What are the heating tubes used during SR-71 airframe manufacturing? I was watching a YouTube video &quot;Titanium - The Metal That Made The SR-71 Possible&quot; when I stumbled upon this frame (at 12:26). 
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?
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 text 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).