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I’m working on a vehicle that can move from planet to planet in my novel in a short time frame (less than 1 day), and it needs a power source. It is made for shuttling heavy cargo (valuable minerals and mined gases), and is about the size and mass of the ISS. It is NOT an FTL vehicle.

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    $\begingroup$ Like you have been told in your other question, we need more details to answer a question. You are giving us 0 details. $\endgroup$
    – L.Dutch
    Commented Jun 14 at 13:38
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    $\begingroup$ Planet to planet in minutes? Everyone is so impatient. Anyway, forget power sources. The required accelerations would destroy the ship and all its contents in an instant. You can only do it with magical science-fantasy engines, at which point you just reach for the magical science-fantasy power sources and call it a day. Trying to reach realism here is impractical, and probably not worth the effort anyway. $\endgroup$ Commented Jun 14 at 13:55
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    $\begingroup$ @Pelinore every planet in the goldilocks zone is less than a light hour away. Also, without getting too legalistic here, something a bit over 60 minutes away may still be worth measuring in minutes. $\endgroup$
    – Nosajimiki
    Commented Jun 14 at 15:41
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    $\begingroup$ This is actually a common issue in harder-sci-fi: space is big, and planets are far apart. Unfortunately without pulverizing the crew you're going to have trip times of at minimum a few hours and at maximum (Earth to Neptune) a few weeks, because getting to relativistic speeds isn't the issue, it's getting there at 1 $g$, because anything above that tends to flatten people pretty quick. $\endgroup$ Commented Jun 14 at 19:22
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    $\begingroup$ Please remember brainstorming is discouraged to the point of being prohibited (see help center). A completely viable answer comes from Anne McCaffrey's novel The Rowan, where "telepathy, psychokinesis and other psychic talents have become scientifically accepted and researched. Telekinetic and telepathic powers are used to communicate and teleport spaceships through space, thus avoiding the light barrier." (Source) I.E., what does "viable" mean on a stack that focuses on imagination? $\endgroup$
    – JBH
    Commented Jun 15 at 3:47

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Anti-matter

All other technical hurdles aside, the most energy dense fuel known to modern science is antimatter. I've heard of Quarkium before which could under certain models of subatomic physics be even more energy dense, but it's not something that has been scientifically proven to exist that I know of.

Antimatter/matter reactions give you more or less 100% mass to energy conversion as opposed to the next best energy source, fusion, which gives you about 8%.

If we assume you have a spacecraft that is 50% matter/antimatter fuel and a 60-70% efficient propulsion system which is standard for rocket like engines, then you theoretically have enough energy stored to speed up to about 0.150-0.175c and then slow back down to a stop.

I say theoretically, because the amount of waste heat produced in the process would turn all known matter that your ship could be made out of so hot that it would itself become a giant plasma explosion if you tried. So, even though a ship could hit those peek speeds, you also have to consider how quickly you can speed up. Radiating heat in space is hard; so, chances are you will not be able to maintain an acceleration of more than a few meters per second squared for any significant period of time meaning that you would have to accelerate over the course of several weeks or months to reach actual relativistic speeds.

So, if you want a ship to hop between planets in a matter of minutes, you need a few non-science based things in your setting. You need an ~100% efferent propulsion system so that your engine does not blow up your ship putting out enough energy to accelerate at insane speeds and you will also need "inertia dampeners" to prevent your ship from being crushed like a pancake accelerating at those speeds

So if we assume these two Clarke Techs exist in your setting, then let's say your goal is to reach a peak speed of 0.15c in 1 minute. That means you need to accelerate at a rate of 750,000m/s^2 to be able to reach Mars in about 30 minutes.

But consider the cost...

There is no way in hell that iron ore will ever be worth moving at relativistic speeds. Antimatter is the most expensive substance known to man. Iron ore is one of the cheapests. The relationship between how fast you go and how much fuel it costs is exponential. If instead of trying to send a shipment of ore in 30 minutes, you try sending a slow boat every few days knowing that they will each take several months to make the journey, you can still maintain a stable supply line at a fraction of the cost. It is just like how we still use giant cargo ships that spend months crossing oceans for bulk goods even though we could fly them over in a day.

The only believable reason a civilization might be able to economically move such cheap materials that far and that fast is if you are working with a non-newtonian propulsion system in which case questions about how energy dense your fuel is becomes meaningless, and the science behind them compleatly unknowable.

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  • $\begingroup$ "The only believable reason a civilization might be able to economically move such cheap materials that far and that fast is if you are working with a non-newtonian propulsion system in which case questions about how energy dense your fuel is becomes meaningless" excellent point [+] $\endgroup$
    – Pelinore
    Commented Jun 14 at 16:24
  • $\begingroup$ @Pelinore Sometimes the most plausible solution for a problem is taking a shortcut through a demon infested parallel universe... and sometimes that is perfectly okay.... $\endgroup$
    – Nosajimiki
    Commented Jun 14 at 16:29
  • $\begingroup$ Just so you know, modern cargo ships cross the Pacific in a couple weeks, it is certainly not a journey of "months". I suppose there are still a few tramp-steamer style vessels that ply a circuitous route, port to port to port rather than there and back, but as I understand modern logistics, those are quite unusual. $\endgroup$ Commented Jun 14 at 16:59
  • $\begingroup$ @SoronelHaetir point still stands that we could fly there in under 12 hours if we really wanted to. But that would be stupidly expensive, so we don't. $\endgroup$
    – Bubbles
    Commented Jun 14 at 17:20
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    $\begingroup$ @controlgroup not at 100% efficiency. Annihilation of the nucleons will be very messy and produce a spray of neutrinos, pions, and other particles which you won't be able to fully extract energy from. Quarkium would just be more dense. Some sort of direct matter-energy conversion might be able to achieve higher useful energy density. But you also have to consider the implications of releasing these amounts of energy within the solar system... $\endgroup$ Commented Jun 15 at 0:02
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No fuel source would work because acceleration would explode the vehicle

Light takes 3 minutes to get from earth to mars on average. Assuming you accelerate to light speed in 10 seconds you'll have a g force of 3 million. A formula one car crash is around 300g, and pulps the car. Any ship, person, vehicle or iron would be pulped if you fired that fast, and you would essentially be firing a shotgun blast at the earth which would cause a significant amount of extinction on the planet.

Frameshift. Use conventional batteries to power a railgun.

You can use whatever power source you like, from coal to nuclear to geothermal. Make a ton of batteries and use it to fire a massive railgun. This will cause the extinction of the other side, but it will get the iron to the location at relativistic speeds.

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Stapled gravity waves

A gravity wave contains an enormous amount of energy. It is not a physical substance, but a contortion of the fabric of space time. These are the things LIGO detects.

Because they are not physical material, they are not bound by the energy-density regime that rules the space of traditional fuels.

So, imagine we've got some handwavium tech that allows us to capture incoming ripples in spacetime, preserving those wrinkles until such time as we choose to release them, like a piece of elastic.

The way I imagine this would work, it would require energy input to capture a gravity wave, so this energy source would be collected on planets or where traditional electrical power is plentiful. The tech on the ship would only be capable of holding and containing the outputs of that, and releasing them on demand. That might look something like a scaled-up version of the Casimir effect.

This is very loosey-goosey sci-fi magic, but I don't see science-based or hard-science on the question. Gravity waves are real and do contain an immense amount of energy. They do move. If you're traveling between planets in minutes, you're already in solidly Clarkean territory.

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  • $\begingroup$ I would be excited to see how a worldbuilder might justify using captured gravity waves to induce the creation of temporary wormholes. +1 $\endgroup$ Commented Jun 14 at 19:20
  • $\begingroup$ Why capture natural gravitational waves? All you have to do is move something massive the right way to make them. And if you can capture gravitational waves and pump loads of energy into them, you ought to be able to just manufacture micro singularities and allow them to evaporate to release the energy on demand. $\endgroup$ Commented Jun 15 at 14:08
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Depends on what you mean by "relativistic speeds".

Getting up into the 0.99c+ realm you're looking at 600+ petajoules per kilogram... That's... A lot...

On the other hand, the sunlight hitting Earth is something like 175 petajoules per second making it to the surface. So a sufficiently large set of highly efficient orbiting mirrors could easily collect enough power to toss your cargo around at high speed. Your limiting factor is going to have more to do with how much acceleration you can pile on your ship before things start to break. And do keep in mind that the ship needs time to stop at the other side, so whatever your maximum acceleration is you get to use it for, at most, half your trip.

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