Update: David suggested that it needs a good starting point to know what to do. So.... I first did an align of the OD of the half pipe to the centerline of the solid round, to get the half pipe into a good starting position. Then I deleted the align, and applied the tangent using the ID if the half pipe and the OD of the SR. Should have been the best of all possible start points, easy movement directly to tangent position, etc. The result was what you see, the tangent pulled the half pipe out at an angle, which does not seem to satisfy the requirements of "tangency".