Tube Diameter vs. Fin Area

[DaveR]

A while back I accquired a large (4") diameter kit along with a bunch of other stuff. It appears to be complete except the fins are missing. I realize this is definately not a model rocket but thought I'd seek help here first as it's not really a kit related question.

I have read that your fin area should be 1 1/2 times the diameter of the body tube. So does that mean your total fin area for all fins should be 1 1/2 times the diameter or each fin should be? If each fin should be, why would having 4 fins be benefical over 3? I'm looking at making 3 fins out of 1/4 inch birch plywood and I'm trying to figure out a fin pattern that will work. I am notorious for "over finning" :rolleyes: and was hoping some of you could provide a little insight.

Also, any suggestions on attaching these fins to the tube? The tube is NOT slotted, but the business end is 3 x 29mm and the overall length is about 6 1/2 '. I guessing that attaching the fins directly to the tube is going to result in less than desirable results very shortly after launch but I could be wrong. I'm not into HP but I was thinking maybe adapting it for 24mm and going that route but not sure I can enough punch to get it off the pad; nevertheless, it would make a really cool display. So, I guess what I'm getting at is, how big should the fins be to look proportionate to the rocket? Comments, toughts, accusations, and "it'll never work" most welcome!

Thanks

[stefanj]

You can slot the tube yourself. Back in the olden days we all did.

I would say an area of diameter ^ 2 for each fin. Think of that as a 4" x 4" square. A simple choice would be a clipped delta, like the Patriot uses.

1/4" plywood is kind of overkill. 1/8" or 3/16" should be fine if you go "through the tube."


I would put in a 29mm mount. There are some 24mm reload kits that would get it off the pad, but not many.

[CPMcGraw]

...I have read that your fin area should be 1 1/2 times the diameter of the body tube. So does that mean your total fin area for all fins should be 1 1/2 times the diameter or each fin should be? If each fin should be, why would having 4 fins be benefical over 3? I'm looking at making 3 fins out of 1/4 inch birch plywood and I'm trying to figure out a fin pattern that will work. I am notorious for "over finning" :rolleyes: and was hoping some of you could provide a little insight...

This is where RockSim comes in very handy. You could try out various shapes and sizes until you reach a satisfying look with enough area to satisfy the margin requirements. You'd also see that five fins strums a happy chord, while six twangs a sour note. Strange but true.

As for your area-rule formula, I'd say each fin needs to have that 1.5x BT diameter area. Three fins would be the minimum without using outriggers of some sort (winglets, or fin-tip fins). Three fins are all you'd need, as you're not trying to align the model with the horizon (airplane mode), you're just trying to keep the nose in front of the tail.

[JRThro]

You'd also see that five fins strums a happy chord, while six twangs a sour note.
How so, Craig?

[CPMcGraw]

How so, Craig?

I suppose it depends on the overall design, but I've found that (a set of) five matched fins tends to have a greater effect over (a set of) four than does six matched fins over five. The amount of improvement levels off at five. In other words, you get a certain percentage of improvement in the margin with each fin being added up to the fifth fin; but the percentage of improvement is outside the trend (on the low side) once you add that sixth fin.

I spotted this on several designs, line Nova and Loadmaster. With larger-diameter tubes, the percentage of improvement may get back into the trend; bit at least up to ST-10 size models, five is the optimum value for max-fins.

My initial observation suggests there is a degree of aerodynamic "blanking out" of the effect of the fins when they start getting too close to each other.

[snaquin]

But six fins are so darned cool .....

Here's six fins mounted in a 4" diameter tube. It's a modified LOC Ultimate with a single 54mm motor mount. If you are interested email or PM me and I'll send you the fin pattern.

It's a proven design, and the original uses 7x29mm motors. The stock kit will fly great on one, three, four or seven motors.

:D

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[DaveR]

You can slot the tube yourself. Back in the olden days we all did.

How exactly do you slot your own tubes?

[stefanj]

How exactly do you slot your own tubes?

A sharp knife. Having a piece of aluminum angle stock to guide the cuts helps a lot too.

[DaveR]

Thanks again for your input. It'll be interesting to see how things turn out.

[snaquin]

How exactly do you slot your own tubes?

