Putting Fins on a Diet?

Just theoretical at this point but am curious as to what you guys think about it.

I saw this idea here somewhere, (a big JayHawk?) where a lot of fin lumber was removed and replaced with foam to reduce weight.

Anyways, picture Endeavour fins, 0.5" Birch Ply, 8" span with 1~1.5" inch holes bored through them in some smart honeycomb fashion. (20~30% wood/weight removal)...Then replace the removed wood with foam and cover the fins with enough fiberglass to regain some of the original strength.

"Theoretically", this could substantially reduce total weight by a few pounds or better if done correctly.

There's probably good reasons why this isn't already common but can't seem to find them.

Here's a quick bar napkin drawing but with too many holes and not quite where they should be.

That should work great, and the concept is quite similar to the honeycomb materials often used for larger fins. Because most of the stress in bending is in the outer layers, removing inner material shouldn't significantly weaken the fin (once the fiberglass skin is added of course). I think the main reason that this isn't done very often is simply because it is unnecessary, and people don't want to add extra work. It should do a great job of cutting weight though - I would say go for it.

I would go farther, replace the entire center section of the fin with foam and make the skins a little bit thicker. What you end up with will be more efficient than the holes, using straight cuts also makes it simple to cut out a piece of pre-made foam to fill the area.

I would use a 4 lb/ft foam, lighter and the tensile strength and toughness get pretty low. Plus 'Needle' the foam, at the day job we use a roller with small spikes on it, for a home job you could use a pushpin and put holes in the foam (just in the surface not all the way through) every 1/2 inch. This helps the peel strength.

An alternative that works well is to cut out the center of the fin like a truss and then bond pre-cured skins to it (leaving the cutouts open - no foam). you can get fancy and router out a recess for the skins or just bevel the edges after the skins are bonded. You might want a small breather hole to each cell. At the 'day job' I have made UAV fuselages this way, cheep, light, and they look very good without a lot of labor

cjl and kramer714,

Thanks for chiming in. This inspires me to go ahead and continue.

On hand, I have:

-1/2" Birch Ply
-1/2" Foam Insulation Panels
-18oz (Bionic/Burlap Bag) weight Fiberglass
-6oz Fiberglass
-enough convolute looking carbon fiber (very thin) tissue for a finish layer over the 6oz glass on the fins.


On instinct, it seems to me, multiple triangular truss style cut-outs might be a happy medium to keep remaining frame Strength + Weight + Labor reduction.

Any caveats or recommendations regarding this plan for brief trips in and out of mach 1.0?

Thanks Again for your input,

Scott

planning on pre-made skins and bond or co-bond with the wood?

18oz glass is pretty thick, sure you want to use that? Usually when you are that thick you only use it for making molds or a layup with high resin content like a boat.

Depending on the size of your fins you might only need two or three cut outs. Keep it simple, what you are trying to do is keep the bending stiffness up, all the wood core webs are doing is to prevent buckling and add some shear transfer from one skin to another.

This has already been put into practice on at least one build thread here:

https://www.rocketryforum.com/showthread.php?t=8020

I think one of the key observations is to put the weight saving holes in randomly, so that you don't create a string of holes in one line that creates a weak point along the fin.

For a bigger rocket, I definitely see doing this, for lighter stuff, not as much.

For the size rocket, I wouldn't worry about it -- I'd just use plywood and accept the weight.

That said, if I were going to do it, I'd use foam core and create them to the desired shape, then apply fiberglass (or CF) cloth over them, and call it a day. I wouldn't bother with pre-made panels.

-Kevin

pcotcher said:

This has already been put into practice on at least one build thread here:

https://www.rocketryforum.com/showthread.php?t=8020

I think one of the key observations is to put the weight saving holes in randomly, so that you don't create a string of holes in one line that creates a weak point along the fin.

For a bigger rocket, I definitely see doing this, for lighter stuff, not as much.

Click to expand...

There's no reason to put the holes in randomly - the lightest way to do it would be not to rely on the core at all for strength. Cutting out the majority of the core and replacing it with something like lightweight foam, and then covering the fin with several layers of CF/fiberglass would give a strong, lightweight fin in which all the bending stress was carried in the outer composite skin, leaving the middle relatively stress-free.

Actually, if you read this thread, the holes were not done to save weight, they were put in to provide more surface area / structure variation to allow the epoxy to bite. I agree with cjl, for weight/strength ratio it is best to core the whole structure out and apply foam (or just use a foam core) wrapped with CF.

pcotcher said:

This has already been put into practice on at least one build thread here:

https://www.rocketryforum.com/showthread.php?t=8020

I think one of the key observations is to put the weight saving holes in randomly, so that you don't create a string of holes in one line that creates a weak point along the fin.

For a bigger rocket, I definitely see doing this, for lighter stuff, not as much.

Click to expand...

troj said:

For the size rocket, I wouldn't worry about it -- I'd just use plywood and accept the weight. -Kevin

Click to expand...

WHOOT! Hold the bus!

