how to restart a fusion reactor in space?

[insane_alien]

fuel cell like they've used since apollo. combines hydrogen and oxygen in an electrochemical reaction that directly produces electricity. you can build them so you can get a few MW out of one the size of a family car.

no need for a fission reactor which would be mostly dead (and radioactive) weight. fission reactors require a crap load of shielding and waste handling equipment while fusion reactors do not require wast handling and the necessary properties of the reaction chamber wall provide sufficient shielding.

 

[the.punk]

fuel cell like they've used since apollo. combines hydrogen and oxygen in an electrochemical reaction that directly produces electricity. you can build them so you can get a few MW out of one the size of a family car.

Ahhhh. I know of those, but don't know the english word for it.

 

[insane_alien]

oh, sorry, didn't notice you were german(the flag SHOULD have been a clue). you're english is very good.

 

[the.punk]

oh, sorry, didn't notice you were german(the flag SHOULD have been a clue). you're english is very good.

Thanks.

In german these cells are named "Brennstoffzelle" and yours would be "Treibstoffzelle" because "Treibstoff" is in english fuel.

---------- Post added at 01:24 PM ---------- Previous post was at 01:16 PM ----------

no need for a fission reactor which would be mostly dead (and radioactive) weight. fission reactors require a crap load of shielding and waste handling equipment while fusion reactors do not require wast handling and the necessary properties of the reaction chamber wall provide sufficient shielding.

Yes. You're right in that point. Fission of uranium produce radioaktive material.

 

[Kozak]

no need for a fission reactor which would be mostly dead (and radioactive) weight. fission reactors require a crap load of shielding and waste handling equipment while fusion reactors do not require wast handling and the necessary properties of the reaction chamber wall provide sufficient shielding.

Combustion engines produce a crap load of products, which require disposal, and need a crap load of fuel
Ideal power output from 1g of U-235 is 80MJ, while for H2 it's 143 kJ.

The combustion products need either to be flushed (partly as a reaction mass), either be decomposed "back to origins". In the first case you'll need a considerable supply of oxygen and hydrogen, and you'll have only a little backup in the case of reactor "flameout". In the second - the energy output of the reactor (in the case of pulse-type reactors) must be at least at 200% level (twice the energy spent to heat up plasma). So that's not so easy.

 

[RisingFury]

When you say fusion reactor are only good when they are big than I think it would be good to have a fission reactor as backup to star the main fusion.


Ah...And what do you mean with "fuelcell" ?

So now you have two reactors, twice as much danger and twice as much weight + two different fuel types. Why not just use a fission reactor then?


The first thing to consider is a fusion reactor that just "flames out" isn't exactly a good design... but if you had shut it down for some reason, I think the best energy storage mechanism would be a magnetic field. Think about it:

A buzzard ramjet needs a powerful magnetic field for operation and a magnetic field can store a large amount of energy and release it very quickly.

W = 1/2 * L * I^2, where L is the inductivity (FF's dictionary doesn't recognize the word - is it correct?) of the coil and I is the magnetic field. Your inductivity can be small, but given that you won't have a problem cooling your coil, you can make it out of a superconductor, allowing you to pump 1000's of amps through it (LHC's magnets can sustain a current of 12 000 A). 1000^2 = 10^6. Even if your inductivity is small you end up with a considerable energy.

Magnetic fields can also collapse very quickly, creating a huge amount of power.

I think such a system would be more then adiquite to restart your reactor if the reactor didn't sustain damage. Might even be enough for multiple restarts, afterall, with resistance of 0, you don't lose any energy and the field only collapses to what you let it to.

 

[the.punk]

So now you have two reactors, twice as much danger and twice as much weight + two different fuel types. Why not just use a fission reactor then?

Because a fission reactor isn't so efficient as a fusion reactor.

 

[insane_alien]

Combustion engines produce a crap load of products, which require disposal, and need a crap load of fuel

The combustion products need either to be flushed (partly as a reaction mass), either be decomposed "back to origins". In the first case you'll need a considerable supply of oxygen and hydrogen, and you'll have only a little backup in the case of reactor "flameout". In the second - the energy output of the reactor (in the case of pulse-type reactors) must be at least at 200% level (twice the energy spent to heat up plasma). So that's not so easy.

fuel cells produce very pure water(usually used for drinking water on space missions) and whats more the hydrogen and oxygen can be recovered when the fusion reactor has started up by simple electrolysis.

pulsed fusion reactors aren't likely to be used on space missions as they are inherently bulky and require mor moving parts which means they are more prone to failure.

continuous fusion should be achievable in an almost solid state confuguration and able to have a higher throttle range from just about supporting itself to where its pretty close to melting the chamber walls. pulsed types haven't shown this range of throttling(yet at anyrate)

 

[the.punk]

And a fission reactor creats more radioactive waste.

 

[jedidia]

as for capacitors storing sufficient energy and releasing it quick enough, well they already can and do. its how most fusion reactors are started due to the extreme load that would be place on the power supply if it was simply hooked up to the mains. when you knock out a few square miles of power grid people get upset.

That's pretty amazing, thanks for the info. So I'd guess a capacitor would be the most likely joice.

So now you have two reactors, twice as much danger

Actually, a fusion reactor is not very dangerous. It doesn't go boom, it only goes flat as soon as the coontainement fails. I think the only way to make some nice damage with it would be to override all saveguards and heat it up until your beryllium core melts away, but not even that results necessarily in an explosion.

