What would a spacefaring race use as fuel for their matter-to-energy reactors?

Question

Imagine that we invent the technology to convert matter directly to energy, and capture most of the output for doing work. We can convert any matter to energy in this way, i.e. the matter does not need to be specially prepared or even homogeneous.

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EDIT: in answer to comments:

• yes, it's straight-up E=mc²
• let's assume these reactors capture 60% of E
• let's assume that the vessel's power needs, including propulsion, are similar to a nuclear submarine, which wiki suggests is "a few hundred megawatts"

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Also imagine that we're space-faring, with hundreds of thousands of vessels capable of interplanetary (but not interstellar -- no FTL) travel, each of which has a matter-energy reactor that powers all ship systems, including a form of propulsion that is entirely electric. (For comparison, in the real world there are ~65K public buses, and ~2,000K semi trucks on American roads these days.)

What would we use as fuel?

The trivial answer is "anything and everything," but I think we won't use just anything. One reason is that if we have any refueling stations far from Earth, we'll have to periodically deliver fuel to them, which means we'll care about efficiency.

What would these interplanetary fuel tankers be hauling? Hydrogen (the lightest element)? Or the heaviest element that's safe (e.g. not dangerously radioactive)? (Or maybe we wouldn't care if it's radioactive, because heavier is just so much more energy-dense?)

We don't have anti-matter, and we can't convert energy back into matter, so we still have to work with the properties and abundance of the matter that is available to us.

EDIT: I am primarily interested in what we'd use for reactors in space (vessels as well as space stations and/or colonies), rather than on Earth. I apologize for not stating this explicitly in the first draft. But I'm definitely interested in answers that present a "whole-ecosystem" perspective. Like, would distant colonies convert their sewage into energy? It seems like they'd want to recycle at least the water... anyway.

Answer 1

Mine tailings:

In your scenario, you can destroy matter. But even waste materials for your civilization out in space are made of the things you find valuable. In space, people are interested in places - asteroids, planets, moons, WHATEVER. Those places are composed of matter. Some of it you are there to mine or harvest, while a lot of it is stuff you don't care about, like silicates. There would be no tankers. You would process the matter that the facilities didn't use for something else.

So if you're extracting minerals from a rocky asteroid, then the rest of the asteroid is fuel. You land on a moon? Whatever you came there for is mined/harvested, and everything else is fuel. The efficiency of the reaction means you need mass to run a civilization, but you don't use very much for power. Over time, everything you didn't use would get consumed as fuel.

The mathematics of fuel will look completely different. Mass is fuel, so ANY mass source is fuel. Just because an asteroid is mineral-poor doesn't mean it isn't worth mining, because the rest of the asteroid represents fuel. A rich mine has lots of minerals, while a poor mine has abundant energy to waste.

Your civilization might spend a lot of effort to convert elements into each other. Do you need gold? Use particle accelerators and the like to produce gold. that's absurdly expensive for us, because it would use a ridiculous amount of energy. For these folks, the more mass they have, the more ridiculous amounts of energy they have. So you're not technically creating matter, just transmuting it using advanced particle physics.

But it will still be more practical to recycle matter in its current elemental form. We recycle aluminum because its expensive to make new compared to reusing the existing refined metal. The same will apply to transmuting elements.

Answer 2

Solar Wind

Converting Mass to Energy creates such a crazy amount of energy that you barely need any mass. One kilogram of matter has ~ 25,000 Gigawatt-hours of energy. Since a normal US house uses 10,000 kilowatt-hours per year, that 1 kg is enough to power 2.5 Million US homes for a full year.

So moving mass around is a waste of time and effort - let the mass come to you.

The solar wind is a stream of charged particles emitted by the Sun. Charged particles can be captured by placing them in a magnetic field, and thus driving them to a collection location.

The Sun emits ~ 1.5 Million tonnes per second of solar wind. That means that every second the Sun emits a billion times more mass-energy than our 1 kg example above.

Magnetic Collectors in Orbit

Place magnetic collectors in orbit directly around the Sun. The solar wind mass-flux is going to die off as 1/r^2 - so closer to the Sun gets more mass, but closer to transit lanes is probably more useful. You may find a bunch of collectors near the Gas Giants are the most convenient, if that's where most of the population lives.

