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Something is rotten in Denmark. If this project achieves a sustained fusion reaction as claimed, that represents a huge breakthrough, and not just for space pro

by lutusp 14y ago

Something is rotten in Denmark. If this project achieves a sustained fusion reaction as claimed, that represents a huge breakthrough, and not just for space propulsion, but for the general topic of fusion power. If it succeeds, it would sweep away all present fusion reaction prototype methods, including the well-tested but so far ineffective( * ) tokamak and laser-fusion approaches. * By "ineffective" I mean none of them has reached the break-even point, that point where more energy is released than is required to initiate the reaction in the first place. The article doesn't say whether the prototype device has actually succeeded in igniting a sustained fusion reaction. If it does, it would quickly move beyond its present goal of producing a more effective source of space acceleration and would answer some longstanding questions about fusion power itself. The fact that this isn't being discussed leads to my statement above -- something is rotten in Denmark. Either the project is overselling its possibilities, and/or it can't really achieve fusion break-even. Speaking hypothetically, if the device could produce a sustained fusion reaction with substantial power, it could be scaled up and used to propel a spacecraft to Mars in much less than 30 days. Assuming a sustained acceleration of 1 g, the hypothetical craft could accelerate for 1/2 the trip, turn around and decelerate for the other 1/2 of the distance, arriving at Mars with zero velocity. Apart from minimizing travel time, this hypothetical profile would prevent the bone loss that accompanies sustained time at zero-g. Making the above assumptions, and assuming that Mars is at a close approach point in its orbit, the travel time could be as little as ... wait for it .. 50 hours. Derivation: 1. Distance d (meters) for acceleration a (m/s^2) and time t (seconds): d = 1/2 a t^2 2. Time t for distance d and acceleration a, assuming 1/2 acceleration and 1/2 deceleration: t = 2 sqrt(d/a) 3. Result for Mars close approach (7.834e10 meters) and acceleration of 1 g: 49.64 hours. Again, speaking very hypothetically. I still think something is rotten in Denmark.

