Batman, back to some of your original questions.... Here's my thought. Laying a bit of glass on the airframe does many things. To wit, fiberglassing:

- Fills the spirals, which is critical
- Gives you a wonderful surface to wet sand to perfection
- Adds mass. You *will* be under optimal mass, so you may as well add it in this fashion
- Strengthens the airframe (Hey, it can't hurt!)

Speaking for myself, the most influential / inspirational rocket I ever built was a PML Nimbus. I say this because it taught me how to surface mount fins, go minimum diameter, and learn dual deploy (all at the same time). I've not built a Cirrus dart, but the similarities are surely there, only on a smaller scale. Virtually every rocket I've built in the last 5 years, and certainly the birds that I've set records with, have more in common than you'd think with the PML Nimbus.

Incidentally, my Nimbus flew three times before I core sampled it. That said, I learned essentially all I know about rocketry from that kit. The dual deploy that they use isn't perfect, but it got me thinkin' and I came up with my own system there.

Finally, I use simple 30 minute epoxy on my fillets. Always have. Even my minimum diameter "M" bird uses simple 30 minute epoxy, with glass laid tip-2-tip over that. My J570 rocket also used 30 minute epoxy for the fillets.

JW

As far as resonance goes, I have to disagree with you about cf vs fg. The nice thing about CF is that it is FAR stiffer than FG, so the resonant frequency of the resulting plate goes way up. Although it will still resonate, it will do so at a much higher frequency, which in many cases means that it would flutter only at a much higher speed. Also, flutter can vary a lot depending on the fin shape itself - something like a clipped delta will be far more resistant than what the Cirrus has, for example, though the Cirrus' fins should be adequate for up through a J350 stock. Honestly, there are a ton of variables, and many ways to prevent flutter - use what you are most comfortable with. If you've used something and it's worked before, by all means use it again.

When you've got an engineering degree and can back up your contention on the shape of fins with facts, figures and money... I'm there... until then, I'll go with my anecdotal experience and what I've seen in rockets like JW and others have built. You're one helluva smart kid Chris... but you don't have the experience yet. Don't take this as an insult, I'm hella impressed with your Duece and some of the other things you've done, but you still aren't legally cert'd and you definitely need to cool you jets once in a while even given how smart you are.

Warren

I'm not saying anything you commented on is wrong. I have no experience with your tapering method, so I can't comment on whether or not it works, and I will definitely have to try it sometime. If anything in that came across as angry or insulting, it was purely unintentional, and I apologize.

I will defend my statement about stiffness being good at eliminating resonance by increasing the resonant frequency though. That is not just based on my experience, but also talks with my mom, who did many years of work with resonances (not in rockets, but other applications) out of college. The stiffness of the CF is exactly why it's a good choice to eliminate flutter in a fin, and this is based on some relatively simple engineering. Now, whether it's better at eliminating flutter than any other method I can't say. As I said, there are many ways of preventing fins from fluttering, and I've only tried a couple. I know there are many more out there that work perfectly that I've never tried, but I can't really comment on those.

As far as the shape itself goes, it's pretty easy to see that a clipped delta will be far more flutter resistant, especially against torsional flutter, than many other fin designs, simply by the geometry of the fin and the areas of the fin that would be stressed during that kind of flutter. The clipped delta is a fairly well known and commonly used design for mach plus flights because of this. I don't know if it's the optimum design, but it is certainly a usable one, and far superior to many other designs if the only purpose is to eliminate flutter. As you mentioned, I can't do all of the equations to come up with a perfectly optimized fin shape, and I never claimed to be able to. I just know of one possible way to get a rocket to survive at least up through mach 2 on a 38mm minimum diameter rocket (though probably not the best way).

Stiffness is good... no question (that almost sounds lewd) Ask Dan Stroud about the tapering method, that's where I learned it. It is KEY to high mach flight survival no matter what the material. The change in thickness from root to tip is the key thing. Even better if you achieve it by using different materials with different inherent resonance frequencies. It is definitely possible to produce a fin that has a resonant frequency that cannot be reached by any possible flight profile. This is one thing I did NOT learn from Wilke...

Warren

Here is my take. I've gone mach 2+ a few times on three separate rockets. I've personally never used carbon fiber, and my fillets are good 'ol 30 minute epoxy (no fillers, no fancy expoxy, etc.) For tip to tip glassing, I use a layer or two of 6 oz. glass that gets mostly wet sanded away.

It is my sincere belief that we over-engineer stuff to the Nth degree, and we can get by with a whole lot less than what we do.

With regard to fins and strength.... I have two stories there. Using surface mounted fins attached using the materials and techniques discussed above, I have core sampled into the salt flats, and the fin section was just fine. I cut off the tube, and flew the same bird again. I have also dropped a rocket from 24K in a flat spin (with a spent motor inside) and the fins were just fine.

