Camera Shroud Modelling (was OR: "Thick fins may not simulate accurately")

[atestani]

I am simulating dual camera holders/shrouds as a trapezoidal fin set and am getting this:


The warning appears if I have the thickness >0.75". Does anyone know any details about this warning, i.e. what does "may" mean and what inaccuracy am I dealing with (if any)?

I have a "fin" thickness of 1.075" and a 0.9" height for these 2 small camera holders/shrouds and they have a fairly large impact on altitude, ~8800 ft. vs ~10000 ft. if removed, hence my query on this.

 

[kjs]

Check your units... i got this but realized I had something like 3" fins (thickness...) You may have 3/4" thick fins or more...

 

[atestani]

Check your units... i got this but realized I had something like 3" fins (thickness...)

They look OK but thanks for the tip.

 

[kjs]

your fins are 1.075 *inches* thick... i did this also - had 6" fin thickness instead of 6mm... - so not "disparaging" you... ;-) just letting you know I was confused by the error message also (and the poor sims)....

 

[Azamiryou]

your fins are 1.075 *inches* thick... i did this also so not "disparaging" you... ;-) just letting you know I was confused by the error message also (and the poor sims)....

I think that's because they're not fins, they're camera shrouds, and they're really 1.075" thick.

I would guess that fins are not the best way to model camera shrouds, but I'm afraid I don't know what the best (or recommended) way is.

 

[bad_idea]

I would guess that fins are not the best way to model camera shrouds, but I'm afraid I don't know what the best (or recommended) way is.

I believe the best way to model the camera shroud in OpenRocket is as a pod-based cone and body tube of the same frontal area as the camera shroud. I do not believe the wide camera shroud will act like a fin, so I think modeling it as a fin is pushing OpenRocket's estimation of the CP unrealistically forward.

 

[atestani]

I have three ways to model the cameras as shown below but I believe how they are actually made should match. I have 3D Print files (stl) for the first 2 and can make the third, the pods approach. For the pods, while the model is "warning free", I wonder if it is the best from an aerodynamic point of view. matters). The first 2 have the thickness warning that I would like to understand.

The question here is what this warning is about and should I worry about it? I am concerned since removing the cameras gives me over 1000 ft. more altitude.

 

[Rschub]

If these drawings accurately reflect the final design of the shrouds, I think they will easily add 10% to the total drag.

 

[Azamiryou]

I believe how they are actually made should match.

Why?

 

[bad_idea]

I have three ways to model the cameras as shown below but I believe how they are actually made should match. I have 3D Print files (stl) for the first 2 and can make the third, the pods approach. For the pods, while the model is "warning free", I wonder if it is the best from an aerodynamic point of view. matters). The first 2 have the thickness warning that I would like to understand.

I am going to restate my thoughts from the other thread again in case they weren't clear.

1) Your camera housing shape is fine. Neither I nor anyone else suggested that you redo the camera housing as a tube with a nosecone.

2) Your camera housing shape is very unlikely to act as a fin aerodynamically.

3) Open rocket treats fins differently from body components when determining the center of pressure.

4) Calling your camera pod a fin though it actually isn't acting as one is artificially moving the center of pressure further forward than it should be.

5) Therefore, modeling the camera housing in OpenRocket as if it were a pod with nosecone and tube of the same frontal area as your camera housing should give you a more accurate and less drastic estimation of the forward shift in center of pressure that your camera housing will have.

No changes to your actual camera housing needed. The only change is how you represent the housing in OpenRocket.

 

[Charles_McG]

I believe the thick fin warning is to let you know you’re violating assumptions built into OpenRocket. Your mileage may vary.

Remember, OR isn’t CFD, nor a physics sandbox.

 

[lakeroadster]

I have three ways to model the cameras as shown below but I believe how they are actually made should match. I have 3D Print files (stl) for the first 2 and can make the third, the pods approach. For the pods, while the model is "warning free", I wonder if it is the best from an aerodynamic point of view. matters). The first 2 have the thickness warning that I would like to understand.

