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Some preliminary design notes:
I seriously considered using graphite for the nozzle. Graphite would be cheaper than my usual choice of steel, lighter and a lot faster to machine. But graphite is inherently weaker, and I was a little concerned about it's ability to withstand the chamber pressure loading from such a large diameter motor. It's pretty much a given that steel will hold up to repeated firings, I'm not so sure the graphite would. I also don't like using up a lot of casing length for a graphite nozzle. So in the end I decided to stick with steel.
For the casing I again stayed with what I've been using, 6061 T-6 aluminum. The casing wall is .125" thick, giving me maximum chamber pressure of about 1,875 psi.
The forward bulkhead will also be 6061 T-6 aluminum, 1" thick. Failure pressure for the bulkhead is calculated at over 4,000 psi.
Both the bulkhead and the nozzle will be retained by (18) 5/16" hardened steel pins or bolts, with a maximum chamber pressure limit of about 3,200 psi.
The initial design calls for (6) grains, 8" long and 5" in diameter with a core of 1.75". With a nozzle throat diameter of 1.2" the Kn should run 416 initial, 472 maximum and 421 ending.
Casing weight: 13.56 pounds
Forward Bulkhead: 2.46 pounds
Nozzle: 7.0 pounds
The miscellaneous parts for the motor have been coming in this week. First was the steel for the nozzle. I ordered the steel from Metal Express, it arrived the next day after I ordered, you have to love that service. So I decided to get started on the nozzle right away, I knew turning 60 pounds of steel into a nozzle would take some time. The steel rod is 5.75" diameter and 8" long. It wasn't a problem chucking it up in my lathe, but the problem came when I tried to turn the convergent taper. I had bored the rod to 3/4" with a drill bit in the lathe, then started turning the 60 degree convergent cone, and I couldn't prevent the lathe from chattering. I used numerous tools, sharpening them to different angles and nothing helped. I used my boring bar with carbide inserts, still no luck. As I moved the tool closer to the inside portion of the cone, the lathe smoothed out. The only thing I can think of is that I'm some 8" from the chuck, and perhaps I have some bearing run out in the lathe itself, that run out being amplified by the distance from the chuck. I think if I had a steady rest large enough, that would help, but my steady rest is only 5" diameter. So it was plan "B".
Plan "B" calls for a graphite nozzle, while not completely giving up on the steel, I did order some 6" diameter graphite rod from The Graphite Store. Fortunately, I had designed the motor with a fair amount of extra casing length, and I should be able to get the full propellant load in the motor even using a 6" long graphite nozzle.
Next to arrive was the Delrin I use for coring rods, ordered from McMaster-Car, great service as always. Another nice find on the web was Yazoo Mills, Inc. They have a great selection of cardboard tubes, even the big ones like I needed. What I really like is they package tubes in boxes UPS will ship, not only does that keep shipping costs down, but it speeds up delivery and I don't have to order dozens of tubes. Shipping is included in the price, another aspect of this company I like.
The only thing I'm waiting on now is the aluminum casing from Online Metals, they ship quick enough, but it's four or five days from the West Coast, so the casing should be here early next week.
With that in mind, I decided it was time to start casting grains for the motor. If I run into any problems with casting I'll find out now, giving me extra time to work it out. I cut one of my cardboard tubes to 8.25", then sanded the edges smooth. I decided to use a plastic disk, like I had used on my 3.5" motors to seal the bottom of the casting tube. A second plastic disk will center the coring rod in the tube, and if I want I could push it slightly into the propellant to smooth the surface some.
Here's the casting set up. Pretty simple really. The cardboard tube is .175" thick and weighs 270.2 grams per 8.25" length.
At a density of .0615 pounds per cubic inch, I'll need 8.47 pounds of propellant per grain. That's a little much for a Presto Multi-Cooker. So I'll use three of them, just so happens that's how many I have. I think I could have just used two, but I had three. So I mixed up three 1330 gram batches of 65% KNO3 and 35% erythritol, one for each cook pot. As each pot melted, I degassed the propellant for about 10 minutes each, pulling a vacuum three times on each pot.
The pouring was easy, a nice big target to pour into! Each pot was given a good stir before pouring into the casting tube.
Here's the grain after casting.
After 3 hours I took a look at the grain, it was still very warm, and the Delrin coring rod was still firmly in place. So I waited until morning, of course the grain was now cool, I gave the coring rod a twist and it slid out with almost no force at all. Then I lifted the plastic caps off the top and bottom of the grain. The bottom of the grain appears to have shrunk just a little, pulling the plastic cap up with it, or perhaps the plastic cap was out of position to start with. I'm not sure, but I have a better idea.
