What is considered as "too many fins"? (L2 high powered build)

[Ben Martin]

I have a 3in thick cardboard tube that I plan on building a rocket around, but to get it stable without an absorbent amount of nose weight, it would require 6 fins on the bottom.

Here are some details:
- 6 1/8in thick fiberglass fins, 2.5in tall (for anti fin flex) might try 3D printing some thicker ones
- 3D printed 10mm thick centering rings with fin alignment notches
- Thick cardboard main body tube (36.25in long, .1in walls, 457g)
- Thick cardboard upper tube and coupler
- 3ft ripstop nylon parachute (or 4ft) with chute release
- Nylon shock cord
- Quick connect links for both the inside U-bolt and the U-bolt on the coupler
- 3D printed nosecone with 4mm thick walls and 300g of bbs and epoxy in the nose
- 3D printed transition piece with screw holes for the motor retention system
- Large enough payload bay for dual deployment, altimeters, GPS, and cameras down the road.

Here is a screenshot of the OpenRocket file so far, and of course I don't plan on flying the most powerful motor it can fit on the first flight, nor do I even think the body tube could withstand it.



Any and all feedback is welcome, this is a future project for after I get my L1.

 

[Rocketjunkie]

Make sure you have the strands go multiple directions. In this video, the shock cord attachment lugs broke off along the strands rather than breaking filaments, dropping the fin unit.

 

[K'Tesh]

When N (Number of fins)/L (Launch) > N/R (Recovery)

[EDIT] I responded after a really stressful couple of days and only a few of hours of sleep.

 

[HASTE]

Go 8 fins and call it "rocktopus"
May be awkward aligning the rail to lugs though.

 

[BDB]

The only thing to watch for is the ability to get the rocket on the launch pad. Too many fins = not enough space between them for the buttons to slide on the rail. (Don’t ask me how I know.)

 

[Andrew_ASC]

Most people and kits stick to 3 or 4 fin designs to reduce plate area and drag force. You can use as many fins as you want as long as it still goes on a 1010 rail, your just making the rocket draggier and more expensive to make.

Try a Delta Trapezoidal shape for more stability.

 

[dr wogz]

too many fins?

When they all morph into a cone or solid mass at the bottom!!

(Actually, there is a point at which the air flow between closely spaced fins come to a stop, essentially acting like a big solid object..)

 

[MikeyDSlagle]

I like lots of fins, I don't like doing fillets on lots of fins however.
Stability wise though, you could make them less swept to increase drag on the leading edge. And you could lose the rear cone.
Find the middle ground, compromise:
Do you want the tail cone? If yes then add fins or nose weight.
Do you want lots of fins? If no then lose the tail cone and/or add nose weight.
Build it how you want it to look or what you are willing to live with for the largest motor you plan on flying regularly. Sometimes rockets we dream up just won't fly. Add nose weight or length for the big motors later if you decide to go that route.

 

[Ben Martin]

I like lots of fins, I don't like doing fillets on lots of fins however.
Stability wise though, you could make them less swept to increase drag on the leading edge. And you could lose the rear cone.
Find the middle ground, compromise:
Do you want the tail cone? If yes then add fins or nose weight.
Do you want lots of fins? If no then lose the tail cone and/or add nose weight.
Build it how you want it to look or what you are willing to live with for the largest motor you plan on flying regularly. Sometimes rockets we dream up just won't fly. Add nose weight or length for the big motors later if you decide to go that route.

It requires a ton of nose weight with only 4 fins.

 

[grouch]

You can get more nose weight by not adding nose weight if you want to help stability. Make the coupler a glued in baffle and use the payload bay to hold the chute. This moved the weight of your recovery train farther north. Also lets you use longer motors is so inclined. Add as many fins as you think looks cool. Multi fins are underrepresented in the hobby.

 

[Nytrunner]

Agreeing with the baffle suggestion. Moving the internals forward better utilizes weight you have to have anyway.

Try increasing the fin height to 3". If you're 3d printing them, it'd be interesting to see a tube to tip tapered design. Maybe 1/4" at the tube where the flutter moment is greatest, to 1/8" at the tips.
Beware print orientation though. If you have the layers running parallel to the tube, the flutter bending will act on the weakest orientation of the print.

 

[Ben Martin]

Agreeing with the baffle suggestion. Moving the internals forward better utilizes weight you have to have anyway.

