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To adjust position on orbit, maintain intended altitude, and deorbit, Starlink satellites feature Hall thrusters powered by krypton. (emphasis added) Most of the electric propulsion systems that I've heard of use xenon.
4 A comment below Why will Starlink satellites use krypton instead of xenon for electric propulsion? links to the 2011 preprint A Performance Comparison of Xenon and KryptonPropellant on an SPT-100 Hall Thruster; IEPC-2011-003 which explains that in this study using a SPT-100 Hall Thruster krypton had a lower utilization fraction than xenon.
Krypton and xenon have roughly the same charge in plasma thrusters (a bit more than the elemental charge) but the mass of xenon is much higher. Thus, for the same accelerating voltage V V, the exhaust velocity and Isp is higher for krypton.
For krypton, you need much more energetic electrons than you would for xenon. This significantly increases the erosion rate, which will shorten the life of the thruster.
Think F = ma, where F in this case is the force of the electric field acting on the ion. If you change fuels (say from xenon to krypton, which is lighter), but don't change field strengths, then F stays the same, while m decreases, which means a (acceleration on the ion) must be bigger.
If the thrusters have a specific impulse of 1500s, that means the krypton atoms are traveling roughly 15 km/s when they exit the nozzle, which exceeds the escape velocity of earth. This may or may not be relevant, depending on how dense the atmosphere / exosphere is where the thruster is fired, and also depending on which way the thruster is pointed. (If the satellite's speed is being ...
Starlink satellites use krypton as propellant. This answer roughly estimates 2.3 kg of Krypton based on a total delta-V of 190 m/s and an exhaust velocity of 20,000 m/s. But how close is that to re...
Krypton is far more common (~1 ppm, over 10x that of xenon), and is produced with similar strategies, so it is a lot cheaper than xenon, at 290 USD per kilogram. SpaceX is launching Starlink very fast, so they can't wait for a suitable xenon supply to show up. Xenon is commonly used only because it has a higher performance.
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