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https://en.wikipedia.org/wiki/SpaceX_Starship_integrated_flight_test_2

On February 26, 2024, SpaceX stated that the most likely root cause was filter blockage where liquid oxygen is supplied to the engines, leading to a loss of inlet pressure in engine oxidizer turbopumps that eventually resulted in one engine failing, resulting in loss of the vehicle.

What is this filter for? Why would they need a filter? What could clog it?

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    $\begingroup$ I've been wondering if the "blockage" they keep talking about is actually an air bubble. $\endgroup$ Commented Mar 1 at 4:10
  • $\begingroup$ I found a Smithsonian Magazine article that briefly mentions feeding a stainless steel nut into the fuel/oxidizer lines for a Merlin engine as part of engine qualification. If I can find a more complete source more related to SuperHeavy I'll try to write a decent answer $\endgroup$ Commented Mar 1 at 5:47
  • $\begingroup$ Also, the Virgin Orbit failure from January of last year was due to a dislodged fuel filter $\endgroup$ Commented Mar 1 at 5:53

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No specific knowledge of Starship, but the shuttle Orbiters had filters in their main propulsion feedlines; the same reasons should apply to Starship.

Namely, keeping loose hardware that was mistakenly left lying around in the propulsion system out of the turbopumps!

What could clog it? Loose hardware / packing material / cleaning rags that was (were) mistakenly left lying around in the propulsion system!

This is a crop of the Space Shuttle Systems Handbook Drawing 10.8 LO2 System. I have highlighted the oxygen prevalve, the engine O2 inlet, and the 1000 micron filter.

enter image description here

These pages are from the MPS Components Manual.

enter image description here

enter image description here

As an aside, when I worked in the shuttle Systems Integration Office, there were always arguments about whether or not to breach the systems to retrieve material seen on the filters in boroscope inspections. Would it be riskier to go after it, or not? The business of Systems Integration.

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    $\begingroup$ Confirming that is a micron filter in a drawing otherwise using US units? The choice of 1000 micron rather than 1mm is interesting but I guess in keeping with general filter terminology so may just be how the OEM described/defined it. $\endgroup$ Commented Mar 8 at 23:53
  • $\begingroup$ @GremlinWranger added some info stating explicitly that it's 1000 microns, and a locater diagram. $\endgroup$ Commented Mar 9 at 0:00
  • $\begingroup$ What kind of / how much "material" was seen on the filters? $\endgroup$
    – ikrase
    Commented Mar 9 at 12:48
  • $\begingroup$ @ikrase It varied. I remember a "metallic particle" and a scrap of tape or some such material (two different incidents). $\endgroup$ Commented Mar 9 at 13:43
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    $\begingroup$ @TylerH I don't seem to have any notes and I don't remember much except that the material that the debris was made out of was a consideration as well as the size. When I worked there, they never ended up going after anything. $\endgroup$ Commented May 27 at 16:51
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Speculative answer (dangerously close to an opinion):

The filters could be there to prevent cryogenic slush from entering the turbine. See Does Starship run on Slushies?

During refrigeration, as liquid oxygen approaches its triple point temperature of -219C, flakes of solid oxygen form at the liquid/gas interface. These solid flakes are 25% denser than the surrounding liquid O2 so they settle rapidly towards the aft end of the tank, especially in the high-g setting of an accelerating rocket.

enter image description here

According to https://www.wevolver.com/specs/spacexs-starship-sn24-bn7, Starship stores liquid O2 at a sub-cooled -207C (colder than the -165C required by the 6bar pressure limit on the tank and only 12c above freezing). Lower operational temperature allows higher fuel density, but slush formation puts a lower limit on the chosen operational temperature.

As the oxygen is consumed during flight, some liquid O2 evaporates to fill the ullage space. The heat of vaporization refrigerates the surface of the (shrinking) volume of remaining liquid. Think of the frost which forms on your BBQ propane tank as it runs low.

Since Starship's liquid O2 is only 12C above its triple point, it is prudent to have a filter to prevent any potential slush flakes from entering the pump impeller.

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    $\begingroup$ "...entering the turbine"? Surely nothing enters the turbines until it's gone through the pump(s) and been combusted. $\endgroup$ Commented May 17 at 23:50

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