Fin shape and rocket stability

[Dane Ronnow]

I'm trying to reduce the amount of fin flex on landing (it's causing hairline cracks in the paint along the fillet) by reducing the size of the fin's trailing end. I started by chopping off 1.25" in OpenRocket, then running simulations that would indicate any reduction in stability, focusing on plots that include pitch rates.

Below are screen shots of the rocket with the unmodified fins. The first is the rocket profile in OR, showing a stability of 1.47 calibers. Next to it are plots with pitch rates—the first deviation being ~21.5 degrees.



Next, screen shots of the modified fins with 1.25" cut off the trailing ends. The stability has decreased to 1.41 calibers, with a pitch rate of ~21 degrees—a slight improvement.



Then I ran simulations with the fins shortened enough to allow the motor nozzle to extend below the trailing edges, thereby absorbing some of the landing impact. I also trimmed the corner of the trailing edge, thinking this might help absorb ground impact by tipping the rocket onto its side quicker than it would with a flat-bottomed fin.

The stability dropped to 1.38 (still good), with a significant reduction in pitch rate—down to about 16.5 degrees.



Is that fin shape workable? OpenRocket tells me it is, but I'm reminded of OR's trouble modeling certain design aspects (unusual transitions or stubby fins up by the nose cone come to mind). I'd hate to cut these fins down, thinking the design will be stable, only to have it do somersaults right off the rail.

I appreciate any input I can get—advice, criticism, suggestions. Thanks in advance.

 

[lakeroadster]

I wouldn't be concerned about stability, as long as your input data is good. What's the ground hit speed? Maybe a bigger chute would be a good modification.

But before you modify the fins, do a swing test of the rocket in "ready for flight" condition: chute, dog barf, motor, etc.

Then, after the fin modification, swing test it again. This should give you some confidence in the simulations and ensure stability before launching it.

Here's a link to Estes TR-1, a great guide on how to perform a swing test.

 

[BigMacDaddy]

Not a comment on the fin shape, but it seems like a pretty low likelihood that the motor in middle takes up any of the impact since rockets are usually swaying or moving laterally when they land.

 

[Dane Ronnow]

I wouldn't be concerned about stability, as long as your input data is good. What's the ground hit speed? Maybe a bigger chute would be a good modification.

Ground hit speed is 10 fps. The parachute is already oversized. I made it that way because I'm launching on dry lake bed—fairly hard surface.

No need for a swing test before the fins are modified. I've already launched this rocket in its present fin configuration. Four flights, all of them straight up, no wavering.

 

[lakeroadster]

Ground hit speed is 10 fps. The parachute is already oversized. I made it that way because I'm launching on dry lake bed—fairly hard surface.

No need for a swing test before the fins are modified. I've already launched this rocket in its present fin configuration. Four flights, all of them straight up, no wavering.

I don't doubt the stability. The reason I suggested that you do the swing test before you modify the fins is it will give you direct feedback on how stable the rocket is. I try to start spinning the rocket backwards, then you'll see how quickly it rotates so the nose is forward. And you can see if it is porpoising, or rocket solid stable.

You can then compare that to the modified fin configuration.

 

[Dane Ronnow]

Not a comment on the fin shape, but it seems like a pretty low likelihood that the motor in middle takes up any of the impact since rockets are usually swaying or moving laterally when they land.

By 'motor in the middle', I'm assuming you mean the second set of screen shots. In that fin configuration, the motor nozzle doesn't reach the ground. But it does in the third configuration. [Edit - I missed what you were saying at first. Sorry.]

In the first two launches of this rocket, the wind was blowing at less than 2 mph, and the impressions in the dirt indicated that the rocket hit the ground nearly vertical. You can see those landings here (first two videos):

https://www.rocketryforum.com/threads/2-6-inch-29mm-mpr-scratch-build.166792/page-3#post-2260049[Edit: I'm not suggesting this rocket will always land nearly vertical, just that it has, in low- to no-wind conditions. It doesn't circle underneath the parachute on descent.]

 

[Dane Ronnow]

The reason I suggested that you do the swing test before you modify the fins is it will give you direct feedback on how stable the rocket is.

Ah, yes. Apples to apples comparison. Good idea.

 

[RocketMike]

For all the math and computer wizardry we have now, sometimes there is still nothing better than the good old fashioned swing test!
Not that the computer isn't very useful though. I have a Rocksim license I paid for a couple of years ago and I hardly ever use it. I really should get more into it.

 

[dhbarr]

Oversized with a spillhole has a lot less swinging, so you tend to land fairly vertically.

