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This is really cool. Electric propulsion for small satellites is a hot field and has been advancing rapidly, and this is no exception. The rising cost of Xenon

This is really cool. Electric propulsion for small satellites is a hot field and has been advancing rapidly, and this is no exception. The rising cost of Xenon is a real problem in the field right now so it's great to see the work being done on alternate propellants (it would be great if we knew more about SpaceX's thrusters, which run on Krypton). Doing some math, their total thrust efficiency seems to be around 29%, which is really good for a thruster of its size being ionized by RF power.

14 5y ago
I was so excited by the possibility of an EmDrive engine for a bit it really seemed like everyone was trying and that there was maybe something to them. But also they would violate laws of physics so remained skeptical. Watched too much Star Trek as a kid I love to look at the skies and imagine humans travelling across the galaxy. One day.
Robotbeat 5y ago
There’s not much to know about SpaceX’s thrusters. They’re basic Hall Effect thrusters that run on a noble gas chemically identical to Xenon but slightly less efficient (since the mass of each singly ionized ion is lower for Krypton, so for the same mass of propellant, you have to ionize more atoms than you would for Xenon… plus the ionization energy per atom may be different) and much cheaper. You could run them on Argon, if you wanted. A Noble Gas is a Noble Gas, to first and second order. There’s no real reason we couldn’t run Xenon thrusters on Krypton except the lower density of Krypton means somewhat heavier tanks for the same mass of propellant.
- dnautics 5y ago
  > A Noble Gas is a Noble Gas Probably not relevant for thrusters, but Xenon does make XeO2 and XeF6, and XeF4, and the other gases don't really do that (krypton does make KrF2)...
- mrfusion 5y ago
  Why is it important to be a Nobel gas?
  - Robotbeat 5y ago
    1) won’t condense on the rest of your spacecraft. 2) won’t corrode your thruster as much. 3) Chemical and fire safety aren’t concerns. 4) Is a fluid already so doesn’t need to be heated to flow. Also, being monotomic makes it easier to model everything. A more complex molecule could have weird chemistry that ends up producing solid particles that impinge or maybe condense on your spacecraft (coating lenses, etc).
    - idiotsecant 5y ago
      None of these are really the primary reason why we use noble gasses. It has to do with the ionization energy of the gas, which tells us how much energy we have to spend per molecule to ionize the gas. https://en.wikipedia.org/wiki/Ionization_energy https://en.wikipedia.org/wiki/Ionization_energy
      - Robotbeat 5y ago
        However noble gases don’t have the lowest ionization energy per unit molecular mass. Bismuth, Cesium, etc do significantly better on those metrics than even Xenon does, and Xenon is better than Krypton. http://aerospace.mtu.edu/__reports/Conference_Proceedings/2007_Ross_2.pdf http://aerospace.mtu.edu/__reports/Conference_Proceedings/20... EDIT: and iodine, ie the subject of this study, is also significantly better than Xenon in this metric: “ A possible alternative is iodine, which is much more abundant and cheaper than xenon and can be stored unpressurized as a solid. In addition, both atomic and diatomic iodine have a lower ionization threshold, and diatomic iodine has a relative mass that is almost twice that of xenon.”
        mrfusion 5y ago
        How do you gasify it if you store it as a solid? Heat?
- archermarks 5y ago
  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.
- taneq 5y ago
  I was confused by "a noble gas chemically identical to Xenon" for a while because isn't the defining feature of a noble gas that it's chemically inert? Why not just call them Krypton thrusters?
  - ashtonkem 5y ago
    I’d argue that being chemically inert is a property that can be compared and marked as “identical”, but that’s just semantics. Technically they’re called Ion Thrusters. They’re just colloquially called Xenon Thrusters because xenon is the most commonly used gas to use due to its weight. Perhaps radon could work too, I’m no expert, but I imagine the radioactivity is an issue.
    - Robotbeat 5y ago
      Usually these type are called Hall Thrusters. Ion thrusters (Gridded Ion Thrusters, the full name) work a bit differently internally than Hall Thrusters, but overall system behaves the same. The general term is “electric propulsion,” as in Nuclear Electric Propulsion or Solar Electric Propulsion.
      - Turing_Machine 5y ago
        There's also thermal (non-ionized) electric propulsion, e.g., arcjet (which heats the propellant with an electric arc) and resistojet (which heats the propellant with a resistance heater). Both arcjet and resistojet have been used on some satellites. The nice thing about those is that they don't rely as much on the physical or chemical properties of the propellant. Just about anything that can be vaporized by an arc or heating element will work. https://en.wikipedia.org/wiki/Arcjet_rocket https://en.wikipedia.org/wiki/Arcjet_rocket https://en.wikipedia.org/wiki/Resistojet_rocket https://en.wikipedia.org/wiki/Resistojet_rocket
gorgoiler 5y ago
…hot field…, hah. The cost of things is usually a function of labour and demand, but Xenon is in sufficiently low supply that building new engines is a better option than improving Xenon farming? Where does it come from?
- ridgeguy 5y ago
  Xenon is a very minor trace component of air. It's collected as a byproduct during fractional cryo-distillation of air to make LOX and liquid nitrogen. [1] [1] https://en.wikipedia.org/wiki/Xenon#Occurrence_and_production https://en.wikipedia.org/wiki/Xenon#Occurrence_and_productio...
  - gorgoiler 5y ago
    > Worldwide production of xenon in 1998 was estimated at 5,000–7,000 m3 O2 annual production is 1e11kg presumably from 5e11 of air (20% of air is O2.) Xenon occurs 1:1e7 so that air should also yield 50,000kg by mass, or 8500m3 by volume. Sounds about right. I’m guessing then O2 buyers don’t care to subsidise the Xe market.

