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Fin cross section in OpenRocket and in RockSim

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MetricRocketeer

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Hi everyone,

It occurs to me that, in both OR and RS, when you change the cross section of fins, the appearance of the fins doesn't really change. But the appearance should change, right?

Thank you.

Stanley
 

K'Tesh

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Hi everyone,

It occurs to me that, in both OR and RS, when you change the cross section of fins, the appearance of the fins doesn't really change. But the appearance should change, right?

Thank you.

Stanley
OR was created, and maintained by unpaid volunteers. They can't be expected to do everything we want. However, you *CAN* give an appearance of multiple surface plains by using "Air Fins" and changing the reflectivity of them. Such as I have done here.

1613696592155.png
 
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MetricRocketeer

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OR was created, and maintained by unpaid volunteers. They can't be expected to do everything we
want. However, you *CAN* give an appearance of multiple surface plains by using "Air Fins" and changing the reflectivity of them. Such as I have done here.
Hi K'Tesh,

First, I meant no criticism of those who created and maintain OR. On the contrary, I acknowledge that they do amazing and wonderful work. I just wanted to make sure that I wasn't missing something.

Second, I don't understand what an air fin is. So what kind of cross section are you showing, please -- a square, rounded, or airfoil cross section?

Thank you.

Stanley
 

K'Tesh

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You're not missing anything there. Theoretically it's possible to model wedge types of fins (often used in scale or military rockets), or those that use plastic fin cans (such as seen in the Guild Heighliner image I posted above), but its really tricky, and I've opted not to do it, and settled for "faking it".

"Air Fins" are the PBTs of fins. They're added to allow someone to recreate the look of the rocket accurately, but are intended to be removed for flight simulations. Lately, I'll use a fin with no decals, or with a common shape to both sides decal that has the right profile (square, rounded, or airfoil), and material, as well as surface (usually smooth paint, or polished) that will be attached to the body tube, or a PBT if necessary due to being attached to a transition. Then I'll create duplicate fin shapes (made of "air", profile airfoil, and with a polished finish) for the decals, these will be offset slightly to prevent the decal image from bleeding through the other side. This way fins, such as those of the Der Red Max, can be shown with accurate decaling. These air fins are attached to a PBT which needs to be deleted for accurate flight characteristics.

1613699158217.png



What *I* wish for would be a way of turning on or off consideration for flight calculations. If an item is turned off (such as an air fin), it could be there, at any size, shape, thickness, material, weight, surface, or cross section, and wouldn't factored into the math. Basically, it's there for appearance only. Likewise, if an item is turned on (such as an internal tube that has been externalized for some reason (launch lugs on standoffs, etc.)), it is added into the calculations.
 

MetricRocketeer

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Hi K'Tesh,

OK. That explains a lot. Thank you.

Incidentally -- this is off-topic from my original thread -- I am noticing in your design the warning about "discontinuity in rocket body diameter." Does that concern you? When I was designing my first rocket, I chose a body tube from the database, which showed an exactly correct diameter for the adjoining nosecone, but I still got such a warning.

Stanley
 

K'Tesh

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Hi K'Tesh,

OK. That explains a lot. Thank you.

Incidentally -- this is off-topic from my original thread -- I am noticing in your design the warning about "discontinuity in rocket body diameter." Does that concern you? When I was designing my first rocket, I chose a body tube from the database, which showed an exactly correct diameter for the adjoining nosecone, but I still got such a warning.
Not a problem.

The discontinuity in rocket body diameter error is something that I just ignore. It's there because of how I simulate the engine hook. It's annoying, but not a big deal.
 

neil_w

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It occurs to me that, in both OR and RS, when you change the cross section of fins, the appearance of the fins doesn't really change. But the appearance should change, right?
There are actually a few things that aren't really represented in the rendering, and you've hit on one of them. I would like to see rounded edges rendered, but it's not that easy to do, and it's not really important, so I wouldn't expect to see this any time... ever. :)

Incidentally -- this is off-topic from my original thread -- I am noticing in your design the warning about "discontinuity in rocket body diameter." Does that concern you? When I was designing my first rocket, I chose a body tube from the database, which showed an exactly correct diameter for the adjoining nosecone, but I still got such a warning.
There are some maddening discontinuity warnings that no one ever seems to know the reason for. If you get one and you can't figure it out, please post the ORK file here and I would like to take a look at it.
 

