Simulations predict an apogee of about 9,924', while I'm not setting 10,000' as a defined goal of the rocket, it should be within reach. The 9,924' is assuming a Cd of .75, with my new launch rail I should be able to get the drag below .75. That, and the fact that I will have plenty of time to finish this project, I should be able to get a better finish on the rocket. I also plan on using no external fasteners on this rocket, again reducing drag.
I decided to stick with what has been working well for me in the past. The rocket body will be made from a minimum of 6 layers of 6 oz. fiberglass cloth using Mr. Fiberglas epoxy to complete the composite. Dual deployment will be via a PIRM2 with drogue and main chutes both deployed through the upper body tube.
One change I do plan to make is the forward closure retainer. I've been using steel retaining pins on both the nozzle and forward closure, the problem is that it increases the diameter of the motor. So I'll use an internal snap ring to retain the forward closure, and possibly the nozzle too.
It's a little early to start a web page on this project, but I've already come up with something I want to share, so here we go...
This is a 4" PVC pipe section 5' long, it forms the mandrel for the body tube. I decided to try something different. I have been using wax paper to cover the mandrel but it is sometimes hard to remove the fiberglass tube when done, and the wax paper tends to stick to the inside of the fiberglass tube. Below the PVC pipe mandrel is a sheet of clear vinyl sheet, it's 8 mil thick and came on a roll 48" wide. I cut the vinyl to overlap about 4 inches. The PVC pipe and the vinyl were both covered with a light dusting of talcum powder.
Here the vinyl is wrapped around the mandrel. I used Scotch tape to hold the vinyl in place at each end, and one spot in the middle. The mandrel was then coated with epoxy, then 6 layers of fiberglass cloth were applied. After allowing to dry overnight...
The newly made fiberglass tube was slid off the mandrel. It was so easy I couldn't believe it! Now for the vinyl inside the tube. I worked from an end, and gently pulled the vinyl off the fiberglass. What a delight, the vinyl peeled right off! The inside surface of the fiberglass tube is perfect, it's just about a mirror surface inside. The fiberglass tube turned out great, and will be used as the lower body tube on the rocket. Finally, a body tube diameter large enough for me to get an arm in!
Notice the wood blocks behind the body tube, that's the start of a nose cone. It'll take four 1x6's glued together to make a block of wood big enough to turn into a nose cone for this rocket. depending on how the weight, center of gravity and center of pressure work out. I may leave this nose cone solid wood. I think I may need the extra weight up there anyway. Simulations show there is only a gain of less than 200' in maximum altitude between an 8 pound and a 12 pound rocket. I think I can gain more than that in altitude from reduced drag if I can make the fins smaller. (More nose weight makes the rocket more stable, so less fin area is required.)
Here is the final draft of the Prelude using AeroLab.
I did a little research on nose cone design, and I came to the conclusion that the Von Karman profile would best suit the Prelude. The Von Karman has good transonic and low end super sonic drag characteristics while maintaining good low speed aerodynamics. I used VCP by Gary A. Crowell, Sr. to design and print out the pattern you see above. The printed pattern was taped to a cardboard box, then the profile cut out with a razor knife.
Here is the nose cone just finished up on the lathe. It really is a lot larger than the 3.5" nose cones used on my last set of rockets. It weighs in at 2.8 pounds. I think I'll end up using this wood nose cone as the master for a mold like I did last time. I can't imagine needing 2.8 pounds of weight on the nose! I could make a composite nose cone that size about 6 ounces.
Here is the nose cone getting it's tip. I use body filler in a little cone made from vinyl taped on to form the tip. I went ahead and ordered more silicone mold compound. I'll use this wood nose to make a silicone mold, then make the rocket nose cone from fiberglass and epoxy like I did with my first series of fiberglass rockets.