The question here is what this warning is about and should I worry about it? I am concerned since removing the cameras gives me over 1000 ft. more altitude.


View attachment 669526

View attachment 669525

View attachment 669524

Use the Cone and BT option... but bring them down into the body, and then manually over-ride the weight and drag.​

​

 

[JoePfeiffer]

Use the Cone and BT option... but bring them down into the body, and then manually over-ride the weight and drag.​

​

Bringing it down into the body makes little or no difference -- OR will still think it's all exposed to the airflow.

 

[lakeroadster]

Bringing it down into the body makes little or no difference -- OR will still think it's all exposed to the airflow.

@JoePfeiffer .... Even if you cut the drag a proportional amount?​

 

[atestani]

I am going to restate my thoughts from the other thread again in case they weren't clear.

1) Your camera housing shape is fine. Neither I nor anyone else suggested that you redo the camera housing as a tube with a nosecone.

2) Your camera housing shape is very unlikely to act as a fin aerodynamically.

3) Open rocket treats fins differently from body components when determining the center of pressure.

4) Calling your camera pod a fin though it actually isn't acting as one is artificially moving the center of pressure further forward than it should be.

5) Therefore, modeling the camera housing in OpenRocket as if it were a pod with nosecone and tube of the same frontal area as your camera housing should give you a more accurate and less drastic estimation of the forward shift in center of pressure that your camera housing will have.

No changes to your actual camera housing needed. The only change is how you represent the housing in OpenRocket.

I see what you are saying but I am having some difficulty with the statement that modelling the housing as a body tube and nosecone (let's call this the bullet shape) with "the same frontal area as your camera housing" will give the equivalent CP effect and drag. Why? Also, what is the frontal area of the bullet and what is the frontal area of my semicircle shaped 1" thick "fin"?

The other thing is once I brought the cameras down near the fin can, the effect of the housing shapes on CP reduced considerably and the shape type (bullet or thick fin) wasn't much of a difference.

 

[JoePfeiffer]

@JoePfeiffer .... Even if you cut the drag a proportional amount?​

Manually setting the drag makes a difference. Whether you've set it into the body still doesn't.

 

[bad_idea]

I am having some difficulty with the statement that modelling the housing as a body tube and nosecone (let's call this the bullet shape) with "the same frontal area as your camera housing" will give the equivalent CP effect and drag. Why?

Re. drag, because this is (so far as I know) the only way within the limitations of OpenRocket to add a non-fin object with drag to the rocket. Re. CP, because the effect of a fin on CP in a Barrowman-derived aerodynamic model is notably different than an object that simply provides drag. Your camera housing is most likely too wide and squat to act as a fin, yet OpenRocket is applying the fin equation to it.

Also, what is the frontal area of the bullet and what is the frontal area of my semicircle shaped 1" thick "fin"?

Frontal area is the 2D area you'd see if viewing a 3D object from dead ahead. The frontal area of a round object (cone, tube, sphere, etc.) is the area of a circle of the same radius, i.e., pi*r^2. You would have to figure out the frontal area of your camera housing.

 

[sr205347d]

Violating Barrowman's simplifying assumptions will invalidate OR results.



That being said, if the rocket without the camera shrouds has a healthy margin of stability, the forward shrouds shouldn't cause your rocket to start skywriting. Just look at rockets that have flown just fine with camera shrouds. However, any greeblies you add forward of the CG will reduce your margin of stability.

 

[neil_w]

I believe the thick fin warning is to let you know you’re violating assumptions built into OpenRocket. Your mileage may vary.

Remember, OR isn’t CFD, nor a physics sandbox.

That's pretty much the long and short of it. So, naturally, I will now expound upon it at length.

Generally the design warnings (those that show in the corner of the rocket figure) are of two flavors:
1) Design violates the assumptions built into OR
2) Something looks suspicious and is very possibly a mistake, e.g. the infamous Discontinuity warning that everyone loves so much.

