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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 mor

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).
  - mchouza 14y ago
    If fusion doesn't provide a substantial energy gain, it's not worth the effort. Existing ion thrusters are quite efficient (60+%) at translating electrical power to exhaust kinetic energy.
    - rbanffy 14y ago
      Indeed, but they offer very low thrust. For manned space travel, you may need something that can give you a higher acceleration, even if at the expense of efficiency. Also, fusion reactors designed for power generation have very different goals than fusion rockets. With power generation, particles leaving the reactor may be considered wasted energy. With rockets, the whole idea is to have particles leaving the reactor in a certain direction taking as much energy as possible with them. You just point the jet at the direction opposite to the one you want to go.
      - mchouza 14y ago
        Well, controlled fusion hasn't reached energy breakeven at all, even taking into account the kinetic energy of the reaction products. But the low thrust of current ion engines is mostly due to power restrictions (https://groups.google.com/group/sci.space.science/msg/0cb3323562c0bc1d?dmode=source https://groups.google.com/group/sci.space.science/msg/0cb332...). If your spaceship has a mass of one ton and you want an acceleration of 1 centigee with an exhaust velocity of 30 km/s, you will need at least: 0.5 * thrust * velocity = 0.5 * (0.1 m/s^2 * 1000 kg) * 30 km/s = 0.5 * 100 N * 3E4 m/s = 150E4 W = 1.5 MW of power.
    - Filligree 14y ago
      If you don't mind waiting a year. Existing ion thrusters don't give enough thrust to get to Mars in 30 days.
    - skew 14y ago
      "kinetic energy" means nothing alone, thrust and specific impulse are better metrics to look at. Doing a lot better than ion thrusters at either without being too much worse in the other and energy efficiency would be interesting. However, the article quotes an exhaust velocity of 30km/s (or ISP of 3000s), and using 200kW doesn't leave much room to beat http://en.wikipedia.org/wiki/HiPEP http://en.wikipedia.org/wiki/HiPEP on thrust without counting on net energy gain. The linked slides also claim net energy gain.
- emp_zealoth 14y ago
  You are completely missing a point of this. It is a propulsion system, not a power generating one. Chemical rocket engine doesn't generate any electrical power either. Main factors in propulsion is specific impulse and reaction mass and this idea promises a huge impulse - basically once in orbit, it could ferry between Earth and anywhere-where-solar-power-is-sufficient because it doesn't need tonnes of fuel, only relatively small load of pellets would get the job done. Of course there is the issue of getting it and it's payload into orbit, but I'd say Skylon will take care of that
  - lutusp 14y ago
    > You are completely missing a point of this. It is a propulsion system, not a power generating one. To represent an advantage over an ion thruster, it must be both. And the NASA documents that describe the project specify a fusion power gain > 1: 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." The phrase "fusion breakeven conditions" means the reaction must produce more power than is requires to start it. This is a requirement to justify the project over other approaches.
    - acomar 14y ago
      As other posters have noted, ion thrusters don't generate very much thrust.
    - emp_zealoth 14y ago
      It is not even supposed to be used to generate electrical power. Citing from the site itself: "Virtually all of the radiant, neutron and particle energy from the plasma is absorbed by the encapsulating, metal blanket thereby isolating the spacecraft from the fusion process and eliminating the need for large radiator mass" No radiators implicitly mean no electricity generation, because it creates waste heat that has to be utilised. http://en.wikipedia.org/wiki/Fusion_rocket http://en.wikipedia.org/wiki/Fusion_rocket Proposed concept is a direct drive, and it actually claims they fixed the problem with neutrons (they are caught by the compressing metal) As for breakeven conditions they merely state that the power that the fusion generates must be at least equal to power used to start/sustain it. http://en.wikipedia.org/wiki/Fusion_energy_gain_factor http://en.wikipedia.org/wiki/Fusion_energy_gain_factor Theoretical limits of efficiency of heat engine is around 70-80% (roughly) http://en.wikipedia.org/wiki/Thermal_efficiency http://en.wikipedia.org/wiki/Thermal_efficiency and you have to use those to convert heat generated in controlled fusion into electricity. To actually achieve net power generation in a power plant, you have to generate a lot more heat that electricity, while for propulsion you merely care about actual raw (so to speak) power. Ion thrusters have their own issues, while their ISP is considerably better than chemical rockets, maximum thrust is abysmal, but it is hard to argue about that due to the lack of data on FDR. However, with ion thrusters there are other issues (for example:wear, charge imbalance)

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).
