Mis-aligned Wrap!

Oops.

I mis-understood one of the alignment instructions. It was plenty clear: I merely blew it.

The S-IVB has two wraps to model the ribs, one on the forward end of the stage, one on the aft end. There's a tunnel cover that stretches between them (overlapping each). So, the alignment mistake—about 20° off, rotated around the body tube—is more than just cosmetic, since there are flat spots in the wrap where the tunnel covers attach.

What to do?

I thought about just cutting the wrap where the tunnel cover needs to go, perhaps using the cut piece as a graft over the molded-in flat spot. I considered cutting the body tube in half, making a stage coupler, and rotating the two pieces to align the wraps correctly. I briefly gave thought to ripping off the wrap and replacing it with a new one.

I decided to try to remove the wrap and reapply it.

Some parts of the wrap came off the body tube very easily, but some were extremely difficult to remove. Where the CyA didn't wick very far under the wrap, it came off easily.

I used a hobby knife, very gingerly, to pry the wrap up, occasionally using CyA debonder to try to dissolve the CyA. (I'd tested it on some wrap scraps.) In a few spots, I had to cut a layer of the body tube away in order to lift the wrap off. There's one bad tear, a nick, and a couple of very small cuts, but the wrap came off in a reusable condition!

It's now back on the tube, properly aligned. Rather than wicking CyA again, I used canopy cement: I didn't want to take a chance on the wrap dissolving or cracking more from the CyA, so I just applied a thin layer of canopy cement to the edges of the wrap, laid it up, and taped it to the tube. Back in business!

Are the Wraps Just Old?

I applied the S-IVB transition wrap this evening. It cracked.

There was a little stress on the wrap, tangentially, in order to keep it tight. The CyA created a few cracks in the forward (small) edge. (Side note: I probably should have just glued this wrap down to the paper transition with canopy glue. I'm using that on the edge joint and on the aft [large] edge: I didn't want to take another chance with this wrap.)

I wonder if these wraps are just a little old. Does the chemistry of the wrap change slightly with age, making them more susceptible to developing these small cracks? I bought the kit about ten years ago.

Of Rings and Sealing Wax

Well, no sealing wax: just sealing thin CyA.

The large tube (S-IC, Interstage, S-II) is done, except for touch-up, masking, and painting. It's on to the S-IVB now. First step is to build the transition between the S-II and the S-IVB (big tube and third stage). This requires building a centering ring-couple set between the small S-IVB tube and the large tube that comprises the S-IC and S-II.

Ideally, this is pretty easy: two centering rings, and a coupler constructed earlier (one of the first steps) by slightly sectioning a small piece of tube that's the same size as the large tube so that it just fits inside the large tube. I made this coupler just a little too big, so out came the sanding block and, since I took off all the outer layers (the ones that coat the cardboard fibers to keep them from fraying), the thin CyA to reseal the coupler. Then, more sanding, since the thin CyA causes the outer fibers of the cardboard to swell, and the surface gets rough.

Just about done, though. Then, it's on to attaching the wrap that gives me the most cause for concern: the third stage wrap, that overlays the transition's taper.

Specialized Techniques, Ullage Rocket Scars

Building the Apogee Saturn V requires all these specialized techniques that I've not used before. Letting CyA wick to seal the edges of the thin plastic wraps, feathering epoxy clay to smooth over joints that don't meet quite perfectly, and who-knows what else in the offing.

I had to come up with one of my own (the two above are in the instructional videos that come with the kit): drizzle a little canopy cement and let it wick—sometimes help it wick—under an edge of a plastic wrap. I used this technique to seal the edges of the scars from the ullage rockets that I removed in order to model Apollo 15 more closely.

The frustrating part is that just about when I get comfortable and competent with one of these techniques, I'm done using it on the rocket!

Fin Fairing Wraps, and Manufacturing Tolerances

Finally: the fin fairing wraps are attached! Unfortunately, some of the manufacturing tolerances are a bit loose. None of the wraps seemed to extend quite as far as I'd expected. It's just a bit of extra craftsmanship to get things looking reasonably good, but it would be nice if the parts were made to better tolerances. (The same issue applies to some of the large tube wraps: they're too short!)

STB-1

I'm working to determine the limits of indirect staging (sometimes known as gap staging): how far apart can the booster and sustainer motors be, and still get reasonably reliable ignition? I built STB-1 (Staging Test Bed #1), pushing the first stage out to about 40 cm, staging an Estes D12 to a B4. The first flight worked perfectly, though there was some interesting tip-off shortly after launch and an odd sort of corkscrew in the flight path.

The second flight wasn't nearly as successful: the sustainer failed to light.

The booster motor mount tube has four vent holes, roughly 3mm diameter, about 4cm or so from the top. These vent into the airframe, with vent holes drilled into the middle and after centering rings (BT-60 airframe). I'd thought that, perhaps, the failure's cause was clogged vent holes in the middle centering ring. Post-flight dissection showed that not to be the case.

The sustainer's motor mount had been dislodged and shoved well forward in the airframe. I couldn't tell if this happened on impact or during staging—I would guess on impact, though. There was a small hole in the forward centering ring, though, to provide clearance for the sustainer's engine retaining hook; after the second flight, it looked to me as though that hole had grown. It's possible that the stages are too far apart for reliable ignition, and it's possible that the hole in the forward centering ring allowed enough pressure to vent forward that the stages separted before sustainer ignition. (That forward centering ring is too far forward: it should not be impinging on the sustainer's engine hook.)

Perhaps more testing to come.

Peeling Warps

After all that work to get the wraps down well, it's time to peel up a few small sections. Some of them are at the gaps between edges of a wrap, and in some of these I didn't go nuts trying to get those edges down, knowing that a tunnel cover would run over that. But, in some cases, it seems that the CyA wicked much further under the wrap than I'd have liked, especially at the forward end of the first stage tunnel cover.

It's painful.

It would have helped, I think, if we'd had some written instructions to browse: it's very difficult to browse quickly through the video instructions. A few more specific hints in the sections on attaching the wraps (e.g., "avoid getting glue under this-and-such part of that-and-such wrap, because you might want to remove that part of the wrap when attaching the tunnel covers") would really have helped, I think.