Thoughts on Super Single Shot

[Chris LaPanse]

I have to disagree with you on this one. I think unglassed, unmodified phenolic should be perfectly capable of taking a 38mm J motor in a minimum diameter rocket. The pressures and forces in play here are not massive, and something like the cirrus is low enough drag that it should easily take the strain. I used a single layer of 1/2 oz glass on my Cirrus for finish purposes only, so we’ll see what it can take at Mayhem…

Honestly, though, I’d be far more worried about the fins than the airframe. The thing that kills more mach rockets than any other is fin flutter, and what would be the best modification for a cirrus would be the carbon fins, or an extra layer of tip to tip.

[denverdoc]

I’ve built 2 Cirrus Darts. The phenolic kit intends that you use one layer of tip to tip across the fins and comes with precut 6oz glass for that purpose. If you’re planning on flying and H or I motor you have no worries about flying the airframe bare. Above that you takes your chances in my mind. The lower airframe comes in 2 pieces. I personally have glassed both my CD airframes with glass – one with 3 wraps of 6 oz. which is patently overkill and the other with just 2 wraps of 3 oz. glass. Both finished very nicely after I filled the weave in the glass with Kilz hi-solids primer and wet sanded.

If you’re planning on flying the SSSS with a Cirrus Dart, make sure you include electronic tracking. I lost my first one on a J285 CTI motor flying on a windy day. After re-simming for the known flight weight, winds and 12″ chute, it put the altitude at 13K plus and the touchdown over 4 miles away. It’s still out there on the prairie somewhere. The second one still has yet to fly. In this case, I’m using a streamer instead and I’ve calculated it to fall at the maximum rate specified by the safety code (don’t remember exactly what that is off the top of my head) so it will basically fall like a rock. The glass will protect the rocket from fracturing. I also always use 3 layers of glass tip to tip to ensure that the fins are solid. I would NOT use carbon tip to tip as it doesn’t like to be bent and unless you’re vacuum bagging the tip to tip stuff down, you won’t be likely to get the carbon to fully suck into the corners between the airframe and fins.

Warren

Gotta love that Kilz primer! As to CF tip-to-tip, depends on the weave, satins and twills will hug corners better than plain, but its best IMO to radius the fillets so as to avoid sharp corners which is where stress gets multiplied–I think John W likes bondo, but man that stuff is hard to sand–unless you’re using a grinder.

I’m real partial to the Aeropoxy filler, mind you this is not for strength–the chopped fabric/epoxy pulp is for that, which is then layered with the filler, using a big dowel or motor casing wrapped in sandpaper to shape it.
Many ways to skin a cat. But laying even heavy CF over that is a snap with or w/o bagging. The other option to bagging is to use a heavy bag of mortar, cat litter, water, whatever to secure the tip to tip. Good thread. But seriously batman check out the Yahoo composites group if you haven’t already. If nothing else you will see some pics of drop dead gorgeous and bullet proof rockets the likes of which I have at least not seen locally. Frigging works of indutrial art.
John S

[denverdoc]

I have to disagree with you on this one. I think unglassed, unmodified phenolic should be perfectly capable of taking a 38mm J motor in a minimum diameter rocket. The pressures and forces in play here are not massive, and something like the cirrus is low enough drag that it should easily take the strain. I used a single layer of 1/2 oz glass on my Cirrus for finish purposes only, so we’ll see what it can take at Mayhem…

Honestly, though, I’d be far more worried about the fins than the airframe. The thing that kills more mach rockets than any other is fin flutter, and what would be the best modification for a cirrus would be the carbon fins, or an extra layer of tip to tip.

Hey speaking of pressures and forces, Chris, do you have any idea how to calculate the dynamic forces on fins as a function of AOA. At zero degrees, its pretty simple. But I am a little perplexed as to how to go about it otherwise. Is it simply a mass flux problem with the velocity of the rocket multiplied by density, fin area, etc times the sin of the aoa–ie pressure times effective area?
John S

[Chris LaPanse]

Well, that should work, but approaching mach, there are several other things to take into account, including pressure waves and such. No idea how to calculate those.

[Warren B. MusselmanModerator]

All I can say is I’ve never shredded a rocket I’ve built for Mach+ flights…. EVER. My first Cirrus handled an I600, a J350 and a J285 and never shredded. My second remains to be seen, but it too has 3 layers of tip to tip glass and the airframe 2 layers – all 3 oz. satin.

I think the key is to kill resonances in the fins. CF is great, but will shred nearly as readily as FG if you don’t do things to kill resonance. With my 3 layers of tip to tip, I run the first layer 1/2 way out the G10 fins, the second 2/3 and the top layer all the way. That alone will kill most of the resonance. I know both of you guys (Chris and John) are also members of the CompositeRockets group and you’ve seen the postings on this. Changing the resonant frequency of the fins along their span is the key to fin survival.

Warren

[Anonymous]

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

[Chris LaPanse]

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.

[Warren B. MusselmanModerator]

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

[Chris LaPanse]

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).

[Warren B. MusselmanModerator]

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

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