|Endo||04/17/2005||3.9 in.||88 in.||9 lb||Aerotech J275||Modified PML Endeavour|
I decided after jumping straight from the Bull Puppy to a scratch built 6" rocket for my Level 2 cert that I wanted to take a step back to something a bit smaller that I could launch more often. Steve gave me an Endeavour kit made by PML. Naturally I wanted to do something different with it, so I layed out all the parts and started thinking about the options. First, I didn't like how flimsy the body tube was where the slots were cut for the fins. With 3 fins stacked in pairs, the fin slots in the quantum tube caused it to warp and become flimsy. So I cut that off.
I also wanted to get away from motor ejection, and thought about how I could incorporate an electronics bay. I figured that if I used the old aft section (now shorter since I cut off the fin part) as the new forward section, and the old forward section (which was slightly smaller) for the new aft section, then I could bolt an e-bay between them. Because I was going to build my own fin can anyway (with the "zipperless design"), I could use both of these tubes for the body.
There was one design goal and another construction goal for the fin can. As for the design, I decided I wanted to provide the option of making this rocket a 2-stage in the future. This involved moving the fins up the body tube about 4 inches to allow a coupler to slip into the back side. Instead of using all 6 fins, stacked in pairs, like the original, I decided to use one set for the top part, the sustainer, and the other set for the booster. However, I had some concerns about stability after both moving the fins forward and removing half of the area. I fired up RockSim and found that if I used 4 fins instead of the original 3 it made a dramatic impact on position of the center of pressure, Cp. I cut a duplicate fin from a piece of G10 I got from HobbyTown USA, and suddenly it was a 4-fin rocket!
One issue with building a 2-stage is how to get the ejection charge current from the electronics bay down to the motor area. There are two charges that need to go off - the separation charge to separate the booster from the sustainer, and the ignition charge for the sustainer. Because both events occur before the sustainer separates, it can all be controlled from the main electronics bay. I had a bunch of ideas about how to run wires down, but after deciding that I wanted a couple of threaded rods running through my centering rings for added strength, I realized that these could also be used to carry the ejection charge. My two rods turned into 4, and with bolts/washers/thumbnuts on both sides, I can attach 4 separate wires to activate the separation charge and sustainer ignition at the bottom of the fin can. While a little heavy, it serves dual purpose in strengthening the fin can and carrying the charges. It looks sweet and is built like a tank!
The construction goal for the fin can was to experiment with vacuum bagging. I purchased a Food Saver for $50 from my local grocery store. I was using kevlar sock and 3 oz fiberglass cloth for my covering. I vacuum bagged each layer separately. After putting the sock on the phenolic tube and saturating with epoxy, I wrapped it in a release layer (plastic with tiny holes that epoxy doesn't stick to), and then a layer of cotton cloth to soak up any epoxy that comes through. I threw that into a bag, and sucked all of the air out. I could quickly see spots of epoxy on the cloth where it had sucked excess through. After curing overnight it had a nice smooth finish. One downside is that where the two halves of the bag meet it creates a seam in the fiberglass that had to be sanded and filled. This could possibly be avoided by wrapping it in a sheet of paperboard or something to prevent the bag from sucking the fiberglass into the seams. It was also important to be sure I smoothed out all of the wrinkles or they would cause the surface to be irregular.
For the paint on this one I decided to go with high quality 2-part auto paint from Sherwin-Williams. For simplicity, I avoided the base-coat/clear-coat scheme and went with a single-step high gloss paint. The smaller aft section of body tube makes a great transition for paint, and the interleaved spike design makes it easy to connect two different colors. It's also a simple design to mask off. Maybe in the future I'll try my hand at some flames.
All in all, this rocket has been a great learning experience. The next step is to build the booster, practically a rocket in itself, and attempt a 2-stage flight.