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

[Ben Martin]

I am assuming that having a ~3in fin span should be fine and won't be inefficient due to the stubby nosecone pushing air around them? Anyone have experience with this? The simulations show that it has a really solid stability margin even with the biggest motors, but I have heard not to always trust the sims.

 

[Nytrunner]

Very interested in how those fins perform.

Where'd you get your carbon rods?

 

[rottenbob2]

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.

View attachment 363478

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

 

[Ben Martin]

Very interested in how those fins perform.

Where'd you get your carbon rods?

Amazon
https://www.amazon.com/dp/B000BQT8QK/?tag=skimlinks_replacement-20

What is your view on my stubby nosecone? It's only 5in exposed. Openrocket shows it as stable but I think that it will mess up with the air flow of the stubby fins.

 

[Nytrunner]

Nose cone's ~1.6 aspect ratio, and you want to fly M1.7 with a 1 diameter fin semispan.

My free and instant opinion is that shouldn't be much of an issue. Look at OR's graph of "stability over time" and pay attention to the CP/CG shift as your speed increases.
My still-free but needs-time opinion will require a single piece solid model of your rocket lol.

 

[Ben Martin]

Nose cone's ~1.6 aspect ratio, and you want to fly M1.7 with a 1 diameter fin semispan.

My free and instant opinion is that shouldn't be much of an issue. Look at OR's graph of "stability over time" and pay attention to the CP/CG shift as your speed increases.
My still-free but needs-time opinion will require a single piece solid model of your rocket lol.

Looks like the stability margin gets pretty low through mach, not sure how much of an issue it would be.

Screenshot of the rocket


I am having issues with OpenRocket and not being able to edit anything. When I try to edit something, the GUI does not show up. I have tried restarting my computer and removing my second monitor but with no luck. It's a very weird issue.

 

[Nytrunner]

I think the fins are fine sizewise, but the balance of the rocket is a bit tight.

There's another plot style that shows CP and CG shift over time as well.
You could probably benefit from some nose weight to shift the CG forward a couple inches.

Your rocket aspect ratio (L/D) is approaching superroc territory (L/D>20) and the CP shift will be greater with AoA changes, so one "caliber" may not be sufficient at high speeds.
I personally aim for a CP-CG separation (static margin) of 12% rocket length. Currently, yours is ~6.6% (that's even below the old NASA sounding rocket range of 8-15% vehicle length)

Have you showed your designs and project to any professors in your program? Sometimes it's interesting to get an outside of hobby perspective (and good networking too)

 

[Nytrunner]

Apologies for double post, but I just realized you're sending this 2-3/4 miles up with only a single parachute.

That could be quite a walk without some sort of dual deployment or chute reefing technique.

 

[Ben Martin]

Apologies for double post, but I just realized you're sending this 2-3/4 miles up with only a single parachute.

That could be quite a walk without some sort of dual deployment or chute reefing technique.

I plan on using chute release.

 

[Ben Martin]

I think the fins are fine sizewise, but the balance of the rocket is a bit tight.

There's another plot style that shows CP and CG shift over time as well.
You could probably benefit from some nose weight to shift the CG forward a couple inches.

Your rocket aspect ratio (L/D) is approaching superroc territory (L/D>20) and the CP shift will be greater with AoA changes, so one "caliber" may not be sufficient at high speeds.
I personally aim for a CP-CG separation (static margin) of 12% rocket length. Currently, yours is ~6.6% (that's even below the old NASA sounding rocket range of 8-15% vehicle length)

Have you showed your designs and project to any professors in your program? Sometimes it's interesting to get an outside of hobby perspective (and good networking too)

I would mess around with nose weight but I legitimately can't edit anything in OpenRocket. Not sure if I will have to uninstall and reinstall it.

 

[watheyak]

I don't think motor deploy is going to be an option, either. Unless there's something I'm missing.

 

[Ben Martin]

I don't think motor deploy is going to be an option, either. Unless there's something I'm missing.

Motor deploy is the plan for pretty much all the launches including the one that will reach 13K+. Even the L1000 is a DMS with adjustable delay. I'll have to make sure that the delay is long enough, if not, I can change out the baffle system with dual deployment. All of this is thought of in the design.

 

[watheyak]

Motor deploy is the plan for pretty much all the launches including the one that will reach 13K+. Even the L1000 is a DMS with adjustable delay. I'll have to make sure that the delay is long enough, if not, I can change out the baffle system with dual deployment. All of this is thought of in the design.

Ah cool. The L1000 is 18 seconds max, which is about 4 seconds short of the sim above. Adding weight to the nose might help shorten the coast a bit as well. Dual benefit!

Double clicking on the components in the tree doesn't bring up a dialog? I guess I'd try deleting and reinstalling OR.

 

[Nytrunner]

If you haven't tried it, check out Neil_w's Openrocket installer (should be in software subforum)

 

[Ben Martin]

Any advice on slotting the airframe? I don't have any fancy tools.

 

[watheyak]

They're Loc tubes, right? I've laid them out carefully with a pencil and cut them out with an Xacto knife. Take your time on the first few light cuts, then you can increase pressure and go a little faster.

 

[Nytrunner]

They're Loc tubes, right? I've laid them out carefully with a pencil and cut them out with an Xacto knife. Take your time on the first few light cuts, then you can increase pressure and go a little faster.

+1 on this, that's how I do all mine.

Print out a fin guide wrap from payloadbay.com instead of trying to lay them out with calipers or angles (youre smart and could do it, but it's a pain)

A big help is buying a shirt length of aluminum angle. Makes scribing straight lines down a tube a breeze

 

[Ben Martin]

They're Loc tubes, right? I've laid them out carefully with a pencil and cut them out with an Xacto knife. Take your time on the first few light cuts, then you can increase pressure and go a little faster.

They are thick "mailing" tubes, but they are really similar.

 

[Ben Martin]

My OpenRocket is still not letting me edit parts, even after a fresh reinstall. This is really weird.

 

[Ben Martin]

Update!

I made some considerable progress on the rocket, including realizing some mistakes I made during the design.

To start off with I epoxied the motor mount and centering rings (excluding rear) into the body tube. I then cut down carbon fiber rods which was a... process. Ended up using a dremel tool on a wet paper towel and a vacuum. The dust was either sucked up or stuck to the towel, keeping my lungs safe. I had safety glasses on of course. Then came the issue of actually getting them into the fins, which I found out that it is NOT a good idea to hammer carbon fiber rods because they still turn into a palm tree really fast. It also ended up cracking the fin which confirmed that the layer adhesion is excellent as it did not separate at the layers. I now need to reprint 2 more fins and drill all them out or print 4 new fins that have bigger holes. Tough decision considering drilling PETG normally melts it but I don't want to waste material on new fins.

On the bright side, good progress was made on the rocket overall and it is starting to take shape. Only things left are the fins, fillets, rear centering ring, motor retainer and the recovery system. All of which are already planned out and ready to go besides the fins.

I plan on making this my L2 certification rocket, I just hope that the baffle doesn't take too much of the energy away from the ejection charge and that the parts hold up.