We're only going to collect a tiny fraction of the solar wind, but it turns out that's enough.

If we assume that the constellation of mass collectors collects 1 particle out of every 100 billion the sun emits, that still collects:

$$ \frac{1.5\text M \tfrac{\text{tonnes}}{\text{sec}} \times 1000 \tfrac{\text{kg}}{\text{tonne}} \times 365 \tfrac{\text{days}}{\text{year}} \times 24 \tfrac{\text{hours}}{\text{day}} \times 60 \tfrac{\text{minutes}}{\text{hour}} \times 60 \tfrac{\text{seconds}}{\text{minute}} }{100\text B} = 470\,000 \tfrac{\text{kg}}{\text{year}} $$

Which is enough energy to support 1.2 trillion US homes.

Answer 3

First it might be a good idea to ballpark how much matter you would need. From Newtonian physics it takes about $62$ megajoules of energy to get a kilogram completely off planet earth, so 2 kilos would need $124$ megajoules, and a station twice the mass of the ISS would need $62000$ gigajoules (which is about the same amout of energy produced by a nuclear power station in 17 hours).

Now if we use Einstein's famous equation $E=Mc^2$ we can find out how much matter that would be needed to get that amount of energy,

$$E=6.2*10^{7}=M(3*10^8)^2$$ $$M=\frac{6.2*10^{7}}{(3*10^8)^2}$$ $$M=\frac{6.2*10^{7}}{9*10^{16}}$$ $$M=7*10^{-10}$$

It would be about $0.7$ nanograms to get a kilogram into space, or equivalently you would need 0.7 milligrams to lift a thousand ton space craft (about twice the mass of the ISS)

Having a "fuel tank" that stores about a ton of material would work well, as it would last for quite some time, as escaping gravity wells uses the most amount of energy in a space craft's flight.

You wouldn't need much of whatever fuel you use, and from this I would propose that your ships have 2 separate fuel systems, water and stone, as these would be present in the solar system in usable quantities.

Hopefully that helps.

Answer 4

Waste.

Landfill sites are expensive to set-up properly. The sites can eventually have housing built on them, but not without extensive plumbing to allow methane and noxious gasses to escape safely. They also create toxic-leakage into groundwater that will continue for hundreds if not thousands of years.

Volunteer to take this waste away in quantity for distribution to space farers and you could turn a tidy profit on the deal ensuring that the planet of our origin stays as pristine as it can given the givens.

Answer 5

The question declares:

the matter does not need to be specially prepared or even homogeneous.

Trash as you would Mr Fusion.

If it is true matter to energy, why wouldn't it be like Doc Brown of 'Back to the Future' refueling Mr Fusion? Ie. using what ever is convenient and low value near by. This would apply to smaller sub GW matter-energy units.

Ram scoops.

For largest of users of energy such as large interstellar ships. Actual planning of fuel use would be needed. I would expect ram scoops would be investigated, that is using whatever dust/gas is in the ships path. Removing things that would damage a ship and using them as fuel would defiantly be useful.

Oort cloud material.

It exists material like this will likely exist at every star system. Additional benefit of supplying water to ships. Automated systems can be deployed to collect this.

Gas giant siphoning.

Again a source that is fairly common so uniform ways of collection of this material can be developed. Easy enough for a large ship to send off an auxiliary ship to collect fuel while making deliveries.

Conclusion: Low value regularity available material.

Hydrogen is the most common element in the universe. I would expect therefor the bulk of matter used to fuel matter energy converters to use hydrogen and hydrogen compounds. (Gas giants, water ices of Oort cloud, interstitial gasses from ram scoops.)

Material that is low enough value that few/no permissions//permits are required to gather and use the material.

Answer 6

Different ships will use different fuels

Much like today's rockets, the principle consideration will be cost. You have to weigh the potential of a fuel against its mass, thrust, cost, and mission parameters. But since all fuels have the same thrust per mass using your matter->energy converter, then you only have a 3-factor problem. And since in space, drag is so negligible, volume is only a minimal concern which further reduces most missions down to just 2 important factors: Mass and Cost. In other words, you want the cheapest possible material per mass that you can fill a fuel tank with as long as the density is not super low.