I slot mine just as Stefan suggested. I have a piece of aluminum angle from Home Depot 1/2 x 1/2" x 72" that I cut down with a hack saw to about 24" long to have a more manageable length. I mark lines for the centers of my slots where the fins should go with the aluminum angle. If the fins are 1/8" thick, I go back and mark another line 1/16" on each side of the first marked line and extend the marks the length of the root edge of the fins. I do this to center the slots/fins and adjust accordingly depending on the fin thickness I'm using.

I use the aluminum angle and a sharp knife to score the lines where I'm cutting the slots. I prefer to use a heavy duty utility knife for this because it doesn't flex and break as much as a #1 blade will and works better in heavy walled cardboard tubing. You can get that Husky knife at Home Depot for around $10 and it's a great investment if you don't already have one. I don't like a knife with a retractable blade, this blade locks and replacing the blade is very easy and secure. Don't try to cut too deep in a single pass, score it a few times gradually increasing the depth of the cut into the cardboard. After a few passes I drop the aluminum angle and just keep scoring in the groove by hand until the blade cuts completely through. Notch out the ends of the slots with a #1 blade. Sand any flash out of the inside of the tube after so it doesn't interfere when you push the centering rings and motor mount up past the slots on the inside surface.

If your slots are a little sloppy, don't fret. Fillets on the fin to body tube joint will cover the small spots if you do happen to cut a little crooked here and there. The original 4" diameter BSD Little John had to have the fin slots cut. Notice on this model I didn't slot to the end of the tube. The fins on this model mount slightly forward of the rear of the body tube so as not to interfere with the centering rings. You may consider this on your design as well before you cut your slots, depending on how your fins will mount - so they don't interfere with centering rings, ect. The fins extend inside all the way to the 54mm motor mount on this model and the rear centering ring butts up against the rear of the root edge of the fins on the inside. Fillets finished it out well.

.

[Gus]

You can also use a Dremel to cut fin slots on larger tubes.

[snaquin]

Dave,

Not sure how long your 4" rocket will be but I do have RockSim files for several 4" flight proven designs I can post fin patterns or files for you if you would like to see a few fin examples for ideas.

Mad Pup 4" dia. about 72" long with a boatail, swept fins (Dimensions same as Dangerous Dave's Composites kit, Long OOP)

LOC EZI-65 Stock Kit about 4" dia. x 56" long

MODS Ultimate, 4" dia. x 58" long with TTW fins and 54mm mount (Same as the images I posted earlier)

If you have RockSim I'll dig out the files and post here. If not I'll have to try to print and scan the patterns from RockSim to post here. I no longer have access to the full version of Adobe Acrobat to use Distiller or Print To PDF.

Let me know if this will help.

[Solomoriah]

PDF995 does a fine job of creating PDF's, and you can use it for free as long as you don't mind the advertisements.

[CPMcGraw]

...I no longer have access to the full version of Adobe Acrobat to use Distiller or Print To PDF...

Download the latest release of OpenOffice (http://www.openoffice.org). Writer has a built-in "Save As PDF" feature, and produces good output. I used earlier versions to produce some of the BARCLONE instruction sets a few years ago. If you don't have a Java Runtime already installed, get the full installer file with Java included.

I've been very happy with OpenOffice, especially for this feature. The fact that it's priced right (FREE!) doesn't hurt...:D

[Solomoriah]

... but OpenOffice.org won't enable him to PDF from RockSim. I'd have recommended OOo otherwise.

[CPMcGraw]

... but OpenOffice.org won't enable him to PDF from RockSim. I'd have recommended OOo otherwise.

Oh, so you're looking for accuracy, too? :rolleyes:

Always the details!

[snaquin]

PDF995 does a fine job of creating PDF's, and you can use it for free as long as you don't mind the advertisements.

PDF995 - Awesome program! I've been using it all day, works great. I downloaded the three program suite so I can combine .pdf documents into a single file.

And yes I do have OpenOffice and the print to .pdf in that program works great for OO files. When I bought my new PC I loaded OO on it since I only a license at work for MS Office.

Thanks for your help guys - I appreciate it

When I get back to the house later I'm going to find those RockSim files on my back up and print some .pdf pages to post.

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[snaquin]

RockSim files and .PDF files (thanks again!) for a few 4" dia. rockets. Fin patterns are in the .PDF files with basic dimensions, CP and dry CG numbersfrom the program.

Dave - feel free to use the fin patterns as ideas for your own similar sized scratch built 4" dia. design, or just to play with the RockSim files if you have the program or the Demo.

The designs are flight proven, except for the dual deploy Ultimate with LOC Electronics Bay. It was about 75% finished when I lost interest and put it down last year. I'll pick up building it again eventually although I don't fly very much HPR these days .....