Do your fins stick out one body diameter or more? The overall shapes look fine.

 

[Dane Ronnow]

Oversized with a spillhole has a lot less swinging, so you tend to land fairly vertically.

I'm sold on the spill hole. Admittedly, my launches have been in very light wind, but the descent, every time, is with the rocket hanging straight underneath the canopy, all the way down. The landing is not vertical, but very near to it.

Do your fins stick out one body diameter or more? The overall shapes look fine.

Not quite—2.265" on a 2.60" tube. [Edit - I moved that outboard tip back to its original position. It's now 2.75" on a 2.60" tube.]

 

[Dane Ronnow]

For all the math and computer wizardry we have now, sometimes there is still nothing better than the good old fashioned swing test!
Not that the computer isn't very useful though. I have a Rocksim license I paid for a couple of years ago and I hardly ever use it. I really should get more into it.

I will definitely be swing testing these fins. As far design and simulation programs go, I relied on OpenRocket heavily. Building the rocket, every part was weighed and measured. I had a mass object that accounted for paint and adhesives, which was adjusted for weight and location relative to CG as the build progressed through primer and painting. OR was very accurate in both CG and CP (I calculated CP manually and found it to be less than an eighth of an inch difference).

By the time it flew, I knew pretty much how it would perform in terms of altitude and length of delay. The FlightSketch altimeter showed me on the first launch how accurate my simulations were—pretty much dead on with the exception of altitude (1559 feet actual, compared to 1636 in OR).

In the end, OR took most of the guesswork out of it.

 

[RocketMike]

I played with OR too before I bought the Rocksim license. I found it to be perfectly fine so I don't even remember what the moment of weakness was that made me invest in Rocksim. I definitely need to start using it more though. Might as well get my money out of it.

 

[bjphoenix]

Fin AREA and fin POSITION both affect stability. The fin is acting to push the back end of the rocket back into the line of movement, so the larger fin exerts a larger force, and the fin placed farther back exerts more leverage. If you can move the fins forward but reshape them slightly that would help. I typically paper both faces of my fins and I think that gives them more strength to resist impacts, but also most of the rockets I've been building lately don't have any part of the fin extending beyond the rear airframe.

 

[Dane Ronnow]

I played with OR too before I bought the Rocksim license. I found it to be perfectly fine so I don't even remember what the moment of weakness was that made me invest in Rocksim. I definitely need to start using it more though. Might as well get my money out of it.

I had the Rocksim trial before I downloaded OpenRocket. For my use, the differences were insignificant.

 

[Dane Ronnow]

Fin AREA and fin POSITION both affect stability. The fin is acting to push the back end of the rocket back into the line of movement, so the larger fin exerts a larger force, and the fin placed farther back exerts more leverage. If you can move the fins forward but reshape them slightly that would help. I typically paper both faces of my fins and I think that gives them more strength to resist impacts, but also most of the rockets I've been building lately don't have any part of the fin extending beyond the rear airframe.

I can't move the fins anywhere. All I can do is remove fin area from the trailing ends. And, as you can see in that first OR profile in my leading post, those ends—or ears, as I call them—are quite large. The fin in the third profile down is what I'd like to get to, which is basically the same fin minus the ears. OR says the stability is good, but they look awfully small to me.

@lakeroadster suggested I do a swing test before cutting anything to set a benchmark for tests after cutting. I'm having a little trouble keeping that much weight moving fast enough to get reliable results. At this point all I can say is the rocket doesn't fly backward.

Regarding papering the fins, I did, with a single sheet of 50 lb. Avery label paper wrapped around the leading edge, then trimmed. I can I say without reservation that without the paper, I would have lost the fins in this rocket's third flight, when the chute fouled and the rocket came in hard and fast. I can't say how fast, but watching the video, I'd guess 50 fps or faster.

These fins are strong. It's the paint cracking along the fillet that is the problem. And I know that comes from flexing on impact. It came down to reinforcing the joint along the fillet, or removing those long tails on the fins to reduce leverage.. I chose the latter, because the former would just add weight, and I don't want any more in the aft end of this rocket. Now I've just got to get enough nerve to lop the ears off, clean up the ends, and launch it. That's my confirmation of stability. Definitely a heads-up launch.

 

[OzHybrid]

Whenever I do rearward fins, I use fiberglass sheet. They can take a lot of abuse. Anything less will result in breakage somewhere.

 

[Dane Ronnow]

Whenever I do rearward fins, I use fiberglass sheet. They can take a lot of abuse. Anything less will result in breakage somewhere.

Knowing what I know now, I'd do the same. Next build.