MetricRocketeer

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There are some maddening discontinuity warnings that no one ever seems to know the reason for. If you get one and you can't figure it out, please post the ORK file here and I would like to take a look at it.
Hi neil_w,

Then here is one.

The long, aft body tube is a LOC BT-7.51 which I took from the database, and whose length I shortened. It should fit the transition in the front, but I get a discontinuity warning. The only way around it was to select a preset, which I guess means a customization.

Stanley
 

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Buckeye

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OR was created, and maintained by unpaid volunteers. They can't be expected to do everything we want.
The OP asked a legit question. Don't bite his head off. If OR and RS render different nose cone shapes, it is reasonable to assume they would render different fin sections, too.

To the OP, you need to understand what these software do behind the scenes. They do not simulate air flow around the shapes you display on the screen. If they did, then they would be CFD codes that cost thousands of dollars an would need lots of computing power.

OR and RS predict CP and CD with empirical data and equations for basic rocket shapes. What is shown on the screen is a CAD representation to help you design the rocket. What you conjure up in the graphics window may violate the aerodynamic assumptions of the solvers, OR especially. All the fake parts needed to make more realistic renderings will produce unreliable results for flight prediction.
 

K'Tesh

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The OP asked a legit question. Don't bite his head off. If OR and RS render different nose cone shapes, it is reasonable to assume they would render different fin sections, too.
One of the greatest failures of the internet is the failure to capture "tone of voice" in typed messages. The tone that I had in my head when typing the message was not that of "KILL The Ignorant", it was of a gentle reminder that we can't always get what we want, when we want it. That's why I immediately followed the "offending comment" with a description on how to try to get something like the desired effect, along with an example.
 

MetricRocketeer

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Hi K'Tesh,

I took no offense.

Buckeye, thank you for coming to my defense -- that was very nice of you. But I saw that K'Tesh was just trying to help me understand by explaining the situation.

I learn a lot from all of you knowledgeable rocketeers.

Stanley
 

neil_w

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Have you had a chance yet to look at my rocket design and analyze why I am getting a discontinuity warning?
Sorry, lost track of that. It's the phantom body tube at the back, OD is 7.677 instead of 7.675 (sorry I am not metric. :)) Not sure why you need that phantom tube at all; you can just as easily attach those fins to Aft (long) body tube.
 

MetricRocketeer

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Hi neil_w,

OK. That explains it.

I was using the phantom body tube so that I could combine all the fins as a sub-assembly.

I am posting the file containing the final design of my first OpenRocket.

Thanks so much with your help.

Stanley
 

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MetricRocketeer

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Hi everyone,

I was looking at my OR design again -- right above, in Post #19 -- and now I am confused about something.

Consider, for example, the Forward (short) body tube. Its outer diameter is 14 cm and its inner diameter is 13.6 cm. So how can its wall thickness be only 0.2 cm? It would have to be 0.4 cm, right?

As another example, look at the Aft (long) body tube. Same situation -- its outer diameter is 19.5 cm and its inner diameter is 19.1 cm. So how can its wall thickness be only 0.21 cm? Again, the wall thickness would have to be 0.4 cm, right?

What am I missing, please?

Thank you.

Stanley
 

neil_w

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Hi everyone,

I was looking at my OR design again -- right above, in Post #19 -- and now I am confused about something.

Consider, for example, the Forward (short) body tube. Its outer diameter is 14 cm and its inner diameter is 13.6 cm. So how can its wall thickness be only 0.2 cm? It would have to be 0.4 cm, right?

As another example, look at the Aft (long) body tube. Same situation -- its outer diameter is 19.5 cm and its inner diameter is 19.1 cm. So how can its wall thickness be only 0.21 cm? Again, the wall thickness would have to be 0.4 cm, right?

What am I missing, please?

Thank you.

Stanley
There are two walls to be counted in the diameter.
 

MetricRocketeer

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Hi neil_w, Charles_McG, and anyone else,

First, Charles_McG, right -- your statement about radius versus diameter was helpful.

Next, maybe I am overthinking this. But I do need to be sure, please.

Consider again the Aft (long) body tube. The difference between its outer diameter is 19.5 cm and its inner diameter is 19.1 cm, and that subtracts to 0.4 cm, which we divide in half to get 0.2 cm. Notice, however, that the wall thickness measures 0.21 cm. So that must be a rounding error. Perhaps the outer diameter is not exactly 19.5 cm and/or perhaps the inner diameter is not exactly 19.1 cm and/or perhaps the wall thickness is not exactly 0.21 cm.

Right?

Stanley
 

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