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I was reading about different sources of propellant for ion thrusters, Xenon being the most common. However, in more recent articles I've found more and more mentions of Solid Iodine being used as ion thruster propellant.

More specifically:

https://en.wikipedia.org/wiki/Iodine_Satellite

https://en.wikipedia.org/wiki/Hall-effect_thruster

A key advantage to using iodine as a propellant is that it provides a high density times specific impulse, it is three times as fuel efficient as the commonly flown xenon, it may be stored in the tank as an unpressurized solid, and it is not a hazardous propellant. 1U with 5 kg of iodine on a 12U vehicle can provide a change of velocity of 4 km/s ΔV, perform a 20,000km altitude change, 30° inclination change from LEO, or an 80° inclination change from GEO. During operations, the tank is heated to vaporize the propellant. The thruster then ionizes the vapor and accelerates it via magnetic and electrostatic fields, resulting in high specific impulse.

I guess my questions are:

  • How can you use solid Iodine as a stand-in for Xenon, do you have to sublimate it?
    • Are there any complications which arise from using a solid fuel?
    • What makes Iodine so efficient to store as a solid fuel?
  • How much more efficient will this be in terms of storage, what are the advantages?
    • How much better is the specific impulse than: Xenon/Krypton/Argon?
    • How much easier is it to store than: Xenon/Krypton/Argon?
    • How much less risky is having Iodine than: Xenon/Krypton/Argon?
    • How much less costly is it than: Xenon/Krypton/Argon?
  • Is this Iodine Satellite the first one to test this stuff out?

Let me know if those questions should be broken out more, I can make multiple to handle specific questions on the iodine propellant, but I figured I'd start with one broad stroke question.

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    $\begingroup$ The answer to "How can you use solid Iodine as a stand-in for Xenon, do you have to sublimate it" is given in the quote: "During operations, the tank is heated to vaporize the propellant (Iodine)". Iodine has a Melting point (I2) 386.85 K ​(113.7 °C, ​236.66 °F) and a Boiling point (I2) 457.4 K ​(184.3 °C, ​363.7 °F). It does sublimate at room temperature, but it is possible to melt it under rapid and strong temperature rise. If the vapor pressure of 35 Pa at 298 K produces a sufficient mass flow rate, sublimation may be used alone. $\endgroup$
    – Uwe
    Nov 11, 2019 at 16:59
  • $\begingroup$ @uwe true, that's a pretty good broadstroke on it. However, I was wondering how you'd heat a tank to cause that though-- will you heat, cool, then heat it again? Will it be maintained at a constant temp? Was hoping for someone to explain in more detail how they planned to heat the tank and move it out of the solid phase into something usable possibly with a diagram if available. $\endgroup$
    – Magic Octopus Urn
    Nov 11, 2019 at 17:28
  • $\begingroup$ Have a look at What are the advantages of solid iodine propellant and how is it used for ion propulsion? as well as What are the parameters of the new Iodine electrical rocket engine developed by RSC Energia? and also Molecular propellant in ion engines It's possible that some of these are already answered. $\endgroup$
    – uhoh
    Nov 11, 2019 at 23:40
  • $\begingroup$ Iodine is also mentioned in MARS-CAT; What is a Cubesat Ambipolar Thruster and how does it work? $\endgroup$
    – uhoh
    Nov 11, 2019 at 23:42
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    $\begingroup$ @uhoh the first link is this question :P. The other links however, I'm reading now. $\endgroup$
    – Magic Octopus Urn
    Nov 12, 2019 at 4:06

1 Answer 1

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I think there is a bit of confusion in the terms people use for iodine in the space propulsion community. For more detailed information I recommend having a look in one of these scientific articles Dietz et al. (2019) and Szabo et al. (2013). I will base my answers on them.

Iodine is not used in solid state for electric propulsion (EP). People like to use the word "solid" to advertise that the propellant is stored in stolid state, instead of a pressurised tank of a gas as xenon or krypton. Yes, you have to sublimate it by heating the iodine container.

Cons: You have to spend energy to heat the container and get iodine in gas form. Also, for iodine, another disadvantage is that it is highly reactive/corrosive.

Pros: Iodine in solid state is much more dense than, for example, a noble gas, occuping less volume with the same amount of propellant. Another clear advantage is that you do not need all the high-pressure pipes and valves that you usually need for a system using a noble gas.

Despite the possibility of storing it in solid state, iodine is also attractive as a propellant for EP because of two main reasons:

  • its atomic mass which is very similar to xenon (126.90 amu vs 131.29 amu), which is important to generate a high thrust;
  • and its ionization threshold which is slighly lower than xenon, which makes one spend less energy to maintain the plasma inside the thruster.
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