ars 14y ago
It's not continuous, it's pulses. But that doesn't actually matter, attach this thing to a big wheel and pulse it to run a generator. Or a few of them pulsing in sequence. But I wonder if the radiation exhaust might be to high to be practical on earth.
- arethuza 14y ago
  The fusion products are presumably fairly hot - so why not use them to heat water and drive a steam turbine generator combination? [NB This is in reply to the idea of using whirling fusion generators as a means of generating power, not part of the discussion on using it as spacecraft propulsion!]
  - lifeisstillgood 14y ago
    I think the suggestion is that the fusion is achieved only around the tiny pellet, and that the fusion acts against the magnetic containment to actually propel the pellet out the back. That's my guess - which slightly beats the idea they have achieved sustainable net positive energy fusion but cannot be bothered to solve all the worlds energy problems and become trillionaires.
    - lutusp 14y ago
      If this idea had been reduced to practice, it would mean the end of the National Ignition Facility, which after decades of effort, has yet to produce more power than is required to start the reaction: http://en.wikipedia.org/wiki/National_Ignition_Facility http://en.wikipedia.org/wiki/National_Ignition_Facility If the described method actually worked, it would force a complete reevaluation of the other approaches to fusion, none of which have actually worked (in a practical sense) after decades of effort. That's why I doubt that the described method has moved beyond the theoretical phase. If this isn't true, then people would try to create a continuous-power version of the technology, in which prior energy releases and high temperatures would be used to sustain new fusion energy releases. I've been reading about fusion power research for decades, I know the problems, and I remain skeptical.
      - sebbi 14y ago
        I think you are missing the point that this kind of fusion requires huge amounts of power. It isn't self sustained. The benefit being that you can electricity instead of chemicals to propel your craft. Mankind has been able to fuse atoms for quite some time now, the only problem remaining - for power plant usage - is becoming energy positive.
abdullahkhalids 14y ago
"It's very scalable; we can achieve fusion at a much smaller scale," he said. "We could run the designed engine off 200KW of solar panels, which is about the same power as generated by the panels around the International Space Station" So this is hot fusion, not cold. Hot fusion has been around for a long time. In fact I remember a couple of high school kids made a hot fusion device once.
- arethuza 14y ago
  My understanding is that "hot" fusion is actually fairly easy - it's generating a surplus of energy from fusion[*] outside of a nuclear weapon that is the tricky part. http://en.wikipedia.org/wiki/Fusor http://en.wikipedia.org/wiki/Fusor [Note: Typically in most H-bomb designs most of the energy still comes from fission - the fast neutrons from the fusion in the secondary igniting fission of the DU tamper round the secondary].
  - lutusp 14y ago
    > My understanding is that "hot" fusion is actually fairly easy ... Unless you're talking about a thermonuclear weapon, it's not easy at all. In fact, decades of research have yet to produce a break-even reaction in the laboratory.
    - nkoren 14y ago
      I think you needed to read the second half of that sentence before replying.
- lutusp 14y ago
  > Hot fusion has been around for a long time. Only in the stars and in thermonuclear weapons. It has not been achieved in a break-even sense anywhere else, in spite of decades of research. Many very large projects are dedicated to achieving it, including the National Ignotion Facility: http://en.wikipedia.org/wiki/National_Ignition_Facility http://en.wikipedia.org/wiki/National_Ignition_Facility The linked article shows that, in spite of vast amounts of money being spent, the project has yet to achieve break-even (i.e. produce more power than required to start the reaction). > In fact I remember a couple of high school kids made a hot fusion device once. No, that did not happen. It's an urban myth.
chrisfarms 14y ago
I guess the point is less about the fusion method being efficient (and getting past break-even) but more about propulsion. ie. for 200KW of input do you get more propulsion from initiating a fusion reaction than using an ion ejection method. Their aim is to move a heavy object with as little fuel as possible not necessarily produce perfect fusion. (although obviously that would be nice for everyone!)
- lutusp 14y ago
  > I guess the point is less about the fusion method being efficient (and getting past break-even) but more about propulsion. But "break-even" means yielding more energy than is required to start the reaction. Obviously anything less than break-even is not worth having -- a chemical rocket would be more efficient. > Their aim is to move a heavy object with as little fuel as possible not necessarily produce perfect fusion. This misses the point that present laboratory fusion reactions require more power than they release. It's not a question of "perfect fusion" but any fusion that has a net positive energy yield.
  - ceejayoz 14y ago
    > Obviously anything less than break-even is not worth having -- a chemical rocket would be more efficient. Why not? The energy to start the reactor comes from solar power, so they didn't have to carry it up with them.
  - rbanffy 14y ago
    > Obviously anything less than break-even is not worth having No. Not at all. We are talking propulsion, using fusion reactions to pass on energy to the propellant and convert that into thrust. Although it would be desirable, it's not required to make it a net-positive reaction - just giving the propellant more energy than a chemical reaction is enough to be more efficient than a chemical rocket. As the article mentioned, you can power this rocket with an ISS worth of solar panels (which is quite a lot of mass). Or, as it didn't mention, a very small fission reactor (provided you could negotiate putting a 200 KW reactor in space).
deleted 14y ago
[deleted]
DennisP 14y ago
The same guy (John Slough) started Helion Energy, which is attempting to build a power-producing fusion reactor. Last I heard they had a 1/3 scale device with promising results, and needed $20 million for a full-scale test. But he got funding for the rocket first. The rocket project may actually be easier. It only pulses once per minute, just runs for three days, and doesn't have to deal with the economic considerations of a practical power plant. Incidentally, Helion is not the only alternative fusion project in the works. Others include Sandia's MagLIF, Lockheed's recently-announced project, Tri-Alpha, General Fusion, Focus Fusion, and IEC/polywell.
- lutusp 14y ago
  > Last I heard they had a 1/3 scale device with promising results, and needed $20 million for a full-scale test. Research takes time, the potential payoff is huge, but in the final analysis, the fusion reaction has to produce more power than it requires. It has to achieve what's called "break-even". A fusion reaction with a power gain < 1 is endothermic -- its temperature is less than that of its power source. A fusion generator by definition produces more power than it requires for initiation -- it has a power gain > 1, and it is exothermic. > Incidentally, Helion is not the only alternative fusion project in the works. Others include Sandia's MagLIF, Lockheed's recently-announced project, Tri-Alpha, General Fusion, Focus Fusion, and IEC/polywell. This is a good example of long-shot science, applied research. None of these ideas show very much promise, but the potential payoff is so big that (IMHO) the research money is well-spent.
  - vectorjohn 14y ago
    >A fusion reaction with a power gain < 1 is endothermic -- its temperature is less than that of its power source. You are trolling! Or you just don't get it. The energy does not disappear. If you put in 1, and the reaction gives you .5, you have 1.5. You do not have to maintain a stable reaction, in the same way a hydrogen bomb doesn't have to produce however many terajoules of electrical energy to be more useful than TNT. It just has to be more total. They are not making an electric generator, they are making a rocket. They are not the same thing.
wazoox 14y ago
