6 ms·

The Quest for a Fusion Drive

83 points by tectonic 4y ago

pfdietz 4y ago
The concept described there sounds like BS. If you're not achieving breakeven, it's probably much easier to just heat plasma with the drive energy and expel that (VASIMIR, if you've heard of that). The plasma stays much cooler than a fusion plasma, so radiation from it should be less -- and this radiation is a big problem, since it heats the vehicle, and that heat has to be radiated, and radiators are big and heavy. The problem of cooling is the bete noir of all high Isp propulsion systems. This is why I liked laser propelled systems, since it's possible to cool an object with a laser beam (anti-Stokes scattering).
Symmetry 4y ago
In terms of fusion drivers I've always liked NASA's PuFF[1] or Pulsed Fission Fusion. The idea is to use a normal z-pinch pulsed fusion setting, which won't normally be self sustaining. But if you surround the D-T mix with depleted uranium it'll adsorb a lot of the fast neutrons from the fusion and trigger a round of fission generating more energy. The recoil against the magnetic compressor/nozzle should be enough to generate enough energy for the next pulse and the reaction byproduct are emitted to become your propellant. [1]https://www.nasa.gov/puff https://www.nasa.gov/puff
- zabzonk 4y ago
  we don't have fusion power on earth (except for bombs) so why could we magically create it (and launch it) in space?
  - deleted 4y ago
    [deleted]
  - yummypaint 4y ago
    Fusion on earth (that people talk about) is about generating electricity. Fusion on spacecraft is about shooting out exhaust gas as quickly as possible and storing energy at high density. The rocket equation relates the exhaust rate to the propelent mass requirements, which determines the feasibility of the spacecraft. It's a completely different optimization problem, and one that has historically favored nuclear energy sources in general.
    - seiferteric 4y ago
      To clarify further, from TFA: "While early versions of the Helicity Drive wouldn’t exceed the Lawson criterion (more energy out than in) and would require external (presumably solar) power, the energy added to the exhaust from fusion would increase the system’s exhaust velocity and ISP (giving somewhere in the neighborhood of 10,000 s isp, vs a maximum of ~400 for chemical rockets)" So it is not able to generate more energy than it takes to operate, just like the fusion devices we can already create on earth, but can create really fast exhaust which makes the rocket more efficient.
  - sbierwagen 4y ago
    In 1998, Robert A. Freitas published a detailed design of an artificial red blood cell with several useful properties, (stores 236 times as much oxygen as a natural red blood cell, indefinite shelf life) but with the minor downside that it would require atomically precise diamond fabrication to actually construct. https://www.tandfonline.com/doi/pdf/10.3109/10731199809117682 https://www.tandfonline.com/doi/pdf/10.3109/1073119980911768... Was that paper a waste of time?
    - causi 4y ago
      It's always shocking to me how inefficient hemoglobin is. Less than 1/500th the mass of oxygenated hemoglobin is oxygen.
      - neltnerb 4y ago
        I'm similarly always a little surprised at just how inefficient photosynthesis is compared to a solar cell. Once I realized it was more than an order of magnitude I lost a lot of interest in biomass (much less ethanol) as a fuel source for sure...
        brookst 4y ago
        Solar cells do one thing. Plants do many things.
  - DennisP 4y ago
    For a power plant, you need to get more energy out than you put in. The best we've done is output that's about 70% of the input energy. But a plasma rocket works without any fusion at all. If you get that same 70%, then you're doing 70% better than a plasma rocket without fusion. On top of that, a power plant needs to run reliably for thirty years or so, withstanding intense neutron radiation, and breeding new tritium fuel from lithium. A fusion rocket might only need to fire for several days for a trip to another planet, and won't breed its own tritium.
  - mr_toad 4y ago
    > we don't have fusion power on earth We do though, you can build a fusion reactor in your garage https://en.wikipedia.org/wiki/Fusor https://en.wikipedia.org/wiki/Fusor What’s hard is making a fusion reactor that produces more energy than it consumes, and does so economically.
    - 1letterunixname 4y ago
      It's a fusor, a glorified toy for making neutrons, not a fusion reactor.
  - credit_guy 4y ago
    The NASA Puff proposal would be more aptly described as a boosted fission engine. From their intro [1] This synergy has been observed in the development of other fission-fusion devices. You may be wondering what "other fission-fusion devices" were ever developed. The answer is of course, nuclear bombs. The first fission-fusion device was not a thermonuclear bomb, but a boosted fission one [2]. In such a device, there's a little bit of Deuterium and Tritium in the center of the Plutonium pit. When the Plutonium starts undergoing fission, the temperature increases to tens of millions of degrees, and the D-T fusion reaction happens; the energy produced by that reaction is negligible. What the reaction does is release a huge amount of neutrons that go and split nuclei of Plutonium, therefore boosting the yield of the fission in the bomb. The idea with Nasa's Puff proposal is the fusion is triggered first, using a plasma confinement device, similar to a Tokamak, but linear, not