Adding stabilizing weight to rockets is a common occurence.  Some of the challenges involved include ensuring it stays in place and allowing for it to be removed or adjusted as needed.  So having some way to change it without grinding and drilling is a big help. Heavy motors tend to move the rocket's center of gravity aft.   Adding noseweight moves it forward, thus restoring the center of gravity to a point where the rocket will be stable.  So when flying larger motors in a given rocket, it is sometimes desirable to add weight without it being permanent. The most common technique is to attach the weight to the screw eye at the base of the nosecone. In the pic below, a nickel, about 5 grams, is attached to the nosecone of my Sprint clone.

One problem with using an ordinary screw eye with balsa is that the hole soon gets stripped with more than a few insertions and extractions.  Screw eyes are usually best mounted permanently in balsa using a little glue to secure them in the holes, but that prevents the very goal we seek. So what I did here was to first insert a wooden dowel into the base of the cone to receive the screw eye.  The hardwood dowel can withstand many insertions and extractions without losing grip on the screw eye.  I cut off a ¾" length of ½" dowel, then bored a ½" hole in the base of the cone to receive it.

I didn't set out to make fun of President Jefferson, but I thought this image was an earful

Here's another cone done the same way.  It goes on my Midget55.  This time I got a tighter fit between the dowel and the hole in the balsa.  The discoloration is from the lead weight rubbing off on the balsa.

With the noseweight attached, here's the same cone as above.  In this case, I cast the weight by melting some fishing sinkers.  (The white paint on the screw eye is due to it being recycled ) One problem with small screw eyes is that the shanks tend to be kinda short.  You can't stack too many coins or washers on them before there's not enough shank left to screw securely into the dowel.  So other attachment methods are necessary when more weight is needed.

To get a longer shank without a big increase in diameter, machine screw eyes are a good solution.  Items 2 and 3 below are typical of screw eyes used with balsa nose cones.  Items 6 and 8 are small machine scew eyes with #6 and #8 threads, respectively.  They provide additional length without the increase in girth as depicted with item 4.

The next challenge then is to rig the nosecone base to mate with the machine threads. The first solution I tried was a T nut.  But these cannot be mated directly to the balsa cones; an interposer of sorts is needed.  A piece of thin plywood works well with the T nuts.  In the pic below, the plywood is 1/8" thick.  Not content with the bond between the ply and the balsa, I drilled four holes and sunk four dowels into the cone to act as piers to ensure the plywood remains affixed to the cone.

A threaded insert is an option to the T nut.  But these need a thicker base to be mounted in.  A thicker piece of plywood will work, at the expense of consuming still more space in the rocket.

In several of my rockets, space is at a premium, so I used the hollowed out nosecones to move the weight forward and allow some extra space for the recovery system.  This has the added benefit of needing less weight to stabilize the rocket; with the weight farther forward, the moment arm is longer, so less weight is needed to achieve the same effect on the center of gravity.

In this case, the base of the nosecone was cut out.  A hot knife works well on the styrene, followed by some sanding to remove the flashing.  The T nut was mounted in the plywood disk.  But first, the disk was cut to mate with the cone at the desired depth.  the edge of the disk was angled slightly to help contour it to the nosecone.  The inside of the cone was then scuffed up to help the epoxy adhere better.  In the pic above, the three metallic spots around the screw opening are from the barbs on the T nut.  After it was mated to the plywood, I grinded them down flush. With the T nut affixed to the plywood, I glued the assembly into the cone using some epoxy with some thickeners in it.

Here's the nosecone above with the weight - a fender washer - in place along with the screw eye.

Here's another cone, this one for a Quest T-40, which has received the identical treatment, from cutting the base of the cone out to epoxying the plywood disk in place. In the pic below is the screw eye for this cone.  When needed, fender washers are sandwiched between the forward nuts, then the screw eye is inserted into the T nut.  Since the eye is recessed well into the cone, the kevlar cord is included for attaching the parachute.  Of the three nuts, the two aft ones are jammed together to ensure they turn with the screw.

Here's an alternative to hollowing out the nosecone and having to build up the plywood disk.  This is another Quest T-40 cone.  In this case, a small access hole is cut into the nosecone base, then a nut is inserted to mate with the screw eye.  Some needlenose pliers are needed, but no special tooling is required. This method takes up more space in the rocket, but the advantage is that, if the nosecone gets separated, it will tend to tumble in rather than come in ballistic as is the case for the tip weighted cones.  That is, in the event of an anomaly, the tail weighted cone poses less danger than the tip weighted cone.

Here's a BT-80 example from my Fat Boy.

Below is another BT-80 example.  The stack of washes on the right is the noseweight needed to keep my upscale Midget80 stable.

My Big Daddy cone (3") is in the pics below.  At right, the ring is where the shock cord and parachute attach.

The BT-60 cone below has had its base bored out, then a plywood disk with T nut was inserted.  At right, the ballast washers just slip into the bore.

This BT-55 cone goes on my Tuber.  The threaded insert was mounted into a piece of plywood which was then mounted in the bored out base of the nosecone.

This is the cone for my Quest DC-Y Space Clipper.

This 2" nosecone goes on an upscale Midget. The base was hollowed out to make precious space for the recovery system.

These last two cones depict not noseweight but alternative attachment methods.  Normally, a plastic cap is glued onto the base of these cones, and the shock cord attaches there.  But I wanted to use the cone for holding some of the shock cord, so I mounted the screw eye up in the cone.

This is the same cone as in the pic above.  A wooden dowel was sanded to fit down in the cone where it was epoxied in place.

This cone was done just like the one above.  A foot or so of shock cord fits in the cone.  That little difference helps keep from having to push the chute too far down in the rocket.

9-5-2010