The thick fin warning falls clearly into the first bucket, and is triggered whenever the fin thickness is greater than n% of the diameter of the body tube it's attached to (where n is a value I don't know off the top of my head.) The challenge when dealing with such warnings it to guess in what way are the results likely to be inaccurate. The program doesn't say because it doesn't know. I would guess in this case that the CP effect of the slab sides of the fin is real. There is possibly additional unaccounted CP effect of the *edge* of the fin (which is now thick enough to create lift of its own), but that would be additive. So, if anything, I would say that the CP effect of the thick fin is probably a bit underestimated. As for drag... who knows. But that's not what we care most about here, so I think we can safely ignore it.

There's one other thing unaccounted for. When only a single fin is in the set (as is the camera shroud), I would think the rocket would rotate until the shroud is behind the body with respect to the airflow, and at that point, it should have much less impact on stability. OR is always reporting worst case margin, with the fin/shroud fully in the airflow.

So what does it all mean? I think the shroud will have some effect on CP (pulling it forward). Beyond that... make your own call.

 

[atestani]

Re. drag, because this is (so far as I know) the only way within the limitations of OpenRocket to add a non-fin object with drag to the rocket. Re. CP, because the effect of a fin on CP in a Barrowman-derived aerodynamic model is notably different than an object that simply provides drag. Your camera housing is most likely too wide and squat to act as a fin, yet OpenRocket is applying the fin equation to it.

Frontal area is the 2D area you'd see if viewing a 3D object from dead ahead. The frontal area of a round object (cone, tube, sphere, etc.) is the area of a circle of the same radius, i.e., pi*r^2. You would have to figure out the frontal area of your camera housing.

So... whether I use a flat plug or a nose cone on the body tube, both giving the same frontal area, the effects on the rocket are the same?! That's non-intuitive, at best.

 

[atestani]

This thread was started to try to understand the thick fin warning and morphed into the best way to model camera shrouds. I'm not complaining since that is what I am trying to do. I am mostly concerned with the effects on CP and, therefore, stability on this long, thin rocket I will fly with an L1000! I have flown the same rocket with I and J motors multiple times with various nose cone weights and with the cameras were near the nose cone. When I move the cameras to the fin can area, the need for nose weight is going way down (which is a good thing) but I am now more than ever concerned with accurately modelling this.

 

[Charles_McG]

The accuracy you desire may be beyond the scope of OR.

 

[bad_idea]

So... whether I use a flat plug or a nose cone on the body tube, both giving the same frontal area, the effects on the rocket are the same?! That's non-intuitive, at best.

No, both the shape of the nose cone and the frontal area have an effect (as does the length of the tube), which is why I suggested you model the camera shroud as a nose cone and tube, with the nosecone having approximately the same aspect ratio as your fairing.

 

[Buckeye]

This thread was started to try to understand the thick fin warning and morphed into the best way to model camera shrouds. I'm not complaining since that is what I am trying to do. I am mostly concerned with the effects on CP and, therefore, stability on this long, thin rocket I will fly with an L1000! I have flown the same rocket with I and J motors multiple times with various nose cone weights and with the cameras were near the nose cone. When I move the cameras to the fin can area, the need for nose weight is going way down (which is a good thing) but I am now more than ever concerned with accurately modelling this.

As mentioned several times, you want accuracy on something that OR can't do. Barrowman modelling only comprehends nose cones, transitions, and simple fins. That is it. Some hacks have been made to use these 3 components to simulate other stuff like rings and pods, but who knows if they are correct? Barrowman assumed that fins are "thin", for which there is no definition other than the arbitrary error trigger in OR.

If you want to put arbitrarily shaped appendages on the rocket and see more certain aerodynamics results, then you need CFD analysis:

https://www.rocketryforum.com/threads/cfd-with-freecad.184708/

Your rocket is not crazy long and skinny. Also, many people attach cameras willy-nilly on rockets and they still remain stable and fly just fine.

