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The 2018 arXiv preprint The New Horizons Kuiper Belt Extended Mission by the New Horizons team says in section 6, pp. 21-22:

Nevertheless, throughout the 2020s, there is significant potential for valuable planetary science (and even astrophysics), with New Horizons. Models indicate some 7-10 kg of usable hydrazine (not including the estimated trapped, ~2 kg) will remain at the end of the first extended mission to facilitate unique observations.

Question: Why does New Horizons have ~2 kg of unusable "trapped" hydrazine? Where is it trapped?

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Using Wikipedia:

The hydrazine tank of New Horizon has an elastomeric diaphragm separating the liquid from the pressurizing gas helium within zero gravity. There are 16 thrusters with redundant plumbing to the tank.

The diaphragm prevents helium entering the plumbing, so all hydrazine within the tubes is trapped. If the elastic diaphragm does not fully align to the "lower half of the tank" some hydrazine may be trapped within the tank too.

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