THE COLDWALL EQUIPMENT 

When I decided to make a YF-12A kit in NASA livery as used with the Coldwall heat transmission project, I tried to find a diagram of this equipment.

Whilst there are several photos of the plane carrying the equipment and videos of ground testing and one of flight testing of the insulation release, I couldn't find any diagrams[1]. NASA was helpful, refering the matter to Langley, but nothing was forthcoming.

Therefore, I had to create the drawings from the available photos. I used three of these,

1 Two YF-12 aircraft in flight
2 YF-12A and YF-12C in flight[2].
3 YF-12A in flight with Coldwall experiment
and the movie
Coldwall movie

Photo 3 shows the most detail but is taken from slightly below the plane, (or it's in a shallow bank), and so vertical lengths will be foreshortened by the tilt of the image.

 Absolute size 

The length of a YF-12A is quoted as 101' 8", (≡ 101.7'). Measurements of the relative length of plane and Coldwall cylinder from various photos gave ratios of 1: 0.097 ± 0.003, which gives a cylinder length of 9.86 ± 0.21'.

There is a reference which quotes the length as 13' but this cannot be correct for the cylinder but might be the length when covered in insulation.

The value used for 1:48 scale was 2.6", (65mm).

 Dimensions 

The measurements were taken from a cropped area of photo 3 which was enlarged to be at 1:16 scale. This value was chosen as the largest simple multiple of 1:48 which fits onto an A4 sheet of paper. Whilst vertical distances are foreshortened on this photo, the diameter of the tube isn't, (being circular), so a factor was found for the correction of the vertical measurements from an enlarged section of photo 2 which seems to be the one most nearly in the horizontal plane of the camera, (allbeit a little above this).

 Photo 3 
Coldwall equipment

The height of the pylon at the rear of the tube was measured as equal to the diameter of the tube. This led to the following conversion factors for heights and widths of measurements made on photo 3.

t = angle of tilt
Cos t = 23/28 = 0.821, hence t = 34.772°

mht = measured height, ht = actual height
ht = mht/Cos t = mht x 1.2174

mwd = measured width, wd = actual width
wd = mwd/Sin t = mwd x 1.7534

Photo 3 was then measured, the corrections made and the drawings below produced.

1:16 plans
Click the image for the full-sized version

The values are sizes in mm at 1:16 scale with the 1:48 scale value in brackets.

On photo 3 there appears to be something angled back from the base of the fin at the rear of the cylinder. However, as this can't be seen on other photos, (perhaps due to scale or distortion?), I've not included it. It may be the remnants of the primer cord used to remove the insulation although I wouldn't have thought bits would be left hanging about at mach 3+! There also seems to be a lump of something where the fin joins the tube. This has also been ignored.

There are a number of guesses!

The pylon appears to be faceted rather than smoothly curved. Four facets have been drawn on the basis of the shadowing on photo 3 but there might well be one or two more. It may also be that the leading facet is slightly concave, (see reflections).

The first three facets are bare metal whilst the last is matt black. The photo shows a distinct difference in colour between the pylon and the cylinder. If it were not for the fact that the cylinder is always described as being made of 'stainless steel', I would have thought it was titanium by the slight yellow tinge and I intend to paint it with 'Titanium' mix.

Whilst the thickness of the rear of the pylon can be measured, the widest part is a complete guess, as is the actual shape of the section where the pylon joins the fuselage.

Having produced the 1:16 drawings, the dimensions were then reduced to one-third to give the 1:48 scale sizes at which the actual parts were made.

Parts plans
Click the image for the full-sized version

The tube was made by winding a triple layer of 0.2mm styrene around a piece of 9mm dowel, coating it with liquid poly as I rolled. This is a bit oversize even though the dowel was sanded down a little. The fin below the tube was built out of 1.00mm styrene for rigidity although this might be too thick for scale, (~2"?).

The pylon was built from a basic sheet of 2mm styrene with 1mm fillets added at the widest part, liberally padded with putty and then filed/sanded to shape.

The wedge was cut from 2mm styrene and sanded to shape. This was made a separate part as I decided it would be easier to create the wedge this way rather than try to cut it back as part of the main pylon.

The pylon support was cut from 2mm styrene, sanded to shape and then the pylon slot cut out.

[1] There are diagrams of the shaker vane equipment tested on YF-12As and the LASRE pod on an SR-71 for anyone interested in building these versions.
[2] The 'YF-12C', was actually an early SR-71 on loan to NASA.