Try increasing the fin height to 3". If you're 3d printing them, it'd be interesting to see a tube to tip tapered design. Maybe 1/4" at the tube where the flutter moment is greatest, to 1/8" at the tips.
Beware print orientation though. If you have the layers running parallel to the tube, the flutter bending will act on the weakest orientation of the print.

I will be printing them horizontally and with PETG (best layer adhesion)

How do you think the baffle and all the 3D printed parts are going to hold up to mach?

 

[Nytrunner]

The baffle's internal and should be fine as long as you make it strong enough (don't print it super thin, use ~20% infill, etc....You probably have more experience with the strength of your prints and the material)

Aero-heating shouldn't be an issue till ~Mach 2.6,so you should be fine for the most part. If you're nervous, put some aluminum tape on the leading edges.
The transonic region ~.8-1.2 is where you'll get the nasty turbulence and shock that the fins have to survive. Suggestion is to choose a motor that will punch you through that region quickly, or keep you below it.

Very interesting field of research, you won't regret reading up on transonic forces and effects.

 

[Ben Martin]

The baffle's internal and should be fine as long as you make it strong enough (don't print it super thin, use ~20% infill, etc....You probably have more experience with the strength of your prints and the material)

Aero-heating shouldn't be an issue till ~Mach 2.6,so you should be fine for the most part. If you're nervous, put some aluminum tape on the leading edges.
The transonic region ~.8-1.2 is where you'll get the nasty turbulence and shock that the fins have to survive. Suggestion is to choose a motor that will punch you through that region quickly, or keep you below it.

Very interesting field of research, you won't regret reading up on transonic forces and effects.

Is the shock cord connected to the baffle or somewhere else in the rocket? So pretty much the election charge would just shoot off the nosecone for parachute deployment?

 

[Ben Martin]

The baffle's internal and should be fine as long as you make it strong enough (don't print it super thin, use ~20% infill, etc....You probably have more experience with the strength of your prints and the material)

Aero-heating shouldn't be an issue till ~Mach 2.6,so you should be fine for the most part. If you're nervous, put some aluminum tape on the leading edges.
The transonic region ~.8-1.2 is where you'll get the nasty turbulence and shock that the fins have to survive. Suggestion is to choose a motor that will punch you through that region quickly, or keep you below it.

Very interesting field of research, you won't regret reading up on transonic forces and effects.

And what about the strength of cardboard tubes at mach? It's pretty thick but nothing compared to fiberglass.

 

[Nytrunner]

Shock cord can be connected wherever you decide to put it. Remember, there's no one perfect way to build these things.

What kind of cardboard tubes are you using? thick mailer tubes, LOC tubes, madcow? LOC and Madcaow have flown past Mach 1. Look up Eric Cayemberg's "LOC N-load" build as a good example.

You're an engineering student right? Analyze, design, experiment! Use this as a venue to reach beyond where you are in your classes and apply what you're learning.

 

[Ben Martin]

Shock cord can be connected wherever you decide to put it. Remember, there's no one perfect way to build these things.

What kind of cardboard tubes are you using? thick mailer tubes, LOC tubes, madcow? LOC and Madcaow have flown past Mach 1. Look up Eric Cayemberg's "LOC N-load" build as a good example.

You're an engineering student right? Analyze, design, experiment! Use this as a venue to reach beyond where you are in your classes and apply what you're learning.

I'm going to "science the **** out of it" - Matt Damon

And they are thick mailer tubes, OD of 3.23in, ID of 3.03in. Very thick for cardboard tubes.

 

[T-Rex]

I know that a standard 3" LOC tube with 6 fins easily fits on a 1010 rail.

 

[MikeyDSlagle]

From my understanding mailing tubes are wrapped different and not as strong as tubes my for rocketry. I have no experience building or flying them. It is just what I was told by several people. But I was talking about a ~7" tube on an M motor.

 

[Ben Martin]

From my understanding mailing tubes are wrapped different and not as strong as tubes my for rocketry. I have no experience building or flying them. It is just what I was told by several people. But I was talking about a ~7" tube on an M motor.

Not certain if it is a mailing tube as it used to have 3M tape wrapped around it. Pretty much a giant roll of scotch tape.