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[snaquin]

RockSim 3D Image Exports for the designs .....

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[DaveR]

Dave - feel free to use the fin patterns as ideas for your own similar sized scratch built 4" dia. design, or just to play with the RockSim files if you have the program or the Demo.

The designs are flight proven, except for the dual deploy Ultimate with LOC Electronics Bay. It was about 75% finished when I lost interest and put it down last year. I'll pick up building it again eventually although I don't fly very much HPR these days .....

.

Sorry for the delay in responding, been out of town. Thanks for posting the fins patterns, I will definately be taking a look at them. I'm just looking for some fin patterns that will be proportionate to the tube size. I don't do HP, (nothing against it, just can't afford it :o ) so she'll probably never fly, but ya never know. Thanks again for all the insight, actually I'm looking forward to attempting to cut my own slots, but I'm thinking I'd should practice on some old tubes first.
Any ideas on how or where to find a fin marking guide for a 4" tube, either 3 or 4 fin?

[barone]

Sorry for the delay in responding, been out of town. Thanks for posting the fins patterns, I will definately be taking a look at them. I'm just looking for some fin patterns that will be proportionate to the tube size. I don't do HP, (nothing against it, just can't afford it :o ) so she'll probably never fly, but ya never know. Thanks again for all the insight, actually I'm looking forward to attempting to cut my own slots, but I'm thinking I'd should practice on some old tubes first.
Any ideas on how or where to find a fin marking guide for a 4" tube, either 3 or 4 fin?
Dave,

I think I added some tools for you to use on the discs I made for you. One of them is a program for making tube marking guides by entering the diameter of the BT and the number of fins. If you can't find it, try this link.....

http://www.42nd-dimension.com/rocketry/templates.html

[DaveR]

Dave,

I think I added some tools for you to use on the discs I made for you. One of them is a program for making tube marking guides by entering the diameter of the BT and the number of fins. If you can't find it, try this link.....

http://www.42nd-dimension.com/rocketry/templates.html

Thanks Don, but I've already tried that. I get an error that says, "Template is too large for selected paper." I even tried using legal paper, same results.

Edit: Also tried using landscape paper orientation with both letter and legal paper, same results

[barone]

Well Dave......here's how I used to do things when I didn't have a template or guide. And I didn't use geometry because I had to think too hard. Piece of string wrapped around the body tube and marked where they come together (circumference). Measured and laid out on paper. Then divide by the number of fins. This gives you a line edge for your guide plus the dimensions for your fin placement. Just need a parrallel line and right angles to complete your template. By math....C=d*pi. Again, find circumference and divide by the number of fins for placement along the circumference. For a 4" diameter BT, the circumference would be about 12.57" (that's why it wouldn't work on 8.5X11 paper). For three fins, they would be seperated by about 4.19" and four fins would be seperated by about 3.14".

[DaveR]

I've been working on this on and off for almost 4 years. I chickened out and surface mounted the fins versus going TTW, but t has some serious epoxy fillets..don't matter much though she'll probably never fly.

Anyway, finally painted her yesterday--now I just need some decals.

I give you the "Der Kolossal Red Max"..........

[Solomoriah]

Probably never fly... why?

[barone]

Because he forgot the motor mount..... :chuckle:

Just kidding. That looks great Dave. Well worth the four years. Now just design some decals and get Gordy to make them.

I've got an 8' rail we can launch it with if you've got some rail buttons. I've got some somewhere for my plastic conversion but haven't been able to locate them yet.

[DaveR]

Probably never fly... why?
I guess I'll fly it someday, it's just condsiderably larger than what I'm used to. It weighs over 2 pounds empty.

Because he forgot the motor mount.....

Just kidding. That looks great Dave. Well worth the four years. Now just design some decals and get Gordy to make them.

I've got an 8' rail we can launch it with if you've got some rail buttons. I've got some somewhere for my plastic conversion but haven't been able to locate them yet.

Nah, it's got plenty of motor mounts - 3X29mm, and a launch lug, rail buttons may be a better option though.

Decals are another story, it'd take a ream of decal paper....

[Mark II]

I guess I'll fly it someday, it's just condsiderably larger than what I'm used to. It weighs over 2 pounds empty.



Nah, it's got plenty of motor mounts - 3X29mm, and a launch lug, rail buttons may be a better option though.