- mchouza 14y ago
  If fusion doesn't provide a substantial energy gain, it's not worth the effort. Existing ion thrusters are quite efficient (60+%) at translating electrical power to exhaust kinetic energy.
  - rbanffy 14y ago
    Indeed, but they offer very low thrust. For manned space travel, you may need something that can give you a higher acceleration, even if at the expense of efficiency. Also, fusion reactors designed for power generation have very different goals than fusion rockets. With power generation, particles leaving the reactor may be considered wasted energy. With rockets, the whole idea is to have particles leaving the reactor in a certain direction taking as much energy as possible with them. You just point the jet at the direction opposite to the one you want to go.
    - mchouza 14y ago
      Well, controlled fusion hasn't reached energy breakeven at all, even taking into account the kinetic energy of the reaction products. But the low thrust of current ion engines is mostly due to power restrictions (https://groups.google.com/group/sci.space.science/msg/0cb3323562c0bc1d?dmode=source https://groups.google.com/group/sci.space.science/msg/0cb332...). If your spaceship has a mass of one ton and you want an acceleration of 1 centigee with an exhaust velocity of 30 km/s, you will need at least: 0.5 * thrust * velocity = 0.5 * (0.1 m/s^2 * 1000 kg) * 30 km/s = 0.5 * 100 N * 3E4 m/s = 150E4 W = 1.5 MW of power.
  - Filligree 14y ago
    If you don't mind waiting a year. Existing ion thrusters don't give enough thrust to get to Mars in 30 days.
  - skew 14y ago
    "kinetic energy" means nothing alone, thrust and specific impulse are better metrics to look at. Doing a lot better than ion thrusters at either without being too much worse in the other and energy efficiency would be interesting. However, the article quotes an exhaust velocity of 30km/s (or ISP of 3000s), and using 200kW doesn't leave much room to beat http://en.wikipedia.org/wiki/HiPEP http://en.wikipedia.org/wiki/HiPEP on thrust without counting on net energy gain. The linked slides also claim net energy gain.
emp_zealoth 14y ago
You are completely missing a point of this. It is a propulsion system, not a power generating one. Chemical rocket engine doesn't generate any electrical power either. Main factors in propulsion is specific impulse and reaction mass and this idea promises a huge impulse - basically once in orbit, it could ferry between Earth and anywhere-where-solar-power-is-sufficient because it doesn't need tonnes of fuel, only relatively small load of pellets would get the job done. Of course there is the issue of getting it and it's payload into orbit, but I'd say Skylon will take care of that
- lutusp 14y ago
  > You are completely missing a point of this. It is a propulsion system, not a power generating one. To represent an advantage over an ion thruster, it must be both. And the NASA documents that describe the project specify a fusion power gain > 1: 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." The phrase "fusion breakeven conditions" means the reaction must produce more power than is requires to start it. This is a requirement to justify the project over other approaches.
  - acomar 14y ago
    As other posters have noted, ion thrusters don't generate very much thrust.
  - emp_zealoth 14y ago
    It is not even supposed to be used to generate electrical power. Citing from the site itself: "Virtually all of the radiant, neutron and particle energy from the plasma is absorbed by the encapsulating, metal blanket thereby isolating the spacecraft from the fusion process and eliminating the need for large radiator mass" No radiators implicitly mean no electricity generation, because it creates waste heat that has to be utilised. http://en.wikipedia.org/wiki/Fusion_rocket http://en.wikipedia.org/wiki/Fusion_rocket Proposed concept is a direct drive, and it actually claims they fixed the problem with neutrons (they are caught by the compressing metal) As for breakeven conditions they merely state that the power that the fusion generates must be at least equal to power used to start/sustain it. http://en.wikipedia.org/wiki/Fusion_energy_gain_factor http://en.wikipedia.org/wiki/Fusion_energy_gain_factor Theoretical limits of efficiency of heat engine is around 70-80% (roughly) http://en.wikipedia.org/wiki/Thermal_efficiency http://en.wikipedia.org/wiki/Thermal_efficiency and you have to use those to convert heat generated in controlled fusion into electricity. To actually achieve net power generation in a power plant, you have to generate a lot more heat that electricity, while for propulsion you merely care about actual raw (so to speak) power. Ion thrusters have their own issues, while their ISP is considerably better than chemical rockets, maximum thrust is abysmal, but it is hard to argue about that due to the lack of data on FDR. However, with ion thrusters there are other issues (for example:wear, charge imbalance)