with the power source is also necessary.

Examples of power sources

  • moon manufactured solar panels with batteries, and an electrically powered propulsion system.
  • hydrogen, oxygen, and rocket engines manufactured on the moon.
  • space ready nuclear-powered engine made on the moon

Excellent answers will provide evidence for how well developed the power source currently is. Evidence for how well-developed drives which can work with that power source are. Evidence for ores of any crucial raw materials on the moon.

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch ♦
    Commented Nov 20, 2018 at 20:16
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    $\begingroup$ One propellant combination that can be sourced from the moon is powdered aluminum and oxygen. For background, see the following NASA Technical Memo: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19940017287.pdf $\endgroup$
    – Jim
    Commented Nov 20, 2018 at 20:29
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    $\begingroup$ As the OP I would prefer the question to remain hard science. The best answers on this question I feel meet that standard - with links to credible sources for all major points. As the person asking the question I am biased towards liking answers even if they don't meet the hard science standard - but I think that hard-science is the most appropriate standard for the question. $\endgroup$
    – user42528
    Commented Feb 17, 2019 at 9:07

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An Aluminium-Oxygen drive burns Aluminium and Oxygen (as the name would suggest), achieving a mediocre specific impulse (the primary measure of fuel efficiency for rockets) of about 285 seconds. Normally this would be of no interest for use as a rocket fuel since current Hydrogen-Oxygen drives such as those used for the Space Shuttle's main engines can achieve specific impulses of around 450 seconds, far superior to the pathetic specific impulse provided by Aluminium-Oxygen combustion.

However, the advantage of an Aluminium-Oxygen engine is the fact that you can make the fuel out of nothing more than regolith (lunar dirt and rock) and electricity. According to this paper, reducing Aluminium Oxide, which is present in the lunar regolith, requires high temperatures (above 1832 K), which can be provided by solar power, as well as Carbon and Iron Oxide, both of which are available in the lunar regolith (although getting enough Carbon will require processing a lot of ore, and it would be reclaimed from the CO2 after the reaction to as great an extent as possible). This reduction was performed for the linked paper above, so it's clearly in the "Already Implemented" stage.

Actually building a rocket that uses Aluminium and Oxygen as fuel will be fairly simple, as it's essentially just a fairly inefficient hybrid-propellant rocket, which is a type of rocket we've already built and flown a number of, and are easily capable of building more of if the need should arise (the need hasn't arisen because liquid fuels have higher specific impulses, but the various liquid fuels in use on Earth are much harder to produce on the moon, so using Aluminium and Oxygen is a viable alternative). So this is somewhere between the "Prototyped" stage (as we don't have an Aluminium Oxygen drive specifically yet) and the "Currently Implemented" (because we do have many hybrid rockets of other types) stage.

Therefore, spacecraft manufactured on the moon could be propelled by an Aluminium-Oxygen drive, using propellant manufactured on the moon with nothing but lunar dirt, a solar farm, and some already-built equipment.

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    $\begingroup$ That's probably best system for just ferrying stuff to lunar orbit $\endgroup$
    – Mranderson
    Commented Feb 16, 2019 at 12:43
  • $\begingroup$ @Mranderson I must admit that it's abysmal specific impulse does make it unsuited to longer missions, yes. The higher stages of long missions (anything interplanetary, with the possible exception of Mars) would likely be better serviced by something like H2/LOX rockets with fuel made from lunar ice (see ShadoCat's answer). $\endgroup$
    – Gryphon
    Commented Feb 16, 2019 at 18:36
  • $\begingroup$ How would this compare to using hydrogen peroxide, if there was a lot of water ice available? $\endgroup$
    – Innovine
    Commented Aug 5, 2019 at 4:57
  • $\begingroup$ @Innovine as stated in the answer, the specific impulse of this kind of drive is abysmal compared to every variety of rocket currently in use. The abundance of fuel, no matter where on the lunar surface you go, is the main benefit. There is water ice available on the moon, but it is nearly entirely present at the poles, the worst possible location to launch a rocket from. Additionally, there are significant problems with fuel storage and reliability for hydrogen peroxide drives (they like to blow up) that make hydrogen-oxygen fuels far superior in the vast majority of situations. $\endgroup$
    – Gryphon
    Commented Aug 15, 2019 at 13:30
  • $\begingroup$ Interesting, because when I asked a separate question about that, I was informed that h2o2 was a much better fuel due to issues in storing high pressure hydrogen. $\endgroup$
    – Innovine
    Commented Aug 15, 2019 at 15:37
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A brute force concept using 1970s technology and lunar materials:

Use lunar ores to make reflective materials and Stirling engines. Use these materials to build solar-thermal power plants on the moon. Store the energy using molten-metal batteries or batteries that melt-and-refreeze metals or salts, also made using lunar material.