Rather than delving straight into costs of stuff here on Earth, lets start by discussing what will make one form of matter cheaper than another. The big thing is of course how available it is. Whatever you can scoop up from next to your space port and load straight onto the ship in bulk will always be cheaper than stuff that needs refining or transportation. Renewability is also a key factor. If you have to keep digging deeper or farther to get something, then it is more scarce than a resource that just keeps coming to you.

The second factor of course will be how easy it is to get onto the ship. Gases are always a bad choice because so much work will need to go into compressing it down into a storable substance, and then you need to transport it under high pressures which has safety concern. So, while it is self-renewing because air will always flow in to take the old air's place, it has a high refinement and storage cost. Solids are generally going to be much more dense and safer in transit, but they come with an even higher refinement cost to break apart enough to move around, and perhaps more effort to grind into dust if it has to be fed through any sort of fuel injection type of system. Solids are also not self renewing: once you take a solid from your environment, no new matter will flow in to take its place. This means that liquids will, almost without exception, be the cheapest fuel source to put onto a ship. They are fairly dense, they flow to replace used up sources, and don't need any refinement before you can pump them into a holding tank. So, any liquid that is also common will generally be the preferred fuel on any given world.

Here on Earth we have oceans of a particularly plentiful liquid called water... salt water, lake water, does not matter for your engine too much since its all just fuel anyway. To get an idea of just how cheap water is, the average cost of 100% unpurified irrigation water to a farmer is about 880,000 tons of water per dollar... now this assumes horizontal or down-hill transportation, but so what? If you build a space port at the foot of a dam, you have no need to even pump the water. Just attach a hose to the fuel tank, open a valve, and let gravity do the filling for you. Beyond the initial cost of the damn and filling station, this makes your fuel practically free and infinite.

.. well not truly infinite, but with ~1.5 quintillion tons of water here on Earth too pull from, it would take a LOT of ships have any noticeable effect on the planet's total water reserves. Since matter has about 9e16 J of potential energy per kg. This means the Earth's total water fuel reserves would be about 1.4e38 J. This is about the total power output of the sun over the next 4.4 million years.

So if water is such a good solution, then why will different ships use different fuels? Simply put, not everywhere in space has liquid water. Go to Titan and all the easily accessible water is frozen solid, but instead you get vast lakes made of liquid methane which would provide a way cheaper fuel source. Or if your space port is on a planet where there is more carbon than oxygen, you will likely see vast oceans of liquid asphalt instead. Or if you are on a more Venus like world, you have the chance of a form of liquid carbon dioxide. If you find an especially young world you might find large exposed oceans of lava: which admittedly might be a bit harder to collect than water, but if you're on a such a world, it's probably still a much easier substance to harness than liquid water.

Answer 7

If I’m reading your question correctly, the answer really depends on what you choose for the characteristics of your engines and generators; their efficiency, waste products, energy requirements, operating lifetimes, and so on.

If your conversion process is fast, clean, efficient, and truly fuel-agnostic, then it doesn’t matter what you use - you won’t need much of it anyway. Depending on the service life of your generators, you might even buy one with a lifetime of fuel already built in. At 100% efficiency, 1kg of mass converts to 9e18 J using the usual formula. That’s more than a 2 gigaton nuclear bomb, which is a problem if you decide to crash at full speed, but that’s another discussion. Thermodynamics will most likely limit how much of that you can use for propulsion, but its still a ridiculous amount. The current annual world energy consumption could be produced by converting just 61kg of mass if you assume perfectly efficiency (1).

You mentioned a fueling infrastructure - tankers, stations and fuel deliveries. That implies that while people could use anything and everything as fuel, at least some people don’t. Somebody goes to the trouble and expense of operating a fleet of tankers and filling stations in a world where fuel is free, so we need a reason for that. The tankers also imply that this fuel comes from a limited number of locations and has to be transported rather than being manufactured everywhere it’s used - again, we need a reason.

Consider a real-world example as a point of reference: A conventional 3000MW fission reactor converts pretty close to 1 kg of mass to energy every year (2). To accomplish this, it consumes 25 tons of enriched uranium fuel and produces 1 kg less than that in waste. That’s only 0.004% efficiency give or take, but it is genuine mass-energy conversion. The energy takes the form of heat, which we use to generate electricity. We could theoretically convert that same mass and generate that same heat using lots of other, cheaper processes, but we use uranium fission because it produces energy at a good rate, is a controllable reaction, it isn’t too short lived, isn’t too rare, and so on. These same considerations could apply to your fuel. What characteristics could make a good fuel worth transporting if you can have other fuel for free?

