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I'm not going to go into great detail of the rocket construction on this page, but I will throw in a few pictures and some notes where I may have made some construction changes. I'll post more details as the rocket nears completion.
A while ago I purchased a circle cutter. It really helps cutting out bulkheads and centering rings. Here you can see some 1/2" plywood being cut for a motor centering ring. I have cut the outside diameter, now the inside hole is being cut. Notice the small nails holding the plywood to a scrap 2 x 6.
Here is the lower body tube, the motor mount tube, centering rings and fins. The fins are 1/4" birch plywood and were given a coat of glass on each side before assembly. The body tubes are the strongest I've made to date, using 6 layers of 6 oz. fiberglass cloth and Mr. Fiberglass epoxy.
Here you can see how I mount the fins. The lower centering rings are tacked in place with quick set epoxy, then each fin is aligned to the marks on the paper, held perpendicular with the squares and tacked in place with epoxy. Once the tacked epoxy spots cure, I used two layers of fiberglass cloth and structural grade epoxy to retain the fins to the centering rings and the motor mount tube. The upper centering ring is not yet attached, it will mount at the very top of the motor mount tube and be epoxied to the thrust bulkhead.
Here is a new PIRM2. I made this one entirely from 1" 6061 T-6 stock. It's slightly smaller and lighter than the original PIRM2 I made from cast aluminum.
Here's the lower body tube assembled. The fins were glassed tip to tip with two layers on each surface. I just finished installing the coupler. It's 10" long. I make the couplers from a length of body tube, then cut out a length to reduce the diameter. The cut section is then installed in the body tube to hold it to the proper shape, then I butt glued it and added two layers of glass cloth inside to reinforce the joint. I paid special attention to make the coupler very strong, and to very close tolerances. It's a very snug fit in the body tubes. If there's a weak spot during flight, it's the coupler, that's the reason I took extra care here. The 5" length that goes into the lower body tube was epoxied on the entire surface.
The upper body has also been made. Now I need to build a new electronics module. I'll use one of my homemade altimeters with a timer as backup. I'll also use a recovery transmitter and an audible locator on this rocket.
Here are images of the front and back sides of the new electronics module.
I'm using two magnetic switches on this module. One switch will be used to start the back up timer, the other will "key the mike" on the FRS radio. The switches close on liftoff as a magnet is pulled away from the body tube at launch.
The main power switch turns on separate power to the timer and to the altimeter by means of a single throw, double pole switch. The FRS radio will have to be turned on prior to assembling the rocket. One battery powers the timer, another battery powers the altimeter and the last battery is power for the deployment charges from the altimeter.
I ground tested the new electronics system numerous times, it seems like all I've been doing is making initiators (it takes three initiators per test). This was after I rewrote the software to control the altimeter, I also changed the way I was making my initiators, one ground test used a weak 9 volt battery and the initiator was slow to ignite. So I shortened up the nichrome wire on the initiators for faster ignition. I also made up a new batch of pyrogen for the initiators, I wanted the pyrogen physically stronger to make sure it wouldn't come off the wire.
I also cut my propellant grains yesterday. It had been 8 or 9 days since they were cast and the propellant was now fully cured. I'm not sure if I'll use these grains or not. I used the granular form of KNO3, and when using the granular form of KNO3 the propellant seems to contract a lot as it cools. Not only pulling in from the ends, but also from the outside in. This resulted in slight deformation of the outside portion of some of the grains. It also left some uninhibited areas on the outside surface as it pulled away from the thin cardboard lining. If I do use these grains, I'll have to coat the outside in epoxy to reseal the cardboard to the propellant, then finish with foil tape as a final inhibitor. Another problem with using granular KNO3 is that the propellant is not as strong, I noticed that early on, but didn't think it was a real problem. It may not be a problem, but the contraction of the propellant does seem to be a problem.
So I cast new grains yesterday using powdered KNO3. The problem with powdered KNO3 is that the melted propellant is very thick, and requires spooning into the casting fixture and tamping. While that's easy enough to do, especially since the xylitol based propellant sets up so slowly, I do worry about voids when using a thick propellant. So I heated some metal tools to 250 F. to aid tamping the propellant. Then spooned the melted propellant into the mold, at about 1/4 increments I tamped and prodded the propellant in the mold, then used a detail sander to vibrate the base and mold to further settle the propellant. When the grains are cured and cut to size we'll see how well it worked, but I really think they will be very good grains. I cast this batch in one melt, 2,200 grams in all. I almost forgot how nice it is to cast xylitol propellant as the pot life is almost unlimited, it's slow to set up, and melts like a dream.
3 June, 2005:
Here you can see the rocket with the first color coat on. The LW-3 motor is laying next to the stand. The masking tape marks on the top mark the center of gravity and the center of pressure.
This is a capture from Aerolab showing the fin details.
I decided to do a full weight load in the rocket to check its center of gravity and center of pressure. The center of gravity lies 53.25" from the tip of the nose, while the center of pressure is 71.3" from the nose. Leaving a stability margin of some 18". So, I guess I missed the mark there, as the rocket is well into the over stable category. I ended up with a lot more weight in the electronics module than I expected, pushing the center of gravity forward. Now I need to decide what to do about the over stability problem. I could cut the fin span down some, or add more weight to the motor, or just leave it as is and not fly on a windy day. Which may be the best option anyway.
The rocket body with nose cone weighs in at 4.6 pounds.
The electronics module weighs 2.0 pounds.
The parachutes, piston, recovery harness weigh 1.6 pounds.
Total Rocket weight less motor is 8.2 pounds.
Motor empty weight is 2.2 pounds.
Propellant weight should be about 3.7 pounds, thermal lining about another .4 pounds, resulting in a liftoff weight of about 14.5 pounds.
Performance: Assuming a liftoff weight of 14.5 pounds, a drag coefficient of .55 and an Isp of 125 at a burn time of 2.1 seconds the maximum altitude should be around 7,650' and a maximum speed of 1,035 fps.
Now, should I push the motor to its full capacity of 3.86 pounds, the Isp should improve and the burn time be reduced. The result should be a maximum altitude of 8,230' and a maximum speed of 1,143 fps.
In the above picture you can see the drogue chute and nose cone. This isn't a new chute, but I wanted to show how it was made. This should be a rock solid chute, as it has eight heavy shroud lines attached by grommets to a reinforced hem line, with four of the shroud line/grommets attaching to 1" nylon that goes all the way across the top of the chute to its opposing grommet. The other four grommets go into a 3" length of the 1" wide nylon. This chute has been used on the last several flights of the rebuild fiberglass rocket with great success. The idea is for the chute to survive a high speed deployment should the rocket take on a arching or horizontal flight path, or the deployment be late/early for one reason or another. The shock cord is 9/16" tubular nylon, and has a nominal strength of 1,500 pounds.
The rocket has been assembled and ready for flight.
Propellant Weight: 3.659 pounds
Total Motor Weight: 6.3 pounds
Rocket (no motor) weight: 8.2 pounds
Lift off weight: 14.5 pounds
Cosmo 2 had successful first flight in launch test 97.
It's really had to explain to someone the sense of satisfaction that comes with the successful conclusion of a project such as this. From the development and testing of a new xylitol based propellant, construction of the air frame, sewing the parachutes, to building and programing the electronic recovery system. When it all works, it's really a lot fun and very exciting. While the project is insignificant in terms or EX rocketry in general, it's a wonderful step forward for me personally.
The Cosmo 2 rocket probably won't fly again until this Fall. By then perhaps I can get a really good, smooth finish on the body, squeeze in a tad more propellant and go for a new personal altitude attempt.