> Something is rotten in Denmark. If this project achieves a sustained fusion reaction as claimed, that represents a huge breakthrough, and not just for space propulsion, but for the general topic of fusion power. Achieving fusion in a tokamak or with lasers has been a solved problem for a couple of decades. The problem with fusion power is generating more energy from the fusion than consumed to power the lasers or tokamak. There is no such problem here as I understand it; the system isn't supposed to sustain itself from fusion, but instead will rely on solar panels or some other energy source to power the fusion reactor.
- sebbi 14y ago
  Exactly. The OP is mixing things up ... this fusion drive is just a means to convert electric energy to velocity, not a fusion reactor to generate power.
- lutusp 14y ago
  > Achieving fusion in a tokamak or with lasers has been a solved problem for a couple of decades. I should have been more clear -- I mean sustained fusion generation, meaning a net energy gain over that required to start the reaction in the first place. That hasn't been achieved. > There is no such problem here as I understand it; the system isn't supposed to sustain itself from fusion ... Not according to the NASA documents (see below). If that were true, there would be no point in using the system. If the fusion reaction produces less power than it requires, the designers would be better off using the source electrical power to drive an ion thruster. The claim being made is that the fusion scheme creates more power than is required to start it -- by using electrical power to initiate a fusion reaction that produces more power than it requires. If this were not true, the amount of converted solar energy described in the project (i.e. 200 KW) is not enough to propel the relatively massive spacecraft to Mars in 30 days using other methods. Here's the evidence: Link: http://www.nasa.gov/directorates/spacetech/niac/2012_phaseII_fellows_slough.html http://www.nasa.gov/directorates/spacetech/niac/2012_phaseII... Title: "The Fusion Driven Rocket: Nuclear Propulsion through Direct Conversion of Fusion Energy" Quote: "an in-depth analysis of the rocket design and spacecraft integration as well as mission architectures enabled by the FDR need to be performed. Fulfilling these three elements form the major tasks to be completed in the proposed Phase II study. A subscale, laboratory liner compression test facility will be assembled with sufficient liner kinetic energy (~ 0.5 MJ) to reach fusion breakeven conditions." [emphasis added] Which brings us back to square one. If this project were to succeed, it would instantly replace the existing approaches (i.e. tokamak and laser inertial confinement) as the most promising candidate for large-scale fusion power generation. As I said before, something is rotten in Denmark -- why isn't this project being described as a candidate for earthly fusion power generation, given that it must achieve break-even to accomplish its mission?
moron4hire 14y ago
it's not about having net energy gain, it's about converting electrical and nuclear energy into kinetic energy.
- lutusp 14y ago
  > it's not about having net energy gain ... That's not what the NASA documents say, the documents that describe the project: http://www.nasa.gov/directorates/spacetech/niac/2012_phaseII_fellows_slough.html http://www.nasa.gov/directorates/spacetech/niac/2012_phaseII... Quote: "an in-depth analysis of the rocket design and spacecraft integration as well as mission architectures enabled by the FDR need to be performed. Fulfilling these three elements form the major tasks to be completed in the proposed Phase II study. A subscale, laboratory liner compression test facility will be assembled with sufficient liner kinetic energy (~ 0.5 MJ) to reach fusion breakeven conditions." {emphasis added] Without a net energy gain, the project would be better off using an ion thruster, which has the advantage of already existing and being quite efficient (but with a power gain < 1).
  - moron4hire 14y ago
    To repeat what others have told you several times in this thread: Why do we still use chemical rockets instead of ion thrusters for boosters? Answer: thrust vs. specific impulse. Ion-thrusters are efficient in use of (high specific impulse), but are weak in how much thrust they create. Your ship would have to be mostly ion-thruster to get it anywhere, thus ruining the efficiency advantage with respect to payload of the engine. http://en.wikipedia.org/wiki/Ion_thruster http://en.wikipedia.org/wiki/Ion_thruster "Given the practical weight of suitable power sources, the accelerations given by ion thrusters are frequently less than one thousandth of standard gravity." What does linear kinetic energy have to do with fission reactors? It's not as simple as "attach a turbine to the kinetic energy conversion process." Or rather, it is that simple, and that's why it doesn't work, because it's another point at which one loses power to inefficiency. But if all you want is kinetic energy in the first place, then you don't lose efficiency to that extra step.
    - 14y ago
jff 14y ago
It's not a sustained reaction. It's just a bunch of short fusion reactions, one after another. We've been capable of short, uncontrolled fusion reactions for decades--it's just that, being bombs, we didn't have much luck generating electricity from them. A hydrogen bomb uses a relatively small quantity of conventional explosives to start a reaction which ultimately releases orders of magnitude more energy than the original input. This engine claims to use a small input of electrical energy to trigger a reaction which releases a lot of energy. Then, much like a car getting blown off the road by a nuclear test, they just position their ship near this reaction and get shoved away. The proposal specifically says they will not convert the energy to electricity, but will use the reaction directly for thrust.
- vectorjohn 14y ago
  Actually hydrogen bombs don't use conventional explosives. They use fission. See http://en.wikipedia.org/wiki/Pure_fusion_weapon http://en.wikipedia.org/wiki/Pure_fusion_weapon (a hypothetical bomb).
  - lutusp 14y ago
    > Actually hydrogen bombs don't use conventional explosives. They use fission. Sort of, but because the fission trigger requires conventional explosives, one could argue that conventional explosives play a key role. In temporal sequence: 1. Conventional explosives compress the fission core. 2. At maximum compression, a special neutron source releases a burst of neutrons to start nuclear fission. 3. The fission trigger delivers a huge pulse of energy to the mixed hydrogen isotope / fission secondary, compressing and heating it. 4. The secondary begins to fuse, releasing much more energy than the fission trigger could produce on its own. The above is somewhat oversimplified: http://en.wikipedia.org/wiki/Thermonuclear_weapon http://en.wikipedia.org/wiki/Thermonuclear_weapon It's our weak imitation of a star.
danmaz74 14y ago
Also speaking very hypothetically, why do you suppose that all the input electrical power is wasted for the propulsion goal? Each fusion mini-explosion will have a total energy that is the sum of the input (ignition) energy and the fusion itself, not just that of the fusion. Edit: This would mean that the thrust could be higher than the electrical input power even below break-even for the controlled fusion. If you only consider the electrical input, you could have a >1 ratio for the electrical energy/kinetic energy conversion (thanks to the fusion).
- lutusp 14y ago
  > Also speaking very hypothetically, why do you suppose that all the input electrical power is wasted for the propulsion goal? Each fusion mini-explosion will have a total energy that is the sum of the input (ignition) energy and the fusion itself, not just that of the fusion. Yes, technically true, but consider this hypothetical scenario: 1. A power source, let's say 1000 watts, can be used for any purpose, or can be used to heat a fusion plasma. 2. A fusion reactor requires 1000 watts to sustain fusion but delivers 250 watts of fusion power. 3. The total power in the system is now 1,250 watts. 4. But that total power is not available for external uses. 1000 watts of it must perpetually be applied only to the task of heating the plasma. 5. That leaves 250 watts for other uses. That means the fusion reaction uses more power than it produces, only 250 watts is available for other purposes, and there is no remedy except to not use the fusion reactor. That's why break-even is an essential precondition for this project's viability.