I agree John, its way easy to get carried away with this stuff. the assumption that then perpetuates the overkill is that it was key to the success when less robust methods were never tried.

In other words it is based on a false assumption and then spread by word of mouth. In one of these uphill battles, I have long questioned the assumption that ultra expensive epoxies with high Tg's are necessary to prevent heat soak from the motor softening up the joints during long burning and/or fast flights. Even the observation that rockets never shred during coast (ok maybe there's an exception out there--"I" have never seen one shred) is batted down by guys like Dan Stroud who are under the belief that subsequent flights shred because of the effects from heat soak from prior flights--ie it was fine during the first 3 flights, then shredaroo on number 4. He might be right, but I have seen no proof of this.

For small very sleek rockets that fly upright, like those under consideration here, and aren't subject to wind shear (the Bermuda triangle at Balls say), you can get away with simpler build technique because the total force just isn't there. IMO its the larger stuff with big flapper fins that requires most of these more advanced techniques...Like the Blackula vid. At least thats my take on it at the moment.

Also I have seen some extremely well built stuff at Nevada shred, presumably from wind shear torquing the rockets apart. Two years ago at Balls and the subsequent weekend during XPRS, following thundershowers, a lot of debris rained down on the playa. I remain convinced it was a wind shear related phenomenon.
John S

That must be one helluva wind shear if it's shredding rockets when they're already at near mach or greater speeds 😯

I agree. But the fact that it happened on two subsequent weekends at nearly identical altitudes during nearly identical weather conditions, forces me to this otherwise implausible conclusion. I've seen it at North site but usually just deflects the rocket off path like when we flew the pink panther on the redline I. Here rocket parts were literally raining down, and well built ones at that that had flown before. I think there was a big debate on the TRA site about the Playa phenomenon. Not sure if any conclusions were reached. I think the speed was a set-up if you will since the rockets were flipped to a high angle of attack and the subsequent forces just tore them apart--usu just above the fin can coupler IIRC.
JS

I have flown SkyScraper at BALLS many times, and it is a 3" diameter rocket w/ 1/8" fins, and the fins are big as elephant ears. Like my others, it is basic expoxy / surface mount fins / glass tip to tip. I've flown the RATT M900 and the RATT L -- both very, very long burn motors. The rocket flies with 25 second mach delays, so you know how fast I think it is going and for long it is above mach. The rocket is 13' tall, and a whoppping 3 inches in diameter.

In short, I can't possibly think of a more stressed scenario for an airframe. Bad aspect ratio, hot motors (hybrids burn notoriously hot), long burns, many joints in the airframe, big flappy fins, thick fins, supersonic, surface mounted fins, Black Rock, blah blah blah. The first time it flew, we waited for the winds to dip below 20MPH. I couldn't have put it in a more stressed situation if I wanted to.

It will be first off the pad at MHM, and it will go to 18K plus again. And again after that. My point is that we overengineer stuff.... which ain't all bad, but it is worth knowing.

agreed, but sometimes it's just plain fun playing amateur rocket scientist...Personally I think with tip to tip glassing you can do a lotta things wrong and live to tell the tale. If theres a take home message for Batman in all this discussion that would be it,IMO, esp if doing surface mount fins--glass at least the fin can (but my as well do it all) to provide a good substrate for the tip to tip. I think Blackula was unreinforced 1/8 or 3/16 G10 IIRC.
John

Brilliant discussion guys. This is exactly what I was hoping to read, and getting a variety of ideas and experiences is perfect. Warren, be sure to put my name in the hat for SSSS, I'll have my $20 at the next launch.

I 'm really wanting to try some homemade carbon fiber fins like what you guys described earlier, but dang, that stuff is really getting spendy. I did order a Cirrus to start with, and I plan to do something like 1 wrap of light glass on the airframe just for finishing an durability, and probably 2 layers on the fins, with a stagger as Warren suggested.

Should I consider DD? I was planning to go single deployment with streamer recovery to keep the rocket a bit shorter, since I have no idea how tough it is to recover something like this from 10K+ (hopefully). Also, how big is the in rocket part of a tracking system? And what types have you guys had good experiences with? I may invest in one of those if I get excited about these altitude birds.

Again, thanks for taking the time to share your knowledge and experience. This club is awesome.

Batman, try Aerosleeves. They have carbon fabric for $3/ft.

I second that: Aerosleeves are a great way to start--seamless finish in 30 minutes or less (plus the cure time). Soller Composites also has the sleeves in everything from CF to hybrids to basalt.
JS

Here's an idea on attaching the fins. I've never tried this but it may work OK, but I'll leave the analysis to the experts. Rather than mounting the fins directly to the airframe using a conventional surface mount, how 'bout a modified TTW approach. Since this is a minimum diameter rocket, you could only go TTW, the thickness of the wall. Not only would the fillets add strength (esp. if using glass or CF), but so would the area of contact between the fin and tube along the entire inside perimter of the slot.

Just a thought...