1. Performance: some materials could convert more efficiently than others. If it takes several tons of cheap junk to produce the same power as one gram of good stuff, it will be well worth it for anybody who needs better acceleration, longer flight times, or bigger payload capacities.
2. Safety: some materials might be riskier than others - maybe you can control the reaction rate better with some things than others, or maybe using the wrong thing risks damaging your generator or worse. Sure, you can save a few credits by connecting your generator to the san, but they used cheaper fuel rods at Chernobyl too and see where that got them.
3. Convenience: some materials could produce nastier waste products than others, making waste storage, handling and disposal a real show stopper. Perhaps pure rocket fuel converts cleanly and completely, while random junk will convert just fine and get you where you’re going, but it spews out loads of hard radiation in the process and leaves you with a residue of poisonous radioactive acid potion smeared all over the engine room.
4. Legality: Depending on your setting, smart devices and materials could be a consideration. Your generator might refuse to accept any other fuel than the genuine original manufacturer’s special blend. A government could even issue fuel that carried an encrypted signature so that your generator would only burn it while on a registered flight path. That might serve as a solution to the Kzinti Lesson(3) or the aforementioned 2 gigaton bomb problem as well.

On the other hand, maybe those tankers and fuel stations don’t really dispense fuel at all, and are called that for historic reasons. If those electric engines you mentioned are regular reaction engines you’ll need to take on reaction mass from time to time. That's what the tankers and stations are all about. For fuel, you just siphon off a few grams of reaction mass to feed your generator since you have tons of it with you anyway.

(1) https://www.bp.com/en/global/corporate/energy-economics/statistical-review-of-world-energy.html

(2) https://www.nuclear-power.com/nuclear-power-plant/nuclear-fuel/fuel-consumption-of-conventional-reactor/

(3) http://www.projectrho.com/public_html/rocket/spacegunexotic.php#id--Propulsion_Systems

Answer 8

Water

1. Common and cheap.
2. Easy to contain.
3. Easy to move from place to place in variable amounts according to need.
4. Minimally reactive.
5. Useful for biologic lifeforms on the ship.
6. Useful as shielding against micrometeorites / cosmic rays.
7. Useful for phase change engine (e.g. steam engine) to capture heat from matter transformation and make it into electricity.
8. Makes soothing splashy sound.

Answer 9

Well, if you are really serious about scientific accuracy then you should at least be aware of the law of conservation of baryon number. If it is strictly true, then the only way to convert whole atoms of matter into energy is to annihilate them with antimatter. But there some small doubt around that law, so there may be a loophole.

You should also think about how the energy comes out. In matter-antimatter annihilation you get mostly gamma rays, which might be hard to use as a propellant. To be clear on terminology, "fuel" and "propellant" are often treated as synonymous, but they have different roles. The role of the propellant is to be accelerated out the back, and the role of the fuel/oxidizer mix is to accelerate the propellant, i.e., itself. But, in this rocket, the fuel is the matter that gets converted to gamma rays, and the propellant is water or whatever that gets heated and sent out the back.

I said that gamma rays don't make a good propellant, but that is debatable. If you can get them all pointed the way you want, they arguably make the best possible propellant. But they are so energetic that managing them is very hard. I remain skeptical about high-thrust photon rockets even in the far future, but Edgar Bonet (whose comment triggered the addition of this paragraph) may disagree. You certainly can't just use a parabolic mirror like a headlight reflector to collimate the photons - I think something fancy would be required.

So I think they would use whatever was cheap for the fuel but probably water for propellant since it's plentiful.

Answer 10

Iron: the poop of stars

I would presume that you'd want something that provided consistent energy output. Having a reactor that sometimes fizzles and sometimes explodes is a bad model. Thus, you're probably looking for an elemental material.

You would want it to be reasonably dense. If you're E=MC^2, then you always get the same energy per weight. Having more dense materials allow for less storage infrastructure for the same mass.

It should be common. Iron is the 4th most common element in the Earth's crust, and we presume that other rocky planets are that way, too.

