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henriquefd
2010-Jan-12, 01:56 PM
I dont think it will take too long before we are mining helium3 at the moon. One of the challenges in interstellar travelling is the energy factor. It is said that 100tonnes of helium3 is enough to power Earth for a whole year and that the Moon would have enough helium3 to power Earth for 10.000 years. So, lots of helium3 there.

Now, lets say that in a few decades, after mining helium3 we develop a sublight speed propulsion device that could take us to 0.5c. Would helium3 be a good fuel for it and would it take a lot of space? Considering only 100tonne is enough to power Earth for a year, it does seem like a fuel source that would occupy little space. Sure, helium is a gas and thus the vollume it takes to have a tonne of helium gas is a lot bigger than the vollume needed for a tonne of iron. Which leades me to another question. I read that we could compress helium3 into crystals, so they occupy less cargo space.

Anyway, I will leave you guys with those questions. Thanks. =)

Hornblower
2010-Jan-12, 02:30 PM
I dont think it will take too long before we are mining helium3 at the moon. One of the challenges in interstellar travelling is the energy factor. It is said that 100tonnes of helium3 is enough to power Earth for a whole year and that the Moon would have enough helium3 to power Earth for 10.000 years. So, lots of helium3 there.

Now, lets say that in a few decades, after mining helium3 we develop a sublight speed propulsion device that could take us to 0.5c. Would helium3 be a good fuel for it and would it take a lot of space? Considering only 100tonne is enough to power Earth for a year, it does seem like a fuel source that would occupy little space. Sure, helium is a gas and thus the vollume it takes to have a tonne of helium gas is a lot bigger than the vollume needed for a tonne of iron. Which leades me to another question. I read that we could compress helium3 into crystals, so they occupy less cargo space.

Anyway, I will leave you guys with those questions. Thanks. =)

My bold for reference. Who said it? I never heard of such an idea as using He 3 as a source of energy.

01101001
2010-Jan-12, 03:10 PM
I never heard of such an idea as using He 3 as a source of energy.

Wikipedia: Helium-3 :: Power generation (http://en.wikipedia.org/wiki/Helium-3#Power_generation)


There have been many claims about the capabilities of Helium-3 power plants. [...] The reality is not so clear-cut.

So if it is soon -- for long values of soon -- mined for use as a fuel, then yeah, sure, use it for fueling space craft.

IsaacKuo
2010-Jan-12, 04:22 PM
Now, lets say that in a few decades, after mining helium3 we develop a sublight speed propulsion device that could take us to 0.5c. Would helium3 be a good fuel for it and would it take a lot of space?
No, helium3 would not be a good fuel for reaching 0.5c. Due to the relatively low specific energy of fusion fuel, fusion rockets would be more suitable for speeds of perhaps 0.01c up to maybe 0.05c. For faster speeds, you would want an externally powered stardrive, like relativistic particle beam, sailbeam, or relativistic kinetic impact powered rocket. Any of these can be powered using relatively inexpensive and inexhaustible solar power rather than expensive and limited He3 fusion fuel.

The theoretical appeal of helium3 is that D-He3 fusion reactions would be mostly aneutronic. With the much easier D-T and D-D reactions, most of the output energy is in fast neutrons. Fast neutrons can't be directly utilized for energy or thrust, you have to first absorb them and then use an inefficient heat engine. In contrast, most of the energy from a D-He3 reaction is in fast protons, which can be magnetically deflected for thrust or electromagnetically manipulated for efficient conversion to electrical power.

The problem? D-He3 fusion is hard to do. It's not clear that we can ever turn it into a useful rocket or reactor engine.

EDG
2010-Jan-12, 05:14 PM
I'm kinda wondering if antimatter power might be easier to master than fusion. Haven't we already generated antimatter in tiny amounts in particle accelerators? That said, it's probably a bit too horrifically dangerous if things go wrong :)

grant hutchison
2010-Jan-12, 05:29 PM
My bold for reference. Who said it? I never heard of such an idea as using He 3 as a source of energy.For interstellar flight, the idea of using helium-3/deuterium fusion as a power source dates back at least as far as the British Interplanetary Society's Project Daedalus study in the mid-70s. But peak velocity for that mission was well short of 0.5c.

Grant Hutchison

Grey
2010-Jan-12, 05:37 PM
I'm kinda wondering if antimatter power might be easier to master than fusion. Haven't we already generated antimatter in tiny amounts in particle accelerators?Unfortunately, since we don't have an available source of antimatter, we'd have to create it ourselves. The process isn't particularly efficient, so that the amount of energy we use creating antimatter is orders of magnitude larger than the amount we could get back out using it as an energy source, and the total amount we've created is tiny.


