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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

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.

lutusp 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. No, it has not. Apart from stars and thermonuclear weapons, there are no fusion reactions that yield more energy than they require, i.e. the achieve break-even. It has not happened. > Thus, if you can do fusion at all, you will end up with more total energy than you started with. You just changed the subject. Apart from stars and weapons, existing experimental fusion reactions produce much less energy than is required to create them. For example, all the laboratory experiments to date. Also, very important, in a fusion reactor at less than break-even, the input power must be used to perpetually sustain the reaction (the state of the plasma), so that power is unavailable for any other purpose. Only the fusion reaction's power can be exploited. So in a hypothetical reactor that requires 1000 watts to sustain fusion but produces 250 watts of fusion power, only the 250 watt fraction is expoitable -- the original power must be reserved for heating the plasma. That's why break-even is essential. >> 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. Yes, it would. Given a power gain > 1, it would be child's play to generate steam and spin a turbine, as just one example. > And research into fusion generators that work pretty much like that has been and is being done. To date, there have been no -- that's NO -- laboratory fusion generators that produce more power than they require for initiation.
- gliese1337 14y ago
  No, it has not. Apart from stars and thermonuclear weapons, there are no fusion reactions that yield more energy than they require, i.e. the achieve break-even. It has not happened. That's true, but irrelevant. Producing more energy from the reaction than was required to start the reaction is a different thing from ending up with more total energy in the system than you started with. Even stars don't do that- they just have sufficiently good containment that they don't lose all of the initial ignition energy, and thus don't need additional power inputs to replace non-existent losses. You seem to be confusing the total power available in a fusion system with the total power available at the output terminals of a generator. The first is relevant to a rocket. The second is not. To date, energy recovery inefficiencies for fusion reactors have always been high enough that the energy lost to neutrinos / waste heat / etc. is large than the amount produced by the reaction, meaning that the power available at the output terminals is less than the input power. Break-even does not necessarily mean that the reaction itself produces more power than the ignition apparatus- it means that the total useful power you can extract from the system, whether you put it there to begin with or not, is larger than the power required by the ignition apparatus. But a rocket doesn't care about recovery. Any power produced by the fusion reaction counts as a gain. So in a hypothetical reactor that requires 1000 watts to sustain fusion but produces 250 watts of fusion power, only the 250 watt fraction is exploitable -- the original power must be reserved for heating the plasma. That's why break-even is essential. You are implicitly assuming that some of the 250W surplus can be extracted, but that none of the original 1000W can. That's a physically indefensible assumption. If you put in 1000W and the reaction generates 250W, then there's a total of 1250 indistinguishable watts running through the system, and you need to be able to harness at least 1000W to keep the system in steady state. If you can extract more than that, you've got a generator. To date, there have been no -- that's NO -- laboratory fusion generators that produce more power than they require for initiation. I made no claim that there were. I said that research into this kind of fusion generator has been done, not that it has resulted in a working generator yet. See, e.g., focus fusion, or magnetoplasmadynamic generators.

lutusp 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. No, it has not. Apart from stars and thermonuclear weapons, there are no fusion reactions that yield more energy than they require, i.e. the achieve break-even. It has not happened. > Thus, if you can do fusion at all, you will end up with more total energy than you started with. You just changed the subject. Apart from stars and weapons, existing experimental fusion reactions produce much less energy than is required to create them. For example, all the laboratory experiments to date. Also, very important, in a fusion reactor at less than break-even, the input power must be used to perpetually sustain the reaction (the state of the plasma), so that power is unavailable for any other purpose. Only the fusion reaction's power can be exploited. So in a hypothetical reactor that requires 1000 watts to sustain fusion but produces 250 watts of fusion power, only the 250 watt fraction is expoitable -- the original power must be reserved for heating the plasma. That's why break-even is essential. >> 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. Yes, it would. Given a power gain > 1, it would be child's play to generate steam and spin a turbine, as just one example. > And research into fusion generators that work pretty much like that has been and is being done. To date, there have been no -- that's NO -- laboratory fusion generators that produce more power than they require for initiation.

gliese1337 14y ago
No, it has not. Apart from stars and thermonuclear weapons, there are no fusion reactions that yield more energy than they require, i.e. the achieve break-even. It has not happened. That's true, but irrelevant. Producing more energy from the reaction than was required to start the reaction is a different thing from ending up with more total energy in the system than you started with. Even stars don't do that- they just have sufficiently good containment that they don't lose all of the initial ignition energy, and thus don't need additional power inputs to replace non-existent losses. You seem to be confusing the total power available in a fusion system with the total power available at the output terminals of a generator. The first is relevant to a rocket. The second is not. To date, energy recovery inefficiencies for fusion reactors have always been high enough that the energy lost to neutrinos / waste heat / etc. is large than the amount produced by the reaction, meaning that the power available at the output terminals is less than the input power. Break-even does not necessarily mean that the reaction itself produces more power than the ignition apparatus- it means that the total useful power you can extract from the system, whether you put it there to begin with or not, is larger than the power required by the ignition apparatus. But a rocket doesn't care about recovery. Any power produced by the fusion reaction counts as a gain. So in a hypothetical reactor that requires 1000 watts to sustain fusion but produces 250 watts of fusion power, only the 250 watt fraction is exploitable -- the original power must be reserved for heating the plasma. That's why break-even is essential. You are implicitly assuming that some of the 250W surplus can be extracted, but that none of the original 1000W can. That's a physically indefensible assumption. If you put in 1000W and the reaction generates 250W, then there's a total of 1250 indistinguishable watts running through the system, and you need to be able to harness at least 1000W to keep the system in steady state. If you can extract more than that, you've got a generator. To date, there have been no -- that's NO -- laboratory fusion generators that produce more power than they require for initiation. I made no claim that there were. I said that research into this kind of fusion generator has been done, not that it has resulted in a working generator yet. See, e.g., focus fusion, or magnetoplasmadynamic generators.