Here's the nose cone with a color coat on it. The strange looking tin thing behind it will be what the silicone mold compound is poured into. The idea is to get a form that holds the nose cone with a minimum waste of silicone mold compound. I made a paper form, cut out the paper and transferred the shape to the tin. Then the tin was cut out, riveted and soldered together. I added some caulk to make sure it was well sealed. I'll still have to make a wood frame to hold everything, the nose cone needs to be securely held in place or it will float up in the silicone.
Work progresses in other areas as well. My new 3.5" aluminum tube arrived today, and I managed to cut a snap ring groove in the casing. At the moment, the casing is wrapped up in fiberglass that is curing, this tube will be the motor mount tube. The 3.5" snap rings are huge, so I ordered a new snap ring pliers today. I should have it tomorrow, so we'll see how motor assembly goes.
My new order of erythritol arrived today as well. I'll start casting grains tonight. It's within the realm of possibility I'll be testing the motor in a day or two. I'll perform the first test with a light load, and see how it goes from there.
The first test of the new SBS-1350 motor has successfully been completed. With the motor test going so well, I'm starting to get excited about the possibilities of the erythritol based propellant. A 3" diameter grain had a steady burn for over 6 seconds. I did a software simulation of the Prelude flying with the motor just tested, the software predicts a motor burnout at an altitude of around 3,500'. That would really be fun to see!
Moving back to the rocket construction.
Here is the stack of four fins. They were rough cut from 1/4" birch plywood, then tacked together with small nails and finish sanded to final dimensions. By sanding them as a stack, there is no doubt they will all be the same final shape and size.
Here the two slots have been cut out of the stack for the centering rings.
Cutting a good chisel shape on the leading and trailing edges is never easy. This may be my most brilliant, or my dumbest idea in some time. Now you kids at home, don't try this with Dads power planer or you'll likely end up with short stubby fingers. You get the idea from the picture, a power hand planer clamped in a vise. Then a 45 degree angle created by hot gluing two wood blocks and steel ruler to the deck. Honestly, it does take a sense of adventure to do this. See the inset in the lower left for an enlargement of the leading edges of the fins. Still not perfect, but not bad.
A full picture of my fin alignment set up.
Here's a close up of the business end.
I think I need to explain myself here. On the top picture, you can see a support above the motor mount tube. I got to thinking, if the motor mount tube wasn't perfectly cut, it wouldn't sit on the plywood and be square, it could tip just a little in one direction. It wouldn't do any good to have all the fins perfectly aligned to a crooked motor tube. So the support above the tube holds the tube perfectly perpendicular to the plywood base. To keep the bottom of the tube in place, I hot glued a short piece of the motor casing tube to the plywood. Of course, everything was drawn out on the plywood with drafting tools before I started.
The fins were all tacked in place with epoxy. Once set, the fins were fiberglassed to motor mount tube. I used a jig I had built to draw the fin slot grooves on the body tube. The grooves were cut out with a rotary tool and a cut off blade.
Here I am inserting the fin/motor tube into the lower body tube. This is by far the best alignment I have ever had. The fins have to be within a few thousandths of perfect alignment.
The body tube was then glued to the motor tube.
Now the fin to body tube fillets were applied. I thickened epoxy with some talc to bond and fill the small crack between the body tube and the fins. Notice the inset picture, I really didn't make your standard fillet, this joint is pretty well squared off. The tip to tip glassing of the fins should provide the needed support without adding a large drag inducing fillet.
Now begins probably the most time consuming aspect of the rocket construction. The fins are glassed tip to tip. I'm doing two layers at a time to save time and epoxy. As each set cures, I'll roll the body tube 90 degrees and do another pair.
Here is the bottom of what will be the thrust bulkhead. I have drilled it out to hold two 5/16" nuts and washers. More on why next.
