4 ms·

Oh I'm well aware, I work on this stuff, but I'm still curious about their system, and any on-orbit data they might have. We're working on krypton Hall thruster

Oh I'm well aware, I work on this stuff, but I'm still curious about their system, and any on-orbit data they might have. We're working on krypton Hall thrusters right now in our lab and trying to characterize the differences in operation. There's some non-obvious scaling factors we're working to understand in order to try and better optimize thrusters for krypton operation. We have a recent paper (free pdf) about this at pepl.engin.umich.edu/pdf/2021_JoAP_Su.pdf.

Robotbeat 5y ago
Interesting! Thanks for the paper link.
jcims 5y ago
Assuming a spherical cow, would there be any advantage to building ion thrusters to accelerate the particles to speeds where relativistic effects become meaningful?
- pfdietz 5y ago
  It would be hugely energy inefficient. Generally, you want the exhaust velocity of your rockets to be close to the total mission delta-V (or, better, the total delta-V up to that point in the mission.)
  - londons_explore 5y ago
    > the total delta-V up to that point in the mission. Doesn't this give ION drives a big advantage... they can have variable exhaust velocity...
    - pfdietz 5y ago
      The space charge limit on the ion current increases with voltage, as does the momentum per particle. The result is that thrust/area of a given ion engine will scale as the square of the voltage. So, one doesn't want to reduce the voltage too much. Electromagnetic rockets, like Hall thrusters, do not have the same space charge limit, and are typically operated at lower exhaust velocity.
- Robotbeat 5y ago
  No, because thrust per unit power drops as exhaust velocity increases. You’ll be increasing your power supply and thruster mass far more than any propellant you save. It’s also energy inefficient to use too high of an exhaust velocity. The mass-optimal exhaust velocity is approximately equal to the square root of (twice the thrusting time times overall power-and-propulsion system specific power times electrical efficiency). Read more here: https://ocw.mit.edu/courses/aeronautics-and-astronautics/16-522-space-propulsion-spring-2015/lecture-notes/MIT16_522S15_Lecture3-4.pdf https://ocw.mit.edu/courses/aeronautics-and-astronautics/16-... Assuming no dry mass (spherical cow!), energy optimal Exhaust velocity is equal to the total elapsed mission velocity at any point. If you can’t adjust exhaust velocity, it’s proportional to approximately to 60-65% of the total mission delta-v (off the top of my head). More discussion of where this energy-optimal exhaust velocity comes from: https://www.youtube.com/watch?v=ogKKjpQvfuM https://www.youtube.com/watch?v=ogKKjpQvfuM
  - btilly 5y ago
    Indeed, one of the memorable stories from https://www.amazon.com/Ignition-Informal-History-Liquid-Propellants-ebook/dp/B076838QS2 https://www.amazon.com/Ignition-Informal-History-Liquid-Prop... was when the author wrote a joke paper analyzing the optimal amount of mercury (!) to mix into rocket fuel for maximal thrust. It takes away from the power, but since mercury is heavy it slows the exhaust, and therefore can increase thrust. To the author's surprise, the general evaluating the crazy ideas that the lab came up with failed to recognize that it was meant as a joke, and ordered that it actually be tested. So some unfortunate test site had to actually set up the test, and verify the theory. Luckily the military came to their senses about then and never actually built real rockets using the principle. (For those who are confused, mercury may be heavy, but is a nasty contaminant. Organic compounds including mercury have a disturbing tendency to be neural toxins. See https://en.wikipedia.org/wiki/Karen_Wetterhahn https://en.wikipedia.org/wiki/Karen_Wetterhahn for a famous example of how little is needed to be lethal. You really don't want to be spewing it over the countryside in a flaming ball.)
    - nkoren 5y ago
      Stories like this make Charlie Stross' "A Tall Tale" seem that much more believable: https://www.tor.com/2012/07/20/a-tall-tail/ https://www.tor.com/2012/07/20/a-tall-tail/
- ben_w 5y ago
  Not really. As the ion energy approaches relativistic, the engine also starts approaching the behaviour of a photon thruster. If your ion drive is solar powered, the system as a whole then starts to behave like a solar sail made from grey paper rather than shiny mirrors.
- jhgb 5y ago
  > would there be any advantage to building ion thrusters to accelerate the particles to speeds where relativistic effects become meaningful? Probably not; why wouldn't you just shine a light in one direction? It's effectively the same thing.
  - pmontra 5y ago
    Don't photons have zero mass?
    - jhgb 5y ago
      Photons have mass equal to their energy equivalent. And relativistic ions have most of their mass in the same form. So propulsion-wise, the two are almost equal...except that a light emitting diode is much simpler than a particle accelerator. Both would have very poor thrust, of course, and with existing energy sources, the waste mass from the energy production would be disproportional so neither would really be used for anything practical.
    - at_a_remove 5y ago
      Photons have zero rest mass. That sounds like hair-splitting but it is not.
    - deleted 5y ago
      [deleted]