toroidal. The device is well below breakeven. But you don't care about the energy produced by the fusion, only that enough neutrons are released to go on an split a bunch of fissile nuclei. Few people are aware, but not only Uranium and Plutonium (and other heavy nuclei) can undergo fission, but also Lithium. Lithium can't sustain a chain reaction, but if external neutrons (or protons) are provided, it is happy to split and release a respectable amount of energy in the process. Pound per pound about as much energy as Uranium and Plutonium. The end result is a very hot plasma of millions of degrees that is expelled and provides huge thrust and specific impulse. Of course the ejected plasma will be radioactive, and that's why this design can't be used on Earth to produce energy, or other useful things. But in space, blowing some radioactive stuff is not a problem: it will simply spread out and go on and on forever and ever. 99.9999999% of it will not encounter anything for billions of years. The few particles that will, by luck of the draw, encounter something will be no different than other cosmic particles coming from supernovas, or other sources. [1] https://www.nasa.gov/puff/intro https://www.nasa.gov/puff/intro [2] https://en.wikipedia.org/wiki/Boosted_fission_weapon https://en.wikipedia.org/wiki/Boosted_fission_weapon
  - 1letterunixname 4y ago
    This is the critical leap into fantasy. We have to get some form of self-sustaining fusion, i.e., producing energy, before we have a shot at making an engine out of it.
    - PhasmaFelis 4y ago
      We don't. An engine doesn't need to be energy-positive, it just needs to produce thrust.
  - traverseda 4y ago
    Well, you don't need to care about safety nearly as much.
    - zabzonk 4y ago
      safety is not what is limiting fusion development on earth - we simply can't do it effectively
      - DennisP 4y ago
        And even if we could, safety isn't a concern for fusion power plants.
      - ClumsyPilot 4y ago
        Ofcourse we can - that's what a hydrogen bomb does! The Teller–Ulam design and the NIF do the exact same job - they heat up and compress hydrogen to trigger fusion. Its just that NIF and has to fuck around with lazers that are perfectly focused on tiny gold pellets with micrograms of hydrogen. The hydrogen bomb deals with literal tons of material at once. If you dont care about safety and environment, you concrete over one of the great lakes, heat them up by exploding thermonuclear bombs and use the steam to generate elctricity.
  - zizee 4y ago
    There is value in creating designs that are feasible if some technological breakthroughs is made. If we manage to make fusion work, then someone can then capitalise on the existing designs. It's also a great motivator for people to work on those hard problems that could enable many fantastic possibilities. It's also just fun to let your imagination not be constrained by what is possible today, but what might be possible in the future.
bobsmooth 4y ago
NASA needs to stop being wimps and bring back Project Orion https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...
- cjtrowbridge 4y ago
  There are a lot of reasons why this is a bad idea, not least of which is the effects of the resulting electromagnetic pulses on satellites, the earth, and the equipment on board the vehicle itself. There is also the radioactive fallout that would affect human populations all over the world. Detonating nuclear weapons at high altitudes or in space would also disturb or damage the van allen belts and expose the surface of the earth to high levels of radiation from solar winds and cosmic rays. And of course there is the fact that it's a violation of international law to bring nuclear weapons to space or to test or detonate them in the atmosphere or in space.
  - politician 4y ago
    > Detonating nuclear weapons at high altitudes or in space would also disturb or damage the van allen belts and expose the surface of the earth to high levels of radiation from solar winds and cosmic rays. Could you please elaborate on the effects of high nuclear altitude testing on the belts? Preferably, a link to a paper.
    - cjtrowbridge 4y ago
      The belts are filled with captured charged particles which deflect cosmic rays and protect the earth from them. If you blow them away, those rays will get through. One of the Project Orion proposals outlined in the linked wiki is about how to mitigate the impact somewhat by launching from unpopulated areas so that humans are not affected by the resulting radiation that gets through the hole you punch in the earth's protective charged particle shield.
      - politician 4y ago
kappuchino 4y ago
Am I the only one to admit thinking "Odd, It makes no sense returning to hybrid disk + ssd drives(1)". Anyway. Way more cool. (1) https://de.wikipedia.org/wiki/Fusion_Drive https://de.wikipedia.org/wiki/Fusion_Drive

83 points by tectonic 4y ago

The concept described there sounds like BS. If you're not achieving breakeven, it's probably much easier to just heat plasma with the drive energy and expel that (VASIMIR, if you've heard of that). The plasma stays much cooler than a fusion plasma, so radiation from it should be less -- and this radiation is a big problem, since it heats the vehicle, and that heat has to be radiated, and radiators are big and heavy. The problem of cooling is the bete noir of all high Isp propulsion systems. This is why I liked laser propelled systems, since it's possible to cool an object with a laser beam (anti-Stokes scattering).