Here the bottom 1/2" disk of the thrust bulkhead is epoxied to a 3/4" plywood disk. Now you see the reason for the two nuts and washers. They hold two 5/16" 6061 aluminum threaded rods. The threaded rods will extend all the way from the thrust bulkhead, into the upper body tube and through a 3/4" bulkhead, then through the electronics module. A couple of washers and nuts on top of the electronics module will tie the rocket lower and upper sections together.
In this picture the thrust bulkhead and coupler have been epoxied in place.
It seems like there are a thousand little things to epoxy. There you can see the lower body tube in back, the upper body tube with its bulkhead pinned and epoxied in place, the start of the electronics module to the right, and a 1/4" plywood board that has been covered with two layer of glass on each side.
Here is basic electronics module finished. The threaded rod from the lower body tube goes through the aluminum tubes on either side of the electronics mounting board. The aluminum tubes provide support for the electronics module when the nuts are tightened on the threaded rod to assemble the rocket.
When I sat down to design this rocket, I only knew one thing for sure, that was that I wanted no outside screws holding the rocket together, and no outside screws retaining the electronics module. I thought about many possible configurations, a single, larger threaded rod would have worked as well. But I didn't like the idea of a rod running through the middle of the electronics module. Nor did I like the idea of screwing the upper and lower halves of the rocket together. So I ended up with the two threaded rods. I debated using four, but in the end, two seemed like they should be adequate. The threaded rods for the most part only have tension loading on them, so they should be more than able to handle the load.
With the electronics module assembled, I was now able for the first time to assemble the rocket. I wanted to get an idea of the stability of the rocket. So I loaded the motor with 10.8 pounds of sand to simulate a propellant load. The rocket was assembled, I just hand tightened the nuts, and picked up the rocket by the upper body tube. It seems to be very strong, there was no flexing in any part of the rocket at all.
After running the new numbers from the rocket in Aerolab, I marked the center of pressure on the body tube. Then I laid the rocket on a 1" diameter wood dowel and rolled the rocket back and forth to find the center of gravity. With no actual electronics or batteries, no parachutes or harness and no nose cone, the center of gravity was about 5" ahead of the center of pressure. With the wood nose cone installed, the center of gravity was about 10" ahead of the center of pressure. That's a good sign, it looks like when I get the electronics and recovery system loaded, I'll be able to use a fiberglass nose cone and still have about 2 caliber's of stability in the rocket.
I've been working on the finish of the rocket, after nearly three tubes of spot putty, several coats of primer and many hours of sanding I think the body of the rocket is nearly ready for the finish coat of paint.
The nose cone has proven to be a problem. I had it ready to go, to the extent of using the wood nose cone as a master for a silicone mold. The first problem was the silicone mold compound itself, I had ordered the stiffest silicone compound the company sold, and they sent me the softest compound. I also calculated the volume of the mold compound, and it was less than I expected. I guess my measurement of 1 quart is different from their measurement of 1 quart. So I knew I'd be a little short on the mold compound, with the mold only reaching up about two inches past the shoulder of the nose.
After casting the mold compound, it was apparent the soft silicone I used was much more difficult to demold than the mold compound I had used on the 3.5" mold. It was also going to be way to flimsy, even with an outer hard shell, the mold would sag and collapse. So I decided to try another approach, to make a fiberglass mold from the wood nose cone.
I applied 6 coats of carnuba wax, and then several coats of a release agent. I had a tip from a friend that Aqua Net hair spray worked great as a release agent. When I applied the hair spray, it didn't look like it was going to work very well, in fact, I think it ate through the wax and soaked into the paint. I went ahead with coating the wood nose cone with fiberglass, about two layers. After the resin was set up, I tried removing the fiberglass. I made a razor slit down one side, and tried peeling up the glass coating. It was a real mess, the fiberglass (or the hair spray) had soaked into the paint. Of course I ruined the fiberglass in the process.
