Iodine - ideal electric propellant for Starlink
While looking at Starlink satellite design information, I remembered that SpaceX implemented Hall-effect thrusters using Krypton instead of Xenon. Although Krypton is not so efficient due to higher ionization energy requirements, it is significantly cheaper (by almost order of magnitude). Based on the low cost target for Starlink satellites and low worldwide production rate of Xenon gas, switching from Xenon to Krypton was a simple decision. Noble gases (Xenon, Krypton and Argon) have been used interchangeably on several electric thrusters, especially from Busek, who is the leading provider of electric propulsion for US satellites.
Noble gases have to be stored in deeply cryogenic vessels, or more commonly, in high pressure vessels. Essentially both spectacular Falcon 9 failures (CRS-7 and especially AMOS-6 mission) were related to failures of high pressure vessels containing Helium, another noble gas. So SpaceX must have considered other options. Battery explosions and high pressure gas ruptures are the worst kind of failures that can happen on a satellite located in densely populated orbit.
Ideal solution funded by USAF and NASA is switch to Iodine as the main propellant. Unlike Xenon or Krypton, Iodine is a halogen that is solid under a room temperature. So an iodine tank can be unpressurized (or lightly pressurized) vessel which can fit any available space on the satellite. Furthermore, it represents no risk during launch or operations as there is no stored energy in the vessel. How it can evaporated? Simply by rather moderate heating to around 80 degrees Celsius, which creates enough evaporation pressure to directly feed the thrusters.
In fact, the Iodine fueled thrusters and satellites have been under direct development for the last decade in USA, EU and China. Busek, leading manufacturer of HET engines, actually markets its main BHT-200 thruster as being able to run on Xenon, Krypton AND Iodine. Actual flight is scheduled on iSAT mission, using the same thruster. There are number of papers published here, here and here that compare iodine to to its noble gas counterparts. And SpaceX electric propulsion feed system lead came from Busek electropropulsion team, where he was leading LISA Pathfinder electric propulsion implementation.
So why did SpaceX skip Iodine and chose Krypton? First, Krypton is a noble gas, so any Xenon or Argon storage, feed and propulsion device can be easily converted to Krypton. Iodine requires different vessel, where pressure containment is replaced with heat/temperature control. Other issue is material compatibility. Noble gases are not reactive with almost any material, while Krypton is a halogen. So there is corrosion/oxidation issue that needs to be addressed and certified for long term operational time span. Due to development time constraints, SpaceX likely chose a quick path. But with their general attitude for iterative development approach, it is highly likely that SpaceX will switch to Iodine electric propulsion. That enables much better "packaging" of fuel into the available volume for each Starlink satellite, which can quadruple propellant storage density. Is it necessary for Starlink? No.
But consider this: I estimated that current Starlink has 33kg of Krypton within 30kg high-pressure vessel. The same volume could pack 61kg of Iodine within (near vacuum) low pressurize vessel. 6kW of Solar power would allow adding two additional 2kW thrusters. Maybe even more to allow thrust in different axis...as it is just 7kg per thruster. All of this would be achieved within the SAME volume of 3.2 x 1.6 x 0.2 m2.
With just 60kg of iodine and Isp od 1800s, each Starlink satellite would be capable of 4.6km/s of deltaV (I estimated 2.4km/s for current version with Krypton). That is enough to reach all targeted orbital inclinations (53.2°, 70.0° and 97.6°) from a single launch. Such satellites could be launched to GTO and transfer to low lunar orbit to provide lunar Starlink. They could even reach low Mars Orbit from launches targeting the lunar gateway. Such high deltaV Starlink satellites could be co-manifested on regular paid launches as secondary payloads. We are talking about at least one mission per year to the lunar gateway and at least three missions per year to GTO. Such modified Starlinks can easily be "flight proven" on existing LEO missions, and then gradually co-manifested with most BEO missions. Their cost would be low, since they are not mission critical and their loss or malfunction just delays the network establishment date.
