Hello everybody,
BTW, some friends in the
Raumcon forum did mean that this flat wavy ring structure was just an optical effect, but this was a three-dimensional effect, as
Craig Capdepon has confirmed.
"The SOFI (spray on foam insulation) was applied with automatic spray gun sets on vertical moving rails I think in 3 locations 120 deg. apart around the tank. The tank would rotate and the guns traveled down the rail to apply the desired thickness. The 3 locations were required because the overlap max. time was 45 seconds. We desired the "as sprayed" finished condition because it was more durable when exposed to sunlight and the weather conditions in Florida. There was no attempt to create a "pattern" to the finish. We tried for a smooth uniform finish.
The wave like patterns were unique to each tank as a result of the variations in the spray foam insulation process. The spray guns pattern control tip would accumulate a build up of foam causing a less than perfect pattern. There were engineering drawing requirements limiting the peaks and valleys of this finish. The critical area near the nose cone was often machined to obtain a smooth surface using a tool we nicknamed the "pencil sharpener".
I found this photo, on which one can see this tool during the repair work of the hail damage on the
ET-124 during the start preparations for the
STS-117, in which he was involved too.
Source: NASA
Of course I had asked further questions, including a term
"Barber pole", which I found that characterizes the SOFI mode.
"It was a
non-stop procedure. The tank was rotating as the guns started spraying a predetermined amount of SOFI. Carriages holding the spray guns traveled (down the rail) at a set speed until the end of the spray operation.
The correct tank rotation, travel of spray carriages, temperature of SOFI components, pressure of component supplied to guns, substrate temperature, cell environment, (temp. & humidity) and the pattern at the gun pattern control tip all determined the thickness and as sprayed condition of the finished product.
"Barber pole" was a shop term to generally refer to the method. The travel of the gun carriages was slow because of the large area (diameter) of the tank. If a smaller cylinder was sprayed (i.e. feed lines) this same method may produce a "spiral-effect" surface."
That these rings actually have a three-dimensional structure can also be seen in this video (unfortunately without sound) which shows an SOFI piece of the
ET-93 used in the fateful last
Columbia mission STS-107.
Craig commented about this video:
"This was not an optical effect. I would have to see a larger cross section to know exactly where this piece is from. It looks like part of an automatic spray possibly from the aft dome of the LH2 tank. I see two knit lines and it appears the three passes were approx. 1 inch each. SOFI is pliable for about 5 seconds after components are mixed and exit the spray gun. The rough surface in this picture is an example of "roll over" caused when SOFI is formed by an outside force soon after application such as excessive air movement from the direction of the gun."
Furthermore, I have learned from him, that he has worked
on every shuttle that has flown,
what I found next to this photo.
"Endeavor the newest shuttle STS-134 (16.05.2011) on the pad. This is ET-122 - we did the mod in February. I've worked on every shuttle that has flown."
A very interesting contemporary witness from that time, I have met in this way.
