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I'm making a small game about travelling deep space on limited resources. One thing I'm interested in having is the idea that the craft is scavenging what it can from planets as it travels through entirely uninhabited locations.

The idea is this is a sufficiently advanced society that they could feasibly extract elements out of common materials to make a fuel from what they came across. So my question is, what element would they design around finding the most? I know earth's atmosphere is largely Nitrogen but if scientists were to make their best assessment, what would they bank on being the most copious element available to fuel the craft?

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Hydrogen and helium make up most of the universe, in fact, hydrogen accounts for 90% of atoms in existence. What makes hydrogen significant? Fortunately, liquid hydrogen is literally rocket fuel when you set fire to it. It's also used in nuclear fusion, that is, smashing the atoms together to release tremendous amounts of energy. It's what fuels stars, which are giant fusion reactors.

So I guess a spacecraft could harvest hydrogen from nebulae (as is discussed in this thread). Then it could either be compressed and burnt up for thrust, or used as a nuclear power source for an entirely different type of engine. Since your question isn't focused on how to extract/obtain hydrogen, I'll leave the rest to your imagination.

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    $\begingroup$ Hydrogen cannot "be compressed and burnt up for thrust" unless you also start harvesting oxygen. Finding a biologically active world (to produce free oxygen) may be a bit iffy. If you propose to find water and extract the oxygen, this will also produce exactly the right amount of hydrogen, so there's no need specifically to extract it from nebulae. $\endgroup$ – WhatRoughBeast Oct 5 '15 at 22:26
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    $\begingroup$ @WhatRoughBeast extracting it from water takes the same amount of energy as burning it as fuel $\endgroup$ – Varrick Oct 6 '15 at 13:43
  • $\begingroup$ @WhatRoughBeast, we must be careful with our terms here. H2, by itself, is not a chemical "fuel" but it makes a wonderful propellant. If you used a nuclear thermal rocket, ion drive, etc. for propulsion then you could just tank up with H2 and use your reactor to drive your ship. No need for oxidizer. If you consider the possibility of refining H2 for D2 (deuterium) in a fusion drive, then it could be both propellant and fuel. $\endgroup$ – Jim2B Oct 6 '15 at 13:44
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Hydrogen is by far the most common element in the universe, and can be used as a propellant (if you have an oxidizer) or as an energy source (either in combustion with an oxidizer, in nuclear fusion, or in a fuel cell). If you are technologically advanced enough to be a space-faring civilization, you should probably have the means to use hydrogen in a large capacity

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  • $\begingroup$ Welcome to the site costrom, nice first answer. $\endgroup$ – James Oct 5 '15 at 16:34
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    $\begingroup$ In fact, there are many theoretical engines for traversing large regions of space which involve a large ram-scoop to collect interstellar hydrogen, an then fuse it for propulsion (generating more propulsion than the scoop's drag). $\endgroup$ – Cort Ammon - Reinstate Monica Oct 5 '15 at 16:45
  • $\begingroup$ You might want to add some discussion about how you would scavenge hydrogen to your answer. Hint: there's plenty on gas giants like Jupiter if you're not convinced that ram scoops are feasible. $\endgroup$ – Brythan Oct 5 '15 at 18:39
  • $\begingroup$ @CortAmmon There's no drag in space... $\endgroup$ – Rob Watts Oct 5 '15 at 20:50
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    $\begingroup$ @RobWatts yes there is... depends on what you want to call the total force of the impacts between your craft and the interplantery/stellar/galactic medium (which could be generously described as a rarefied gas). there is absolutely a retarding force, and it can add up dramatically if you are using a large enough scoop to provide a useful amount of thrust $\endgroup$ – costrom Oct 5 '15 at 22:01
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Water. Unless you are using hydrogen for fusion, you need an oxidiser. Water can be split using solar power giving hydrogen and oxygen to be used as rocket fuel. It can also be electrolysed to give hydrogen peroxide, a monopropellant, what you use for steering your ship. If you are using a nuclear thermal rocket (heat up stuff up with a uranium reactor so it shoot out a novel fast propelling your ship) Hydrogen is the best fuel to use, and you can go on to breathe the oxygen. You can use the water for drinking and growing plants. Hydrogen-oxygen fuel cell batteries can be used for power.

TLDR; Water has all the uses of hydrogen and far more

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The form of propulsion using least resources would be a light sail.

The Planetary Society launched a test ship in May 2015 to try out some of the technology with limited success. Further missions are planned.

A lightsail uses the pressure of solar radiation as a propulsion method and essentially uses no fuel at all.

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  • $\begingroup$ This is fascinating! I'm going to Google the phenomenon but are there any particular resources you recommend for reading on this? $\endgroup$ – SuperBiasedMan Oct 6 '15 at 13:28
  • $\begingroup$ That makes sense for travel within a solar system, but wouldn't the intensity of light be far too weak and the directions it comes from far too random in interstellar space? $\endgroup$ – Karmic Slingshot Oct 6 '15 at 16:03
  • $\begingroup$ Possibly it needs to use 2 solutions, a lightsail to accelerate to the point where a ramscoop is usable. The lightsail could also be used to deaccelerate. $\endgroup$ – user23614 Oct 7 '15 at 9:45
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have a magnetic field that funnels any and all particles the ship finds in its path into the ship's engines. The engine separates the hydrogen out of what it grabs, fuses it into helium. From the fusion reaction it obtains the energy it needs to power all ship systems, plus the heat to superheat the rest of the elements it grabbed and send them out the "back", sort of like a jet engine. I have seen hints of this type of ram engine in fiction stories I have read.

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