Stability effects of vertical surface at the end of a fin

[Duderino]

Very interesting stuff, thank you again.

This is prompted the question:





Grumman XSSM-N-6 Rigel

https://www.cradleofaviation.org/hi...s/exploring_space/grumman_xssm-n-6_rigel.html
I am thinking something similar to the ESTES Ramjet with that booster.

 

[JoePfeiffer]

OK, just spent a little while going through the CP code in OR, and it turns out this result is correct. OR attempts to allow for both interference between fins and the body (which makes the fins more effective -- calling it interference is arguably confusing, but it is the term), and interference between the fins (which makes them less effective). It also counts the number of fins on a body tube when calculating the body-fin interference.

So what's happening is when you've got both the main fins and the body tube test fins, the fins are less effective because of fin-fin interference. Lots of math later (waving my hands), the test case with fins-on-fins has CP slightly further back than fins-on-tube.

 

[BABAR]

OK, just spent a little while going through the CP code in OR, and it turns out this result is correct. OR attempts to allow for both interference between fins and the body (which makes the fins more effective -- calling it interference is arguably confusing, but it is the term), and interference between the fins (which makes them less effective). It also counts the number of fins on a body tube when calculating the body-fin interference.

So what's happening is when you've got both the main fins and the body tube test fins, the fins are less effective because of fin-fin interference. Lots of math later (waving my hands), the test case with fins-on-fins has CP slightly further back than fins-on-tube.

Interesting. I am not a sim-dude or a math-dude, and I understand that sometimes reality is NOT intuitively obvious. I have read that for staged rockets, with equilaterally places fins on booster and sustainer, that the rocket is MORE stable with the booster rotated for maximum MISalignment of the forward sustainer and rear booster fins compared to “in-line” alignment (think Apogee’s new SkyMetra as pictured), as “in-line” alignment places the rear fins in the “wash” of the forward fins.

Still, I figgered that putting the “vertical” fins on the lateral tips of the “horizontal” fins moved them further from the mid line of the rocket, so I figgered they would have a greater Moment of force than if the were directly placed on the body tube surface. Oh well. I had some very satisfying flights on the T-Bolt. Had the added benefit that descends with the fins “up” so doesn’t break the fins.


https://www.rocketryforum.com/threa...fin-design-think-c-141-kevlar-failure.124493/

 

[BABAR]

Very interesting stuff, thank you again.

This is prompted the question:


View attachment 415544


Grumman XSSM-N-6 Rigel

https://www.cradleofaviation.org/hi...s/exploring_space/grumman_xssm-n-6_rigel.html
I am thinking something similar to the ESTES Ramjet with that booster.

That would be seriously cool. Would be do able as a gap stage, EXCEPT the booster will literally “Go Ballistic” after staging unless you rig some sort of recovery device.

 

[JoePfeiffer]

Interesting. I am not a sim-dude or a math-dude, and I understand that sometimes reality is NOT intuitively obvious. I have read that for staged rockets, with equilaterally places fins on booster and sustainer, that the rocket is MORE stable with the booster rotated for maximum MISalignment of the forward sustainer and rear booster fins compared to “in-line” alignment (think Apogee’s new SkyMetra as pictured), as “in-line” alignment places the rear fins in the “wash” of the forward fins.

Something I found really interesting was a rocket Jim Jarvis built for *really* high altitude that had the booster on ball bearings, so it could rotate freely to minimize effects on it from sustainer.

Still, I figgered that putting the “vertical” fins on the lateral tips of the “horizontal” fins moved them further from the mid line of the rocket, so I figgered they would have a greater Moment of force than if the were directly placed on the body tube surface. Oh well. I had some very satisfying flights on the T-Bolt. Had the added benefit that descends with the fins “up” so doesn’t break the fins.

View attachment 416132View attachment 416132View attachment 416133
https://www.rocketryforum.com/threa...fin-design-think-c-141-kevlar-failure.124493/

My guess (which I haven't verified) based on experience with torque wrenches and the eternal "what angle does one put the wrench when tightening an AR-15 barrel nut" question is that moving the fin off center doesn't have any effect, since the location on a plate where you apply torque doesn't affect how much torque is applied. But I haven't really thought about it.

