This marks what I consider the end of my Rookie year, although it's been about 2 years since I started in EX rocketry. The PVC body tubes were a fast and inexpensive choice, as was the EMT steel motor casings. Now I need to move on to lighter and stronger materials.
This project will be my first attempt at a fiberglass body scratch built rocket. This is a fundamental change from my previous rockets, as all my past rockets used PVC tubing for the body, with screws and bolts holding things together. The fiberglass body should result in a stronger, more ridged rocket. I will also be able to tie the fins into the body tube with fiberglass, making them much stiffer and less prone to flutter. Lastly, the rocket should come in significantly lighter.
While I'm at the point of changing things, I might as well go for a lighter motor as well. I have ordered some 2.25" OD aluminum 6061 T-6 tubing for a new motor. I'll detail the motor construction in another page.
I decided not to go with a cardboard or phenolic tube as the base for the rocket body. That may be a mistake but I wanted to try it with just fiberglass. The mandrel was a 3" (3.5" actual OD) schedule 40 PVC pipe. I wrapped a layer of wax paper over the mandrel to keep the epoxy from sticking to the PVC. I used 6 oz/yard fiberglass cloth, the cloth you see sold about everywhere for car body repair. The section I was making was 24" long, and had a circumference of about 12.5 inches.The first batch of epoxy I mixed was 150 grams, not sure how much I would need it turned out to be too much, as I only used 67.3 grams.
The mandrel was coated with epoxy using a foam brush. After the mandrel was coated the cloth was laid on the mandrel and I started working the cloth into the wet epoxy, taking care the cloth was totally saturated with epoxy and all wrinkles and bubbles were worked smooth with a gloved hand or the foam brush. The mandrel was turned slowly to roll the cloth onto the mandrel. There was about a 1.5" overlap with the cloth. Once the cloth was in place and saturated, I went over the entire surface and smoothed the epoxy, eliminating any glossy areas or puddles of epoxy by dabbing it with a clean paper towel.
After about 12 hours the epoxy was cured, and I sanded the entire surface, removing any high spots and roughing the remaining areas. The surface was then cleaned with acetone and allowed to dry.
For the second layer, I knew I wouldn't need as much epoxy as the first coat, so I mixed a batch of 70 grams, when done, I had only used 46.1 grams of epoxy on the second coat. For the second layer, I used an identical piece of fiberglass cloth, and worked it the same as the first layer.
I repeated the process for the next three layers, only instead of sanding between the layers I added the next layer to a slightly uncured previous layer. I also used a dual bulb heat lamp about 20 inches from the tube to speed up the curing process. The finished tube now has 5 layers of glass, and a thickness of about .07". I'm not sure if that is strong enough yet, but I'm going to give it a try. I think for a really large project, I may go with an initial layer or two of heavier fiberglass cloth.
Here is a picture of the first fiberglass tube I made. It will be the lower body section that holds the motor.
The fins are being made from 1/8" birch plywood. If you look closely you can see two nail heads in the square stack of plywood. I drilled two small holes through all four fin blanks, then used small finish nails to pin them together. The fins were sanded as a set in my 12" disk sander.
Once the fins were sanded to form, I used my table saw to dado out two grooves. The fins will slot into two centering rings. Below the fins you can see one of the centering rings being laid out. I'm going to wait with cutting out the center of the centering rings until my new motor casing material arrives.
Here are the fins slid onto the centering rings. At this point I think I need to make a jig to hold the fins and centering rings. Everything needs to be held in perfect alignment before epoxy is applied.
Here is the mandrel set up for the upper body tube section. The mandrel has been cleaned, and a layer of waxed paper rolled over the tube.
Here is the tube after the first layer of glass has been applied. It is a messy job!
Between layers of glass and cutting out the fins, I decided to use the free time to make a couple on new parachutes. It's a job I truly dread, I hate sewing. I suppose the fact that I'm not any good at it doesn't help. I do manage to get the fabric sewn together, but any real tailor would cringe at the sight. The cost of large parachutes is high, and it is something I can do, so I do it. I lost both of my better chutes on the Cosmo rocket at the IEAS launch. They were looking pretty tattered anyway, as they had gone through quite a few launches. I liked the X-form chute, I didn't sway or spin at all. So for the main chute I wanted a large X-form, or sort of semi X-form. I wanted a large enough diameter to bring the rocket in slowly, but I wanted those X-form side slots to spill the air. Each finished panel is 69" long by 39.5" wide. I decided to go with dual deployment on future flights, so a small, strong drogue chute was needed also. I cut the drogue as a simple flat circular chute with a finished diameter of 28".