So I decided to just use the wood nose cone. Now, had I made that decision at the outset, I would have saved myself a lot of work (and money). Because now I would have to dig the wood out from inside of the nose cone, where I could have bored it out on the lathe earlier. The nose cone weighed 2.8 pounds at the start, and after much drilling and chiseling I reduced it to 750 grams or about 1.6 pounds. The 1.6 pounds is a nice compromise, the nose is still very strong and gives me some extra weight up there for added stability.
I decided to try not using a piston in this rocket. With the increased diameter of this rocket, I don't see why I can't get by using a deployment bag on the main chute, and leave the drogue chute loose above the main. Providing I use enough length on the shock cord between the drogue and the nose cone, the deployment charge gases should quickly dissipate out the open upper body tube, allowing the drogue chute to be pulled out by the ejected nose cone.
So I did a couple of quick deployment tests. I figured now was the time to do it before I had a finish coat on the rocket. I tried 3.2 grams in the first test, the nose cone didn't quite extend the shock cord all the way. For the second test I used 4.2 grams, that seemed about perfect, the nose cone ejected to the full length of the shock cord without a lot of snap back when it reached the end.
I've been working with the FAA to get a launch wavier for the Prelude flight, with a possible altitude of over 10,000' I was going to need a new launch site. It's been a little frustrating, as the first 3 locations I came up with were not approved. Quite frankly, I was running out of people I knew that had land in appropriate areas for a launch.
Then I remembered a friend of mine from my scuba days that had moved to a farm outside Sioux City. After talking with my FAA contact again, he said this area was in the Sioux City airports approach control area, and permission would be needed from the Sioux City airport as well. The FAA guy not only gave me the name and number to call in Sioux City, but he made contact with the airport before I did, to fill them in on what I was needing. As it turns out, the Sioux City airport had no problems with the area, since this location was really on the fringe of their airspace anyway. I must say, dealing with the FAA and the airport has been a pleasant experience. I'm sure these people have a lot better things to do than to help me get a rocket in the air, but they have really been helpful.
Here is a picture from the road of the launch area.
The way it sounds, I'll be launching from the little valley you see in the middle back of the photo. This really is a very good place for a launch. As you can see, this is rough terrain with few crops to worry about. The land that is owned by my friends family is 1/2 mile wide by 1 mile in length. So the size of the area is excellent. The only bad part is a few trees, a couple of water hazards and the hills. The best part is that it should be possible to do summer launches here. The lack of roads and people/houses are another positive.
I've got the Prelude painted. It's not fancy, I wish I had the sense of style to put some sort of great color scheme together. Heck, I have a hard enough time coming up with a name for a rocket. But a fancy color scheme doesn't make the rocket fly any better, so I opted to paint it a nice bright red and yellow. That should make finding it a little easier. While the rocket doesn't look like million bucks, it is by far the best finish I've had on a fiberglass rocket.
Now that I brought up finding the rocket. I ordered a transmitter to help locate the rocket. I'll use both the FRS and this new transmitter. The new transmitter is very tiny, I'll locate it in the nose cone. The battery is supposed to last over a month, that should be plenty of time... I didn't order the receiver for the transmitter, I'm going to see how my scanner works with it first. But I'd bet money I'm going to be ordering the receiver too. I just picked up a new GPS too, my all the toys I think I need.
Here's my fleet, from left to right: Comso 2, Prelude, Rebuilt Fiberglass Rocket and the Dual Outboard Rocket.
Here's another capture from AeroLab. This graphic shows the estimated drag coefficient of the rocket from 0 to Mach 3.
In the past my Cd has been fairly high, in the .7 to .75 range on the flights approaching Mach .8 and greater. Of course a lot of the construction techniques I used in this rocket are intended to lower the drag on the rocket. The only way I'll find out is to fly the rocket and see how high it goes. If I had an average Cd of .5, the predicted maximum altitude of the rocket is over 12,000'. I'm guessing I'll come in with a Cd of about .6, giving me an expected apogee of about 11,388'. That is assuming a full propellant load of 10.8 pounds, which I likely will not have.