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Given that temperatures in the combustion chamber are crazy high, are there any pressure sensors rated for the high temperature and high pressures located there? Or is there a cooling mechanism for the pressure sensors?

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    $\begingroup$ Whenever you think "too anything for sensors" consider if it can be held together by a solid structure that struggles to withstand it, a plain tiny cheap strain sensor can measure it. Just affix it (or several) to the structure and measure how much it yields to the pressure, expands thermally, bends under the force, strains under acceleration or whatever. Strain guages can convert a bridge into a scale to weigh trains, an airplane wing into a guage of lift force, a beam of an industrial blast furnace into thermometer. $\endgroup$ – SF. Jun 27 '18 at 13:48
  • $\begingroup$ @SF That aproach can be useful, but in rocket combustion chambers there are typically too many physical effects at play to give consistent results. $\endgroup$ – Rikki-Tikki-Tavi Jun 29 '18 at 8:34
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The pressure sensors do need to be shielded somehow from the full fury of the combustion chamber.

Here's a schematic showing the connection of the Space Shuttle Main Engine chamber pressure (Pc) sensors to the combustion chamber.

enter image description here

You can see that the Pc sensor is not connected directly to the chamber, but interfaces with it via a hole drilled up to one of the acoustic cavities, which is cooled by fuel flow. Also note the "thermal isolator".

This arrangement was not foolproof, the "purge orifice" aka "lee jet" caused several failures during development and testing. You can read about them in the reference.

I don't have details, but the sensor was certainly "rated for high temperatures and pressures" even with this arrangement.

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    $\begingroup$ Excellent schematic and explanation! $\endgroup$ – Uwe Jun 27 '18 at 11:35
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I assume you talk about average static pressure, not total pressure or high frequency acoustics. If this is untrue then please clarify the question.

A boundary layer gets it's pressure from the main flow. Therefore it is not necessary to hold a pressure tap into the main flow. A thin tube (a capillary) is attached radially to the side wall. The longer the tube, the more fluctuations in pressure will be attenuated, but the cooler the gas is at the sensor. It is important to make the tube long enough that the sensor doesn't get burned.

There is very little gas moving in or out of the capillary, so typically no special cooling is needed.

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    $\begingroup$ If the mass of the gas in the thin tube (a capillary) is much smaller than the mass of the tube itself, the end of the tube will be much cooler. But the point were the tube is attached to the side wall of the combustion chamber need cooling like the wall. If there are pressure fluctuations inside the chamber, the long tube will influence the measured pressure. Pressure waves in the tube will be reflected at the end. Resonances and standing waves should be avoided. $\endgroup$ – Uwe Jun 27 '18 at 10:32

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