Use high-power ablation technology for the rockets. The bottom of the ship is a large shaped piece of metal. Send an intense energy beam from the launch site to the bottom of the ship. Boil off the metal, to provide thrust. Earth-based launchers using this concept would need about 3 GW of power. (Per "Halfway to Anywhere", in A Step Farther Out.) Since the moon has about 1/6 of Earth's gravity, 500 MW would suffice.

Bonus points if the "intense energy beam" is a laser beam, mounted on a stuffed shark. (The sharkskin would probably need to be imported from Earth.)

Include a small-scale solar-thermal system on the spaceship, along with a small-scale version of the battery system. Send a modest energy beam from the moon to the ship's solar-thermal system to power the spaceship.

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    $\begingroup$ Do you have any references for the availability of suitable ores for molten metal batteries on the moon? $\endgroup$
    – user42528
    Commented Nov 20, 2018 at 17:27
  • $\begingroup$ The molten-metal battery concept is very flexible. A wide range of metals are suitable. The original concept was to use aluminum. Aluminum is an element in much of the moon's rocks. $\endgroup$
    – Jasper
    Commented Nov 20, 2018 at 17:33
  • $\begingroup$ @Ben - Another concept that could be called a "molten metal battery" is to melt a metal with a low melting point (such as sodium or aluminum) to store energy, and let it re-freeze to release energy. $\endgroup$
    – Jasper
    Commented Nov 20, 2018 at 17:47
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    $\begingroup$ en.wikipedia.org/wiki/Geology_of_the_Moon for elements $\endgroup$
    – Artemijs Danilovs
    Commented Nov 20, 2018 at 17:50
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    $\begingroup$ This is not a hard science answer. Add more technical evidence that this system will work. $\endgroup$
    – kingledion
    Commented Nov 20, 2018 at 22:36
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"The Japanese Kaguya spacecraft, which was launched in 2007, detected uranium with a gamma-ray spectrometer. Scientists are using the instrument to create maps of the moon's surface composition, showing the presence of thorium, potassium, oxygen, magnesium, silicon, calcium, titanium and iron."

https://www.space.com/6904-uranium-moon.html

Thus the obvious solution is to build a nuclear fission reactor on the moon and build a uranium mine. Nuclear fission (and fusion) do not require oxygen, so no atmosphere is needed on the moon.

Nuclear thermal rockets were prototyped and had (non-flight) tests from the 1950s to 1970s.

To date, no nuclear thermal rocket has flown, although the NERVA NRX/EST and NRX/XE were built and tested with flight design components. The highly successful U.S. Project Rover which ran from 1955 through 1972 accumulated over 17 hours of run time. The NERVA NRX/XE, judged by SNPO to be the last "technology development" reactor necessary before proceeding to flight prototypes, accumulated over 2 hours of run time, including 28 minutes at full power. The Russian nuclear thermal rocket RD-0410 was also claimed by the Soviets to have gone through a series of tests at the nuclear test site near Semipalatinsk.

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  • $\begingroup$ This seems like a good start to an answer, but how can this be used to power spacecraft launched from the moon? $\endgroup$
    – user42528
    Commented Nov 20, 2018 at 17:15
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    $\begingroup$ @Ben: in very terrifying way I'd guess. $\endgroup$
    – PTwr
    Commented Nov 20, 2018 at 17:30
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    $\begingroup$ Or an even more terrifying way. $\endgroup$
    – Skyler
    Commented Nov 20, 2018 at 18:47
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    $\begingroup$ This is a nice answer. Unfortunately it seems like these uranium supplies are unlikely to be in high enough concentration to be usable space.com/8644-moon-map-shows-uranium-short-supply.html $\endgroup$
    – user42528
    Commented Nov 20, 2018 at 21:38
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    $\begingroup$ This is not a hard science answer. Where are these deposits? Are they economically retrievable? What is the list of components that a nuclear thermal rocket uses? Can you launch one without irradiating the environment? $\endgroup$
    – kingledion
    Commented Nov 20, 2018 at 22:38
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The simplest method of obtaining a rocket fuel from the Moon is to mine the ice on the poles. This gives rocket fuel from a single mining source.

