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neil_w

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Greebles, @neil_w , Greebles.
Nope, my fleet is 100% greeble-free. Not that I have anything against them, just not my thing (or rather: I don't really know how to do them). My decorative crap of choice is pointlessly complex fin/tube/ring arrangements.
 

BABAR

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Nope, my fleet is 100% greeble-free. Not that I have anything against them, just not my thing (or rather: I don't really know how to do them). My decorative crap of choice is pointlessly complex fin/tube/ring arrangements.
One man's crap is another man's gold.
 

Mike Haberer

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Note, the total area of my fins and ring is about 17 square inches. That is about the equivalent area of 4 fins, 2" x 2.13".
The square inches of the fins is relevant to calculating the center of pressure, but not the drag on the rocket. The drag is based on the frontal area going into the airstream. The includes the nose cone and the leading edges of the fins. You have six fins and the entire ring as leading edges, several times the frontal area of a simple 3 fin design.

I have a very draggy rocket where the it flew 213' on a C5, 246' on a C12 and 317' on a D16, all measured with a Fightsketch Mini altimeter. Total impulse did not make a big difference in altitude, drag won the day...
 

Dotini

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The square inches of the fins is relevant to calculating the center of pressure, but not the drag on the rocket. The drag is based on the frontal area going into the airstream. The includes the nose cone and the leading edges of the fins. You have six fins and the entire ring as leading edges, several times the frontal area of a simple 3 fin design.

I have a very draggy rocket where the it flew 213' on a C5, 246' on a C12 and 317' on a D16, all measured with a Fightsketch Mini altimeter. Total impulse did not make a big difference in altitude, drag won the day...
Please don't be too quick on your assumptions, Mike. My ring is only 0.020" in thickness. If my calculations are correct, the ring is 0.188 square inches in frontal area. My fat but rounded edge 1/8" fins are 0.83" in length, so total frontal area for them is 0.623 square inches, giving a total ring + fin frontal area of 0.81 square inches. If that were distributed over 3 1/8" thickness fins, that would require fins of 2.16". Please check my calculations.

But guess what? I possess the technology and the materials, if not the need, to now make my fins .020" in thickness. Just for fun, I will do this on my next model.
 
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BABAR

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Please don't be too quick on your assumptions, Mike. My ring is only 0.020" in thickness. If my calculations are correct, the ring is 0.188 square inches in frontal area. My fat but rounded edge 1/8" fins are 0.83" in length, so total frontal area for them is 0.623 square inches, giving a total ring + fin frontal area of 0.81 square inches. If that were distributed over 3 1/8" thickness fins, that would require fins of 2.16". Please check my calculations.

But guess what? I possess the technology and the materials, if not the need, to now make my fins .020" in thickness. Just for fun, I will do this on my next model.
I am far from an expert, but it is a bit more complicated than just the surface area of the fins and the thickness of the fins and ring.

don’t get me wrong, thinner is generally better, but I don’t think the drag reduction between thin and REALLY thin fins (or rings) is directly proportional to the change of thickness.

your design is elegant (and cool), but may have another disadvantage. The 6 fins in combination with the ring divides the air stream into “channels” each bordered by a segment of the ring, two fins, and a segment of the body tube. This may also create an unintended “tube fin” effect. Tube fins definitely work, but the are known to be generally less efficient than regular fins (reference Larry Brand’s page for detail, especially


my biggest concerns for your design are two fold, both based on my experience with BackSliders AND Horizontal Spin.

first and most definite, pure BackSliders when they work come in at a mild to moderately steep glide. The effect on flight duration is not the issue (nobody expects these to be competition duration models and you and I both are fans of small field recoveries, we don’t WANT rockets if any sort to have a long “hang time”, we want them back safe, sound, and NEARBY!). What IS an issue with these is that in the process of descent hey develop a significant linear forward trajectory, TWO of my stable of my THREE SUCCESSFUL BackSliders (hey, I only built 4;)) have been arrow straight on descent, I would estimate their lateral travel was about equal to their apogee altitude, so they were moving laterally (literally speaking “Backward “) as fast as they were descending. This is gonna literally impact on you fin can, and in this case the “Ring” won’t help as much as it does with HORIZONTAL spin, although it may help simply because a ring with 6 fins will certainly be tougher Than one or two fins alone. This IS a solvable problem, one solution being Asymmetric fin attachment which I believe I have proved works well, at the cost of at lest a minimally squirrelly but completely same boost (which if you are like me, is no cost at all:bravo:!)

