I'm in John. Let's discuss when we get together to v-bag your fins. I even have a Kevlar conical we can use.

Warren

Honestly, I'd say CF should easily hold up if done right. I'm thinking of something like Jim Jarvis's LDRS 25 rocket (4" MD on an N4000), scaled down slightly. I'd be pretty surprised if you got too much beyond M3 as well, although I guess it's possible. Actually, just using some quick and dirty assumptions (ISP=200s, burnout mass=4.5kg, no drag), I get a max velocity of 1360m/s, or almost exactly Mach 4. Considering that that is the zero drag case, I'd be pretty surprised if you exceeded mach 3 by any significant amount (unless you somehow managed to make the rocket, including motor casing and recovery gear, weigh significantly under 4.5kg).

Cool. I'm interested. You would definitely want a Von Karman cone for it. I can ask my friend who did the mandrel for my 38mm VK cone to do the same thing in 75mm. The length may exceed what he can do with his lathe setup, though.

Aerodynamically, there's a pretty big advantage to skipping the double-wall setup and just using high-temperature epoxy for the fins. With mostly unidirectional carbon, the fins can be surprisingly thin and still plenty strong.

That definitely sounds like the way to go, Adrian. A VK cone plus single thickness tubing would probably have a pretty significant gain compared to the double wall with a conical (though if I remember right, the conical does do better at high mach than it does at transonic).

John,

I suspect you already have a 75mm VK cone from PF. My take would be to lay up low mass fins out of honeycomb and multiple layers of CF, Kevlar and perhaps glass as a sanding veil like my L3 fin can. I see no reason why that can't hold up at Mach 3+.

I suggest we take this discussion off forums.

W

I suspect that for this kind of rocket, drag is a larger problem than a few ounces - you'd probably get higher with solid carbon fiber fins made out of layers of uni aligned in different directions than you would with honeycomb - the honeycomb would be lighter, but the pure CF would be thinner for the same strength and stiffness.

It's not the weight for the sake of weight, it's the moment arm of having high density, high mass fins out far from the centerline trying to lever the fins off the body. Use low mass material - hell I bet we could even do foam core - with high stiffness and a mix of materials to prevent resonant fin flutter.

W

If one would be fun, two would be better, so tell you what. You and John make the booster, Chris and I will make the sustainer, and we'll hit 100 kft together. 😆

It's not the weight for the sake of weight, it's the moment arm of having high density, high mass fins out far from the centerline trying to lever the fins off the body. Use low mass material - hell I bet we could even do foam core - with high stiffness and a mix of materials to prevent resonant fin flutter.

W

You can prevent flutter with solid carbon fiber - it just needs to be stiff enough to drive the resonant frequency above the region that it will ever hit in normal flight. The unidirectional helps too, if you align it properly. As I said, to make it strong enough, you would need heavier fins, but they would still end up thinner than the foam or honeycomb core. I would still think that making CF plate from scratch would work fine (with a couple layers of T2T). I could experiment with some CF plates - I've had good luck laying them up between two layers of glass and then post curing them at 180 degrees. Of course, the resin I have is only rated as good for up to 220F (roughly equivalent to Aeropoxy), so if you think more than that is necessary, then I'd probably look into different resin. I'd definitely be interested in helping out though.

Actually, come to think of it, Adrian's idea about the 2-stage is intriguing - the staged M record could be within reach if this was used as the booster with a ~1100Ns 38mm J motor as the top stage (CTI J530 Imax or AT J570, probably). If you get it going to ~M3 or so, then separate instantly upon burnout, the top stage could coast for quite a while before lighting. A J570 lit in a good, optimized rocket at 20k and M1 or so could almost definitely break the current ~44k M staged record. Of course, I haven't simmed anything, so all of this is guesswork at the moment.

Seems like before you get to the fin material, elasticity, strength, etc, you need to figure out if you are going to keep the fins behind the mach cone (71 degree sweep at Mach 3) or stick them into the supersonic flow, in which case you want a sharp LE and relatively mild sweep back. This will dictate how much leverage is out there and nail down one degree of freedom for you flutter analysis.

I'm just throwing out questions, I don't have answers, but what about a pronounced spanwise taper to the fins, like .300" at the base and .004" at the tip, with a lightweight core? That would hurt a bit in the subsonic realm, but would add strength with little additional drag/flow compression at supersonic speeds.

They should definitely be within the mach cone, IMHO. Even if you assume a ridiculously short rocket (say, 1 foot longer than the casing), that allows for something like a 17 inch semi span before you hit the shock wave at the base of the rocket. Of course, the flow will still be supersonic at the fins, even with them inside the shock, and you'll still want them to have a nice bevel to them.

As far as material is concerned, there's also premade CF plate - I'd bet this 1/8" plate would work fine (with perhaps a single t2t layer to hold it on):

http://cstsales.com/carbon_plate.html

Of course, you could get better strength per thickness/weight by making it yourself out of unidirectional CF oriented in specific directions as needed for maximum strength only where it is needed.

Well, I've put together some rough sims, and even with some fairly bad assumptions, I'm able to get some sims as high as 80k (with something like 40 seconds supersonic) on a 2-stage M3000 to J570, and 42k for a single stage M3000 shot. Admittedly, the 80k is probably a bit higher than is reasonable, but I'd bet the current 45k staged record is well within reach. Of course, with final burnout occurring at 27k and mach 2.8, it would definitely get up there pretty nicely.

I think the leading edge of the wing creates its own shock wave, and not enough sweep creates a lot of wave pressure drag. By sweeping the fin back, the effective mach number- what is perpendicular to the leading edge- is subsonic.

The trade off is a very inefficient lifting surface and more parasitic drag.

Then again, I'm not a REAL rocket scientist.

I believe that is accurate, though we only touched on supersonic flight in my Vehicle Design and Performance class last semester. Once school starts up again, I'll be taking 3000 level aerodynamics, and I could ask my professor about that to see what the best way to go would be.

Oh, and here's my rough 2-stage model: