> 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.
> 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.
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?
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.
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.
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.
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.
>> 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.
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.
>> 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.
That's the topic of discussion. Therefore it is relevant.
> Break-even does not necessarily mean that the reaction itself produces more power than the ignition apparatus ...
That is exactly, precisely what it means. That is how break-even is defined.
Quote: "The fusion energy gain factor, usually expressed with the symbol Q, is the ratio of fusion power produced in a nuclear fusion reactor to the power required to maintain the plasma in steady state. The condition of Q = 1 is referred to as breakeven."
Any questions?
> You are implicitly assuming that some of the 250W surplus can be extracted, but that none of the original 1000W can.
I am not implicitly assuming anything. In the example, because 1000 Watts is required to sustain the plasma in a fusing state, none of that power is available for any other purpose -- it might as well not exist. An attempt to harvest any part of that power will extinguish the fusion reaction. This leaves 250 watts. Those are the facts.
> That's a physically indefensible assumption.
Okay, you need to learn basic physics. One cannot harvest more than 250 watts from the hypothetical system without extinguishing the reaction. The original 1000 watts is unavailable -- it might as well not exist.
>> 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.
Yes, you did. That was your claim -- that fusion reactors produced more power than they required for initiation. Here's what you said:
> Thus, if you can do fusion at all, you will end up with more total energy than you started with.
It is false. While the reaction is underway, you do not have more total energy than you started with, you have less. I have explained why this is so, very clearly.
You simply are not even reading. Nobody said anything about maintaining any steady state. That's needed for a generator. This project is talking about doing one off detonations.
Have you heard of a hydrogen bomb? If they lost energy to creating the fusion in a hydrogen bomb, what would be the point? If you removed the fusion part of a hydrogen bomb, would you get a bigger explosion? No. You get MORE ENERGY using fusion. However, we don't have a way to harness that to make electricity yet to maintain a steady reaction. But maintaining a steady reaction is not what this project proposes doing. Do you get it yet?
Edit: smaller explosion -> bigger explosion. So many explosions.
And your evidence is that I quoted everything that I replied to, word for word, and quoted from the original NASA project documents as well?
> Nobody said anything about maintaining any steady state.
I did, and so did NASA. You missed its significance. Pulsed systems have an average power level, and a peak power level. Both need to be analyzed.
> That's needed for a generator.
Yes, and NASA wants a net generator of energy, something better than break-even, otherwise it's not worth doing. And they say this in their documents about the project.
> This project is talking about doing one off detonations.
I can't believe you missed the significance of NASA'a remarks about break-even. Don't you understand that, pulsed or not, break-even still has a meaning, and if they can't get to break-even, the project makes no sense?
A steady state generator either does or doesn't achieve break-even. A generator that consists of a series of pulses also does or doesn't achieve break-even. Here's what NASA has to say about this:
Quote: "A subscale, laboratory liner compression test facility will be assembled with sufficient liner kinetic energy (~ 0.5 MJ) to reach fusion breakeven conditions."
I can't believe NASA thinks break-even is an essential program goal. Maybe they should hire you as a consultant, so they won't waste taxpayer dollars trying for break-even, after all, according to you, because the output is pulsed, break-even has no meaning.
> But maintaining a steady reaction is not what this project proposes doing.
You very clearly do not understand the relationship between peak and average power. The device being described generates a series of pulses, but for there to be a point to the exercise, the average output power must exceed break-even.
A radar has a peak output power of two megawatts and a steady-state input power of ten watts. Does the radar violate the principle of energy conservation? Yes or no?
A fusion reactor has a peak output power of two megawatts and requires an average plasma sustaining power of 200 KW. Such a generator either does or does not achieve break-even over time, and as quoted above, NASA cares very much which is so.
> But maintaining a steady reaction is not what this project proposes doing.
You need to learn the relationship between peak and average power. Stop embarrassing yourself.
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.