According to Wikipedia:

In March 2010, it was reported that the Mini-SAR on board Chandrayaan-1 had discovered more than 40 permanently darkened craters near the Moon's north pole that are hypothesized to contain an estimated 600 million metric tonnes (1.3 trillion pounds) of water-ice.

Then you just need a bit of heat, a lot of electricity and the ability to compress and to separately store H2 and O2.

You melt the water and use Electrolysis to split the water.*

Wikipedia link for if you don't know what that is:

This technique can be used to make hydrogen gas and breathable oxygen. As hydrogen is an important industrial commodity, by far most industrial methods produce hydrogen from natural gas instead, in the steam reforming process.

Then you just use cryogenic compression to convert the gases to liquid form for storage.

If the ice is not where you want to be launching rockets from, it is easy to transport the ice to the launch area. I would recommend that over transporting the O2 and H2 gases. For one thing, if the transport breaks down, ice will evaporate much more slowly than cryogenic liquid gasses.

*When did they change the term from cracking water to splitting water? A search on "crack water" yielded a whole bunch of links that I wasn't looking for.

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  • $\begingroup$ Although water ice is a good solution, it has a number of annoying properties, including only being found at the poles, which are the worst place to launch from and land on, meaning you have to either haul ice to the lunar equator or launch ships into polar orbits. Another annoying thing is that its only found in permanently shadowed places, where you can't get solar power to crack the ice, so you have to haul it out of the crater first, and even then the solar power at the poles is far lower than that at the equator, so it takes a long time to split the fuel. $\endgroup$
    – Gryphon
    Commented Feb 13, 2019 at 18:47
  • $\begingroup$ @Gryphon, that is why I talked about transporting the ice. Also, the poles might be the best place to generate electricity. Just as there are craters that have permanent shadows, there are peaks that have close to continual sunlight. That would allow splitting almost year round. $\endgroup$
    – ShadoCat
    Commented Feb 13, 2019 at 18:51
  • $\begingroup$ There's also the issue that the ice (at least the ice observed so far) is at a rather low concentration. You'd need some substantial refinement before you even start splitting the water. $\endgroup$
    – Gryphon
    Commented Feb 13, 2019 at 18:56
  • $\begingroup$ Another issue is that water on the moon seems likely to be a highly valuable resource in such a scenario - so using it as rocket fuel might not be economical. $\endgroup$
    – user42528
    Commented Feb 13, 2019 at 20:16
  • $\begingroup$ @Ben with 600 million tonnes of it, it's not like there isn't enough to go around. If my math is right, that's enough to launch a Space Shuttle every day for several millennia. Of course, the extraction rate does have to be taken into account, but on sheer quantity available, we wouldn't run out anytime soon (if you accept the 600 million tonne estimate, but I doubt it's off by more than an order of magnitude or so). $\endgroup$
    – Gryphon
    Commented Feb 13, 2019 at 20:59
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Solar power combined with ion engines and a mass driver:

Power Source

Solar power could conceivably be used to power the craft, providing both the onboard power and propulsion.

Solar power is already widely implemented, with solar panels sufficient to supply 227 Gigawatts of electricity having been installed globally by 2015.

The main component of most photovoltaic cells is silicon. This is the second most abundant element on the lunar surface, however, it exists in various ores rather than the relatively pure form used for solar panels on earth. A process to extract silicon from these ores would be required. Such a process has been suggested.

Propulsion in space

For propulsion while in space solar panels could be combined with ion drives - this is a technology which has already been implemented. Ion drives require a propellant, for this a range of elements have been used or proposed, including xenon, argon, iodine, mercury, and bismuth. Designs such as VASMIR could theoretically use practically any material for propellant. Thus it should be possible to find a suitable propellant on the moon.

Propulsion to launch

Ion drives do not, however, provide sufficient thrust to escape lunar gravity. This could be achieved by accelerating the craft on a track using linear motors, as implemented in maglev trains on earth. There are many implementations for transportation on earth, but so far this has not been used to propel a vehicle to lunar escape velocity. Such a launch system has been proposed for use on earth, where air resistance and a much higher escape velocity pose challenges not encountered on the moon.

Summary

Solar panels could be used to power propulsion systems which can be run on electricity. Ion drives provide such a propulsion system for use in space, and mass drivers provide such a system for launch.