I used brass grommets to attach the shroud lines for both parachutes, and heavy braided nylon for the shroud lines as well. I like the strength of the heavy braided nylon, and its resistance to fouling.
The parachute material is lightweight nylon. I go to Wal Mart and pick up a few yards now and again. Just so I have some on hand. It's very inexpensive, only $1 to $2 per yard.
Here are the chutes finished and rolled up.
Here are the chutes opened up.
Here is a capture from Areolab showing the early design. Not knowing the finished weight of many items, I may have to adjust the CG by using a wood nose cone vs. a fiberglass nose cone, or I may trim the fins slightly shorter. New Note: With the rocket more or less complete, I measured the CG with a fully loaded motor and a simulated nose weight. The CG was 49" from the nose. Leaving me with a nice 2.2 caliper margin of stability. That was with a nose weight of 223 grams. I should be able to make the nose cone lighter than that, bringing my estimated caliper of stability margin closer to 2.
The rough upper body is now complete with 5 layers of 6 oz fiberglass. Still waiting for my motor casing material I decided to work on the electronics canister. I pondered for some time as to what configuration I wanted it. I decided to go with a fairly small cylindrical fiberglass tube with fiberglassed plywood ends. I would use eye bolts at either end, the eyebolts would connect inside the canister through a threaded coupler. The top end canister eye bolt would hold the main parachute, and the lower eye bolt would thread through a bulkhead in the upper body tube and attach at the threaded coupler, the lower eye bolt would then attach to the drogue parachute. Since all recovery forces would go through the electronics canister, I decided to use fairly heavy 5/16" stainless steel eye bolts.
Here is a picture of the electronics canister ends. They are 1/4" birch plywood with a layer of fiberglass on each side. After the resin and glass cloth were applied, I put a layer of waxed paper above and below, then laid it on a heavy piece of flat iron, then a piece of iron on top and some weights to really compress it. After it cured I pulled the waxed paper off. What a nice, smooth finish it left. Now I see why people use vacuum bagging. I'm just finishing cutting the excess cloth off. Then I'll sand the edges smooth. Just above the disks you can see the rough cylindrical portion of the canister.
Here is a shot of what I hope will be a deployment charge canister. It's a .6" diameter aluminum tube. I have filled it with about 1/2" of epoxy, once the epoxy is cured I will drill a small hole through the epoxy for the initiator wires. To use, the initiator will be inserted from the open top with the wires passing into the electronics canister. The desired amount of black powder will be added, then wadding to use up extra space. To finish I'll simply tape the top of the canister closed.
Here is the canister almost finished. The thick bulkhead to the right will be mounted into the upper body tube. The electronics canister will be dropped in from the top of the upper body tube until it rests on the bulkhead. Then the lower eye bolt will be screwed into the coupler inside the canister. The main parachute will attach to the top eye bolt, the drogue will attach to the lower eye bolt.
Here is the canister assembled, the black arrow shows the plywood bulkhead that will be mounted permanently to the upper body tube.
Here is what it looks like slid into the upper tube. It will go in several inches more to allow room for the body tube coupler. You can see the open end of the ejection charge canister.
Here is the upper tube with the bulkhead installed. The bulkhead is retained by (7) 1/4" hard wood dowels epoxied in place. Once the epoxy has fully cured, I'll cut them off flush and sand smooth.
Here is a completed deployment initiator. I used a christmas tree bulb with the top of the bulb ground off. Then a small amount of BP was used to fill the bulb cavity, the tip of the bulb was then sealed with a drop of hot glue. I also used hot glued on the exposed wire ends once the lead wire was soldered to the bulb. The hot glue keeps the wires from shorting, and also provides some stress relief.
Click Here for a very short video of the first ground test of the deployment canister.
Here is the deployment canister after the test. No damage, at all.
Just about out of things to do until my motor casing material arrives, I decided to cut out the lower body tube fin slots. I carefully measured the diameter, divided by four, then laid out the marks on a piece of paper, then transferred those marks to the body tube. I use a large piece of angle iron to draw parallel lines on the body tube. Once the lines were drawn, I used my rotary tool with a cutting wheel to cut out the fin slots. From what some people have said, I thought it was going to be about like brain surgery, either I could be a brain surgeon, or (more likely) it wasn't all that bad. In the picture I inserted the fins into the body just to see what it would look like. I'm no where near being ready to mount the fins.