Kevin, you are absolutely right.

I figured I'd put my money where my mouth is, so I stopped by Harbor Freight this afternoon after some out of town training and picked up some goodies including a cheapie scale.

My secret rocket construction helper elf and I were amazed at how little the fin set actually weighs compared to the "Aircraft Plywood (Birch)" I had selected as fin material in RS9. My bad, I have seriously miscalculated the weight of my fin material.

The whole upper and lower fin set weighs roughly 2.2lbs x 3. No actual need for a fin diet.

My apologies for starting this thread based on erronous factual data, but many thanks to all that responded with some really great techniques.

EDIT: Obviously, I need to weigh the parts as I go for realistic RS9 sims from here on out.

S. Broderick

When you started this thread it really intrigued me. A couple of friends and I are going to build a half scale patriot and were hopeing to break 10,000'. We are planning on a L1115 and airstarting 2 J410s. With the rocket weighing 25lbs, it sims out to 9400'. A little short, and I am assuming the sim would be about 500-1000' off on the low side. So I was looking at ways to save weight without sacrificing strength. So, this might be an option for us.

Dave

One caveat: in many cases, drag is more important than weight when it comes to high-altitude, high speed flight, and although the method in the OP would save quite a bit of weight, it would also necessitate several layers of fiberglass reinforcement, increasing the thickness (and therefore the drag) of the fins. That's why I went with solid carbon fiber plating for my L3 fins - after much simulation and calculation, I came to the surprising (at least to me at the time) conclusion that 1/8" thick fins of solid carbon fiber would allow the rocket to go higher than 1/4" fins with a honeycomb core and CF skin, despite a nearly 3 pound weight penalty for the solid fins (out of 24lbs total).

Of course, this heavily depends on how fast your rocket will be going. I'd say if your peak velocity is less than about mach 0.6 or so, then the weight probably makes a bigger difference than the drag, but I'll be the first to admit that my number here is only a guess. I haven't run detailed sims to see where the crossover is for a variety of fin shapes and thicknesses.

Scott Evil said:

My apologies for starting this thread based on erronous factual data, but many thanks to all that responded with some really great techniques.

Click to expand...

Absolutely no need to apologize -- it's a good discussion.

-Kevin

We used this technique for our X-15 build last year. The fins were substantially lightened by removing lots of material. We didn't fill them back in with foam though. We just laminated luan door skins over the exterior surfaces with yellow carpenter's glue.

I think I may be a little late, but why not just use the aerospace composite fin stock that can be found here? https://www.giantleaprocketry.com/hpdefault.asp
same basic concept.

SD

PunkRocketScience said:

Click to expand...

What are you going to use all the centering rings for?

You did cut out a size that can be used as CRs in a standard size tube, right?

Handeman said:

What are you going to use all the centering rings for?

You did cut out a size that can be used as CRs in a standard size tube, right?

Click to expand...

Those look like they would be BT-5 to T300 (3"), maybe? For something like an oversize Egg Brute Renegade? Hey, it could work, especially if the CRs themselves were drilled with several holes to lighten them, and then covered with cardstock skins.

MK

We've established that reducing fin weight is negatable considering the gross weight of this rocket.

But..."If" the fins were to (resonate?) through mach, would I be better off with solid glassed over solid wood fins or would it be better to have them lightened-up/multi-materialed to decrease inertia at mach flutter.

The fins pictured below are (as we speak?) having 7.5 degree/1.25 inch (or so) bevels cut on the leading and trailing edges of all six fins. (thanx Tigger)

The rest of the build will go in its own thread, but currently I'm at the stage of getting the fins as dialed in as I can. Second pic demonstrates some psychotic velocities.

Inertia doesn't matter all that much for flutter (well, it does, but I'm oversimplifying here). The biggest thing you want to prevent flutter is to have very stiff fins, and if you don't care about the weight, the solid fins will be the better way to go. They will be stiffer for a given amount of reinforcement. I would add a bit of reinforcement with that fin shape though, without a doubt, as the tip of your forward fins will be somewhat prone to fluttering at that kind of speed.

If it were me and I was going for pure performance, I would not put a break in the fins. I would keep the existing profile and make it one, solid fin structure. You are not going to be adding much weight, but you are going to be reducing some drag (and perhaps some flutter propensity if it goes transonic).

That said, the fin break is cool looking.

Greg

cjl said:

Inertia doesn't matter all that much for flutter (well, it does, but I'm oversimplifying here). The biggest thing you want to prevent flutter is to have very stiff fins, and if you don't care about the weight, the solid fins will be the better way to go. They will be stiffer for a given amount of reinforcement. I would add a bit of reinforcement with that fin shape though, without a doubt, as the tip of your forward fins will be somewhat prone to fluttering at that kind of speed.

Click to expand...

Thanks for clarifying the physics CJL, the forward fins are kinda problematic aren't they. I'll focus towards some rigid inlays. Some ideas already coming to mind.