That said, it's probably a bit too horrifically dangerous if things go wrong :)Well, we'd need a lot more antimatter before it became dangerous. Right now, if we could take all the antimatter that has ever been produced, annihilate it with matter, and harness the energy produced with 100% efficiency, we'd have enough energy to light a 100 watt bulb for a few minutes.

henriquefd
2010-Jan-12, 05:44 PM
Interesting. I never thought of antimater as a energy source for spaceships. I'm still new to all this.

Suposing we had the technology to create antimater in a cost-effective way. Would it take too much space in a spaceship? Also, on another note, wouldn't it be possible, in that case, to create some sort of antimater shield to protect the spaceship against micrometeorites?

Oh, and thanks to all for the explanations so far! It seems I have found the right forum to learn!

SolusLupus
2010-Jan-12, 07:11 PM
Unfortunately, since we don't have an available source of antimatter, we'd have to create it ourselves. The process isn't particularly efficient, so that the amount of energy we use creating antimatter is orders of magnitude larger than the amount we could get back out using it as an energy source, and the total amount we've created is tiny..

Yes, it isn't efficient, but if it gives you long missions into space...

Outside of that, not useful here on Earth.

Ronald Brak
2010-Jan-13, 06:23 AM
I dont think it will take too long before we are mining helium3 at the moon.

No need to go to the moon. Helium-3 is a decay product of tritium. If helium-3 is wanted it would be much cheaper and easier to make it here.

henriquefd
2010-Jan-13, 09:10 AM
No need to go to the moon. Helium-3 is a decay product of tritium. If helium-3 is wanted it would be much cheaper and easier to make it here.

Obtaining h3 from tritium is uneconomical:

"Due to the rarity of helium-3 on Earth, it is manufactured instead of recovered from natural deposits. Helium-3 is a byproduct of tritium decay, and tritium can be produced through neutron bombardment of lithium, boron, or nitrogen targets. Current supplies of helium-3 come, in part, from the dismantling of nuclear weapons where it accumulates;[23] approximately 150 kilograms of it have resulted from decay of US tritium production since 1955, most of which was for warheads.[24] However, the production and storage of huge amounts of the gas tritium is probably uneconomical, as tritium must be produced at the same rate as helium-3, and roughly eighteen times as much of tritium stock is required as the amount of helium-3 produced annually by decay (production rate dN/dt from number of moles or other unit mass of tritium N is N γ = N ln 2/t where the value of t/(ln 2) is about 18 years; see radioactive decay). If commercial fusion reactors were to use helium-3 as a fuel, they would require tens of tons of helium-3 each year to produce a fraction of the world's power, implying the same amount of tritium production, and 18 times this much total tritium stock.[25] Breeding tritium with lithium-6 consumes the neutron, while breeding with lithium-7 produces a low energy neutron as a replacement for the consumed fast neutron. Note that any breeding of tritium on Earth requires the use of a high neutron flux, which proponents of helium-3 nuclear reactors hope to avoid"(Wikipedia)

IsaacKuo
2010-Jan-13, 12:29 PM
Interesting. I never thought of antimater as a energy source for spaceships. I'm still new to all this.
The big deal-killer for antimatter is how stupendously expensive it is to produce. The only known way to practically create antimatter is to smash particles together and every once in a while you're lucky and an anti-proton is created instead of a menagerie of other particles. This is inherently extremely inefficient. There are also challenges in other aspects of antimatter storage and use, but these don't look too bad compared to the stupendously hopeless case of antimatter production.

In contrast, something like relativistic particle beam propulsion can be perhaps 60% efficient.

Suposing we had the technology to create antimater in a cost-effective way. Would it take too much space in a spaceship?
There's no shortage of space in outer space. But seriously, long term antimatter storage is a non-trivial problem. We can't even store hydrogen fuel long term, much less anti-hydrogen fuel long term.

Also, on another note, wouldn't it be possible, in that case, to create some sort of antimater shield to protect the spaceship against micrometeorites?
No. Antimatter would convert relatively nonthreatening micrometeorites into dangerous penetrating ionizing radiation.

JustAFriend
2010-Jan-18, 07:55 PM
Interesting. I never thought of antimater as a energy source for spaceships. I'm still new to all this.


So you're saying that you haven't ever watched Star Trek over the past 50 years?????

:shifty:

henriquefd
2010-Jan-19, 11:53 PM
So you're saying that you haven't ever watched Star Trek over the past 50 years?????

:shifty:

Exactly. I mean, i have heard about it and I remember watching Kirk and Spock on tv a couple of times when I was a kid, but by that time anti-matter wasnt part of my vocabulary.

I have just recently started watching Star Trek. I'm starting with Next Generation. A friend of mine says it's a good starting point. I am liking it so far. =)

Elukka
2010-Jan-21, 03:28 AM
It's probably a good thing you're not taking Star Trek as a guideline of what may or may not be possible in real life. ;)