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    $\begingroup$ Other points which would improve this answer, but which I haven't got round to include: info on the feasibility of manufacturing batteries on the moon, and info on the elements required to build an ion engine. Also, are there any usable sources of xenon, argon, iodine, mercury or bismuth on the moon? $\endgroup$
    – user42528
    Commented Nov 21, 2018 at 21:40
  • $\begingroup$ Exactly. Why bother with expensive chemical propellants when you can just build a space catapult to launch a ion-driven spaceship directly into orbit? However, the catapult still needs to be quite big, as you need to accelerate to more than 6000km/h, and the ion drive needs to be powerful enough to raise the pericenter of the orbit out of the danger zone within the first two hours of flight. The higher the end speed from the catapult, the more time the ion drive has to do its work. $\endgroup$
    – cmaster - reinstate monica
    Commented Mar 30, 2019 at 18:09
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Magnetic propulsion. For 6 days every month.

earth's magnetic tail

https://www.nasa.gov/images/content/222898main_orbit2_20080416_HI.jpg

The moon moves through the Earth's magnetic field in the course of its orbit. Once in the field, moon-based spacecraft could move via electromagnetic propulsion. This is not science fiction.

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

Electrodynamic tethers (EDTs) are long conducting wires, such as one deployed from a tether satellite, which can operate on electromagnetic principles as generators, by converting their kinetic energy to electrical energy, or as motors, converting electrical energy to kinetic energy.1 Electric potential is generated across a conductive tether by its motion through a planet's magnetic field.

Spacecraft with batteries (charged by solar panels during the other 24 days of the month) charge up their long tethers and use them to propel themselves about, pushing against the Earths field during its monthly visit.

Longer and more energetic tethers might be used all month long, pushing against the relatively weaker electromagnetic field of the sun and the charged particles of the solar wind.

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  • $\begingroup$ While this is an interesting idea for maneuvering in space, I'm fairly sure that it will not provide enough thrust to lift off from the lunar surface (feel free to correct me if I'm wrong). What mechanism would you propose for achieving orbit in the first place? $\endgroup$
    – Gryphon
    Commented Feb 15, 2019 at 16:30
  • $\begingroup$ Electrostatic propulsion is something different, but it is apparently enough for dust to lift off the surface of the moon and scoot around. space.com/35240-moon-dust-levitates-nasa-study.html So to lift off - electrostatic repulsion from the moon. For flying around: electrodynamic propulsion. $\endgroup$
    – Willk
    Commented Feb 15, 2019 at 17:24
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I strongly recommend two books that came out in the '70s

G. Harry Stine "The Third Industrial Revolution"

and

Heppenheimer's "Colonies in Space"

I see both in used book stores on a regular basis.

Stines notions was that you build orbital power satellites that would beam microwave energy down to receiving antenna on Earth. You use phased array to keep the beam narrow. No, the energy isn't enough to cook you if you are on the receiving antenna.

Material is mined on the moon and launched to a Lagrange point by a rail gun. There it is broken down using kilometer diameter solar mirrors. Much of the waste is silica -- which can be foamed and used as structural infill. Aluminum is the main structural material. Some silica is broken down to silicon (solar cells) and Oxygen (breathing) Hydrogen is in short supply. But if you can make oxygen, then you only have to ship up 1/8 the amount of rocket fuel you did before. And maybe those polar craters on the moon do have water in them.

Stine is convincing. He has an engineering background and had access to various think tank reports from the likes of the Rand Corporation.

Colonies in space is a bit further out and is more about establishing more than a work camp in zero-G.

Iron seems to be in short supply on the moon. H. proposes a nuclear rocket. Build a nuclear reactor that gets hot enough to turn gravel into hot gas. You can move asteroids then by landing such a rocket and a gravel crusher. At this point, I don't think we can make a nuclear engine that directly operates at those temperatures. Make electricity, make a plasma. Electrically accelerate the plasma. You can get huge specific impulse this way. It's not hard to get plasma up to a respectable fraction of light speed. It's more efficient however to accelerate more mass to a lower speed.

Be sure of your trajectories. Don't want to drop a 3-mile rock on the Earth by mistake.

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The easiest propellant to make on the moon would be ALICE, or an aluminum nano-power mixed with ice. No need to separate out the oxygen and hydrogen from the water.