Here is the grain after removing the core and ends caps. To the right of the grain is a new casting base (the better idea) I made.
I thought about a better base for the casting stand, I considered wood, and plastic. But wood would have to be covered to prevent sticking, and I didn't have plastic thick enough. So I decided on aluminum, some 6061 would sure turn nice on the lathe, but I didn't have any. So I dusted off the old foundry and fired it up. It didn't take long, from the start of setting it up to pouring the mold I was done in an hour. I had a nice 6 inch diameter by 1.5" thick slug of cast aluminum. So I chucked it up in my big South Bend lathe, faced it, turned a step for the casting tube to set into, then bored out the center to hold the coring rod. I'm not sure if the propellant will stick to the aluminum or not. I did manage to get it pretty smooth considering it was cast aluminum.
Here is the second grain I cast using the new aluminum casting stand base. This new base worked even better than I could have hoped for, a virtually perfect grain.
I've just cast the fourth grain, and I wanted to get some of thoughts down before I get too far into this.
For the fourth grain I tried using two casting pots rather than three, and not degassing with the vacuum pump. I wasn't happy with the density of the finished grain, so I cut it open and remelted it. I did learn a couple of things by cutting the grain open. First, the propellant adheres the the cardboard casting tube very tenaciously. Without reheating it is impossible to separate the two once cured. I also learned the propellant isn't shrinking away from the casting tube at all, as I found no indications of disbonding.
The Delrin coring rods I'm using continue to perform flawlessly. The grain needs to be completely cooled, then the rod falls out. The rod is 1.75" diameter, and swells to 1.77" when the melted propellant is poured into the casting tube. When the propellant and Delrin rod cool, the rod returns to it's original diameter of 1.75", leaving the propellant core at 1.77" and easy removal of the rod.
I believe the KNO3 in the propellant is settling out to a degree in the casting tubes. The KNO3 is denser than the erythritol, so that does seem to make sense. I've noticed at the top of the grains after a pour, the very top .1" or seems very clear, then just below the surface you can see the whiter KNO3 particles. Grinding the KNO3 to a powder may help, but I'm not entirely convinced of that either. I think what may help, and I've suspected this for a while, is that I'm running the propellant a little fuel rich at a 65/35 ratio. I'm thinking 70/30 may be more appropriate. I'm not going to make any changes now, but it is certainly something I think I need to run some tests on in the future. I also would like to cast a few long grains, and perform burn rate tests from sections cut from different positions along the grain length.
|Grain Number||TubeLength||Tube Weight||Total Grain Weight|
|Total||51"||1668gr./ 3.677 lb.||53.8 Pounds|
Propellant Weight: 50.123 pounds
Average Propellant Grain Length: 7.9"
Propellant Diameter: 5"
Core Diameter: 1.77"
Density: 0.0616 pounds per cubic inch
Kn: 382, 430, 382
Assembled Motor Weight: 79.6 pounds
Here's the start of the graphite nozzle. The convergent cone has been cut to 45 degree half angle, and the initial bore has been made to 3/4". Two o-rings grooves are cut as well.
Here we are looking into the convergent end, you can see the throat has been opened up to 1.25" now, with the 3/4" initial bore remaining in the divergent cone area.
Here is the finished graphite nozzle with a 1" thick 6061 T-6 aluminum retainer. The retainer started out as a solid disk like the one to right, which will be the forward bulkhead.
A couple of things about the nozzle. To keep it shorter, I used a 45 degree half angle on the convergent cone rather than a 30 degree half angle I normally use. The actual angle of the convergent cone has little effect on performance, so I opted for the shorter nozzle vs. narrower angle. The divergent exit diameter is 2.65", which may seem small, but running at a chamber pressure of around 500 psi that's optimal.
The retainer is aluminum, I had wanted to go with 1/2" steel, but the steel plate I had ordered hasn't arrived. I do wonder how fast the graphite will transfer heat to the aluminum, if I have a 9 second burn, I may weaken the aluminum too much. I think I'll make a fiber gasket to act as an insulator layer between the nozzle and the retainer just to be on the safe side.
The surface area of the step on the nozzle that holds the retaining ring is 8.963 square inches, at a chamber pressure of 1,000 psi the load is about 2,900 pounds per square inch. Both the graphite and the aluminum should easily handle the loading. A greater concern is usually the divergent cone area, if a graphite nozzle is going to break it seems that's the area where it happens. That's why I left that area fairly thick, at it's thinnest right at the exit the graphite is still .88" thick.
