Perhaps my Crowning Achievement?

So, this will be kind of an odd little build thread. But I can't say I've seen this "technique" used anywhere else (although I'm sure someone has tried it).

Here's my problem. I have this rocket, but for its next flight, I need it to have larger fins. I really don't want to build a new rocket, and I would prefer to be able to return to the current fin size after the next flight is completed. So, I'm going to try to "crown" the existing fins such that the crowns can be removed when the flight has been completed.

I've made some progress on this project, but I'm not done yet, so I don't know how it's going to turn out. I'll post some pics of what I've done so far, and then we'll see how it goes.

The rocket in question is in the first pic. The idea is to put a 1" frame around the perimeter of the fins (second pic) and then add a skin of some sort. The third pic is just a prototype where the skin is a piece of balsa wood. As it turns out, the existing fins are exactly 1/4" thick. So, I got some 1" wide x 1/4" thick basswood pieces and some 1" x1/4" G10 pieces for the frame. I really hope the basswood will work, because the G10 is very heavy. For the skins, I have two, 2'x2' pieces of G10. One is 1/32" and the other is 3/64". Again, I hope the thinner material will work out. The fins will be exposed to a maximum speed of Mach 1.2, so the crowns will need to be reasonably strong.

That's it for now. Stay tuned .....

Jim

Now that is a challenge!


Sent from my iPhone using Rocketry Forum

Just curious. What is the reason for the larger fins? Stability?

Wouldn't it be easier to add nose weight? There are ways to have removable nose weight.

So if they stay on long enough to burn some propellant weight off, then shed (not shread) would your flight work?

That would be pretty neat. Probably not recommended.

nh4clo4 said:

Just curious. What is the reason for the larger fins? Stability?

Wouldn't it be easier to add nose weight? There are ways to have removable nose weight.

Click to expand...

Well, you might suspect this is for a multi-stage flight, so nose weight stays attached for the entire flight whereas the larger fins and their associated weight don't hang around for very long (unless you're talking about removable nose weight during the flight).

Jim

cavecentral said:

So if they stay on long enough to burn some propellant weight off, then shed (not shread) would your flight work?

That would be pretty neat. Probably not recommended.

Click to expand...

Yes, that occurred to me, and you're right. By the time the velocity is high enough stress the crowns, they are no longer needed.

Jim

qquake2k said:

Very interesting challenge. I'm curious, why do you need the fins to be larger for just one flight?

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Well, I would like them to be larger for the one flight, but for several reasons, I doubt the flight would be repeated. This is a flight that is going to be difficult to get approved, and I suspect that difficulty will increase every year going forward.

Jim

What if instead of the basswood you used a few more layers of carbon? That might cut down on thickness a bit.

Alex

Aksrockets said:

What if instead of the basswood you used a few more layers of carbon? That might cut down on thickness a bit.

Click to expand...

It has to be removable so there could be a CF sheath that was built around a mold of the existing fins. I'm just curious about attachment. I am assuming a series of some time of small pins that extend through the fin and fin sheath, but I don't know where Jim is going with this.

Aksrockets said:

What if instead of the basswood you used a few more layers of carbon? That might cut down on thickness a bit.

Alex

Click to expand...

The basswood is only for the frame. Perhaps you mean instead of G10, use a few more layers of carbon? I considered making some thin carbon plate and using that for the skins. I think if I were starting over (hope I don't need to), I might do that, but choosing the right thickness would be a guess and I don't have a particularly good way of making flat plate. One advantage would be the ability to include a peel ply surface for bonding to the frame.

Jim

dixontj93060 said:

It has to be removable so there could be a CF sheath that was built around a mold of the existing fins. I'm just curious about attachment. I am assuming a series of some time of small pins that extend through the fin and fin sheath, but I don't know where Jim is going with this.

Click to expand...

I still have to figure out how to attach these. Pins through the fin are one of several possibilities.

Jim

JimJarvis50 said:

Well, I would like them to be larger for the one flight, but for several reasons, I doubt the flight would be repeated. This is a flight that is going to be difficult to get approved, and I suspect that difficulty will increase every year going forward.

Jim

Click to expand...

Iiiinteresting.

One particular flight which is a) harder to get approved, and b) needs more stability for the full stack and not necessarily for the sustainer....hmm...

You've got me stumped.

How about a ring the crowns are attached to, fit very snugly against the outside of the BT and resting just above the primary fins?

CarVac said:

Iiiinteresting.

One particular flight which is a) harder to get approved, and b) needs more stability for the full stack and not necessarily for the sustainer....hmm...

You've got me stumped.

Click to expand...

Really? This is an easy one.

Jim

Zeroignite said:

How about a ring the crowns are attached to, fit very snugly against the outside of the BT and resting just above the primary fins?

Click to expand...

Well, let's get just a bit further on the build and then figure out the attachment method. How it turns out will matter I think. Time to go fly some rockets.

Jim

JimJarvis50 said:

Really? This is an easy one.

Jim

Click to expand...

Is it?

I mean the "easy one" would be N5800-N5800 or N5800-O3400, but I don't see why that needs additional stability over a 6-grain sustainer motor.

CarVac said:

Is it?

I mean the "easy one" would be N5800-N5800 or N5800-O3400, but I don't see why that needs additional stability over a 6-grain sustainer motor.

Click to expand...

Maybe he got one of those Castor 120s in the Estes sale...

Trooper said:

Maybe he got one of those Castor 120s in the Estes sale...

