Just How Important Is Fin Symmetry

[Funkworks]

A project I have is to build a two-stager using Loc's 38 mm MMT Terrier booster. It has 4 plywood fins. The instructions don't mention anything about sanding the fins (I think) but I did so anyway and I did it by hand with sandpaper over the last 6 months (4 fins x 4 edges x 2 sides = 32 surfaces to sand, so about 1-2 per week), because I'm turning into a city-boy bookworm with no room for a shed or heavy machinery. Anyway, here's what the fin tips look like today:

And here's what the fin roots look like (2-inch smurf for scale):

It's for a 2-stage rocket, so my concern is that if anything about the fins isn't perfectly symmetrical, it will spiral or veer off the vertical, and then the 2nd stage won't light up when it's supposed to. I'm expecting to reach over 4000 ft.

My question: Is this sanding job good enough to keep a boosted rocket flying straight until the second stage lights up? Or is it worth going the extra mile and use a planer while I might have free access to one for a few weeks.

 

[teepot]

They look good to me.

 

[David Schwantz]

I think they will be fine. But if worries you, you could always sand the bigger ones down to the smaller size.

 

[Zeus-cat]

I have a rocket that looks like it already crashed. All 4 fins are different sizes and shapes. It flies straighter and has less rotation than most of my rockets.

 

[Banzai88]

You'll be fine. Lots of folks do WAY WAY worse, and their rockets go up and down just fine, too.

 

[BABAR]

Your rocket will fly just fine without absolutely perfectly symmetrical airfoils. About the only time in model Rocketry if makes a big difference is competition birds and rockets with cameras, the latter makes the footage entertaining to some, nauseating to others.

The following pics go with the END of this post, but system puts them here.

Many REAL sounding rockets used intentional spin stabilization, including booster in this multistage designed to lift a vehicle for hypersonic testing (no, I don’t think the BOOSTER was designed to go hypersonic)


The vehicle is accelerated by the S30 motor for 28 seconds until burnout at an altitude of 17 km. At burnout of the first stage the vehicle has already reached a velocity of 930 m/s, respectively Mach 3.17. The fins of the 1st stage are canted and induce a spin rate of approximately 1 Hz for stabilization.

From
https://core.ac.uk/download/pdf/11103973.pdf
Also

https://www.apogeerockets.com/education/downloads/Newsletter228.pdf

Finally, here are a couple of model rocket asymmetric fin boosters, the first with additional outboard asymmetric thrust, that flew fine.


https://www.rocketryforum.com/threa...rt-sustainer-15-gap-stage-paint-scheme.60249/


Scroll to post 4
https://www.rocketryforum.com/threa...w-sr-73-raven-glider-and-new-gyskelion.38883/

 

[shockie]

If the fins aren't exact duplicates of one another, from an aerodynamic point of view, there will be small force imbalances created during the flight, that could affect the trajectory. Is that what you are asking? The same would be true for differing surface finishes on identical flat plates for example, rough versus smooth.

But then the boundary layer could be tripped forward of the fins and then it wouldn't probably matter.

As long as they are on straight, that would go a long way in ensuring a good flight with little disturbances.

 

[jqavins]

My question: Is this sanding job good enough to keep a boosted rocket flying straight until the second stage lights up? Or is it worth going the extra mile and use a planer while I might have free access to one for a few weeks.

Why not both? Plenty of others who know better that I have stated what I would have, which is that it will fly fine if you use them as is. The rocket is more likely to spin with those fins than it would be with perfectly symmetrical and identical ones. So the question isn't "Are they OK?" but rather "Are they OK with you?"

Go ahead and plane them to pefection if you it would make you feel netter about them. In your shoes I probably would, all the while silently asking "Why the heck am I bothering to do this?"

 

[BABAR]

Not to disparage Smurfs, but if you make things TOO perfect it just won't be Smurf-Like. Sort of like some furniture and blue jeans are intentionally "weathered" before they hit the markets

 

[jqavins]

The smurf was included only as a reference for scale. This is a (very!) different project. (He did say it's a booster with a 38 mm motor mount and aiming for 4000 ft. That would be smurfy beyond the smurfy smurftitude that smurfulations indismurf for the smurf rocket.)

 

[BABAR]

The smurf was included only as a reference for scale. This is a (very!) different project. (He did say it's a booster with a 38 mm motor mount and aiming for 4000 ft. That would be smurfy beyond the smurfy smurftitude that smurfulations indismurf for the smurf rocket.)

