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What engines and fuels would interstellar species use for space travel?


Emrys42
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I have been trying to determine(for a game I am designing) what fuels and engines would a technologically advanced interstellar species most commonly use for space travel assuming they want as much energy output as possible for the smallest volume and mass possible. It seems as far as fuel goes hydrogen is the only combustible fuel that makes sense, other then that they might use antimatter, fission and fusion. Are there any other forms they might want to use and why?

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Converting nuclear energy to electricity that accelerates ions is quite inefficient. One better uses alpha radioactivity to expel directly the helions or the remaining nucleus. This has already been proposed, by coating a thin film with an alpha emitter.

 

Or even more efficient, let fission expel a half-nucleus, and catch the other half to obtain thrust. Though, no convincing setup has been proposed for that one.

 

Worry: even with such methods, travel to the nearest stars remains very long, and these engines have unacceptable drawbacks with all imagined setups. A fission half-nucleus gets 100MeV or c/24 in random direction, so the original nucleus brings c/95 specific impulse. So if the spaceship attains quickly c/100 (and brakes upon arrival, which leaves 1/8 of the inital mass) it still takes 500 years to the nearest star - using technology known to be impossible to us now.

 

Fusion isn't much better. Only antimatter would improve, among the forms of energy we imagine - but no-one can even propose a research direction to try to store it in any amount.

 

At c/100, which is still too little, hydrogen and chemical fuels, slingshot and Oberth effect, Solar thermal engines and Solar sails are insignificant.

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It might be done with chemical rockets and fission power, according to The Millennial Project: Colonizing the Galaxy in Eight Easy Steps by Marshal Savage. However, it will take millennium. Basically, people live in space, inside giant domes built of materials mined from asteroids and comets. People gradually migrate outward toward the stars using the asteroid belt and smaller moons within the Sol system, then using materials from the Kuiper belt, the Oort cloud, and other small interstellar bodies. Since water can be split into H2 and O2 and appears to be plentiful in space, fuel for chemical rockets is available as long as fissionable material is abundant.

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Converting nuclear energy to electricity that accelerates ions is quite inefficient. One better uses alpha radioactivity to expel directly the helions or the remaining nucleus. This has already been proposed, by coating a thin film with an alpha emitter.

Agreed. It was a poor choice of words. I never proposed to generate electricity with a fission (or fusion) reactor to power an ion thruster. My proposal was to use either an ion thruster (using whatever to power it... solar power?) or, alternatively, use fusion or fission.

 

Or even more efficient, let fission expel a half-nucleus, and catch the other half to obtain thrust. Though, no convincing setup has been proposed for that one.

 

Worry: even with such methods, travel to the nearest stars remains very long, and these engines have unacceptable drawbacks with all imagined setups. A fission half-nucleus gets 100MeV or c/24 in random direction, so the original nucleus brings c/95 specific impulse. So if the spaceship attains quickly c/100 (and brakes upon arrival, which leaves 1/8 of the inital mass) it still takes 500 years to the nearest star - using technology known to be impossible to us now.

 

Fusion isn't much better. Only antimatter would improve, among the forms of energy we imagine - but no-one can even propose a research direction to try to store it in any amount.

 

At c/100, which is still too little, hydrogen and chemical fuels, slingshot and Oberth effect, Solar thermal engines and Solar sails are insignificant.

We are discussing some "technologically advanced interstellar species", who may just have a life expectancy (and the necessary patience) that is required for space travel. Who knows, they just may think that 5000 years is a short time.

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Solar panels and ion thrusters have propelled a very nice JPL mission to pass by several asteroids. Chemical engines wouldn't have done it. Though, the acceleration is faint and provides too little speed within the time spent in good Sunlight to gain a good speed.

 

There are species for which 5000 years and more are acceptable: bacteriae, fertilized spores... Some seem to survive in a favourable space environment, say within snow. Hence the opinion that life may spread among planets and even Solar systems. Though, the thread was rather about intentional travel. An intelligent mushroom maybe? Some (dumb ones) are reportedly thousand years old.

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The periodic table would be the same everywhere. The laws of physics would also be the same. So they would use the propellants - if any - that we use. They would encounter the same action=reaction reality that rocket travel is based on, so it follows they would develop some sort of rocketry. How long they stayed with that inefficient of a technology depends only on their imagination and socio-economic system. Specifically, do they have a means of acquiring the resources to make it happen.

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The periodic table would be the same everywhere. The laws of physics would also be the same. So they would use the propellants - if any - that we use. They would encounter the same action=reaction reality that rocket travel is based on, so it follows they would develop some sort of rocketry. How long they stayed with that inefficient of a technology depends only on their imagination and socio-economic system. Specifically, do they have a means of acquiring the resources to make it happen.

It seems they wouldn't use most the fuels we use, since most of what we use is not even as good as liquid hydrogen. Seems to me liquid Hydrogen would be the worst fuel they might use, while antimatter is the best fuel. Although, if these were hypothetically the worst and best(energy to volume ratio) fuels they are willing to use, then couldn't they have more dense versions of these fuels as well? For example if hydrogen gas is good then a more compressed hydrogen would be better?

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It seems they wouldn't use most the fuels we use, since most of what we use is not even as good as liquid hydrogen. Seems to me liquid Hydrogen would be the worst fuel they might use, while antimatter is the best fuel. Although, if these were hypothetically the worst and best(energy to volume ratio) fuels they are willing to use, then couldn't they have more dense versions of these fuels as well? For example if hydrogen gas is good then a more compressed hydrogen would be better?

Logical. I imagine they would have the awareness to appreciate antimatter over Hydrogen. . . They may have better insight as to how to manage antimatter as a propulsive energy source. A caveat: We may be talking about several intelligent species and there may be vast differences in how they approach the matter, technologically speaking. That said, they all still have to work with the same elements and physical laws we do. Maybe there is an 'all roads lead to antimatter' reality we just have been a little slow in appreciating. . .

Edited by Moonguy
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  • 1 year later...

I have been trying to determine(for a game I am designing) what fuels and engines would a technologically advanced interstellar species most commonly use for space travel assuming they want as much energy output as possible for the smallest volume and mass possible. It seems as far as fuel goes hydrogen is the only combustible fuel that makes sense, other then that they might use antimatter, fission and fusion. Are there any other forms they might want to use and why?

 

I could see safety reasons as being a cause to include an emergency/backup source. How most of our own oceangoing vessels are designed. Would also need to consider a ship engaged in maneuvering(docking, entering atmosphere, etc).

 

 

It's likely that they would use free energy because their elite probably doesn't suppress such technology unlike here on Earth.

 

My simulated creatures frequently discovered sources of free energy. Their population reached astounding heights, then the program crashed...

 

Let us assume it does not exist in our Universe and fervently hope that this is so.

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It seems they wouldn't use most the fuels we use, since most of what we use is not even as good as liquid hydrogen. Seems to me liquid Hydrogen would be the worst fuel they might use, while antimatter is the best fuel. Although, if these were hypothetically the worst and best(energy to volume ratio) fuels they are willing to use, then couldn't they have more dense versions of these fuels as well?

 

Liquid hydrogen requires liquid oxygen..

Result of reaction between these two is water.

This is true for the most of fuels that burn- they need source of oxygen.

On the Earth they take oxygen from air. Spacecraft doesn't have such luxury.

 

Energy density table

http://en.wikipedia.org/wiki/Energy_density

Edited by Sensei
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High energy lasers focussed over interstellar distances on a spacecraft's light sail to accelerate it are a reasonable extrapolation of current technology.

 

With one at each end of the journey efficiency would be much better than any onboard fuel.