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More likely that CarVac thinks there are certain things that I'm smart enough not to do. He is wrong. I'll fess up soon enough, and the flight really isn't the point of this thread, but I did put a clue in Post 14 here:

https://www.rocketryforum.com/showthread.php?62175-2-stage-to-minimum-diameter&p=661205#post661205

Jim

JimJarvis50 said:

More likely that CarVac thinks there are certain things that I'm smart enough not to do. He is wrong. I'll fess up soon enough, and the flight really isn't the point of this thread, but I did put a clue in Post 14 here:

https://www.rocketryforum.com/showthread.php?62175-2-stage-to-minimum-diameter&p=661205#post661205

Jim

Click to expand...

Hmmm 98mm staged to a 75mm wouldn't be pushing anything significantly higher than you already have gone, unless you are using some crazy ex motors. For the sake of a thought analysis, lets say you aren't. I think I remember you mentioning somewhere that your second stage was limited to a 6G motor due to the length of the stage. As you're not wanting to build a new rocket, you are going to use all the stages/parts you have already to make something work that will push the altitude to something crazy high. This means 98mm first stage (O3400 most likely), 98mm 2nd stage (N1100 again or something with higher initial impulse?), and a 75mm 3rd stage (has to be a long burn). The bigger fins would be needed on the first stage to make everything fly straight and reduce any possible weather cocking. Getting warmer?

Phil

babp1011 said:

Hmmm 98mm staged to a 75mm wouldn't be pushing anything significantly higher than you already have gone, unless you are using some crazy ex motors. For the sake of a thought analysis, lets say you aren't. I think I remember you mentioning somewhere that your second stage was limited to a 6G motor due to the length of the stage. As you're not wanting to build a new rocket, you are going to use all the stages/parts you have already to make something work that will push the altitude to something crazy high. This means 98mm first stage (O3400 most likely), 98mm 2nd stage (N1100 again or something with higher initial impulse?), and a 75mm 3rd stage (has to be a long burn). The bigger fins would be needed on the first stage to make everything fly straight and reduce any possible weather cocking. Getting warmer?

Phil

Click to expand...

Not bad. Actually, N5800/N2501/M745 - still Class II. And not crazy high, just 130K. The fin crowns take the stability from 1.8 calibers to about 3.6 calibers. I'm open to comments on the subject, but 3.6 just feels better to me for a very long, skinny rocket. The powers-that-be will really constrain the flight, so the most likely outcome is that I'll still own one or two motors at the end. Lots of technical issues, but I think it could work.

Jim

You should pop one of the up-and-coming Loki 54mm L's on the top there...

...since when is 130kfeet "not that high"?

Zeroignite said:

...since when is 130kfeet "not that high"?

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When the waiver at BALLS was 491k feet.

CarVac said:

When the waiver at BALLS was 491k feet.

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AKA "just please don't go to space today"

Zeroignite said:

...since when is 130kfeet "not that high"?

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I just meant that this flight adds a few more Ns to the flight relative to my two-stagers, but also adds a bit more weight. The result is a somewhat higher altitude prediction, but not "crazy" high.

One thing I learned from the balsa prototype as that there was too much wiggle for the frame in the front (and the back too). These fins have a lot of bevel in them, starting several inches from the edge of the fin. So, I decided to add some support along the perimeter of the fin. The pic shows the wood strips I added with double-sided tape around the permimeter. I built a little sanding jig so that I could sand them to a with of 1/4 inch to match the thickness of the fins. This really firms things up.l

I made one set of crowns using the thinner 1/32" G10. The first one I made with Aeropoxy laminating resin and milled glass. It seemed fine and the 1/32" G10 seemed strong enough to do the job (although certainly not over-designed). I ran out of Aeropoxy, so I decided to do the second and third crowns with some leftover Cotronics 4525 (have to use it up before it goes bad). Those seemed fine too, except when I tested the strength of the bond between the G10 and the basswood, the G10 peeled off much too easily. Dang! This was after scratching up the G10 pretty thoroughly and cleaning both the G10 and wood with solvents. My theory is that the glue is too thick to get into the scratches in the G10 and doesn't soak in to the wood, so the joint isn't very strong. Very surprising to me. I don't know if this would happen with any thick epoxy or just the 4525.

So, I got more Aeropoxy and decided to make the second set of crowns with the 3/64" G10. A few pics of the results are attached, along with a profile of the cross section of the crown. These will add about 1.5 pounds to the 115 pound pad weight.

Next steps are to fill in the area where the frame meets up with the body tube and fillets, and then do a little beveling.

Jim

The next step is to cover over the bottom of the frame (where it meets the tube) and also the tapered area at the front and back of the fin. I covered most of the area with a piece of wood. Then I tried to fill in the remaining area with some aeropoxy light. It worked, but the filler material is so thin that I'm sure it won't hold up. So, I'm going to stop with the little wood pieces and declare victory.

Jim

Looks great! Is the aeropoxy light the only method of bonding the new covers to the original fins or did you add something else?

At what altitude/speed are you expecting the crowns to cease being part of the rocket?

babp1011 said:

Looks great! Is the aeropoxy light the only method of bonding the new covers to the original fins or did you add something else?

Click to expand...

It's difficult to see in the picture (the last one), but the aeropoxy light just extends the length covered by the wooden piece a little. In the picture, it's the reddish brown material. It is only intended to cover the part of the fin that was originally beveled (i.e., to make the crown a little more aerodynamic). The aeropoxy light didn't really accomplish that very well - too fragile, so I am not using that for the covers. The aeropoxy light is not intended as a means of attaching the crowns to the fins. I still haven't decided how I'm going to do that yet.

Jim