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Comment by wtallis | Hacker News Reader

The krypton isn't being used as a chemical reactant, and in fact being largely chemically inert is a benefit for ion thrusters. I suspect they're using krypton instead of xenon due to the latter being more expensive, even though the higher atomic mass of xenon would be beneficial.

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Lower atomic mass gives better Isp, although for power-limited situation you usually want to maximize thrust instead.

In the pre-flight press conference the topic of krypton thrusters came up, and at first he made a superman joke, but then said the real answer was the lower cost of krypton.

What does "Isp" stand for?

Probably meant specific impulse.

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

It stands for Specific Impulse, which is a measure of fuel economy for rocket engines.

Specific impulse. It is a convoluted method of measuring the exhaust velocity, and thus the efficiency, of a reaction engine.

Lower atomic mass gives better Isp for chemical rockets, for which the exhaust velocity is proportional to the temperature of the exhaust divided by the square root of the atomic mass of the exhaust product. However, these satellites are using ion propulsion. In ion engines, the exhaust velocity is constant, so a higher atomic mass allows each atom / molecule to carry more energy. This makes higher mass fuels like Xenon better for ion engines. (Radon would be better still but the radioactivity poses problems for various systems.)

Why is the exhaust velocity constant? I would naively expect the engine to impart roughy equal energy into each atom, so that lighter atoms would be faster. But I know little of the specifics of ion engines.

Thrust power = Isp * thrust. For the same amount of input power, increasing molecular size raises thrust and reduces Isp.

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