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I was reading about Akatsuki and how it failed its insertion burn to Venus orbit. In the source I was reading, it was mentioned that:

At a press conference on 10 December, officials reported that Akatsuki's engines fired for less than three minutes, far less than what was required to enter into Venus orbit. Further research found that the likely reason for the engine malfunction was salt deposits jamming the valve between the helium pressurization tank and the fuel tank. As a result, engine combustion became oxidizer-rich, with resulting high combustion temperatures damaging the combustion chamber throat and nozzle.

I was wondering what would potentially cause salt buildups in the craft, especially in such a location as "the valves" and which valves? Was the salt a by-product of the specific combustion materials? It quotes an additional event "like this one" as:

A similar vapor leakage problem destroyed the NASA Mars Observer probe in 1993.

I guess my three questions here are:

Were these due to the same reaction? What reaction caused salt deposits on Akatusuki? And why would salt deposits cause the engines to run oxidzer-rich?


Note: Akatuski performed a recovery burn and reached its intended objective after the failure.

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Partial answer addressing

why would salt deposits cause the engines to run oxidizer-rich?

The propulsion system is shown in this drawing.

enter image description here

At top is a helium pressurant tank, at left is the fuel tank, at right is the oxidizer tank.

The "salt" deposits partially clogged the check valve CV-F restricting the flow of helium to pressurize the fuel tank. The oxidizer tank had full helium flow. This resulted in higher pressure in the oxidizer tank, more oxidizer flow than fuel flow, and therefore an off-nominal mixture ratio.

enter image description here

Drawings, and info sourced from, the excellent article here. Apparently at the time of writing, the cause of the flow restriction was not known.

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    $\begingroup$ Note that 'salt' has been used in the chemical sense, which includes much more than NaCl. $\endgroup$ – Hobbes Sep 21 '19 at 6:43
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    $\begingroup$ I think it's a translation error, or at least an infelicity. $\endgroup$ – Organic Marble Sep 21 '19 at 11:06

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