But the argument goes further. There is no fundamental limit for having just 60kg of the propellant on board. Since Iodine is not contained in a high pressure vessel, it is pretty easy to have any shape desired and distributed on four edges of the satellite. So SpaceX could easily redesign Starlink satellite to have...200kg? 300kg? 500kg of propellant? In the same volume! But that is not all. Beyond LEO, the solar irradiation is continuous, enabling full use of 6kW of electric power. By distributing six additional thrusters they could have thrust to any direction required while getting constant solar irradiance. One or two axis could get two additional thrusters, providing full thrust of 300mN at 6kW at Isp of 1800. With that amount thrust and propellant, Starlink could go to low Mars orbit. Get something. And then get back to LEO. Or Lunar Gateway. With some samples? Which leads me to the next application of Starlink.
Ideal application of such Iodine based Starlink is low cost space tug. One near term implementation is deorbiting spacecraft (StarCatcher) where communication payload would be replaced with robotic arm capable of capturing independent object and dragging it to lower orbit for reentry. And since Starlink should be able to operate at Mars distance (with a bit less electric power available) AND have robotic arm....maybe it could rendezvous with Mars Sample Return (MSR) mission. It would require just ascent vehicle providing 4km/s in a single stage. Easily achieved with MMH/NTO or solid booster. Once in orbit, StarCatcher would locate it and grab it with robotic arm. Starting slow ion propulsion back to Earth. And why would it land there? It has enough deltaV capability. It can come to lunar gateway or ISS. You know, just in case.
MSR is just one possible application. Even easier is Asteroid Sample Return. As long as the asteroid gravity is smaller than maximum SEP acceleration from Starlink (sorry, Phobos and Deimos are out of the question), the same robotic arm could possibly.....try to catch some rock from the Asteroid? A single Falcon Heavy could launch a set of 10-20 such satellites. So what if one fails. There are nine others. Let them pick different samples. And get back. to ISS. Or Lunar Gateway. I am sure astronauts would love to pick up rocks with their xEMUs.
Iodine propulsion offers great capabilities that can be quite easily applied to Starlink and make it a platform of choice for variety of other missions. It enables many different capabilities AND reduces risk during launch and operations phase.
Noble gases have to be stored in deeply cryogenic vessels, or more commonly, in high pressure vessels. Essentially both spectacular Falcon 9 failures (CRS-7 and especially AMOS-6 mission) were related to failures of high pressure vessels containing Helium, another noble gas. So SpaceX must have considered other options. Battery explosions and high pressure gas ruptures are the worst kind of failures that can happen on a satellite located in densely populated orbit.
Ideal solution funded by USAF and NASA is switch to Iodine as the main propellant. Unlike Xenon or Krypton, Iodine is a halogen that is solid under a room temperature. So an iodine tank can be unpressurized (or lightly pressurized) vessel which can fit any available space on the satellite. Furthermore, it represents no risk during launch or operations as there is no stored energy in the vessel. How it can evaporated? Simply by rather moderate heating to around 80 degrees Celsius, which creates enough evaporation pressure to directly feed the thrusters.
In fact, the Iodine fueled thrusters and satellites have been under direct development for the last decade in USA, EU and China. Busek, leading manufacturer of HET engines, actually markets its main BHT-200 thruster as being able to run on Xenon, Krypton AND Iodine. Actual flight is scheduled on iSAT mission, using the same thruster. There are number of papers published here, here and here that compare iodine to to its noble gas counterparts. And SpaceX electric propulsion feed system lead came from Busek electropropulsion team, where he was leading LISA Pathfinder electric propulsion implementation.
