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I was watching a YouTube video "Titanium - The Metal That Made The SR-71 Possible" when I stumbled upon this frame (at 12:26). enter image description here

I could see these heating tubes sort of things under the airframe. What exactly are these and what benefit do they add during the manufacturing process to the airframe?

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    $\begingroup$ What video is this out of? $\endgroup$
    – Carlo Felicione
    Jul 25, 2020 at 19:32
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    $\begingroup$ My guess is they do not participate in the manufacturing itself. The SR-71 was mostly made of titanium alloy, so composite material curing should be out of the way. However, the blackbird was built to sustain tremendous the amount of heat stemming from the air friction. These heating tubes could very well be a component of a heating stress test that was used to certificate the aircraft, or to verify a plane before release to the air force $\endgroup$
    – BambOo
    Jul 25, 2020 at 20:38
  • $\begingroup$ How do you know they are heating tubes? $\endgroup$
    – pericynthion
    Jul 25, 2020 at 20:58
  • $\begingroup$ @CarloFelicione I found the video, edited link into question. $\endgroup$
    – Jpe61
    Jul 25, 2020 at 21:29
  • $\begingroup$ @pericynthion They seemed red, and seem to be radiating heat, hence I concluded they must some sort of heating tubes. $\endgroup$
    – Salmonbeing00
    Jul 26, 2020 at 11:24

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It's basically an open air test oven.

The SR71's skin temperatures averaged around 5-600 deg F, up to 1000 deg F in places. The titanium structure had to accommodate a lot of thermal expansion from all this heat, and used special skin joints and corrugation of the larger-area skin panels to accommodate the expansion.

The heating system in the clip would have been a test fixture with an array of radiant heating elements to subject a skin assembly to operating temperature to study the effects and validate the design (go to Page 29 of the linked PDF).

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