 

[insane_alien]

That's pretty amazing, thanks for the info. So I'd guess a capacitor would be the most likely joice.

well, its not capacitors on their own, but capacitors allow you to store up power from whatever low wattage source you have and then discharge it all at once.

like in a camera, the battery is utterly incapable of getting the flash bulb to light up on its own. so a capacitor is used to store a few seconds output of the battery and release it all in something like a millisecond. much higher power.

 

[Kozak]

fuel cells produce very pure water(usually used for drinking water on space missions) and whats more the hydrogen and oxygen can be recovered when the fusion reactor has started up by simple electrolysis.

You're forgetting about:
1.Fusion reactor minimal energy scale. Even with "throttling" - unless having a constant heat dump (= engine exhaust), you'll have to have radiators of corresponding performance. That is, most likely you can't have it constantly running, that is, you'll have to have another energy source
2.Mass ratios. A fission reactor weighs less than chemical fuel that is required to start a fusion reaction once by your scheme, and provides practically unlimited number of attempts to do it.

So, unless you're talking about "generation ships", this is unsensical.

pulsed fusion reactors aren't likely to be used on space missions as they are inherently bulky and require mor moving parts which means they are more prone to failure.

When did light in laser confinement reactor became a "moving part"?..

 

[the.punk]

well, its not capacitors on their own, but capacitors allow you to store up power from whatever low wattage source you have and then discharge it all at once.

like in a camera, the battery is utterly incapable of getting the flash bulb to light up on its own. so a capacitor is used to store a few seconds output of the battery and release it all in something like a millisecond. much higher power.

Oh yes. Capacitors are also used on some laboratory lasers. They are charged a few days on an atomic power plant and then they will release all the power on an giant laser beam.

Yes. An capacitor would be an option.

 

[insane_alien]

You're forgetting about:
1.Fusion reactor minimal energy scale. Unless having a constant heat dump (= engine exhaust), you'll have to have just huge radiators. That is, you can't have it constantly running, that is, you'll have to have another energy source

assuming your going to use this for thrust anyway, you're going to need huge radiators anyway as not all the heat will go into the exhaust.

2.Mass ratios. A fission reactor weighs less than chemical fuel that is required to start a fusion reaction once by your scheme, and provides practically unlimited number of attempts to do it.

not so, the fuel used to start the fusion reactor can be easily and practically recovered and used again and again.

also, you only need ~3200 kg of fuel (hydrogen + oxygen) to start the fusion reactor by fuel cells. which you'll get back as hydrogen and oxygen. show me a sub 10 tonne nuclear reactor.

When did light in laser confinement reactor became a "moving part"?..

you'll need moving parts to inject the fuel pellets. you'll also need moving parts to focus the lasers(they need recalibration every now and then) and i'm sure there are others. not to mention the lasers tend to produce some nasty waste products.

 

[Kozak]

assuming your going to use this for thrust anyway, you're going to need huge radiators anyway as not all the heat will go into the exhaust.

This "huge" is 50-95% smaller than previous.

also, you only need ~3200 kg of fuel (hydrogen + oxygen) to start the fusion reactor by fuel cells. which you'll get back as hydrogen and oxygen. show me a sub 10 tonne nuclear reactor.

http://en.wikipedia.org/wiki/Tupolev_Tu-119
Most small reactors weigh less than 10 tons, it's the surrounding machinery what makes them heavy - and this machinery is going to be present with combustion cycle also.

not so, the fuel used to start the fusion reactor can be easily and practically recovered and used again and again.

What if a start attempt fails?..

 

[insane_alien]

any ship that massive will be carrying enough fuel to make multiple attempts.

and the Tu-199's reactor was heavier than 10-tonnes IIRC. and the shielding was a bit crap.

 

[the.punk]

also, you only need ~3200 kg of fuel (hydrogen + oxygen) to start the fusion reactor by fuel cells. which you'll get back as hydrogen and oxygen. show me a sub 10 tonne nuclear reactor.

I think you are talking about electrolyse. A fuel cell creates energie with hydrogen + oxygen, but what you get is NOT hydrogen and oxygen. It water. Sure, water is composed of hydrogen and oxygen, but you need energy to get the hydrogen and the oxygen split again.
Because when you get energy from something (it is not necesarry what), you never get the same as before.

 

[insane_alien]

well, in previous posts i had made it clear that once the reactor was started, it would be feasible to electrolyse the water and recover the hydrogen and oxygen from when the fuel cells were running.

i was not suggesting that fuelcells produced hydrogen and oxygen and did not think it was required to reiterate that the water was electrolysed at a later time.

 

[the.punk]

Ah..ok. I missed that.

 

[RisingFury]

Actually, a fusion reactor is not very dangerous. It doesn't go boom, it only goes flat as soon as the coontainement fails. I think the only way to make some nice damage with it would be to override all saveguards and heat it up until your beryllium core melts away, but not even that results necessarily in an explosion.

Keep in mind that the reactor itself is still radioactive. It gets bombarded by neutrons. In the highly unlikely event of a detonation, you can safely assume that radioactive fragments would be sprayed all over the place.




@the.punk: Sure, a fusion reactor may be more efficient, but the extra weight needed for the fission reactor would pretty much insure that any benefits you get from efficiency are knocked off by slower acceleration.

The main reason you'd wanna use a fusion reactor is because you can pick your fuel up on the way, the fuel that you had to store (yes, yes, difficult to store hydrogen, but you can store water and use electrolysis to break it up) is much lighter then the fuel that powers a fission reactor, the radioactive wasted left over by the fusion reactor is almost insignificant compared to that of a fission reactor and it's far safer - there's no chance of an uncontrolled nuclear reaction.

Ultimately, you're talking about the efficiency of nuclear reaction and not power production. Power production would most likely rely on a steam engine and those usually around 20 - 30% efficient.