It should be easy to handle. Liquids might be better in this category because you can flow them into whatever conversion you're using. Maybe mercury, but mercury isn't common and it's quite toxic.

Iron is at the bottom of the nuclear energy well. When huge stars get past their hydrogen burning phase, they work their way up the periodic table until they reach iron. Everything iron and above takes more energy to make than you get out of it, so starting fusion into iron is the deathknell of any star. All of the heavier elements only exist because they were created in supernova.

So, yea, pure iron bars would be my suggestion.

Edit: I'd like to put in a second vote for "ram scoops." Fuel that you need a gas tank for is usually inferior to fuel you can pick up on the way. That's why all explorers are also hunters/prospectors.

Answer 11

Ceremonial tea cakes

Your edited post says:

• each vehicle needs a few hundred MW
• 60% of energy is captured

Using E_captured = 0.6 * mc^2, this means m = E_captured / (0.6 * c^2), and m/t = E_captured/t / (0.6 * c^2). E_captured/t is the energy per unit time, which we'll take to be 500 MW. This means the mass per unit time is 0.00927 milligrams per second. That comes out to 292 grams per year.

Logistics of obtaining and carrying that much fuel are not a concern. Enough for years of operation would fit in your carry-on luggage. Thus, the fuel can be chosen for whimsical reasons; whatever the crew finds amusing.

Once a year, the crew gathers in the reactor room for a ceremony. Tea and cakes are served. Anyone with something to say about the ship - thanking her for carrying them this past year, grousing about her frequent mechanical failures, apologizing for scraping her hull in a bumpy landing - stands up and says it. The ship, being just a ship, doesn't say anything, but the crew feel better for getting it off their chests. At the end of the ceremony a tea cake is placed in the reactor, of a mass sufficient to power the ship for the coming year.

Answer 12

It depends on what you mean by "convert matter directly to energy". If you have something that can generate positrons and antiprotons and, with few beams, you totally annihilate the matter at this point sorry for the pun, but it doesn't matter.

If instead you have a fusion reactor that can fuse any light element to iron and a fission reactor that can break any heavy element to iron obviously I would exclude iron and close elements. Then I would say chondrite.

Well, chondrite is not an element, it is a mix of elements, but it is mainly composed by light elements (Si, Mg, O, C etc.) that could be put in a fusion reactor to create iron. The advantage is that you can take them from any small object you find in space and you don't have to lift them from the surface of a planet. Going against the gravity takes energy. Furthermore refuelling might be easy. You can pick up small asteroids along the way. Actually having a fusion reactor that burns everything to iron means that you can also pick up icy rocks along the way.

Answer 13

Normally they would use the highest density materials they could find because physical space is a valuable commodity in outer space.

Lead comes to mind.

Second if they had any dangerous materials like spent nuclear fuel, you might as well consume it instead of keeping it around to poison the crew.

Out of necessity anything, but hydrogen and helium are the most abundant materials in all of space. They can be stored in a liquid form to increase density a bit.

The other fuel that would be amazing if you could get it, material from a black hole. Even material near the event horizon is probably 10x more dense than normal matter.

Clarification "Black hole material" is any matter compressed sufficiently by a black hole. I will leave the exact compression ratio for the OP to decide.

The dangers of the matter suddenly expanding as you move away from the black hole would always be there. So special packing and skills to keep it expanding at a manageable rate would be necessary.

In theory, you might only fill the fuel tank 50% of black hole matter to allow for the matter to expand. In fact you might have to only put 10% of black hole material into the convertor at a time.

It would be a constant tug of war between cramming this black hole matter into the fuel tank and the black hole matter expanding. Can you use enough of it up at once to prevent it bursting the tank.

A relatively small amount of black hole matter properly managed could fuel your ship, maybe for decades.

True blackhole material you might have enough energy in a cubic foot of material to travel to the nearest galaxy at 100x speed of your poor hydrogen consuming competitors. Also you could probably carry significantly more cargo as your fuel tank might be the size of an oven.

The people harvesting and managing this material would probably have the highest paying jobs as the danger of getting trapped in a black hole or ship exploding due to taking on too much of it at once and it suddenly expanding when you get it away from the black hole.

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"No, no I need it 2nd day delivery"

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