by lutusp 14y ago

ars 14y ago

It's not continuous, it's pulses. But that doesn't actually matter, attach this thing to a big wheel and pulse it to run a generator. Or a few of them pulsing in sequence. But I wonder if the radiation exhaust might be to high to be practical on earth.

arethuza 14y ago
The fusion products are presumably fairly hot - so why not use them to heat water and drive a steam turbine generator combination? [NB This is in reply to the idea of using whirling fusion generators as a means of generating power, not part of the discussion on using it as spacecraft propulsion!]
- lifeisstillgood 14y ago
  I think the suggestion is that the fusion is achieved only around the tiny pellet, and that the fusion acts against the magnetic containment to actually propel the pellet out the back. That's my guess - which slightly beats the idea they have achieved sustainable net positive energy fusion but cannot be bothered to solve all the worlds energy problems and become trillionaires.
  - lutusp 14y ago
    If this idea had been reduced to practice, it would mean the end of the National Ignition Facility, which after decades of effort, has yet to produce more power than is required to start the reaction: http://en.wikipedia.org/wiki/National_Ignition_Facility http://en.wikipedia.org/wiki/National_Ignition_Facility If the described method actually worked, it would force a complete reevaluation of the other approaches to fusion, none of which have actually worked (in a practical sense) after decades of effort. That's why I doubt that the described method has moved beyond the theoretical phase. If this isn't true, then people would try to create a continuous-power version of the technology, in which prior energy releases and high temperatures would be used to sustain new fusion energy releases. I've been reading about fusion power research for decades, I know the problems, and I remain skeptical.
    - sebbi 14y ago
      I think you are missing the point that this kind of fusion requires huge amounts of power. It isn't self sustained. The benefit being that you can electricity instead of chemicals to propel your craft. Mankind has been able to fuse atoms for quite some time now, the only problem remaining - for power plant usage - is becoming energy positive.

