SEMROC Little Joe II Building Thread
 

This thread is meant to parallel the similar thread for the Saturn 1B, since both models use the same Apollo Capsule kit. It is not meant as a competitor to that thread, but as a complement since both models are of the same vintage and historical importance. While a great deal of time is spent working on the Apollo portion of the model, there are other aspects of the LJ-II that have similar requirements, such as the application of wraps and the built-up fins. What I want to try to do is document the construction using some techniques I found helpful, and any pitfalls I may run across, during the process.

Construction Begins! [Bandwidth Alert!]
 

This first section begins where most models begin -- with the motor mount. The use of laser-cut components makes this process so much easier, and Carl is to be congratulated for the accuracy of these parts. I found them to slip together with just enough friction to keep them from excessively moving around while gluing, but not so much that I needed to sand the rings to make them fit over the motor tube.

The first image show the mount in partial profile, after initial fitting and glue-up:



Beginning with the assembly of these parts, I made a change from the instructions. I wanted a tighter fit of the rings over the engine hook, but without crushing the tube by trapping the hook under the ring. To get this fit, I sanded some shallow flat notches in both rings. The next image shows the tool I use to sand these notches:




Using one of these twigs, the same width as the hook, I carefully sand a shallow flat notch in each of the rings, with the one in the normal lower ring directly opposite the pre-cut engine hook opening. This allows the rings to fit more snugly over the hook, holding it in position better. Next, the former lower ring will become the top ring:



What I am now able to do is tie the Kevlar thread around the motor tube, and run it through the larger slot:



I use these CA applicators to put the glue to the joints:



I am using Titebond II for this project, which is my glue of choice for building most models. The glue is applied thin -- about 50-50 with water -- and this allows the glue to wick into the joints and get deep into the "roots". When the first layer is dried, apply a second layer right over it and this will secure the parts without the typical waste of glue. It really doesn't take that much glue to work with these paper components. Once the glue dries, these parts are rock-solid.

Now, I attached a snap swivel to the free end of the elastic chord:



Here is the snap swivel attached:



And now, the motor mount subassembly is complete, ready for insertion into the body tube:



This ends the first part of the project. Next message will deal with the fins.

Wonderful pictures!
Keep it going.

Great build pics! But, I wanted to see what the Little Joe II looks still "in-the-bag" before
the build begins... (not too late for that, isn't it?)

Uhhh, 'fraid so...

Sorry. I'll have to get another one for bag shots...:D

I'm sure someone has an unbuilt kit.

I guess I'm the only one who builds a house from the top down?????

I wish I had seen this before I started building!!!

Seriously, You are truly skilled and experienced rocketeer. Your pictures and descriptions are worthy of a "How to build rockets" handbook.

But I get sooooo excited I sometimes seal the cone, mount the fins and paint and decal, without even a mmt or recovery given a thought. On one occasion, I had to glue a centering ring down a rockets throat with shock cord on a year old build while prepping for the next day's launch.

I must follow the directions next time!

Bob

You've only had to do this once??? And you call me experienced! :o

Fin Assembly
 

The fins are very lightweight, both in mass and in construction. As shown in the original instructions, you build a frame around the outside edges of one side of the folded cardstock skin, then fold the other side over and glue it to the frame. Carl's tight laser cuts a clean line for these pieces, and the wedge-shaped pieces are accurate, but the pieces themselves don't quite fill up the gaps at the corners. The long straight piece is a tad short; meaning you wind up with two pieces of balsa just barely touching on the inside corners, and a big gap to fill on the outside corner. A better fit could be achieved if the long piece was about a 16th inch longer, and bevels were sanded into the ends to meet the short straight piece and the root-edge long wedge.

A second issue is a product of the overall frame design. The large central piece of balsa that ties the frame together between the wedges is glued to the skin on one side only. This leaves one whole skin side unsupported, and thus allows for acceleration-induced collapsing of the skin during launch. On a high-power "C" launch, this could lead to a vectoring of the slipstream by the fin shape, causing a roll in one direction. While this may not be a big issue, and the resulting spin may add some stability to the model, I think in a future build I might have to use a thicker piece, like 3/32" or even 1/8", and sand it to a matching wedge shape before fitting it into the frame. This would allow both skins to have contact with this member and the fin would retain its shape better.

These issues are simply a result of following the original version closely, and not a criticism of Carl or his work. These issues existed in the original kit 37 years ago, and they remain today.

First image shows the four fins together, one completed:



Here's the assembled fin, all nicely closed up:



Here's one of the still-open fins, showing the frame gaps:



In this last image (sorry for the blur -- inside shooting, flash, low-light, close proximity...) you can see how the large member fits against one side of the fin but doesn't touch the other. The skin on this side is supported solely around the edges. This piece needs to be thicker, and sanded to match the wedges. Doing this will allow the skin to be supported along the center.

I used full-strength Titebond II applied with a small artist's brush. Lay down a thin layer onto the skin first, then apply a layer to the wood piece before setting it in place. The full-strength glue has the "tack" you need to hold things in position.

You will note the fins are sitting on a full sheet of skin images. After building the first fin with thinned glue, I found the moisture deformed the skin too much. I wound up having to cut new pieces from scratch for one fin, and needed at least one fresh skin to finish the work.

Next: Attaching things to the body...

I thought the same thing about the center balsa piece. At first I wondered why the original was done that way. I then thought that some builders wouldn't be able to get a good fit and would end up with a bulge across the length of the fin. I doubt I'll ever fly mine on a C motor, so I'm happy with it like it is so far.

I've built the escape tower legs and I'm now on the center support structure. I got tired of doing the tiny detail work so I ut it away for a while and I've built a couple of regular rockets like the Nike-X to rest my eyes. :)

I'll get back to the LJ II in a few days.