Decals are another story, it'd take a ream of decal paper....Three F motors should get it going good. It wouldn't need high power, just mid power.

If you want decals for a 4" diameter rocket, vinyl is the better way to go.

http://www.stickershock23.com/Der_Red_Max.html

[GregGleason]

Three F motors should get it going good. It wouldn't need high power, just mid power...



The aggregate of three F's, though mid-power motors, might put it into the HPR realm. I'd have to look at Doug Sam's handy-dandy graphic to be sure though.

For AeroTech RMS, a G71R (max liftoff mass of 2135.124 grams) or a G76G (max liftoff mass of 2564.162 grams) should do the trick. It won't fly high, but should be safe AND impressive.

For AeroTech SU, a G80T would work (max liftoff mass of 1966.163 grams).

I would sim those 3 motors and see how they do.

The liftoff mass is per my calcs. YMMV.

Greg

[Doug Sams]

The aggregate of three F's, though mid-power motors, might put it into the HPR realm. I'd have to look at Doug Sam's handy-dandy graphic to be sure though.I too was thinking about it being HPR. A full F is 80Ns. Using the nominal woosh factor for APCP (~specific impulse) of 2Ns per gram, then a full F would be 40g of propellant. So three of them is getting close to the max limit of 125g. If the motors use any of the milder compositions, then it could well be over 125g. OTOH, many F motors are well shy of the max impulse, so, even if they use one of the milder formulas, they may still be under 125g. Gotta look at the mfgr spec sheets or TMT/NAR cert docs to be sure.

What should be of greatest concern is getting three APCP motors lit at the same time. It's a whole lot harder than lighting three BP motors. My two attempts at siimultaneously lighting composite motors weren't very good. I got one of two Road Runner F's lit for an airstart with no ill effects. But the 2 of 4 G's that lit on my upscale Ranger left the rocket retired on its maiden flight :(

I haven't really mastered this yet, so I can't offer a lot of advice. But ignitors with bridge wires were suggested to me.

I've wondered about but haven't tried effecting the Cesaroni style ignition using an e-match to ignite a stationary BP pellet at the top of the motor. Not sure the kosherity of doing it that way ;)

Doug

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[DaveR]

I too was thinking about it being HPR. A full F is 80Ns. Using the nominal woosh factor for APCP (~specific impulse) of 2Ns per gram, then a full F would be 40g of propellant. So three of them is getting close to the max limit of 125g. If the motors use any of the milder compositions, then it could well be over 125g. OTOH, many F motors are well shy of the max impulse, so, even if they use one of the milder formulas, they may still be under 125g. Gotta look at the mfgr spec sheets or TMT/NAR cert docs to be sure.

What should be of greatest concern is getting three APCP motors lit at the same time. It's a whole lot harder than lighting three BP motors. My two attempts at siimultaneously lighting composite motors weren't very good. I got one of two Road Runner F's lit for an airstart with no ill effects. But the 2 of 4 G's that lit on my upscale Ranger left the rocket retired on its maiden flight :(

I haven't really mastered this yet, so I can't offer a lot of advice. But ignitors with bridge wires were suggested to me.

I've wondered about but haven't tried effecting the Cesaroni style ignition using an e-match to ignite a stationary BP pellet at the top of the motor. Not sure the kosherity of doing it that way ;)

Doug

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That's why I don't think it'll ever fly, way too much thinking there...makes my head hurt just reading your post. :p I'm a KISS method kinda guy.

As Greg was saying, If there's a lone SU or even a reload that would have enough "umph" to get her going without requiring a cert, I'd probably go that route. Really just haven't done any homework on this yet.

Hadn't considered vinyl decals, good idea. :cool:

[Doug Sams]

As Greg was saying, If there's a lone SU or even a reload that would have enough "umph" to get her going without requiring a cert, I'd probably go that route. Really just haven't done any homework on this yet.If you have the single motor option, yes, go for it. Reading the thread fast and loose, I was under the impression you had only the 3x29 option.

It shouldn't be too difficult to find a 29mm G motor with enough thrust for a 2+ pound rocket.

Doug

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[DaveR]

If you have the single motor option, yes, go for it. Reading the thread fast and loose, I was under the impression you had only the 3x29 option.

It shouldn't be too difficult to find a 29mm G motor with enough thrust for a 2+ pound rocket.

Doug

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I apologize, I wasn't clear on the motor mount. It's 3x29mm but they are "inline" so there's a central mount.