abdullahkhalids 14y ago

"It's very scalable; we can achieve fusion at a much smaller scale," he said. "We could run the designed engine off 200KW of solar panels, which is about the same power as generated by the panels around the International Space Station" So this is hot fusion, not cold. Hot fusion has been around for a long time. In fact I remember a couple of high school kids made a hot fusion device once.

arethuza 14y ago
My understanding is that "hot" fusion is actually fairly easy - it's generating a surplus of energy from fusion[*] outside of a nuclear weapon that is the tricky part. http://en.wikipedia.org/wiki/Fusor http://en.wikipedia.org/wiki/Fusor [Note: Typically in most H-bomb designs most of the energy still comes from fission - the fast neutrons from the fusion in the secondary igniting fission of the DU tamper round the secondary].
- lutusp 14y ago
  > My understanding is that "hot" fusion is actually fairly easy ... Unless you're talking about a thermonuclear weapon, it's not easy at all. In fact, decades of research have yet to produce a break-even reaction in the laboratory.
  - nkoren 14y ago
    I think you needed to read the second half of that sentence before replying.
lutusp 14y ago
> Hot fusion has been around for a long time. Only in the stars and in thermonuclear weapons. It has not been achieved in a break-even sense anywhere else, in spite of decades of research. Many very large projects are dedicated to achieving it, including the National Ignotion Facility: http://en.wikipedia.org/wiki/National_Ignition_Facility http://en.wikipedia.org/wiki/National_Ignition_Facility The linked article shows that, in spite of vast amounts of money being spent, the project has yet to achieve break-even (i.e. produce more power than required to start the reaction). > In fact I remember a couple of high school kids made a hot fusion device once. No, that did not happen. It's an urban myth.

chrisfarms 14y ago

I guess the point is less about the fusion method being efficient (and getting past break-even) but more about propulsion. ie. for 200KW of input do you get more propulsion from initiating a fusion reaction than using an ion ejection method. Their aim is to move a heavy object with as little fuel as possible not necessarily produce perfect fusion. (although obviously that would be nice for everyone!)

lutusp 14y ago
> I guess the point is less about the fusion method being efficient (and getting past break-even) but more about propulsion. But "break-even" means yielding more energy than is required to start the reaction. Obviously anything less than break-even is not worth having -- a chemical rocket would be more efficient. > Their aim is to move a heavy object with as little fuel as possible not necessarily produce perfect fusion. This misses the point that present laboratory fusion reactions require more power than they release. It's not a question of "perfect fusion" but any fusion that has a net positive energy yield.
- ceejayoz 14y ago
  > Obviously anything less than break-even is not worth having -- a chemical rocket would be more efficient. Why not? The energy to start the reactor comes from solar power, so they didn't have to carry it up with them.
- rbanffy 14y ago
  > Obviously anything less than break-even is not worth having No. Not at all. We are talking propulsion, using fusion reactions to pass on energy to the propellant and convert that into thrust. Although it would be desirable, it's not required to make it a net-positive reaction - just giving the propellant more energy than a chemical reaction is enough to be more efficient than a chemical rocket. As the article mentioned, you can power this rocket with an ISS worth of solar panels (which is quite a lot of mass). Or, as it didn't mention, a very small fission reactor (provided you could negotiate putting a 200 KW reactor in space).

deleted 14y ago

[deleted]

DennisP 14y ago

The same guy (John Slough) started Helion Energy, which is attempting to build a power-producing fusion reactor. Last I heard they had a 1/3 scale device with promising results, and needed $20 million for a full-scale test. But he got funding for the rocket first. The rocket project may actually be easier. It only pulses once per minute, just runs for three days, and doesn't have to deal with the economic considerations of a practical power plant. Incidentally, Helion is not the only alternative fusion project in the works. Others include Sandia's MagLIF, Lockheed's recently-announced project, Tri-Alpha, General Fusion, Focus Fusion, and IEC/polywell.