the SECOND one, which I will post in more detail separately, is that unfortunately I think PURE UNASSISTED BackSlide (no streamer or other physical rocket configuration change at apogee, just a forward eccentric port ejection charge which transiently “stalls” controlled Barrowman flight Trajectory) is not 100% reliable. While nothing else is 100% either, my experience has been 80%, and the 20% failures have been I believe random. Meaning I am not convinced it is truly fixable. I hope I am wrong. I will explain in a separate post. As opposed to reliability of traditional methods of chutes and streamers, which I expect experienced rocketeers would put at may 95%, and of the 5% failure rate I’d guess experienced HONEST rocketeers would attribute 95% to operator error (poor packing) and most of the rest to equipment failure (broken shock cord, bad ejection charge, electronic Gremlin) with a very small percentage (if any) being truly random. I think @Steve Shannon would agree, in the vast majority of chute recovery failures, the Flyer messed up. For parachute and presumably streamer rockets, ballistic recoveries should be close to a “never” event.

so earlier I may have “dissed” your chute or streamer idea, now I think it is a good idea, first with a full chute to test stability of the rocket on boost (although your design easily passes my “mindsim” analysis, it’ll fly straight and true as long as you have a decent thrust to weight ratio and appropriate delay.). Second, I think a small streamer may indeed be required to get to 100% (or at least practically 100%, which is where a non experimental rocket must be. In other words, I can’t recommend to friends or a kit manufacturer to build a design that lawn darts 80% of the time, the Estes Cosmos Mariner, Estes MIRV, and the Holverson foamy Swinger aside:angiefavorite:.)

on the good side, the exception of one of my four models which was unstable on boost (for the record, I launch these is isolation, start with small motors, and nobody is within 300 yards), all the other FIRST flights were good.

so looking forward to your flight report!
 
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Dotini

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I am far from an expert, but it is a bit more complicated than just the surface area of the fins and the thickness of the fins and ring.

don’t get me wrong, thinner is generally better, but I don’t think the drag reduction between thin and REALLY thin fins (or rings) is directly proportional to the change of thickness.

your design is elegant (and cool), but may have another disadvantage. The 6 fins in combination with the ring divides the air stream into “channels” each bordered by a segment of the ring, two fins, and a segment of the body tube. This may also create an unintended “tube fin” effect. Tube fins definitely work, but the are known to be generally less efficient than regular fins (reference Larry Brand’s page for detail, especially


my biggest concerns for your design are two fold, both based on my experience with BackSliders AND Horizontal Spin.

first and most definite, pure BackSliders when they work come in at a mild to moderately steep glide. The effect on flight duration is not the issue (nobody expects these to be competition duration models and you and I both are fans of small field recoveries, we don’t WANT rockets if any sort to have a long “hang time”, we want them back safe, sound, and NEARBY!). What IS an issue with these is that in the process of descent hey develop a significant linear forward trajectory, TWO of my stable of my THREE SUCCESSFUL BackSliders (hey, I only built 4;)) have been arrow straight on descent, I would estimate their lateral travel was about equal to their apogee altitude, so they were moving laterally (literally speaking “Backward “) as fast as they were descending. This is gonna literally impact on you fin can, and in this case the “Ring” won’t help as much as it does with HORIZONTAL spin, although it may help simply because a ring with 6 fins will certainly be tougher Than one or two fins alone. This IS a solvable problem, one solution being Asymmetric fin attachment which I believe I have proved works well, at the cost of at lest a minimally squirrelly but completely same boost (which if you are like me, is no cost at all:bravo:!)

the SECOND one, which I will post in more detail separately, is that unfortunately I think PURE UNASSISTED BackSlide (no streamer or other physical rocket configuration change at apogee, just a forward eccentric port ejection charge which transiently “stalls” controlled Barrowman flight Trajectory) is not 100% reliable. While nothing else is 100% either, my experience has been 80%, and the 20% failures have been I believe random. Meaning I am not convinced it is truly fixable. I hope I am wrong. I will explain in a separate post. As opposed to reliability of traditional methods of chutes and streamers, which I expect experienced rocketeers would put at may 95%, and of the 5% failure rate I’d guess experienced HONEST rocketeers would attribute 95% to operator error (poor packing) and most of the rest to equipment failure (broken shock cord, bad ejection charge, electronic Gremlin) with a very small percentage (if any) being truly random. I think @Steve Shannon would agree, in the vast majority of chute recovery failures, the Flyer messed up. For parachute and presumably streamer rockets, ballistic recoveries should be close to a “never” event.

so earlier I may have “dissed” your chute or streamer idea, now I think it is a good idea, first with a full chute to test stability of the rocket on boost (although your design easily passes my “mindsim” analysis, it’ll fly straight and true as long as you have a decent thrust to weight ratio and appropriate delay.). Second, I think a small streamer may indeed be required to get to 100% (or at least practically 100%, which is where a non experimental rocket must be. In other words, I can’t recommend to friends or a kit manufacturer to build a design that lawn darts 80% of the time (the Estes Cosmos Mariner and the Holverson foamy Swinger aside:angiefavorite:.)

on the good side, the exception of one of my four models which was unstable on boost (for the record, I launch these is isolation, start with small motors, and nobody is within 300 yards), all the other FIRST flights were good.

so looking forward to your flight report!
Thanks for your extensive post! Altho ready to launch, my flight and report will have to wait, as my able assistant is vacationing in Montana until nearly Father's Day.
 
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