I wanted to test the mid section deployment separation, but not having the lower body tube done, I'd have to come up with something else. I was going to use a short length of PVC as the coupler between the sections, so why not use a longer length of PVC as a mock lower body tube. I found about a three foot length of PVC and made a bulkhead to fit inside, then screwed the bulkhead into the PVC 9 inches deep, the same depth as the real bulkhead in the lower body tube would be. I then drilled and tapped 3 holes through both sections for the #4x40 nylon screws I would use as shear screws.
I still want redundancy, so I decided to use 2 initiators in the deployment charge canister. Not that I need 2 of them for a ground test, but I wanted the test to be as real as possible. Each initiator holds less than a tenth of a gram. So I added one gram of BP in the initiation canister, for a total of just under 1.2 grams. I used my old launch controller to fire the initiators.
Here's the moment after the charge went off. Click Here for a short video. It was a nice brisk separation, as the 2 sections ended up some 25 feet apart.
Here's a close up of the tubes. The upper body tube is to the left, the mock lower body tube to the right. You can see the small hole in the upper body tube is intact, and the arrow points to the nylon screw after it sheared. There was no damage, and the first test went very well.
I've been weighing the pros and cons of how to start my timers. In the past I have used either a pull wire, a g-switch or a launch rod switch. I'll give some thoughts on each.
So, what's the best answer? I've been thinking about a fourth option. Using a magnetic reed switch. It would be lightweight, self contained in the body tube, and could initiate the piezo buzzer at launch. One small magnet stuck to the launch rail would be all that was required. My concerns are what happens to the internal reed when subjected to high g-forces? Could the contact open again during launch? It probably wouldn't be catastrophic, if the switch did open during the motor burn, it should close again at motor burn out when the g-forces of launch subside. And an extra delay of one or two seconds would be unlikely to damage the rocket.
My next project is the nose cone. My previous nose cones have all been turned from wood, while that's an option, I would like a lighter nose than wood can provide. So a fiberglass nose would be ideal, although not real easy to do. I decided, to get a more or less perfect mold for a fiberglass nose cone, I'd need to turn it in my wood lathe. I normally use clear pine for my nose cones, but since this is just going to be a mold I used some 2"x6" lumber I had, and glued three pieces together to get the required thickness. After allowing the glue to dry overnight, I removed the clamps and cut the block down to 3.75" x 3.75"x 12". This formed the blank I loaded into my lathe.
Here is the start of the turning process.
After about an hour or so, here is the rough nose cone I will use as a master for a mold. This wood was very poor quality, and had many deep indents around the course grain. This was obviously new growth pine, and not nearly as dense as old growth pine. It would require a lot of filling and sanding.
To start, I used some Bondo body filler to fill in the deep cracks and imperfections in the wood. The body putty was then sanded and two coats of primer applied. Once the primer was dry I sanded again. In this picture I have applied more body putty in the low spots. I made a little paper cone to form the tip of the nose, it was filled with bondo and allowed to cure.
While I was working on the nose cone. UPS was kind enough to drop off my motor casing. In the picture above is the casing being used as a mandrel for the motor mount tube. During the course of the day I made 5 wraps of 6 oz cloth. Using the heat lamps really speeds the curing of the epoxy.
Now that the motor mount tube was done. I can start putting the fins on the frame. I debated how to go about this. I was going to make a jig that would hold the fins in place, but the jig would need to be perfect to get the fins in perfect alignment. Plywood isn't necessarily perfect, and to glue it or screw it together perfectly would be difficult too. So I used a 2'x2' plywood base, then taped some paper to the base. I still had my drafting T-square. So I used it to draw out the fin shapes on the paper. I used the paper to align the fins, and a square to make sure the fins were perpendicular to the plywood base. Once everything was as straight as humanly possible, I hot glued each fin at two spots on the lower centering ring, it's hard to see the hot glue in the picture but its there. Once the hot glue was set, I used some fast set epoxy to tack the upper centering rings to the fins.