 

[markkoelsch]

What is the span on your fins? A good rule of thumb is to make the span at least as large as the body diameter. Possibly larger.

 

[Ben Martin]

What is the span on your fins? A good rule of thumb is to make the span at least as large as the body diameter. Possibly larger.

I was told differently if it is going through mach as there's fin flex. It is at 2.5 right now with a 3.1in diameter tube. They will likely be 3D printed so I want to limit the strain on them.

 

[boatgeek]

I don't have experience with this personally, but some other people who have been around the block here have said that if your fin span is less than your body diameter, your fins may not be very effective. If it were my rocket, I'd be shooting for at least 3" fin spans, maybe a little more, then make the fins a little thicker to offset flutter. The fin stiffness varies by something like thickness cubed, so a little more thickness gets you a lot more resistance to flutter.

 

[Ben Martin]

The issue is that I plan on 3D printing them, so a wider span means a significantly thicker fin, but I'll experiment a bit.

 

[Ben Martin]

The updated design with significantly thicker fins to account for the larger span.

Debating rather or not to use the tube I have now and attaching another to the top of it using a coupler or just finding a longer tube to use. Having two separate ones would make assembly much easier, but I am a bit worried about the strength of it.

 

[Nytrunner]

The issue is that I plan on 3D printing them, so a wider span means a significantly thicker fin, but I'll experiment a bit.

Wider fin true, but not necessarily across the whole fin. See my earlier suggestion about tapering from tube to tip.

The updated design with significantly thicker fins to account for the larger span.

Debating rather or not to use the tube I have now and attaching another to the top of it using a coupler or just finding a longer tube to use. Having two separate ones would make assembly much easier, but I am a bit worried about the strength of it.
View attachment 363537

If you go with the coupler, make sure the sliding side has at least one body diameter of length to it. The fixed side can be shorter although I wouldn't go less than half the tube diameter.

If both sides will be fixed through the flight and you just blow the nose, removable rivets or other fasteners can be used to strengthen the coupler connection. Heck, if you rivet it in, you can replace the baffle coupler with an eBay coupler when you want to expteriment with Dual deployment

 

[Ben Martin]

Wider fin true, but not necessarily across the whole fin. See my earlier suggestion about tapering from tube to tip.



If you go with the coupler, make sure the sliding side has at least one body diameter of length to it. The fixed side can be shorter although I wouldn't go less than half the tube diameter.

If both sides will be fixed through the flight and you just blow the nose, removable rivets or other fasteners can be used to strengthen the coupler connection. Heck, if you rivet it in, you can replace the baffle coupler with an eBay coupler when you want to expteriment with Dual deployment

I like the idea of having a replaceable baffle/dual deploy. I could use the plastic rivets to hold the tubes together (4 on top, 4 on bottom), although that could cause some structural issues. Maybe I'm just underestimating the rivets.

Right now the fins are 6mm thick, getting thinner at the edges due to me "airfoiling" them. I'm going to do a test print tomorrow and test their strength. I do like your tapering idea and I incorporated it into the design.

 

[Ben Martin]

Wider fin true, but not necessarily across the whole fin. See my earlier suggestion about tapering from tube to tip.



If you go with the coupler, make sure the sliding side has at least one body diameter of length to it. The fixed side can be shorter although I wouldn't go less than half the tube diameter.

If both sides will be fixed through the flight and you just blow the nose, removable rivets or other fasteners can be used to strengthen the coupler connection. Heck, if you rivet it in, you can replace the baffle coupler with an eBay coupler when you want to expteriment with Dual deployment

Do you know what the diameter and length of the aft closure of a 54mm motor is? I need it for the motor retention system I am designing and I can't find any reliable data from it. All I have found so far is a drawing showing the diameter of an aft closure which happens to be the same as the outside diameter of the tube I plan on using. However, I could not find the length.

My plan is to print the transition with screw holes and lip for motor, then have the motor slide into it and retained by a top plate with 6 M3 screws.

 

[tim cubbedge]

This drawing is for CTI 54mm cases. Not sure how AT differs.
https://www.pro38.com/pdfs/Pro54_dimensions.pdf

 

[Ben Martin]

This drawing is for CTI 54mm cases. Not sure how AT differs.
https://www.pro38.com/pdfs/Pro54_dimensions.pdf

Looks like the diameter differs a little bit, not certain about the length.