My bad

 

[David Schwantz]

What's a Smurf??

 

[jqavins]

It's what you get if you cross a Keebler elf and a blueberry.

https://www.rocketryforum.com/threads/a-rocket-for-a-smurf.159959/

 

[David Schwantz]

Like blueberries !!

 

[Zeus-cat]

Some photos of my FAB rocket. The fins are SLIGHTLY asymmetrical. In the video of the WSR club launch you can see FAB being launched at 11:26. Notice that there is no rotation as the rocket goes up.

WSR Launch

 

[BABAR]

The fins are SLIGHTLY asymmetrical.

Amateur!



https://www.rocketryforum.com/threa...ns-spaced-at-120-degrees.121768/#post-1407277

 

[jqavins]

It wanted to spin, but couldn't figure out which way.

What does FAB stand for?

 

[jqavins]

(Imported post from BABAR's linked thread above)

Skip to Post 22 for Flight Report

In my continuing exploration of asymmetric fin designs (perhaps better described as "How NOT to build a rocket") I came up with a modification of the basic 3FNC design. This is the SlipStream (alternative name was WhackJob, that just didn't have the ring I was looking for.)

The question was, if I did the standard 3 fins, spaced 120 degrees from each other, but put ALL of them on one side of the rocket, would it fly, and would it fly well? For those who think I wouldn't post this if it DIDN'T fly well, I'd have to say they need to look at a few of my past attempts (the Stinker was particularly well named.)

Anyway, feel free to vote in the poll or not. I'll give it a few days, and then post the flight report.
View attachment 251031View attachment 251032View attachment 251033View attachment 251035View attachment 251036View attachment 251037View attachment 251038
As always, comments (positive and negative) appreciated.

OK, here's are two comments. 1) You are sick, twisted rocketeer, and we all knew that already. 2) I like your dog.

 

[BABAR]

(Imported post from BABAR's linked thread above)OK, here's are two comments. 1) You are sick, twisted rocketeer, and we all knew that already. 2) I like your dog.

Hey, what’s simpler than three fins and a nose cone?


EVERYBODY loves Lucy, our Lab.

I go to the bank or the pharmacy by myself, I have to show ID.

I go with Lucy sticking her head out the back window, it’s “Oh you’re LUCY’S dad!” I don’t even have to identify myself.

She’s also a RINO (Retriever In Name Only), she’ll fetch a ball twice and get bored. Interestingly, she never barks with one exception, she barks during rocket countdowns. It’s really weird, that is the only thing she barks at. Rocket launches themselves, both low power and high power, don’t bother her at all.

 

[jqavins]

Is she maybe barking in excitement? "Bark bark. Oo, oo, he's about to launch a ROCKET! Bark!"

 

[Zeus-cat]

...What does FAB stand for?

FAB is from the old Gerry Anderson TV show Thunderbirds. The marionettes always said FAB that sort of meant things were going well.

More FABs than you ever wanted to hear: FAB

 

[David Schwantz]

I have 3 labs. They never stop fetching!

 

[modeltrains]

Interestingly, she never barks with one exception, she barks during rocket countdowns. It’s really weird, that is the only thing she barks at. Rocket launches themselves, both low power and high power, don’t bother her at all.

I don't know what kind of dog logic that is, but I do know that ya gotta love dogs!
Hmm, wonder if similar logic is involved in Georgie cat sitting halfway across the dining room and talking at me while I load or unload the dishwasher?

 

[Sooner Boomer]

Hey, what’s simpler than three fins and a nose cone?

Two fins and a nosecone?
One fin and a nosecone?
Just a nosecone? Lots of cone rockets out there.
Just a body tube?(onna stick) Lots of finless rockets, too (https://www.rocketryforum.com/threads/another-finless-rocket.131953/)
Just a fin? Hmmmm.... Gotta think on that. Monocopter?

 

[Woody's Workshop]

I remember the ACME rocket, I think it was by Jim at Flis Kits?
But looking at that thing, you would think it would have no chance of flying.
I'd be more concerned about stability and recover A LOT more than uneven fins!
Sand em, Glue em, Paint em, Fly em. Enjoy!!!

 

[Funkworks]

Question has been answered. Great to know some asymmetric designs do fly straight. I agree that gluing the fins on straight is probably the most important thing at this stage.

The smurfs might observe but i don’t think they’ll be building this one. ;-)

 

[cbwho]

The dog barks during the countdown as she is doing the count down with you. As you bark out 5 4 3 2 1, she barks too!