ALICE Rocket Fuel Tests

Another option is to use pure water heated with a nuclear reactor, making a steam rocket. This does not have the specific impulse of hydrogen/oxygen, meaning that it does not provide as much momentum change per unit mass of propellant, but it has a number of large advantages: You don't have to worry about handling cryogenic fuels, the spacecraft is simpler, the fuel tank can just be a bladder, etc.

Steam Rocket Powered by Lunar Water

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Sodium in nuclear thermal rockets, or the same sodium as remass in an aluminium oxygen hybrid rocket.

Sodium with an atomic mass of 23, is surprisingly good as an NTR propellant. Working near the melting point of uranium dioxide, the said propellant produced just over 300 seconds of ISP. Equivalent to that of modern storable propellants in term of specific impulse.

If combined with a graphite moderator, the ISP reaches 340 to 350s， high enough for most transfers, and certainly enough to reach nearby planets.

The sodium can be obtained as a byproduct from processing lunar feldspar rocks for aluminum construction material and breathing oxygen.

Sodium in a reaction of aluminum and oxygen greatly increases the thermal property of the plume and produces an ISP rating about 360 to 370s. This is viable for a Jupiter transfer.

Also, you can harvest lithium from lunar rocks and burn it in liquid oxygen. This affords more than 550 seconds of ISP, which is enough to send an interplanetary invasion. Lithium is also good for propellant of (artillery) guns, and as an energetic material in explosives.

Lastly, sodium and lithium can be used as a propellant for electric drives, which can be powered by silicon solar panels manufacturable from moon rocks. Or a fission reactor has to be shipped from the earth.

However, sending just the uranium 233 will not be too expensive, at least not as expensive as sending all the propellants up to the moon. This makes the whole fuel production process economically viable.

There is thorium ore on the moon, so nuclear power will be the go. Build the reactor on the moon, and you will be able to sell space service even back to the earth!

In summary, use alkaline metals to replace the hydrogen, and it shouldn't be too hard to build spaceships on the moon.

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    $\begingroup$ The question has the hard science tag, meaning that answers should be backed up by citations. Can you provide such? $\endgroup$
    – L.Dutch ♦
    Commented Feb 25, 2019 at 12:33
  • $\begingroup$ all the energy / specific impulse profiles specified above is calculated with Children of a Dead Earth , the most realistic space battle simulator, using real world enthalpy of formation/standard entropy data. The cost of the elements/materials are calculated using real world data on lunar soil / solar abundance of the elements. Power generation and refining is assumed to happen on the surface and is therefore irrelevant for this calculation. $\endgroup$
    – Ardas
    Commented Jul 10, 2019 at 8:22
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Straying slightly from currently available technologies to those that are possible, but not yet achieved...

The Lunar surface is rich in Helium-3, so if Helium-3 fusion propulsion is developed, there is abundant fuel for it.

https://www.esa.int/Our_Activities/Preparing_for_the_Future/Space_for_Earth/Energy/Helium-3_mining_on_the_lunar_surface

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  • $\begingroup$ The top few centimeters of the lunar surface contain low-double-digit parts per billion of helium-3. Also, you can get all the benefits of He-3 fusion with p/B-11 fusion, without having to burn a fuel that requires you to scrape up and process billions of tons of lunar regolith. $\endgroup$
    – Christopher James Huff
    Commented Oct 6, 2019 at 13:10
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In all honestly, although there are many feasible answers, there is only one true correct answer.

Tritium

Tritium is one of the best fuel sources in the solar system, by a wide margin. Uranium radioactive and requires massive infrastructure to run. Hydrocarbons have annoying side effects and are some of the least efficient fuel sources for effort put in, and solar takes a lot of work just to get a small amount (of albeit non-fueled) energy.

Tritium is fairly safe as far as fuels go and constantly being spewed off by the sun in massive amounts. It's energy dense, and can largely be 'burned' as is (resulting in hydrogen which... can just be burned again). However, it reacts with Earth's atmosphere, rendering it into old fashioned H2 (aka, hydrogen), so on the surface of Earth, Tritium is useless, and is worth significantly more than it's weight in gold and never gets used as a fuel source.

The moon, however, is a Tritium sponge. In a day's worth of a single person harvesting, they could harvest enough Tritium to pay for the trip to the moon. In a year, it could cover our entire world's space program to date. And this is before you get serious tritium sifting infrastructure going.

It's such a ridiculous gold-rush like opportunity, that every multi-million dollar corporation that's not trying to go to the moon is stupid.

