The AF doesn't need any glass. Its good to have, but IMO the primary reason for glassing small AF's for barely mach flights is the superior finish one can generally get. Also if you roll your own, it can be thinner and lighter than phenolic. A compromise is to lightly glass paper.

Also, I like wetsanding but paper or even PT can be a mess w/o some waterproofing, even assuming a heavy primer coat (YMMV)--another reason, at least two maybe more of the SSS rockets from last year or under snow as we talk. I had a rocket that was phenolic covered with CF sit out there between may and october and little was left. If cows can eat acme fin cans...

Seriously, another good reason is adhesion of the fins. You have no choice but to surface mount the fins. Even a light layer of FG can help here, especially in connection with tip to tip glassing. What I think is good insurance against shred, are good fillets. I make mine from a pulp of chopped kevlar and epoxy usually. FG and CF can also be used.

I have heard but a little faint hearted to try is simply supergluing CF fins to CF. The whole AF doesn't need to be CF just the outermost layer. As wild as this sounds, the guy who recently wrote an article on composites in one of the rocket mags advocates for this, with baking soda and CA fillets. If I'm not mistaken, some of the most extreme MD flights on M and above have been done this way. Again, I'm too faint hearted to try this yet. But will sneak up on it.

The fin material can be homemade if you're really serious about making the strongest, most flutter resistant fins possible.

There is a great Composites group on Yahoo that is all about state of the art building techniques if you're interested.
Best of luck, ask lots of questions. Its not at all hard or mysterious.
JS

I have to say, a PML cirrus is a great choice, and should easily get into the 11k range on a J350. It won't be a winning design, but it will give a good feeling for what an altitude bird is like. I still need to fly my cirrus (if anyone has a tracker I can borrow, I'll fly an I600 in it at Mayhem). All that's really needed for this though is unglassed phenolic, 1/16" G10 fiberglass fins (with some tip to tip glass), and a bit of luck. That's all the Cirrus is made of, and stock it can do around mach 2 fairly easily. If you want some material that is a bit stiffer and thinner, I could make you some CF plate, but if your goal is to break 10k, all that requires is the thin G10 glass with a tip to tip layer. If you will be at the Feb launch, I can bring my Cirrus so you can look it over and get a feel for what that kind of bird should be like.

Chris,

You are more than welcome to borrow my tracker when you need it (assuming in not needing it in that 2hour window when I actually get to fly).

Batman, the Cirrus Dart is a great place to start for the J350 shot. My SSSC flight was a cut down and glassed Cirrus Dart with a Performance Rocketry nosecone. I have had a lot of fun with the high altitude shots and enjoy playing with the different design options. For fins I like to use a longer root and less span. This gives you more surface area area for the glue joints and the smaller span helps cut the fin flutter.

Think about the composites especially for the fins. It's pretty easy to make super thin balsa (or even Kevlar) core fin material. Just lay it out between mylar or wax paper and clamp down with a couple of stout boards. Cut to whatever shape you want once it's cured. Obviously there are more complex ways of doing this, but this simple method works pretty well.

I've seen many thread discussions about paint and polish but have not had time to try on mine. Some of my birds have been pretty rough and still performed well. I think that one extra bump on the launch tower can cost altitude as much as anything.

Chris - I plan to be at every launch and I would very much like to see your Cirrus. Hopefully we don't get snowed out in Feb.

For simulation purposes, what is a reasonable Cd to use for this type of rocket? And is rocksim reliable enough to use for stability calculations? I used it to design my L2 bird but I was pretty conservative with stability because I wanted to play it safe. I'm guessing cutting down fin drag can make a big difference here, so maybe I'd be a bit more aggressive.

And along the lines of what Ed suggested, even if I use the cirrus as a "base" kit, I will probably make my own balsa core carbon fins. I've wanted to try making thin carbon fins for awhile and this seems like a good time. Where do you guys recommend getting glass and carbon cloth? I use west systems for most of my high power builds, do I need a different type for laminating the fins or for the tip to tip glass?

Thanks for the input

Batman,

I'm thinking 0.4 min Cd is a good figure, but as you likely know this will vary with velocity, esp in the transsonic region, so I would not override the one calculated--or you will get way unrealistic results. But you can look at the one rocksim computes as a fx of vel. Again min should be about 0.4. The Rocksim stability computation is a modified Barrowman approach which is likely more accurate for a Cirrus kind of rocket than stock barrowman. The laminating resin for fins should be the same, but the fillets might benefit from using a structural epoxy such as the Aeropoxy product. Where to get glass/CF. Gotta look around for the best price, esp with the CF. CST does not have good fabric prices. Aircraft spruce is very fast, medium priced, soller composites is very good source, esp for sleeving. The list goes on--don't overlook e-bay either.
Hope this helps,
John

As John said, you have to look around for the best price on the carbon. CST has great selection, but you pay a premium for it. ACP isn't much cheaper. Aircraft spruce is OK, as John mentioned. One thing I would recommend for this is to make pure carbon, rather than balsa core. While you can get lighter weight fins with the balsa core, you can make much thinner fins with the pure carbon, and the lower drag matters more than the weight for this. With 6 layers of 5.7oz carbon, you can make a plate of material that is 1/16" thick, at least triple the stiffness of PML G10, and more than adequate for this type of competition. That's what I would use if I entered (seriously considering it right now). I might even drop it to 4 or 5 layers, to make it even thinner, though I don't want to sacrifice too much stiffness.

West system will work for this kind of thing, but the ideal would be a true aerospace epoxy, such as Aeropoxy or Pro-set. They will be significantly stronger, especially when heat cured.

One thing to definitely do with this is tower launch it. Launch lugs should be avoided at all cost. As for motor retention, masking tape is the method of choice for me. My cirrus is set up for friction fit, and that's how I would set up the competition bird too. Less weight and drag than any other method, and perfectly reliable when done right.

I'll bring my Cirrus to the February launch - it definitely has the smoothest finish of any of my rockets 🙂 I can't wait to fly it - if everything goes right, it'll be my highest and fastest flight by a HUGE margin when it goes on that I600. My current record is around 600mph and 5120 feet with my flight last Mayhem on my BBX (K550), but this thing sims to 10500 at 1300+mph. It'll be a walk though... (single deploy)

Batman,

Thats would I did for my G80 SSS--2 layers of kev on the inside for shear resistance and 2 layers of CF on the outside. I need new batteries for my digital calipers--best guess is about 0.05" thick and puts G10 to shame, both in weight and stiffness. Cf all the way thru would be a good option as well. But I think Chris is right on the money here--6 to 8 layers for a MD J.

Where balsa is good IMO is when the fins have to be thicker--say for scale birds. The Nike/BB I'm working on has root thickness of 1" which tapers to 1'2 inch over the span. This is perfect for balsa cores. But LDRS will put everything to the test when those 3 M skids all light.... 😯
John S

Yeah - I used 4 layers of 4.8oz CF for my fins last year, and ended up with something that was about a third the weight and half the thickness of PML 1/16" G10, but at least as stiff if not a little stiffer. You can definitely do some nice stuff with CF, that's for sure 😀

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

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.

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

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

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.

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