[HN Gopher] In-orbit demonstration of an iodine electric propuls...
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In-orbit demonstration of an iodine electric propulsion system
Author : jollybean
Score : 119 points
Date : 2021-11-19 13:35 UTC (9 hours ago)
(HTM) web link (www.nature.com)
(TXT) w3m dump (www.nature.com)
| kunstmord wrote:
| If anyone wants to read about EP in (a lot) more detail, here's a
| great overview article:
| https://aip.scitation.org/doi/full/10.1063/5.0010134
| Scoundreller wrote:
| I can't stop laughing at the company name the authors work at:
| "ThrustMe"...
| Symmetry wrote:
| A company doing similar work that I shared a startup incubator
| with for a while is Accion. They use a liquid salt so their
| propellant is sort of pre-ionized.
|
| https://accion-systems.com/our-technology/
| archermarks wrote:
| 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.
| gorgoiler wrote:
| _...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 wrote:
| 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_produc
| tio...
| gorgoiler wrote:
| > _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.
| Robotbeat wrote:
| 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.
| taneq wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| 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/Resistojet_rocket
| mrfusion wrote:
| Why is it important to be a Nobel gas?
| Robotbeat wrote:
| 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 wrote:
| 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
| Robotbeat wrote:
| 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
| /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 wrote:
| How do you gasify it if you store it as a solid? Heat?
| moh_maya wrote:
| 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..
| archermarks wrote:
| 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.
| jcims wrote:
| 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 wrote:
| 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 wrote:
| > 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...
| Robotbeat wrote:
| 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-...
|
| 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
| btilly wrote:
| Indeed, one of the memorable stories from
| 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 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 wrote:
| Stories like this make Charlie Stross' "A Tall Tale" seem
| that much more believable:
| https://www.tor.com/2012/07/20/a-tall-tail/
| ben_w wrote:
| 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 wrote:
| > 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 wrote:
| Don't photons have zero mass?
| [deleted]
| at_a_remove wrote:
| Photons have zero _rest_ mass. That sounds like hair-
| splitting but it is not.
| jhgb wrote:
| 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.
| Robotbeat wrote:
| Interesting! Thanks for the paper link.
| dnautics wrote:
| > 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)...
| aaronharnly wrote:
| I love that they supplied 4 citations for the opening assertion:
| "Propulsion is a critical subsystem of many spacecraft
| [1,2,3,4]". Glad I didn't have to take that on faith :)
| [deleted]
| tubby12345 wrote:
| I work on compilers and I have lines in papers like "runtime
| performance is important for many heavily used services
| [1,2,3,4]". The reason you do this is to preempt some annoying
| reviewer whose area is some other dimension of the same problem
| (eg correctness).
| tgbugs wrote:
| I look forward to seeing more methods sections which have a step
| that involves launching things into orbit. I bet that at some
| point the exact type of rocket that is used will cause variable
| results due to differences in the vibrational modes modifying
| payloads so that their behavior changes.
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