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Could you provide "reasonably extrapolated" figures about the mirror diameter - or other technology - that focusses light on a sail over an interstellar distance? And about the reasonaly extrapolated mean power of the laser?

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Could you provide "reasonably extrapolated" figures about the mirror diameter - or other technology - that focusses light on a sail over an interstellar distance? And about the reasonaly extrapolated mean power of the laser?

With a lot of work .....

 

The power would depend on how much acceleration the passengers wanted.

 

And of course I'd have to guess the minimum practical mass per unit area of the sail to see how big it could be. It couldn't usefully mass more than about half the payload.

 

With onboard fuel, you first have to accelerate all the deceleration fuel up to maximum speed.

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High energy lasers focussed over interstellar distances on a spacecraft's light sail to accelerate it are a reasonable extrapolation of current technology.

 

With one at each end of the journey efficiency would be much better than any onboard fuel.

 

 

Could you provide "reasonably extrapolated" figures about the mirror diameter - or other technology - that focusses light on a sail over an interstellar distance? And about the reasonaly extrapolated mean power of the laser?

 

 

With a lot of work .....

 

The power would depend on how much acceleration the passengers wanted.

 

And of course I'd have to guess the minimum practical mass per unit area of the sail to see how big it could be. It couldn't usefully mass more than about half the payload.

 

With onboard fuel, you first have to accelerate all the deceleration fuel up to maximum speed.

 

 

I'm interested in the diameter of the mirror. You told "reasonably extrapolated". How much?

 

 

 

 

Please tell us the mirror diameter to provide power at the distance of a remote star. The Esa paper does not envisage it.

The ESA paper does not describe that or alternative optical focussing.

As there is a laser at each star power from each laser is only required for half the distance.

Any calculation of power etc would require an enormous number of assumptions and a few billion/trillion dollars to verify them.

There may be a few billion dollars' worth of relevant classified "Star Wars" research.

Which part of "With a lot of work ....." did you not understand?

This is my last post in this topic.

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In my opinion "propulsion engines" will only be used for short flights between planets in a solar system. Propulsion engines do not seem the way forward for "interstellar travel".

 

When we talk about interstellar travel, I think that will come when we understand the universe better,

There is still much yet unknown, Take the "holographic universe"

 

The Fermi National Accelerator Lab near Chicago, once home to the historic Tevatron atom smasher, has a new toy: the Holometer. As its rather fancy name suggests, the Holometer is for testing whether our universe is actually a 2D hologram that’s painted on some kind of cosmological horizon, popping into 3D whenever we attempt to observe it

http://www.extremetech.com/extreme/188727-pixels-of-the-universe-experiment-begins-to-see-if-the-universe-is-a-2d-hologram,

 

 

If you tear a hologram in half, you can still see the whole image in each piece. The same is true with smaller and smaller pieces

 

 

The holographic principle goes something like this. From our zoomed out vantage point, the universe seems to be a perfectly formed enclave of 4D spacetime. Look around you: Everything is as it should be. What happens if you keep zooming in, though? Past microbes, past protons, past electrons… until you get down to the smallest possible unit that can exist in the universe. Moving closer to an old-style TV until you can see the individual pixels is a very good analogy. The holographic principle suggests that, if you zoom in far enough, we will eventually see the pixels of the universe. It’s theorized that these universal pixels are about 10 trillion trillion times smaller than an atom (the Planck scale, in physics terms).

 

This then could mean that instead of travelling vast distances, We could travel these distances by entering any part of the hologram, basically travelling inwards to reappear anywhere within universe/hologram.

 

It will take a new science to take us to the stars, Not propulsion engines.

Edited by sunshaker
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I agree. Propulsion engines as we understand them today will not suffice for interstellar travel, but if we can develop something that can tap into vast amounts of energy whereby reaching something very close to the speed of light then time will cease to be a limiting factor to the travels as per the Lorentz Transformation formula.

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  • 3 weeks later...

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