What are the heating tubes used during SR-71 airframe manufacturing?
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).