Regardless, coming down with the rudder up is a good thing!

 

[Charles_McG]

Is it mistaken to conflate the moment of lift of an offset plate fin with stability expressed as Cg/Cp?

In my mindsim, the extra momemt would show up as restorative force; which I -think- is different from Cp. Like how small fins on a slender rocket might sim as stable, but ends up coning all over the place.

But I'm willing to learn, if I'm wrong-thinking it.

 

[Senior Space Cadet]

See my post about acid trip inspired fins.
My theory, in building those fins, was that it would help more than it would increase drag.
I have no way to prove or disprove. I'd need to build a windtunnel with lots of sensitive instruments.

 

[BABAR]

I am coming in a little late to this party.

I think “tip-fins” added orthogonal to standard fins are doing a lot more than aircraft winglets. The aircraft winglets help to prevent the loss of Bernoulli effect lift from air “cheating” by going over the lateral wing edge rather than flowing straight from forward to trail edges of the wing.

The “tip-fins” are providing a completely different set of fin forces orthogonal to the fin upon which they are attached. Good example in addition to the Jayhawks is the Galactic Taxi
https://bellevillehobby.com/product...wiJNLrRxI4zd8Y_cVje9ljqqUI9x8NwcaAtF-EALw_wcB
Yes, there is probably some loss at the fin-fin attachment site, but I don’t see why that should be greater than the fin body site. And it would seem intuitively obvious (which as we often find out is not necessarily factually correct!) that moving the surface further from the center line should have a GREATER effect than surface mounting.

 

[58pan]

What about short fins, long winglets?
Drag is surface exposed to a top view, stability would be surface exposed to side view? innit?

 

[Nytrunner]

What about short fins, long winglets?
Drag is surface exposed to a top view, stability would be surface exposed to side view? innit?
View attachment 420233

Conect them together and you have a triangular ring fin! (No reason it shouldnt work if the surface area is right)

 

[BABAR]

Connect them together and you have a triangular ring fin! (No reason it shouldnt work if the surface area is right)

That would be my take as well. What I am NOT so sure about is whether their displacement from the midline axis makes them more or less effective than the same planar surface are mounted directly on the rocket body tube.

My Tank Killer had a square outside fin “ring”. I did something right with that bird, every flight was straight with absolutely ZERO rotation, perfect apogee deployment on every flight on D12-3 but the first one (motor ejected despite a motor hook). Last flight had a shock cord failure, nose got the chute, body did a tumble recovery that while not ballistic still pretty much destroyed the fins. Actually should be an easy rebuild,,,,

Anyhoo, the downside current proposed design is more interfaces/joints/corners which will create a bit more drag, but will still work. So you give up some peak velocity and altitude compared to your typical 3 fin and a nose cone rockets, and you add more weight. So you may need to bit more motor to overcome the additional weight and drag to get it up to speed. On the plus side, I theeeeeenk that as you are approaching a ring fin, it will have a bit less tendency to weathercock.




https://www.rocketryforum.com/threads/tank-killer-caught.157129/

 

[MALBAR 70]

What about short fins, long winglets?
Drag is surface exposed to a top view, stability would be surface exposed to side view? innit?
View attachment 420233

The Fliskits Tri-Glide works on this principal. The three foam gliders make a triangular ring fin during boost.




There are several other Fliskits models that use coplanar fins with smaller winglets attached, such as the Avalear and Stingray

 

[OverTheTop]

One of the reasons you don't see large flat areas away from airframes in commercial craft is strength. With the surface away from the airframe any forces generated get turned into bending moments. The structures between the area and the airframe have to be sufficiently sturdy to withstand these forces. Strengthening things costs mass, which costs fuel, or in the case of rockets payload capacity. The further from the airframe the more difficult this is, adding more mass. So While outboard super-winglets could work, they have other drawbacks which knock them off in the trade studies. They can look great though!

 

[Nytrunner]

They can look great though!

Which is like.....the main driver for a significant portion of sport rocketry

 

[OverTheTop]

Which is like.....the main driver for a significant portion of sport rocketry

Absolutely. Poetic license.

 

[BABAR]

Unless you are in it for competition, altitude, speed, or size records (all of which are worthwhile goals), 3FNC gets old fast.