14 5y ago

I was so excited by the possibility of an EmDrive engine for a bit it really seemed like everyone was trying and that there was maybe something to them. But also they would violate laws of physics so remained skeptical. Watched too much Star Trek as a kid I love to look at the skies and imagine humans travelling across the galaxy. One day.

Robotbeat 5y ago

There’s not much to know about SpaceX’s thrusters. They’re basic Hall Effect thrusters that run on a noble gas chemically identical to Xenon but slightly less efficient (since the mass of each singly ionized ion is lower for Krypton, so for the same mass of propellant, you have to ionize more atoms than you would for Xenon… plus the ionization energy per atom may be different) and much cheaper. You could run them on Argon, if you wanted. A Noble Gas is a Noble Gas, to first and second order. There’s no real reason we couldn’t run Xenon thrusters on Krypton except the lower density of Krypton means somewhat heavier tanks for the same mass of propellant.

dnautics 5y ago
> A Noble Gas is a Noble Gas Probably not relevant for thrusters, but Xenon does make XeO2 and XeF6, and XeF4, and the other gases don't really do that (krypton does make KrF2)...
mrfusion 5y ago
Why is it important to be a Nobel gas?
- Robotbeat 5y ago
  1) won’t condense on the rest of your spacecraft. 2) won’t corrode your thruster as much. 3) Chemical and fire safety aren’t concerns. 4) Is a fluid already so doesn’t need to be heated to flow. Also, being monotomic makes it easier to model everything. A more complex molecule could have weird chemistry that ends up producing solid particles that impinge or maybe condense on your spacecraft (coating lenses, etc).
  - idiotsecant 5y ago
    None of these are really the primary reason why we use noble gasses. It has to do with the ionization energy of the gas, which tells us how much energy we have to spend per molecule to ionize the gas. https://en.wikipedia.org/wiki/Ionization_energy https://en.wikipedia.org/wiki/Ionization_energy
    - Robotbeat 5y ago
      However noble gases don’t have the lowest ionization energy per unit molecular mass. Bismuth, Cesium, etc do significantly better on those metrics than even Xenon does, and Xenon is better than Krypton. http://aerospace.mtu.edu/__reports/Conference_Proceedings/2007_Ross_2.pdf http://aerospace.mtu.edu/__reports/Conference_Proceedings/20... EDIT: and iodine, ie the subject of this study, is also significantly better than Xenon in this metric: “ A possible alternative is iodine, which is much more abundant and cheaper than xenon and can be stored unpressurized as a solid. In addition, both atomic and diatomic iodine have a lower ionization threshold, and diatomic iodine has a relative mass that is almost twice that of xenon.”
      - mrfusion 5y ago
        How do you gasify it if you store it as a solid? Heat?
archermarks 5y ago
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.
taneq 5y ago
I was confused by "a noble gas chemically identical to Xenon" for a while because isn't the defining feature of a noble gas that it's chemically inert? Why not just call them Krypton thrusters?
- ashtonkem 5y ago
  I’d argue that being chemically inert is a property that can be compared and marked as “identical”, but that’s just semantics. Technically they’re called Ion Thrusters. They’re just colloquially called Xenon Thrusters because xenon is the most commonly used gas to use due to its weight. Perhaps radon could work too, I’m no expert, but I imagine the radioactivity is an issue.
  - Robotbeat 5y ago
    Usually these type are called Hall Thrusters. Ion thrusters (Gridded Ion Thrusters, the full name) work a bit differently internally than Hall Thrusters, but overall system behaves the same. The general term is “electric propulsion,” as in Nuclear Electric Propulsion or Solar Electric Propulsion.
    - Turing_Machine 5y ago
      There's also thermal (non-ionized) electric propulsion, e.g., arcjet (which heats the propellant with an electric arc) and resistojet (which heats the propellant with a resistance heater). Both arcjet and resistojet have been used on some satellites. The nice thing about those is that they don't rely as much on the physical or chemical properties of the propellant. Just about anything that can be vaporized by an arc or heating element will work. https://en.wikipedia.org/wiki/Arcjet_rocket https://en.wikipedia.org/wiki/Arcjet_rocket https://en.wikipedia.org/wiki/Resistojet_rocket https://en.wikipedia.org/wiki/Resistojet_rocket

gorgoiler 5y ago

…hot field…, hah. The cost of things is usually a function of labour and demand, but Xenon is in sufficiently low supply that building new engines is a better option than improving Xenon farming? Where does it come from?

ridgeguy 5y ago
Xenon is a very minor trace component of air. It's collected as a byproduct during fractional cryo-distillation of air to make LOX and liquid nitrogen. [1] [1] https://en.wikipedia.org/wiki/Xenon#Occurrence_and_production https://en.wikipedia.org/wiki/Xenon#Occurrence_and_productio...
- gorgoiler 5y ago
  > Worldwide production of xenon in 1998 was estimated at 5,000–7,000 m3 O2 annual production is 1e11kg presumably from 5e11 of air (20% of air is O2.) Xenon occurs 1:1e7 so that air should also yield 50,000kg by mass, or 8500m3 by volume. Sounds about right. I’m guessing then O2 buyers don’t care to subsidise the Xe market.