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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 p

by 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?
- Vivtek 14y ago
  The thing about a fusion rocket engine is that the energy is used to throw stuff out the back. In a fusion generator this would not be considered a feature - if you want a stationary generator that doesn't melt your city you have to contain that plasma, which is very, very energy-intensive.
  - lutusp 14y ago
    Yes, true, but neither scenario has a working model of a fusion generator with power gain > 1. > if you want a stationary generator that doesn't melt your city you have to contain that plasma, which is very, very energy-intensive. Actually, if you think about it, a rocket engine that can create a fusion reaction and direct the energy out "the back" as you put it, and a power generator that also directs fusion energy to a secondary process, are very similar. In the extreme case, you could take the space device, put it in a vacuum chamber, and direct the thrust into a steam generator.
- gliese1337 14y ago
  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. Yes, it has. Fusion produces energy; the energy you have to put in to initiate fusion does not magically disappear. Thus, if you can do fusion at all, you will end up with more total energy than you started with. What has not been achieved is capturing enough of that resultant energy in a form (such as electricity) in which it can be used to keep the reaction going. That step, however, is completely irrelevant to a rocket engine. The energy released by fusion is largely kinetic energy, and that is exactly what you want in rocket exhaust, no need to go through a lossy conversion step. 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. Not so. If the fusion reaction generates any energy at all, then that is better than just using your electrical power source by itself, for exactly the same reason that it's better to use a starter motor to initiate ignition of gasoline in your car than it is to try to run your car off of a weak starter motor. This is true even if you have no way of using the engine to recharge your battery. 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. No, it wouldn't. That would only happen if the additional step were taken of discovering a highly efficient means of capturing the kinetic and radiant energy of the engine exhaust and converting it back to electricity. And research into fusion generators that work pretty much like that has been and is being done. Edit: The slides at http://www.nasa.gov/pdf/636883main_FDR_talk_NIAC_2012_final.pdf http://www.nasa.gov/pdf/636883main_FDR_talk_NIAC_2012_final.... even say: It is assumed that initially FDR employs solar panels for house keeping power Eventually it would be derived directly from nozzle flux compression I.e., "once we do figure out how to turn this into a generator, then you don't need solar panels anymore." But they don't know how to derive energy from the exhaust stream through the nozzle yet, so they have to keep it going with solar panels. And extracting energy from the exhaust would necessarily reduce the ISP of the engine somewhat.

by wazoox 14y ago

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?

Vivtek 14y ago
The thing about a fusion rocket engine is that the energy is used to throw stuff out the back. In a fusion generator this would not be considered a feature - if you want a stationary generator that doesn't melt your city you have to contain that plasma, which is very, very energy-intensive.
- lutusp 14y ago
  Yes, true, but neither scenario has a working model of a fusion generator with power gain > 1. > if you want a stationary generator that doesn't melt your city you have to contain that plasma, which is very, very energy-intensive. Actually, if you think about it, a rocket engine that can create a fusion reaction and direct the energy out "the back" as you put it, and a power generator that also directs fusion energy to a secondary process, are very similar. In the extreme case, you could take the space device, put it in a vacuum chamber, and direct the thrust into a steam generator.
gliese1337 14y ago
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. Yes, it has. Fusion produces energy; the energy you have to put in to initiate fusion does not magically disappear. Thus, if you can do fusion at all, you will end up with more total energy than you started with. What has not been achieved is capturing enough of that resultant energy in a form (such as electricity) in which it can be used to keep the reaction going. That step, however, is completely irrelevant to a rocket engine. The energy released by fusion is largely kinetic energy, and that is exactly what you want in rocket exhaust, no need to go through a lossy conversion step. 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. Not so. If the fusion reaction generates any energy at all, then that is better than just using your electrical power source by itself, for exactly the same reason that it's better to use a starter motor to initiate ignition of gasoline in your car than it is to try to run your car off of a weak starter motor. This is true even if you have no way of using the engine to recharge your battery. 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. No, it wouldn't. That would only happen if the additional step were taken of discovering a highly efficient means of capturing the kinetic and radiant energy of the engine exhaust and converting it back to electricity. And research into fusion generators that work pretty much like that has been and is being done. Edit: The slides at http://www.nasa.gov/pdf/636883main_FDR_talk_NIAC_2012_final.pdf http://www.nasa.gov/pdf/636883main_FDR_talk_NIAC_2012_final.... even say: It is assumed that initially FDR employs solar panels for house keeping power Eventually it would be derived directly from nozzle flux compression I.e., "once we do figure out how to turn this into a generator, then you don't need solar panels anymore." But they don't know how to derive energy from the exhaust stream through the nozzle yet, so they have to keep it going with solar panels. And extracting energy from the exhaust would necessarily reduce the ISP of the engine somewhat.