lutusp 14y ago
> Last I heard they had a 1/3 scale device with promising results, and needed $20 million for a full-scale test. Research takes time, the potential payoff is huge, but in the final analysis, the fusion reaction has to produce more power than it requires. It has to achieve what's called "break-even". A fusion reaction with a power gain < 1 is endothermic -- its temperature is less than that of its power source. A fusion generator by definition produces more power than it requires for initiation -- it has a power gain > 1, and it is exothermic. > Incidentally, Helion is not the only alternative fusion project in the works. Others include Sandia's MagLIF, Lockheed's recently-announced project, Tri-Alpha, General Fusion, Focus Fusion, and IEC/polywell. This is a good example of long-shot science, applied research. None of these ideas show very much promise, but the potential payoff is so big that (IMHO) the research money is well-spent.
- vectorjohn 14y ago
  >A fusion reaction with a power gain < 1 is endothermic -- its temperature is less than that of its power source. You are trolling! Or you just don't get it. The energy does not disappear. If you put in 1, and the reaction gives you .5, you have 1.5. You do not have to maintain a stable reaction, in the same way a hydrogen bomb doesn't have to produce however many terajoules of electrical energy to be more useful than TNT. It just has to be more total. They are not making an electric generator, they are making a rocket. They are not the same thing.

wazoox 14y ago

> Something is rotten in Denmark. If this project achieves a sustained fusion reaction as claimed, that represents a huge breakthrough, and not just for space propulsion, but for the general topic of fusion power. Achieving fusion in a tokamak or with lasers has been a solved problem for a couple of decades. The problem with fusion power is generating more energy from the fusion than consumed to power the lasers or tokamak. There is no such problem here as I understand it; the system isn't supposed to sustain itself from fusion, but instead will rely on solar panels or some other energy source to power the fusion reactor.

sebbi 14y ago
Exactly. The OP is mixing things up ... this fusion drive is just a means to convert electric energy to velocity, not a fusion reactor to generate power.
lutusp 14y ago
> Achieving fusion in a tokamak or with lasers has been a solved problem for a couple of decades. I should have been more clear -- I mean sustained fusion generation, meaning a net energy gain over that required to start the reaction in the first place. That hasn't been achieved. > There is no such problem here as I understand it; the system isn't supposed to sustain itself from fusion ... Not according to the NASA documents (see below). If that were true, there would be no point in using the system. If the fusion reaction produces less power than it requires, the designers would be better off using the source electrical power to drive an ion thruster. The claim being made is that the fusion scheme creates more power than is required to start it -- by using electrical power to initiate a fusion reaction that produces more power than it requires. If this were not true, the amount of converted solar energy described in the project (i.e. 200 KW) is not enough to propel the relatively massive spacecraft to Mars in 30 days using other methods. Here's the evidence: Link: http://www.nasa.gov/directorates/spacetech/niac/2012_phaseII_fellows_slough.html http://www.nasa.gov/directorates/spacetech/niac/2012_phaseII... Title: "The Fusion Driven Rocket: Nuclear Propulsion through Direct Conversion of Fusion Energy" Quote: "an in-depth analysis of the rocket design and spacecraft integration as well as mission architectures enabled by the FDR need to be performed. Fulfilling these three elements form the major tasks to be completed in the proposed Phase II study. A subscale, laboratory liner compression test facility will be assembled with sufficient liner kinetic energy (~ 0.5 MJ) to reach fusion breakeven conditions." [emphasis added] Which brings us back to square one. If this project were to succeed, it would instantly replace the existing approaches (i.e. tokamak and laser inertial confinement) as the most promising candidate for large-scale fusion power generation. As I said before, something is rotten in Denmark -- why isn't this project being described as a candidate for earthly fusion power generation, given that it must achieve break-even to accomplish its mission?

moron4hire 14y ago

it's not about having net energy gain, it's about converting electrical and nuclear energy into kinetic energy.