 

[atestani]

OK... I want to believer

As mentioned several times, you want accuracy on something that OR can't do. Barrowman modelling only comprehends nose cones, transitions, and simple fins. That is it. Some hacks have been made to use these 3 components to simulate other stuff like rings and pods, but who knows if they are correct? Barrowman assumed that fins are "thin", for which there is no definition other than the arbitrary error trigger in OR.

If you want to put arbitrarily shaped appendages on the rocket and see more certain aerodynamics results, then you need CFD analysis:

https://www.rocketryforum.com/threads/cfd-with-freecad.184708/

Your rocket is not crazy long and skinny. Also, many people attach cameras willy-nilly on rockets and they still remain stable and fly just fine.

Here is the thing: For the same rocket and dual "thick fin" camera shrouds using an L1000, if I place the cameras near the nosecone, I need about 35 oz of weight in the nose to get to ~3 caliber/13% (rocket has a fineness ration of about 25). If I move these same camera shroud's down to the top of the fin can, I don't (appear to) need any weight at all for the same approximate stability! The reason is there is a dramatic shift in CP and negligible movement in CG .

I'm not looking for accuracy at all here but a test of reasonableness if I can launch this rocket safely!

As I said, I've launched this rocket multiple times with the cameras near the nosecone on I and J motors with varying amounts of nose weight. The simulations were done using the thick fin shrouds and the nose weight applied to get to around 3 caliber/13%. Flights were always nicely stable. Since I am now planning to fly with a K535 and L1000, even more weight is required and now is beyond the capacity of my modifiable nose weight system.

Whether I model the shrouds as a round thick fin, thick trapezoidal fin, or pod tubes/nose cones, the CP is reasonably close in all cases. However, the CP is drastically different having the cameras at the fin can or nose cone, to the tune of 30+ oz. of nose weight vs. zero nose weight for the same stability margin!

Sure, I will move the cameras and fly with some smaller motors but I am still unclear about this. A quick look says, I will be over-stable with an small I motor if the above results are valid.

 

[Art Upton]

This thread was started to try to understand the thick fin warning and morphed into the best way to model camera shrouds. I'm not complaining since that is what I am trying to do. I am mostly concerned with the effects on CP and, therefore, stability on this long, thin rocket I will fly with an L1000! I have flown the same rocket with I and J motors multiple times with various nose cone weights and with the cameras were near the nose cone. When I move the cameras to the fin can area, the need for nose weight is going way down (which is a good thing) but I am now more than ever concerned with accurately modelling this.

Curious , did you have those cameras on the front of the unstable rocket you posted in a video on the other thread?

 

[Rschub]

Applying a moment ahead of the CG will be destabilizing, which is what a camera shroud mounted off of the centerline of the rocket will do.

It is possible [probable?] that the way you are setting the sim up is overstating the drag aerodynamic effects of the pods.

No one here can tell you the exact effect, and OR is suggesting there is a potential issue. This is a judgement call. It would probably pay to be conservative.

 

[atestani]

Curious , did you have those cameras on the front of the unstable rocket you posted in a video on the other thread?

Yes, but that was when I was weighting for a 1.5 caliber stability which was unstable for this thin rocket. After that I started weighting for 3+ caliber/13%.

 

[atestani]

Applying a moment ahead of the CG will be destabilizing, which is what a camera shroud mounted off of the centerline of the rocket will do.

It is possible [probable?] that the way you are setting the sim up is overstating the drag aerodynamic effects of the pods.

No one here can tell you the exact effect, and OR is suggesting there is a potential issue. This is a judgement call. It would probably pay to be conservative.

I don't think I am setting up the pods or thick fins incorrectly. Attached are three ORK files. You can see the CP is basically the same in all three. Drag is different as would be expected.

 

[sr205347d]

Sure, I will move the cameras and fly with some smaller motors but I am still unclear about this. A quick look says, I will be over-stable with an small I motor if the above results are valid.

Two things about this. The first is that you can't consider OR results valid when violating Barroman's assumptions, which you are doing with camera shrouds.

Second is your concern about being "over-stable." Don't be. Good speed off the rail will minimize weathercocking.