5.5 ATACMS Construction

Sorry, no pictures. Maybe next time.

The model was built from LOC 5.5" tubing and the long LOC 5.5" nosecone. The finished length was about 41". Not very long. It also doesn't have much fin area. This required some interesting construction techniques.

To start with, I was planning on flying this model with the Ellis Mountain L330. I chose this motor rather than the Aerotech 75mm K since the low thrust of the L330 would keep the model subsonic. With all of the weight of the L330 in the back of the model stability was going to be a problem. I knew that getting 1 caliber of margin would be impossible without raising the weight to the point where the L330 could not lift it. I was encouraged by an article that showed that short models do not experience as large a CP shift from winds as longer models. So it looked like it could be done. A check using VCP showed that the CP would be about 15.25 inches from the tail. (the L330 is only 16" long so it would all be behind the CG) I was going to need some nose weight.

I also needed to fit a parachute into the model. The only way to do this was to cut off the base of the nose cone shoulder and put the entire recovery system in the nose. This also meant I needed a different way of attaching the the shock cord to the nose. I was able to solve several problems at once....

I cut off an inch or so of the tip of the nose cone and drilled a whole large enough to pass a piece of Doug Pratt's Tubular Kevlar™ through. I then tied an overhand knot in the Kevlar™. I then applied epoxy to the knot and built the nose tip back up. Some sanding and filling resulted in a decent looking nosecone.

The next step was to add 1 pound of lead fishing weights to the tip of the nose. I first ran some Kevlar™ cord through the weights (the cord went through the main Kevlar™ shock cord), tied off the last weight, and left a foot or so dangle free to tie onto other things. (like more weights :-) I then poured epoxy over the weights in several pours (to limit heat production) until the weights were fully encapsulated. The nosecone construction was then finished by epoxying a piece of body tube to the shoulder. This was done since I was using "anti-zipper" construction. The booster required a bit more effort...

Booster construction started with the fins. The fins were built with a G10 core with 1/8" balsa skins. I then airfoiled the fins. (balsa is MUCH easier to sand than G10) The motor mount tube was 17" of LOC 3" MMT. ( I didn't write any of this down so this is all from memory :-) I glued a 3" to 54mm centering ring and a short piece of coupler in the forward end as a thrust ring. This also served as motor retention for the L330. I used a 3" centering ring with a 1/4" eyebolt through it. The eyebolt threaded into a hole in the forward closure of the motor.

The motor mount ran the full length of the booster. The booster started with boat tail that went from 4.5" to 5.5" over a length of 4". Then a short piece of body tube followed by the coupler that slide into the body tube glued to the nosecone. I was concerned about the strength of the coupler so I glued four thin plywood strips between the coupler and motor mount tube. These ran longitudinally from the forward centering ring to about an inch short of the aft centering ring. (this was recessed in the body tube to allow the fins to be glued to the motor mount tube through short slots in the aft end of the body tube) The boat tail was cut from poster board and then a layer of 2 oz. fiberglass was epoxied to it. This was then quartered and epoxied on after fins were attached.

Since none of the 75mm motors have an ejection charge I needed to put in an altimeter. The only place to put it was between the motor mount and the body tube. So I cut a hatch in the side. The hatch also served as the mounting plate for the altimeter and a backup timer. These were wired to switches so that I could arm the charges on the pad. The switches were DPDT types. This was so I could completely disconnect the ejection charges from the electronics and place a short across the charges. This is a good thing since the flashbulb based ejection charges I was using are very sensitive to static discharge.

Version 2.0

This is a somewhat belated addition describing the second version of this rocket.

I do have a Rocksim file of this design:

The primary difference between the second and first version is that the second has a 54mm motor mount instead of 75mm. This is to head off any recurrence of insanity involving an L330. I didn't take too many pictures during construction and this one shows just about everything I want to talk about so here it is:

This shows the important features. Starting with the fins. These started as before with G10 cores with balsa laminated to both sides. Which was then sanded to something resembling the profile of the real thing. The most notable exception being the lack of a hinge or anything resembling that structure.

The two centering rings shown had shallow groves cut into them for the fins to fit into. This adds a little bit of strength but mainly aids in alignment. There are two holes in each fin tab which serve three purposes. The first should be obvious from the picture as I ran a length of 9/16" tubular Kevlar™ around the fin can in way of a shock cord attachment. I tied a knot in it and that was it. No glue.

The second purpose was as relief holes for when I foamed the fin can. This would allow for movement of the foam between the fins to help even out the distribution. There are also four holes, one for each cavity in the top centering ring.

The third effect of the holes is that it removes just a little bit of weight.

Also visible in this photo is the t-nut and its mounting plate attached to the top of the forward centering ring. This is the mounting point for one of the two rail guides.

The next step in construction is to epoxy the four tail cone sections made from 1/64" plywood between the fins. After waiting a bit for the epoxy to cure, the fin can was foamed with two part urethane foam.

Actually a pretty simple rocket to build. This version ended up about a pound lighter than the first.