The graphite nozzle weighs 6 pounds, and the retainer another 1 pound, making the nozzle assembly exactly 7 pounds. Graphite is a love/hate kind of a material. I spent almost exactly 3 hours from start to finish on the graphite nozzle. It's amazing how fast you can turn it down, and sand it to a nice smooth finish. But what a mess, my poor shop vac is about choked with graphite dust, not to mention me.
I decide to go with .25" retaining bolts in the nozzle and forward closure. I'll use grade 5 bolts with an UTS of 120,000 psi. I didn't see the sense in going with the larger 5/16" bolts, the casing has a burst pressure in the 1,875 psi range, 18 of the .25" bolts will keep the parts retained at chamber pressures over 4,000 psi.
Here's a picture that perhaps gives you an idea of the ludicrous size of this motor. The Prelude rocket to the left, the SBS-6250 grains in front, in the middle left is the SBS-6250 nozzle, behind the nozzle is the "K" class motor used in the Comso 2, middle right is the SBS-6250, and far right is the Cosmo 2 rocket.
The motor is now complete, I finished the forward bulkhead this morning (Oct. 14, 2005). I tapped the forward bulkhead for a transducer port. I decided to just go with a pressure measurement from this test rather than thrust. I do have a 1,000 pound load cell, but I don't have a stand set up for it yet. I'm planning on digging a hole in the ground for testing the motor. That seems a bit safer anyway, and a lot easier to get a pressure line from the bottom of the hole than to bury a load cell.
I was a little concerned about the forward bulkhead sealing. The motor casing tube was .020" out of round at the forward bulkhead end. I was concerned the o-rings would get cut when they went past the bad spot on the casing. As an extra measure of redundancy, I applied silicone RTV to the inside of the casing after the bolt holes. The RTV would get pushed against the bulkhead and form another seal between the bulkhead and the casing as it was inserted.
As it turns out, when I installed the bulkhead I didn't notice the out of round spot at all. The bulkhead went in nice and smooth. Inspecting the bulkhead from inside the casing, it appears the RTV did a good job too. I could see a nice little bead of RTV around the joint. The RTV really is good stuff. I have used it in the past to seal small motors all by itself. In the 3.5" diameter motors I used it to glue the EPDM rubber to the bulkhead, and while the rubber has in some cases ablated away, the RTV is always completely intact. It seems to withstand the temperatures of a sugar motor with no adverse effects.
Here is the forward closure. I added a short length of steel pipe to extend into the casing four inches or so. The idea is to try to prevent the port from getting plugged by combustion residue. I also drilled three holes in the side of the tube in case the opening at the top gets plugged.
The top two grains are seen here, I drilled a hole in the spacer area above the propellant in each grain, you can see a hole above the arrow in grain 2. I still get nervous about the grains cracking from internal pressure in the core. These holes are intended to allow gases to flow behind the grains to prevent core over pressure.
Well, it's done. The motor is assembled and ready to test. Before assembly, I painted my igniter mix on the cores and ends of the upper three grains, and the cores of the lower three grains. I used about 1.5 wraps of EPDM rubber to insulate the casing wall. I had plenty of free diameter, so I let the EPDM overlap.
The nozzle was snug fit, I used the RTV again on the casing wall, the same as I did on the forward closure. I had to use a rubber mallet to get the nozzle in, I cringed at every blow. But you know what? I recall last summer the guys at the IEAS launch were using a sledge hammer to get the nozzle on their "O" motor. So I guess a wimpy little rubber mallet wasn't so bad.
I've been loading things up in my van, getting ready for the test tomorrow. I ran a test of my pressure transducer, and calibrated it to the computer in my van. I made up two igniters, 6' long. I never would have imagined I would have needed a six foot long igniter...
I've been thinking about the "what if's". I guess my biggest concern is heat saturation. The motor should burn at least 8 seconds, perhaps even 10 seconds. The graphite nozzle may well melt the casing, leading to a spectacular nozzle blow out. That wasn't as great a concern when I was planning on using a steel nozzle, steel would have taken more time to absorb and transfer the heat. The o-rings at either end may melt and cause blow by, I'm hoping the RTV helps in that area. I'm not as worried about chamber pressure failure, although this motor has a significantly lower safety margin than most of my motors do.
T minus 14 hours and counting. Look for the results tomorrow on Static Test 109.