Symmetry 4y ago

In terms of fusion drivers I've always liked NASA's PuFF[1] or Pulsed Fission Fusion. The idea is to use a normal z-pinch pulsed fusion setting, which won't normally be self sustaining. But if you surround the D-T mix with depleted uranium it'll adsorb a lot of the fast neutrons from the fusion and trigger a round of fission generating more energy. The recoil against the magnetic compressor/nozzle should be enough to generate enough energy for the next pulse and the reaction byproduct are emitted to become your propellant. [1]https://www.nasa.gov/puff https://www.nasa.gov/puff

zabzonk 4y ago
we don't have fusion power on earth (except for bombs) so why could we magically create it (and launch it) in space?
- deleted 4y ago
  [deleted]
- yummypaint 4y ago
  Fusion on earth (that people talk about) is about generating electricity. Fusion on spacecraft is about shooting out exhaust gas as quickly as possible and storing energy at high density. The rocket equation relates the exhaust rate to the propelent mass requirements, which determines the feasibility of the spacecraft. It's a completely different optimization problem, and one that has historically favored nuclear energy sources in general.
  - seiferteric 4y ago
    To clarify further, from TFA: "While early versions of the Helicity Drive wouldn’t exceed the Lawson criterion (more energy out than in) and would require external (presumably solar) power, the energy added to the exhaust from fusion would increase the system’s exhaust velocity and ISP (giving somewhere in the neighborhood of 10,000 s isp, vs a maximum of ~400 for chemical rockets)" So it is not able to generate more energy than it takes to operate, just like the fusion devices we can already create on earth, but can create really fast exhaust which makes the rocket more efficient.
- sbierwagen 4y ago
  In 1998, Robert A. Freitas published a detailed design of an artificial red blood cell with several useful properties, (stores 236 times as much oxygen as a natural red blood cell, indefinite shelf life) but with the minor downside that it would require atomically precise diamond fabrication to actually construct. https://www.tandfonline.com/doi/pdf/10.3109/10731199809117682 https://www.tandfonline.com/doi/pdf/10.3109/1073119980911768... Was that paper a waste of time?
  - causi 4y ago
    It's always shocking to me how inefficient hemoglobin is. Less than 1/500th the mass of oxygenated hemoglobin is oxygen.
    - neltnerb 4y ago
      I'm similarly always a little surprised at just how inefficient photosynthesis is compared to a solar cell. Once I realized it was more than an order of magnitude I lost a lot of interest in biomass (much less ethanol) as a fuel source for sure...
      - brookst 4y ago
        Solar cells do one thing. Plants do many things.
- DennisP 4y ago
  For a power plant, you need to get more energy out than you put in. The best we've done is output that's about 70% of the input energy. But a plasma rocket works without any fusion at all. If you get that same 70%, then you're doing 70% better than a plasma rocket without fusion. On top of that, a power plant needs to run reliably for thirty years or so, withstanding intense neutron radiation, and breeding new tritium fuel from lithium. A fusion rocket might only need to fire for several days for a trip to another planet, and won't breed its own tritium.
- mr_toad 4y ago
  > we don't have fusion power on earth We do though, you can build a fusion reactor in your garage https://en.wikipedia.org/wiki/Fusor https://en.wikipedia.org/wiki/Fusor What’s hard is making a fusion reactor that produces more energy than it consumes, and does so economically.
  - 1letterunixname 4y ago
    It's a fusor, a glorified toy for making neutrons, not a fusion reactor.
- credit_guy 4y ago
  The NASA Puff proposal would be more aptly described as a boosted fission engine. From their intro [1] This synergy has been observed in the development of other fission-fusion devices. You may be wondering what "other fission-fusion devices" were ever developed. The answer is of course, nuclear bombs. The first fission-fusion device was not a thermonuclear bomb, but a boosted fission one [2]. In such a device, there's a little bit of Deuterium and Tritium in the center of the Plutonium pit. When the Plutonium starts undergoing fission, the temperature increases to tens of millions of degrees, and the D-T fusion reaction happens; the energy produced by that reaction is negligible. What the reaction does is release a huge amount of neutrons that go and split nuclei of Plutonium, therefore boosting the yield of the fission in the bomb. The idea with Nasa's Puff proposal is the fusion is triggered first, using a plasma confinement device, similar to a Tokamak, but linear, not toroidal. The device is well below breakeven. But you don't care about the energy produced by the fusion, only that enough neutrons are released to go on an split a bunch of fissile nuclei. Few people are aware, but not only Uranium and Plutonium (and other heavy nuclei) can undergo fission, but also Lithium. Lithium can't sustain a chain reaction, but if external neutrons (or protons) are provided, it is happy to split and release a respectable amount of energy in the process. Pound per pound about as much energy as Uranium and Plutonium. The end result is a very hot plasma of millions of degrees that is expelled and provides huge thrust and specific impulse. Of course the ejected plasma will be radioactive, and that's why this design can't be used on Earth to produce energy, or other useful things. But in space, blowing some radioactive stuff is not a problem: it will simply spread out and go on and on forever and ever. 99.9999999% of it will not encounter anything for billions of years. The few particles that will, by luck of the draw, encounter something will be no different than other cosmic particles coming from supernovas, or other sources. [1] https://www.nasa.gov/puff/intro https://www.nasa.gov/puff/intro [2] https://en.wikipedia.org/wiki/Boosted_fission_weapon https://en.wikipedia.org/wiki/Boosted_fission_weapon
- 1letterunixname 4y ago
  This is the critical leap into fantasy. We have to get some form of self-sustaining fusion, i.e., producing energy, before we have a shot at making an engine out of it.
  - PhasmaFelis 4y ago
    We don't. An engine doesn't need to be energy-positive, it just needs to produce thrust.
- traverseda 4y ago
  Well, you don't need to care about safety nearly as much.
  - zabzonk 4y ago
    safety is not what is limiting fusion development on earth - we simply can't do it effectively
    - DennisP 4y ago
      And even if we could, safety isn't a concern for fusion power plants.
    - ClumsyPilot 4y ago
      Ofcourse we can - that's what a hydrogen bomb does! The Teller–Ulam design and the NIF do the exact same job - they heat up and compress hydrogen to trigger fusion. Its just that NIF and has to fuck around with lazers that are perfectly focused on tiny gold pellets with micrograms of hydrogen. The hydrogen bomb deals with literal tons of material at once. If you dont care about safety and environment, you concrete over one of the great lakes, heat them up by exploding thermonuclear bombs and use the steam to generate elctricity.
- zizee 4y ago
  There is value in creating designs that are feasible if some technological breakthroughs is made. If we manage to make fusion work, then someone can then capitalise on the existing designs. It's also a great motivator for people to work on those hard problems that could enable many fantastic possibilities. It's also just fun to let your imagination not be constrained by what is possible today, but what might be possible in the future.