Once the fast set epoxy was cured. I made some fiberglass micro strands by chopping up some fiberglass cloth in my coffee grinder. It takes a while but works well once the glass is finely chopped. The micro strands of glass are then added to mixed West Systems epoxy to form a thicker epoxy paste. I inverted the fins and filleted the fins at the joints. Once this is cured the assembly should be strong enough to be handled.
After the end fillets had cured. I started the main attachment of the fins to the motor mount tube. It looks a mess, and it was a mess. I wanted as much strength as possible here, so I cut little strips of fiberglass and epoxied two layers at each fin to tube joint. I also overlapped the fiberglass to the inside of each centering ring. I'm afraid I'll have some clean up sanding to do before the fins go back into the lower body tube, but this should make for a very strong joint.
Here's the nose cone about done. After 5 applications of body putty, and 6 coats of primer. For a mold, I decided to try a two part silicone casting mold resin. From what I understand it's about the best you can use for mold making. It's a little expensive at about $30 a quart, but, if it works well I'll have a mold I can use to make as many new fiberglass nose cones as I could ever need. I had to mail order it, so I'm sure it will be here next week some time. I'm already trying to figure out the best way of fiberglassing the inside of the mold.
I've been working on the new motor, it's about finished. You can see the details here.
Here I've moved ahead a little. The fin/motor tube structure has been attached to the body tube. And all the joints filled with epoxy and glass micro strands. The fins are very strong at this point. You can pick the rocket up and swing it around by a single fin and nothing even flexes. Even with the rocket resting on its fins and sanding on the joints above caused no movement. In the picture above I've started the tip to tip glassing of the fins. Between the top fins in the center of the body tube you can see two little wood dots. These are 3/16" wooden dowels that are drilled into the centering rings, then epoxied in place. There are four of these at each lower centering ring. I doubt they are really needed, as everything is epoxied in place and already very strong. But I wanted to make sure it would be strong enough, so I added the dowels for insurance. The upper centering ring is epoxied to both the motor mount tube and the body tube, and the bulkhead plate that holds the deployment strap D-ring in place is epoxied to the body tube and has (7) 3/16" wooden dowels epoxied through the body tube.
Here's a shot from the other end. I wasn't really sure how to go about the tip to tip glassing of the fins. I wanted the ends covered too, so I used masking tape to hold the glass over the ends of the fins.
The glassing of the fins is complete. One thing I learned is that a lighter weight cloth would help on the fins. It's hard to form the cloth over the contours of the airfoils. I have a small detail sander, that helps greatly in sanding the fins and the body tube between the fins.
I still don't have a body tube coupler, I was going to use a short length of the PVC pipe I used as the mandrel to form the body tube. But that's kind of cheating and it's very heavy for its short length. So I decided to make a fiberglass coupler. I laid up 5 wraps of cloth on a 16" long section today. That should be enough for two couplers, just in case I break one, or decide to start on another body tube.
I've been working on the electronics canister too. Yesterday, I laid up 10 layers of fiberglass cloth to form a sheet of fiberglass. This sheet will be the mounting surface for the electronics.
I mounted the fiberglass board to the lower canister bulkhead with aluminum angle bracket going through the two bulkhead retaining bolts. The board was bolted to the bracket with two bolts. I came up with a rather odd way to mount a battery. The battery rests on a plate that is bolted to one of the bulkhead retaining bolts. The battery can be cable tied to the bolt, then the battery rotated into position. The main deployment link bolt in the center holds the battery from one side, and the outer canister cover (when installed) will hold it from the other side. This side of the board will be for timers, or other electronics.
Here is the outside of the mounting board. It has room for a 9 volt battery at the bottom, and an RRC2 altimeter above the battery.
Here is the rocket with its first coat of primer. I used KILZ which is a high solids primer.
The coupler is done and installed in the lower body tube, much lighter than the PVC I had planned on using. All that's left now is the nose cone. The mold making and casting of the nose cone can be seen here.
Here is a new capture from Areolab. The measured center of gravity is 50.6875" from the nose tip. Which means the rocket is almost stable statically, and has a 8.7525" between the CG and the CP, giving me a 2.44 caliber of stability. That's plenty, and means the rocket may weather cock some, but should be very, very stable in flight.
Here's the rocket pretty much done. I'll do another static test or two of the motor before it flies.
Here's a shot of the business end of the rocket. The motor retainer was formed from some alumimun stock and held in place with a #8 screw. The 12 pins that retain the nozzle are also used to transfer the thrust to the motor tube.