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  • $\begingroup$ I'd like to point out that the reason for the high price of tritium is its rarity. Were a company to go to the moon and bring back tonnes of the stuff, it's price would plunge, and the load would be worth a few dollars a gram, at the most. The Apollo program and its precursor Gemini cost a total of 288 billion in 2019 dollars. Even assuming we manage to cut that cost a hundredfold for a second set of lunar missions, there's no way it would be able to pay for itself by shipping back tritium. $\endgroup$
    – Gryphon
    Commented Sep 28, 2019 at 7:15
  • $\begingroup$ Just a bit of a nitpick on your statement that "every multi-million dollar corporation that's not trying to go to the moon is stupid". Sorry if it turned into a bit of a rant. $\endgroup$
    – Gryphon
    Commented Sep 28, 2019 at 7:15
  • $\begingroup$ @Gryphon It's actually in the same boat as Aluminium. Back in the early days of the US, Aluminium couldn't be manufactured, you could only get lucky and mine it pure. It was literally worth more than it's weight in gold, and was considered one of the most valuable minerals on the planet (which made it a big deal they capped the Washington Monument with it). Then cheap aluminium production was invented, and the price of Aluminium PER POUND plummeted, yes, but the value of the Aluminium INDUSTRY skyrocketed. The reason is because a regularly used substance is a more value industry than rarity. $\endgroup$
    – lilHar
    Commented Sep 30, 2019 at 17:57
  • $\begingroup$ @Gryphon and as an industry grows with an initially-rare resource, the profit-margins are insane, especially the more useful it is. And since tritium is the best fuel, and has other uses, it falls in that category more solidly than anything in history. Demand would outpace production increases. With the extraction method literally involving heading to another world, that means that profit margin AND low competition, which means the profit-margins would make simple rarity commodities seem like a joke. $\endgroup$
    – lilHar
    Commented Sep 30, 2019 at 17:59
  • $\begingroup$ @Gryphon And to top it off, it would be an energy/fuel industry, giving it the demand increase projection of oil. In short, the first company that CAN make Tritium Moon Mining a thing will be THE most wealthy business in the world, within a decade, hands down, no contest. It's a perfect storm of economic opportunity. $\endgroup$
    – lilHar
    Commented Sep 30, 2019 at 18:04
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While not really a propellant, since we as a civilization started to explore the seas using sails, there's no reason why we wouldn't (at least at first) explore space in a similar fashion.

Enter Solar Sails:https://en.wikipedia.org/wiki/Solar_sail

Another idea would be to turn the moon in a 'laser beam hedgehog' and use powerful lasers to propel spaceships across our solar system, in a similar fashion to a solar sail.

Otherwise, at current, the best conventional engine is still the Hydrogen Engine: https://www.nasa.gov/topics/technology/hydrogen/hydrogen_fuel_of_choice.html It (liquid hydrogen), however, would have to be obtained from water from ice mined from the asteroid belt and shipped to the moon.

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  • $\begingroup$ I like the idea of using resources from elsewhere in the solar system. Though not strictly to the letter of the question, it gets the spirit of it. How would the lasers be powered? $\endgroup$
    – user42528
    Commented Nov 20, 2018 at 17:23
  • $\begingroup$ @Ben and answerer: Do note that unlike sailboats (per the analogy), a solar sail driven craft can accelerate only away from the light source. That means a solar sail spacecraft is more limited than a sailboat. A sailboat can, contrary to common sense, sail against the wind - though only at an angle against the wind up to approximately 45 degrees away from the wind, not directly into the wind at 0 degrees from it. This is not an all-stop roadblock and it could still be useful, but it does need to be part of all planning (ie: How are we going to stop when we arrive?) $\endgroup$
    – Loduwijk
    Commented Nov 20, 2018 at 19:42
  • $\begingroup$ Solar sails have a thrust-to-weight ratio far less than 1. Once you get them into orbit, sure, you've got a fuel-free way to go anywhere. But what are you going to use to launch them? $\endgroup$
    – Mark
    Commented Nov 20, 2018 at 22:16
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    $\begingroup$ How do you take off from the surface of the planet with a solar sail? This is not a hard-science answer, add more technical evidence that your plan is feasible. $\endgroup$
    – kingledion
    Commented Nov 20, 2018 at 22:40
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    $\begingroup$ @Aaron, although one can't directly thrust towards the light source, one can "tack" to slow down one's orbit and allow the gravity of the light source to pull one in. $\endgroup$
    – Gryphon
    Commented Feb 14, 2019 at 1:04

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