Last year for Balls, I did a composite-cased 6" motor that wound up working pretty well. One of the advantages of this design as opposed to building a rocket around an aluminum case is the weight saved when the rocket and motor are one, avoiding the redundancy of having an airframe wall and a case wall.
Of course, in the ~5 years since I've been building min dia rockets, I've tended to follow the traditional wisdom of strength, strength, strength to avoid a shred, and it's served me very well. My standard fin can procedure of 1/4" carbon + carbon/kevlar layup x 3 layers went undefeated against some pretty hot motors. It's easy to make something that holds together, but after doing the composite-case project, it got me to thinking -- time to push the limits a little bit with fin cans, walking the line between strong enough and unnecessarily strong and heavy. Much like the determination of how thick or thin to make the composite case -- finding the sweet spot of "just strong enough" such that optimal weight reduction is recognized.
With this in mind, and after reading of Manny's failure last month and the flame war over Ari's L3 shred, I decided to get going on this objective to try and shed some light on the subject. All the "experts" coming out of the woodwork and accusing the guy of recklessness, etc., when one of the goals of the project was to try and fly an M in a stock Magnum (which has been done multiple times with success, BTW), I found frustrating. It doesn't take any kind of knowledge or foresight to beef the $h!t out of something and have it hold together with a big motor, but presumably part of the point of the big motor is to gain performance, and thus the overbuilding largely defeats the purpose. What takes knowledge and foresight is knowing (or discovering through scientific method) what you can and can't get away with, in order to optimize performance.
So, last week I built a 3" min dia with parts I had lying around, with the goal of making it as light as possible. Fins were 1/8" G10, using my usual one-tube build method, electronics in the nose cone, dual deploy with a line cutter made of a spent shell casing and a piston to cut the zip tie around the chute burrito, saving length and drag. In theory, fins are stressed most when misaligned, so I paid extra attention to getting the fins perfectly aligned, and put on regular-sized fillets with regular epoxy. I had toyed with the idea of putting on one layer of cloth, but as I tend to do my most productive building in the evenings (or all-nighters) leading up to a launch, and started that one two days before URRF last week, the layup never happened. This, however, was for the better, so as not to tamper with scientific method. The rocket is long enough for my 3" 10,000ns case. I made a fast-burning 6000ns blue motor to fly it at URRF, imitating the speed of a slower 10,000ns motor while still staying under the URRF waiver. The hot 6000 actually had it going close to Mach 2 in thicker air, so theoretically it was more taxing on the fins than would be a bigger, slower burning motor which has the rocket moving fastest up in the thinner air. Anyway, the flight was arrow straight and it worked perfect, the fins didn't budge, somewhat vindicating the theory. I flew my old 4" (heavy) min dia the same weekend on a motor almost twice the size, but got the same altitude, further underscoring the advantages of minimalistic building. The rocket, ready to fly except for motor, weighed something like 3 pounds.
I'm going to go to Kansas for Sweaty Balls weekend after next and try and take advantage of the great waiver they offer. One flight will be this rocket on the 10,000, but to justify the drive, I wanted another flight. I have a 17,500ns 98mm case, which I have flown a couple times in my old 98mm min dia at Black Rock, getting ~32k each time. I want to build a 4" min dia with this same principle, and fly it on the same motor, and see what kind of a performance boost I get. Sure, one layup would be cheap insurance and wouldn't add that much weight, but the test of the concept went great, so I'm going to give it a shot in pure form. I've got some 4" fiberglass tubes lying around and some scrap G10. I've got a little less than 10 days until I have to leave for the launch, and my girlfriend and I are going to be on vacation for five of those, which leaves just enough time to put this thing together and make the two motors. I'll do something of a build thread here, but there's not much to see, just gluing fins on a tube. My gut is they both work just fine. We'll find out.
Of course, in the ~5 years since I've been building min dia rockets, I've tended to follow the traditional wisdom of strength, strength, strength to avoid a shred, and it's served me very well. My standard fin can procedure of 1/4" carbon + carbon/kevlar layup x 3 layers went undefeated against some pretty hot motors. It's easy to make something that holds together, but after doing the composite-case project, it got me to thinking -- time to push the limits a little bit with fin cans, walking the line between strong enough and unnecessarily strong and heavy. Much like the determination of how thick or thin to make the composite case -- finding the sweet spot of "just strong enough" such that optimal weight reduction is recognized.
With this in mind, and after reading of Manny's failure last month and the flame war over Ari's L3 shred, I decided to get going on this objective to try and shed some light on the subject. All the "experts" coming out of the woodwork and accusing the guy of recklessness, etc., when one of the goals of the project was to try and fly an M in a stock Magnum (which has been done multiple times with success, BTW), I found frustrating. It doesn't take any kind of knowledge or foresight to beef the $h!t out of something and have it hold together with a big motor, but presumably part of the point of the big motor is to gain performance, and thus the overbuilding largely defeats the purpose. What takes knowledge and foresight is knowing (or discovering through scientific method) what you can and can't get away with, in order to optimize performance.
So, last week I built a 3" min dia with parts I had lying around, with the goal of making it as light as possible. Fins were 1/8" G10, using my usual one-tube build method, electronics in the nose cone, dual deploy with a line cutter made of a spent shell casing and a piston to cut the zip tie around the chute burrito, saving length and drag. In theory, fins are stressed most when misaligned, so I paid extra attention to getting the fins perfectly aligned, and put on regular-sized fillets with regular epoxy. I had toyed with the idea of putting on one layer of cloth, but as I tend to do my most productive building in the evenings (or all-nighters) leading up to a launch, and started that one two days before URRF last week, the layup never happened. This, however, was for the better, so as not to tamper with scientific method. The rocket is long enough for my 3" 10,000ns case. I made a fast-burning 6000ns blue motor to fly it at URRF, imitating the speed of a slower 10,000ns motor while still staying under the URRF waiver. The hot 6000 actually had it going close to Mach 2 in thicker air, so theoretically it was more taxing on the fins than would be a bigger, slower burning motor which has the rocket moving fastest up in the thinner air. Anyway, the flight was arrow straight and it worked perfect, the fins didn't budge, somewhat vindicating the theory. I flew my old 4" (heavy) min dia the same weekend on a motor almost twice the size, but got the same altitude, further underscoring the advantages of minimalistic building. The rocket, ready to fly except for motor, weighed something like 3 pounds.
I'm going to go to Kansas for Sweaty Balls weekend after next and try and take advantage of the great waiver they offer. One flight will be this rocket on the 10,000, but to justify the drive, I wanted another flight. I have a 17,500ns 98mm case, which I have flown a couple times in my old 98mm min dia at Black Rock, getting ~32k each time. I want to build a 4" min dia with this same principle, and fly it on the same motor, and see what kind of a performance boost I get. Sure, one layup would be cheap insurance and wouldn't add that much weight, but the test of the concept went great, so I'm going to give it a shot in pure form. I've got some 4" fiberglass tubes lying around and some scrap G10. I've got a little less than 10 days until I have to leave for the launch, and my girlfriend and I are going to be on vacation for five of those, which leaves just enough time to put this thing together and make the two motors. I'll do something of a build thread here, but there's not much to see, just gluing fins on a tube. My gut is they both work just fine. We'll find out.
