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The Viking Lander Terminal Descent Engine (3 per vehicle) was a ~600 lbf monopropellant hydrazine engine with an unusual exhaust system of 18 small nozzles. Multiple sources state that the reason for this design was to prevent exhaust plume erosion of the Martian surface below the lander, to prevent degradation of the science results.

(all annotations mine in these images)

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(Image source)

What is the purpose of this metal tube connected to the center of the aft dome of the reactor chamber?

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(Image source)

This paper shows a cutaway drawing of the engine, but doesn't label the tube.

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(Image source)

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It is a tap to measure Chamber Pressure

You can see similar tubes protruding from other hydrazine engines designed by Rocket Research Corp. Per the source: "A 1/16-inch OD x 0.010-inch wall tube is provided to measure thrust chamber pressure". This reading could've been used as a measure of engine thrust for throttle control feedback loops.

I originally thought it was a tap-off for tank pressurization or turbo-pumps, but this very informative paper revealed Viking used an unregulated helium blow-down system.

Edit:

I have found some images which show that the tap was stoppered onboard the Viking lander. This explains why there is no mention of its use in the Viking control scheme (as noted by Organic Marble's comment). It was likely used for engine qualification testing and perhaps planned to provide engine health telemetry but ultimately never used on-mission.

Viking Far

Viking Near

First Image Source

Second Image Source

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  • $\begingroup$ +1, This was my guess as well, but I'd like to see some documentation for this specific engine. I couldn't find any. $\endgroup$
    – Organic Marble
    Commented Jul 6, 2020 at 21:05
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    $\begingroup$ @Organic Marble, I was also having trouble finding documentation. I couldn‘t even find any mention of the engine’s name, and RRC is also hard to find any information on. I’ll keep looking though. $\endgroup$
    – A McKelvy
    Commented Jul 6, 2020 at 22:31
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    $\begingroup$ @Organic Marble, Added $\endgroup$
    – A McKelvy
    Commented Jul 7, 2020 at 14:04
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    $\begingroup$ OK, that's a test article not the flown spacecraft, but it can be indicative. I found this page crgis.ndc.nasa.gov/historic/Viking_Archives_Collection and if you scroll down to the "Launch Preparations at Kennedy Space Center in 1975" there are photos of the spacecraft being placed in the aeroshell. Some of them don't have the nozzles installed, but in the later ones, I don't see that tube at all. Especially this picture crgis.ndc.nasa.gov/crgis/images/3/3c/Viking-28.jpg So I agree it was a test port. Probably that tube didn't fly to Mars, just a plug. $\endgroup$
    – Organic Marble
    Commented Jul 7, 2020 at 14:22
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    $\begingroup$ Other pictures make it seem like the test port is manufactured as part of the aft dome, so now I think the tube did fly. "test port not used during mission" is still the correct answer I believe. $\endgroup$
    – Organic Marble
    Commented Jul 7, 2020 at 17:00

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