Robotbeat 5y ago

Interesting! Thanks for the paper link.

jcims 5y ago

Assuming a spherical cow, would there be any advantage to building ion thrusters to accelerate the particles to speeds where relativistic effects become meaningful?

pfdietz 5y ago
It would be hugely energy inefficient. Generally, you want the exhaust velocity of your rockets to be close to the total mission delta-V (or, better, the total delta-V up to that point in the mission.)
- londons_explore 5y ago
  > the total delta-V up to that point in the mission. Doesn't this give ION drives a big advantage... they can have variable exhaust velocity...
  - pfdietz 5y ago
    The space charge limit on the ion current increases with voltage, as does the momentum per particle. The result is that thrust/area of a given ion engine will scale as the square of the voltage. So, one doesn't want to reduce the voltage too much. Electromagnetic rockets, like Hall thrusters, do not have the same space charge limit, and are typically operated at lower exhaust velocity.
Robotbeat 5y ago
No, because thrust per unit power drops as exhaust velocity increases. You’ll be increasing your power supply and thruster mass far more than any propellant you save. It’s also energy inefficient to use too high of an exhaust velocity. The mass-optimal exhaust velocity is approximately equal to the square root of (twice the thrusting time times overall power-and-propulsion system specific power times electrical efficiency). Read more here: https://ocw.mit.edu/courses/aeronautics-and-astronautics/16-522-space-propulsion-spring-2015/lecture-notes/MIT16_522S15_Lecture3-4.pdf https://ocw.mit.edu/courses/aeronautics-and-astronautics/16-... Assuming no dry mass (spherical cow!), energy optimal Exhaust velocity is equal to the total elapsed mission velocity at any point. If you can’t adjust exhaust velocity, it’s proportional to approximately to 60-65% of the total mission delta-v (off the top of my head). More discussion of where this energy-optimal exhaust velocity comes from: https://www.youtube.com/watch?v=ogKKjpQvfuM https://www.youtube.com/watch?v=ogKKjpQvfuM
- btilly 5y ago
  Indeed, one of the memorable stories from https://www.amazon.com/Ignition-Informal-History-Liquid-Propellants-ebook/dp/B076838QS2 https://www.amazon.com/Ignition-Informal-History-Liquid-Prop... was when the author wrote a joke paper analyzing the optimal amount of mercury (!) to mix into rocket fuel for maximal thrust. It takes away from the power, but since mercury is heavy it slows the exhaust, and therefore can increase thrust. To the author's surprise, the general evaluating the crazy ideas that the lab came up with failed to recognize that it was meant as a joke, and ordered that it actually be tested. So some unfortunate test site had to actually set up the test, and verify the theory. Luckily the military came to their senses about then and never actually built real rockets using the principle. (For those who are confused, mercury may be heavy, but is a nasty contaminant. Organic compounds including mercury have a disturbing tendency to be neural toxins. See https://en.wikipedia.org/wiki/Karen_Wetterhahn https://en.wikipedia.org/wiki/Karen_Wetterhahn for a famous example of how little is needed to be lethal. You really don't want to be spewing it over the countryside in a flaming ball.)
  - nkoren 5y ago
    Stories like this make Charlie Stross' "A Tall Tale" seem that much more believable: https://www.tor.com/2012/07/20/a-tall-tail/ https://www.tor.com/2012/07/20/a-tall-tail/
ben_w 5y ago
Not really. As the ion energy approaches relativistic, the engine also starts approaching the behaviour of a photon thruster. If your ion drive is solar powered, the system as a whole then starts to behave like a solar sail made from grey paper rather than shiny mirrors.
jhgb 5y ago
> would there be any advantage to building ion thrusters to accelerate the particles to speeds where relativistic effects become meaningful? Probably not; why wouldn't you just shine a light in one direction? It's effectively the same thing.
- pmontra 5y ago
  Don't photons have zero mass?
  - jhgb 5y ago
    Photons have mass equal to their energy equivalent. And relativistic ions have most of their mass in the same form. So propulsion-wise, the two are almost equal...except that a light emitting diode is much simpler than a particle accelerator. Both would have very poor thrust, of course, and with existing energy sources, the waste mass from the energy production would be disproportional so neither would really be used for anything practical.
  - at_a_remove 5y ago
    Photons have zero rest mass. That sounds like hair-splitting but it is not.
  - deleted 5y ago
    [deleted]