GregGleason said:

If it were me and I was going for pure performance, I would not put a break in the fins. I would keep the existing profile and make it one, solid fin structure. You are not going to be adding much weight, but you are going to be reducing some drag (and perhaps some flutter propensity if it goes transonic).

That said, the fin break is cool looking.

Greg

Click to expand...

Greg I agree with you 100% design vs performance wise.

Design surviving performance is the active goal here.

Scott Evil said:

Thanks for clarifying the physics CJL, the forward fins are kinda problematic aren't they. I'll focus towards some rigid inlays. Some ideas already coming to mind.

Click to expand...

Yeah, in general I would worry about fins swept back so the tip is behind the entire root, especially for supersonic flight. It can absolutely be done, but they need significantly more reinforcement than something like a clipped delta design (which is what yours would be without the break). Of course, the break makes it look way cooler

Out of curiosity, what's your max simmed velocity on an N5800?

Scott Evil said:

The fins pictured below are (as we speak?) having 7.5 degree/1.25 inch (or so) bevels cut on the leading and trailing edges of all six fins. (thanx Tigger)

Click to expand...

So you have the "gapped" edges beveled too?

I'm curious what would be different if both gap edges were flat?

Just thinking out loud

bandman444 said:

So you have the "gapped" edges beveled too?

I'm curious what would be different if both gap edges were flat?

Just thinking out loud

Click to expand...

That's an interesting question. I might have to try some simulations later...

If you noticed thats what I did!

All the outside edges were beveled but the gaps were flat.

Interesting.....

cjl said:

Yeah, in general I would worry about fins swept back so the tip is behind the entire root, especially for supersonic flight. It can absolutely be done, but they need significantly more reinforcement than something like a clipped delta design (which is what yours would be without the break). Of course, the break makes it look way cooler

Out of curiosity, what's your max simmed velocity on an N5800?

Click to expand...

Max velocity @1273 mph. Max alt @ almost 15.5K ft. "Gees"...curiously enough, still about 76...??? Seems like some kind of Einsteinian APCP Quantum Gravity Thing that never made it to the chalkboard.:marshmallow:

But that's OK. Trying to build around all the parts weighing 76 times more than what they do in my secret rocket labouritory is challenging enough.

Scott Evil said:

"Gees"...curiously enough, still about 76...??? Seems like some kind of Einsteinian APCP Quantum Gravity Thing that never made it to the chalkboard.:marshmallow:

But that's OK. Trying to build around all the parts weighing 76 times more than what they do in my secret rocket labouritory is challenging enough.

Click to expand...

I wouldn't worry about it too much. When a bunch of sims in Rocksim have curiously high acceleration that stays about the same regardless of motor, it usually means that the 76 gee acceleration isn't occurring during burn. My bet is that it is occurring at main deployment, which is why it doesn't really change with motor type. If you plot the acceleration on a graph, or look at it real time in a 2d flight profile, I'd bet that the max acceleration during burn is much smaller. On my L3 rocket, which is lighter than yours, the peak acceleration on an N5800 is more like 30 gees or so, so I'd guess yours would be less than that.

Some of the data RS displays in the flight summary can be confusing in that itlists the max value for certain items. No matter in what phase of the flight it occurs..To get a real sense of them when you run the 2D launch display is to click open the 'Details' panel and watch the data change there..I usually run it with the details panel closed but if I want to see the 'real' numbers under thrust I open it and run the sim at usually the slowest or next to slowest speed..

I wouldn't worry about it too much. When a bunch of sims in Rocksim have curiously high acceleration that stays about the same regardless of motor, it usually means that the 76 gee acceleration isn't occurring during burn. My bet is that it is occurring at main deployment, which is why it doesn't really change with motor type. If you plot the acceleration on a graph, or look at it real time in a 2d flight profile, I'd bet that the max acceleration during burn is much smaller. On my L3 rocket, which is lighter than yours, the peak acceleration on an N5800 is more like 30 gees or so, so I'd guess yours would be less than that.

Click to expand...

Some of the data RS displays in the flight summary can be confusing in that itlists the max value for certain items. No matter in what phase of the flight it occurs..To get a real sense of them when you run the 2D launch display is to click open the 'Details' panel and watch the data change there..I usually run it with the details panel closed but if I want to see the 'real' numbers under thrust I open it and run the sim at usually the slowest or next to slowest speed..

Click to expand...

Thanx for the RockSim reality check guys, watching closer even with the N5800, this rocket doesn't do much better than 30 G's or so on the up part.
The chutes hit kinda hard but that can be corrected.

So you have the "gapped" edges beveled too?

I'm curious what would be different if both gap edges were flat?

Just thinking out loud

Click to expand...

This comment got my attention as well, at a potential velocity of up to 1.5 mach for few seconds or so, having all six fins beveled is just asking for trouble.

Changes made regarding fin geometry ...The trailing edge of the forward fins will have a simple round over. The leading edge of the trailing fins the same.


Any velocity/temperature/ablation vs. fin material advice would be appreciated as well.

S.E.