lutusp 14y ago
> it's not about having net energy gain ... That's not what the NASA documents say, the documents that describe the project: http://www.nasa.gov/directorates/spacetech/niac/2012_phaseII_fellows_slough.html http://www.nasa.gov/directorates/spacetech/niac/2012_phaseII... Quote: "an in-depth analysis of the rocket design and spacecraft integration as well as mission architectures enabled by the FDR need to be performed. Fulfilling these three elements form the major tasks to be completed in the proposed Phase II study. A subscale, laboratory liner compression test facility will be assembled with sufficient liner kinetic energy (~ 0.5 MJ) to reach fusion breakeven conditions." {emphasis added] Without a net energy gain, the project would be better off using an ion thruster, which has the advantage of already existing and being quite efficient (but with a power gain < 1).
- moron4hire 14y ago
  To repeat what others have told you several times in this thread: Why do we still use chemical rockets instead of ion thrusters for boosters? Answer: thrust vs. specific impulse. Ion-thrusters are efficient in use of (high specific impulse), but are weak in how much thrust they create. Your ship would have to be mostly ion-thruster to get it anywhere, thus ruining the efficiency advantage with respect to payload of the engine. http://en.wikipedia.org/wiki/Ion_thruster http://en.wikipedia.org/wiki/Ion_thruster "Given the practical weight of suitable power sources, the accelerations given by ion thrusters are frequently less than one thousandth of standard gravity." What does linear kinetic energy have to do with fission reactors? It's not as simple as "attach a turbine to the kinetic energy conversion process." Or rather, it is that simple, and that's why it doesn't work, because it's another point at which one loses power to inefficiency. But if all you want is kinetic energy in the first place, then you don't lose efficiency to that extra step.
  - 14y ago

jff 14y ago

It's not a sustained reaction. It's just a bunch of short fusion reactions, one after another. We've been capable of short, uncontrolled fusion reactions for decades--it's just that, being bombs, we didn't have much luck generating electricity from them. A hydrogen bomb uses a relatively small quantity of conventional explosives to start a reaction which ultimately releases orders of magnitude more energy than the original input. This engine claims to use a small input of electrical energy to trigger a reaction which releases a lot of energy. Then, much like a car getting blown off the road by a nuclear test, they just position their ship near this reaction and get shoved away. The proposal specifically says they will not convert the energy to electricity, but will use the reaction directly for thrust.

vectorjohn 14y ago
Actually hydrogen bombs don't use conventional explosives. They use fission. See http://en.wikipedia.org/wiki/Pure_fusion_weapon http://en.wikipedia.org/wiki/Pure_fusion_weapon (a hypothetical bomb).
- lutusp 14y ago
  > Actually hydrogen bombs don't use conventional explosives. They use fission. Sort of, but because the fission trigger requires conventional explosives, one could argue that conventional explosives play a key role. In temporal sequence: 1. Conventional explosives compress the fission core. 2. At maximum compression, a special neutron source releases a burst of neutrons to start nuclear fission. 3. The fission trigger delivers a huge pulse of energy to the mixed hydrogen isotope / fission secondary, compressing and heating it. 4. The secondary begins to fuse, releasing much more energy than the fission trigger could produce on its own. The above is somewhat oversimplified: http://en.wikipedia.org/wiki/Thermonuclear_weapon http://en.wikipedia.org/wiki/Thermonuclear_weapon It's our weak imitation of a star.

danmaz74 14y ago

Also speaking very hypothetically, why do you suppose that all the input electrical power is wasted for the propulsion goal? Each fusion mini-explosion will have a total energy that is the sum of the input (ignition) energy and the fusion itself, not just that of the fusion. Edit: This would mean that the thrust could be higher than the electrical input power even below break-even for the controlled fusion. If you only consider the electrical input, you could have a >1 ratio for the electrical energy/kinetic energy conversion (thanks to the fusion).

lutusp 14y ago
> Also speaking very hypothetically, why do you suppose that all the input electrical power is wasted for the propulsion goal? Each fusion mini-explosion will have a total energy that is the sum of the input (ignition) energy and the fusion itself, not just that of the fusion. Yes, technically true, but consider this hypothetical scenario: 1. A power source, let's say 1000 watts, can be used for any purpose, or can be used to heat a fusion plasma. 2. A fusion reactor requires 1000 watts to sustain fusion but delivers 250 watts of fusion power. 3. The total power in the system is now 1,250 watts. 4. But that total power is not available for external uses. 1000 watts of it must perpetually be applied only to the task of heating the plasma. 5. That leaves 250 watts for other uses. That means the fusion reaction uses more power than it produces, only 250 watts is available for other purposes, and there is no remedy except to not use the fusion reactor. That's why break-even is an essential precondition for this project's viability.