Having completed the S-II stage, I’ve worked my way down the stack as far as the S-II aft interstage—a ring structure interposed between the first and second stages to provide clearance between the S-II’s rocket engines above and the dome of the S-IC’s liquid oxygen tank below. We can see it being dropped astern in this video taken from the second stage of Apollo 4:

Also briefly visible in that video are the ullage rockets on the aft interstage, which were fired to shove the S-II forward just before its own engines fired. The whole stack was in free-fall between the shutdown of the first stage motors and the firing of the second stage engines, and there was concern that liquid hydrogen and oxygen would float around in the S-II’s tanks during this time, allowing gas bubbles to get into the mixture when the S-II engines fired. So the ullage rockets gave a little shove to the S-II, settling its fuel and oxidizer to the bottom of their tanks, next to the drainage piping, before the S-II engines were fired up. (Ullage is a nice old word originally referring to the air-space within a cask or bottle of wine. By extension, the unfilled volume in a rocket’s tanks is called ullage, and ullage rockets are used to make sure the ullage moves to the top, and the contents to the bottom.)

It turned out the S-II didn’t really need this precaution, and the number of ullage rockets on its aft interstage was reduced over time—from the eight visible on the Apollo 4 video, to four by the time of the Apollo 11 launch, and the last few Saturn V launches had no ullage motors at all on this stage.

So that’s the first problem with the Revell kit representation of this interstage—it has eight attachment points for ullage motors, and I only need four for my Apollo 11 launch vehicle. The second problem is that the kit parts for the ullage motors are the wrong size and shape—but I have New Ware resin replacements to deal with that. And the third problem is (predictably enough) that the attachment points are in the wrong places—Revell sites the eight ullage motors exactly on the principal axes of the Saturn V, whereas they were all displaced about 16 inches anticlockwise from those axes (looking down).

So all these mounting points have to go:

It’s a slightly harder job than disposing of the similarly misplaced mounting points on the aft skirt of the S-II, because these are big moulded structures that partially overlap the stringers on either side. So I first had to trim them level with the stringers, and then apply Dymo Tape to guide my razor saw and preserve the remaining stringer structure.

Then I used a 2mm chisel to remove all the excess plastic.

The other thing I needed to do was to mount the lower ends of the various fairings that I’d applied to the S-II. Many of these fairings extended across the flight separation line between the S-II and the aft interstage, and their lower ends were discarded along with the aft interstage (you can glimpse them in the Apollo 4 video, above). So I’d had to chop the ends off the New Ware resin parts when I mounted the fairings on the S-II stage. All these ends went into a pot, individually labelled, and now I was able to fit the interstage to the S-II and reunite all the severed parts (after chiseling away small parts of the stringers to make room for their attachment). There was also a personnel access hatch to be added, provided as a photoetch part by New Ware.

After priming, the final result looked like this:

I attached the interstage to the rear of the S-II again at this point, so that I could ensure continuity of the black stripes which begin on the S-II and extend across the interstage to the forward skirt of the S-IC stage. I got all the masking in the correct place, and then separated the interstage again for painting.

The interior colour of the Apollo 11 interstage is another source of doubt. The only colour images I can find come from early Apollo missions. Although the interior looks yellow (or even pink) in the interstage jettison videos of Apollo 4 and Apollo 6, it’s being subjected to the rocket blast of the S-II’s engines in these images. But there’s a clear view of Apollo 6’s aft interstage during the stacking process on the ground, which shows its interior with a metallic finish.

So that’s what I went with.

I also added a flight-separation junction to the bottom edge of the interstage, using 0.5mm x 1.5mm styrene strip, which you can see in this view.

The lines along which the interstage separated from the S-IC stage below, and the S-II above, were slightly raised ridges, painted white, which provided clear visual lines of demarcation traversing the black stripes, even when the rocket was viewed from a distance:

My original plan had been to add styrene to the top and bottom of the interstage, but it became evident it would be easier and more secure to attach the upper flight separation line to the flanges on the base of the S-II. Here’s the result when the two are stacked together, which I think simulates the appearance of the real thing fairly well:

So now it’s on to the first (and for me, final) stage.

• Go to S-IC stage build log