5 ms·

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 e

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?
        moh_maya 5y ago
        Iodine? Keep it cold and it stays solid. But it doesn’t liquify; it is one of the elements that can sublimate from solid to gas (not sure of the physical chem here, but the transition temp to liquid vs gas from solid is so close that it doesn’t have a distinct liquid phase(?)). So I’m imagining a solid block of iodine (it is in cold space), and when the engine needs thrust, some current from the solar cells is passed through a heating element to drive sublimation + lots of complex rocket science that I do not know of. If you are referring to the noble gases, I don’t think they are cooled to the point of solids; just high pressure liquids..
        randomuser1337 5y ago
        I designed iodine thrusters for a different company. Your description of how you store/flow the iodine is correct and it's no more complicated than that. It's actually an easy propellant to work with due to its easily controlled vapor pressure (through heat) - the main issue with designing these systems is dealing with how outrageously corrosive iodine is. It. Eats. Everything. You can technically liquefy/boil iodine (I've done it a few times). It happens around 110C IIRC.
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

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?
      - moh_maya 5y ago
        Iodine? Keep it cold and it stays solid. But it doesn’t liquify; it is one of the elements that can sublimate from solid to gas (not sure of the physical chem here, but the transition temp to liquid vs gas from solid is so close that it doesn’t have a distinct liquid phase(?)). So I’m imagining a solid block of iodine (it is in cold space), and when the engine needs thrust, some current from the solar cells is passed through a heating element to drive sublimation + lots of complex rocket science that I do not know of. If you are referring to the noble gases, I don’t think they are cooled to the point of solids; just high pressure liquids..
        randomuser1337 5y ago
        I designed iodine thrusters for a different company. Your description of how you store/flow the iodine is correct and it's no more complicated than that. It's actually an easy propellant to work with due to its easily controlled vapor pressure (through heat) - the main issue with designing these systems is dealing with how outrageously corrosive iodine is. It. Eats. Everything. You can technically liquefy/boil iodine (I've done it a few times). It happens around 110C IIRC.

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