Why Dual fins?

What are the reasons for rockets with dual fins? I have been looking through historical books of rockets, and aside from rockets with a second set of forward fins, I see very few with an aft dual fin setup. Is there an aerodynamic benefit from a dual fin setup vs. a single fin setup of similar shape and proportion? Is it only with hobby rockets because it looks cool? Does the forward fin (of an aft dual fin rocket) 'change the rules' for the aft fin? Why?

It's just for looks. Depending on the geometry, you can make some of them whistle - that's cool too. In general, they'll have more drag than something with similarly sized single fins.

Looks cool, and depending on the spacing between the fins and if you leave the edges square between them, they whistle nicely.

Well said!

Thanks. I was wondering about that. They do look cool.

Cool factor, and skill factor. Getting a series of fins to all line up takes a bit of skill & know-how..

dr wogz said:

Getting a series of fins to all line up takes a bit of skill & know-how..

Click to expand...

...or a 3d printer to make a super easy to use fin alignment guide.

GrouchoDuke said:

...or a 3d printer to make a super easy to use fin alignment guide.

Click to expand...

Or a ruler and a couple of small clamps.

Well I have an RB-05A Sport by Wildman, it's a scale missile kit. I suspect there's another reason to alter the center of pressure for missiles to do so for increased agility. Stock kit had a negative stability number before adding a pound of lead to it.

The dual fin setup will have less plate drag in drag force calculations as the "frontal" cross sectional area viewing from nose is less with a shorter fin span for the same amount of total fin area as a non dual fin setup has, is another technical reason I could think of as a mechanical engineer student. Meanwhile the top view or side view has an enlarged skin area from the dual fin setup. The benefits of sweep angles are related to mach number, shockwaves, and more material is from Ch.5 Wing Design you can google some really good aeronautical references to understand the importance of sweep angles and stability traits. Sweep angle will affect lateral stability and reduce stability in other aspects. Increasing sweep angle also reduces induced drag at expense of stability. Used a reference for a senior design project when I was designing fins for first time. From a rocketry perspective the lateral positioning of the dual fins will affect the CP position, which might be beneficial if you want a design to have a higher nosecone payload if you need more stability with a super heavy nose load. If any aeronautical or aerospace dorks want to chim in and bash what I've said... Go ahead.

The question I can't answer is when does skin drag from top area out weigh the plate drag from frontal area. Skin drag is likely heavily influenced by the boundary layers of gas particles acting on the skin surface. There's streaklines, pathlines, and various situations for turbulent or laminar flow. And right now I'm in a compressible gas dynamics course involving supersonic wind tunnel, rocket nozzle, and diffuser design. Didn't cover much of skin drag at all as a mechanical. They wanted more plate drag problems, head losses in pipes, and didn't give a darn about other stuff. Thread needs more dorks.

Andrew_ASC said:

Well I have an RB-05A Sport by Wildman, it's a scale missile kit. I suspect there's another reason to alter the center of pressure for missiles to do so for increased agility. Stock kit had a negative stability number before adding a pound of lead to it.

Click to expand...

What’s all that black crap on your fins & airframe?

That is one impressively messy glue up!

Rocketpoxy G5000 stuck to tape permanently without a s*** ton of sanding. Internal bulkheads are Cotronics 4700. I'll likely redo fillets when time allows. Already flown it on an I300T for an L-1 cert attempt. Airframe survived 5400ft fall and I got all the recovery gear back so it's flying again. Shock cord wasn't up to par. Stability was excellent despite 29mph wind at 5000ft. Built in a day. RSO said he's seen worse and one guy claimed to RSO fins just snap on, that was the feel better story, and I about fell over laughing. It was a side project from two semesters of working on rockets for university, we were going to a launch site and I figured, what the h*** I wanna launch something too. Named it LeadHead. No one laughed when nose smacked ground and stuck about 8" into ground. The thud of the nose hitting ground was louder than the airframe. No one argued about flying a pound of lead for balance versus the negative stock stability! Some stout 3/32" fins for sure. No cracks anywhere. Couldn't even kick nose lose, stuck in mud lol.

Chris_H said:

That is one impressively messy glue up!

Click to expand...

You guys really get OCD about rocket builds. It flew I assure you. Recovery wasn't fine. Professor said we were all going to get killed. University made us sign a bunch of forms because lawyers. We had to teach him about open rocket the day before his cert in a motel room. He had a bunch of team ummm assistance...With sanding. But he even certified on a minimum diameter Blackhawk29 with an H118 that went as far without a tracker in UROC... Aerotech booth yelled Hell YeahZ. Landed 40 yards from the pad while we all walked for miles. These RSOs just thought we were some pretty nutty noobs asking for FAA clearances with yard long multistage rockets on L-1 motors and certify attempts with trashy builds. Cuz Noobs. Project rocket was smooth as glass and still imploded first try. Second project build got top three in SEDS... Cert rockets weren't the top priority on quality dude.

That is not ocd- that is a mess. A few years ago, when my son was 10, he built a rocket. The fillets looked like that.


Sent from my iPad using Rocketry Forum

I appreciate the critism of the build quality. Any remaining issues will be corrected before its next flight and I may post an update in the future for further feedback.

Andrew_ASC said:

I appreciate the critism of the build quality. Any remaining issues will be corrected before its next flight and I may post an update in the future for further feedback.

Click to expand...

Andrew, build your rockets for yourself. If your rockets look like Fido’s butt, but stay together well, that’s what matters. Having an experienced mentor to show you the basics is a real advantage that not everyone has. You’ll get better.

Steve Shannon said:

Andrew, build your rockets for yourself. If your rockets look like Fido’s butt, but stay together well, that’s what matters. Having an experienced mentor to show you the basics is a real advantage that not everyone has. You’ll get better.

Click to expand...

(+1)

He a yoot - give him a break. Some day he will spend all winter building a purty-birdy, but not now while he is in school and time is limited.

I did not mean to be overly insulting. What I am getting at is that in my experience it does not take much additional time to make a fillet that does not need to be redone/cleaned up. Life is busy and redoing thing takes more time.

Figure out the size of the fillet. Run tape along the edge of the fillet. Apply epoxy. Use a rounded object (dowel, pvc pipe, gloved finger) to pull the fillet smooth. Remove tape and let set.


Sent from my iPad using Rocketry Forum