bobsmooth 4y ago

NASA needs to stop being wimps and bring back Project Orion https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...

cjtrowbridge 4y ago
There are a lot of reasons why this is a bad idea, not least of which is the effects of the resulting electromagnetic pulses on satellites, the earth, and the equipment on board the vehicle itself. There is also the radioactive fallout that would affect human populations all over the world. Detonating nuclear weapons at high altitudes or in space would also disturb or damage the van allen belts and expose the surface of the earth to high levels of radiation from solar winds and cosmic rays. And of course there is the fact that it's a violation of international law to bring nuclear weapons to space or to test or detonate them in the atmosphere or in space.
- politician 4y ago
  > Detonating nuclear weapons at high altitudes or in space would also disturb or damage the van allen belts and expose the surface of the earth to high levels of radiation from solar winds and cosmic rays. Could you please elaborate on the effects of high nuclear altitude testing on the belts? Preferably, a link to a paper.
  - cjtrowbridge 4y ago
    The belts are filled with captured charged particles which deflect cosmic rays and protect the earth from them. If you blow them away, those rays will get through. One of the Project Orion proposals outlined in the linked wiki is about how to mitigate the impact somewhat by launching from unpopulated areas so that humans are not affected by the resulting radiation that gets through the hole you punch in the earth's protective charged particle shield.
    - politician 4y ago

kappuchino 4y ago

Am I the only one to admit thinking "Odd, It makes no sense returning to hybrid disk + ssd drives(1)". Anyway. Way more cool. (1) https://de.wikipedia.org/wiki/Fusion_Drive https://de.wikipedia.org/wiki/Fusion_Drive