So why did SpaceX skip Iodine and chose Krypton? First, Krypton is a noble gas, so any Xenon or Argon storage, feed and propulsion device can be easily converted to Krypton. Iodine requires different vessel, where pressure containment is replaced with heat/temperature control. Other issue is material compatibility. Noble gases are not reactive with almost any material, while Krypton is a halogen. So there is corrosion/oxidation issue that needs to be addressed and certified for long term operational time span. Due to development time constraints, SpaceX likely chose a quick path. But with their general attitude for iterative development approach, it is highly likely that SpaceX will switch to Iodine electric propulsion. That enables much better "packaging" of fuel into the available volume for each Starlink satellite, which can quadruple propellant storage density. Is it necessary for Starlink? No.
But consider this: I estimated that current Starlink has 33kg of Krypton within 30kg high-pressure vessel. The same volume could pack 61kg of Iodine within (near vacuum) low pressurize vessel. 6kW of Solar power would allow adding two additional 2kW thrusters. Maybe even more to allow thrust in different axis...as it is just 7kg per thruster. All of this would be achieved within the SAME volume of 3.2 x 1.6 x 0.2 m2.
With just 60kg of iodine and Isp od 1800s, each Starlink satellite would be capable of 4.6km/s of deltaV (I estimated 2.4km/s for current version with Krypton). That is enough to reach all targeted orbital inclinations (53.2°, 70.0° and 97.6°) from a single launch. Such satellites could be launched to GTO and transfer to low lunar orbit to provide lunar Starlink. They could even reach low Mars Orbit from launches targeting the lunar gateway. Such high deltaV Starlink satellites could be co-manifested on regular paid launches as secondary payloads. We are talking about at least one mission per year to the lunar gateway and at least three missions per year to GTO. Such modified Starlinks can easily be "flight proven" on existing LEO missions, and then gradually co-manifested with most BEO missions. Their cost would be low, since they are not mission critical and their loss or malfunction just delays the network establishment date.
But the argument goes further. There is no fundamental limit for having just 60kg of the propellant on board. Since Iodine is not contained in a high pressure vessel, it is pretty easy to have any shape desired and distributed on four edges of the satellite. So SpaceX could easily redesign Starlink satellite to have...200kg? 300kg? 500kg of propellant? In the same volume! But that is not all. Beyond LEO, the solar irradiation is continuous, enabling full use of 6kW of electric power. By distributing six additional thrusters they could have thrust to any direction required while getting constant solar irradiance. One or two axis could get two additional thrusters, providing full thrust of 300mN at 6kW at Isp of 1800. With that amount thrust and propellant, Starlink could go to low Mars orbit. Get something. And then get back to LEO. Or Lunar Gateway. With some samples? Which leads me to the next application of Starlink.
Ideal application of such Iodine based Starlink is low cost space tug. One near term implementation is deorbiting spacecraft (StarCatcher) where communication payload would be replaced with robotic arm capable of capturing independent object and dragging it to lower orbit for reentry. And since Starlink should be able to operate at Mars distance (with a bit less electric power available) AND have robotic arm....maybe it could rendezvous with Mars Sample Return (MSR) mission. It would require just ascent vehicle providing 4km/s in a single stage. Easily achieved with MMH/NTO or solid booster. Once in orbit, StarCatcher would locate it and grab it with robotic arm. Starting slow ion propulsion back to Earth. And why would it land there? It has enough deltaV capability. It can come to lunar gateway or ISS. You know, just in case.
MSR is just one possible application. Even easier is Asteroid Sample Return. As long as the asteroid gravity is smaller than maximum SEP acceleration from Starlink (sorry, Phobos and Deimos are out of the question), the same robotic arm could possibly.....try to catch some rock from the Asteroid? A single Falcon Heavy could launch a set of 10-20 such satellites. So what if one fails. There are nine others. Let them pick different samples. And get back. to ISS. Or Lunar Gateway. I am sure astronauts would love to pick up rocks with their xEMUs.
Iodine propulsion offers great capabilities that can be quite easily applied to Starlink and make it a platform of choice for variety of other missions. It enables many different capabilities AND reduces risk during launch and operations phase.
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