OpenRocket 22.02: A very quick introduction to pods

[mtnmanak]

Hi everyone - having a bit of trouble figuring how to use pods and have them calculated into the CP.

I am trying to model a 4" BSG Colonial Viper.

I am trying to use pods to create the top/sides body tube sections and then model the fins that come off those sections. This is easy to model visually, but adding the components with pods has no effect on the CP.

Here is a illustration below. I am showing it with only one pods to keep this simple:

No "fins":



One "tube fin" with a straight fin:



You can see it models the CG no problem, but the CP stays right at 63.3mm.

The .ork file is attached below.

Am I doing something wrong or do pods not count in the CP calculation?

 

[jmasterj]

Hi everyone - having a bit of trouble figuring how to use pods and have them calculated into the CP.

I am trying to model a 4" BSG Colonial Viper.

I am trying to use pods to create the top/sides body tube sections and then model the fins that come off those sections. This is easy to model visually, but adding the components with pods has no effect on the CP.

Here is a illustration below. I am showing it with only one pods to keep this simple:

No "fins":

View attachment 599669

One "tube fin" with a straight fin:

View attachment 599670

You can see it models the CG no problem, but the CP stays right at 63.3mm.

The .ork file is attached below.

Am I doing something wrong or do pods not count in the CP calculation?

A pod is not modeled as a tube fin. Make a dimensionless pod, add a tube fin to it, and add a fin with an offset to the outside of the tube fin.

 

[neil_w]

That ORK file actually crashes my current version.

Can you provide a screenshot that shows the entire OR window? I would like to see the component tree.

 

[mtnmanak]

That ORK file actually crashes my current version.

Can you provide a screenshot that shows the entire OR window? I would like to see the component tree.

Neil - using version 22.02.beta.05

Screenshot:

 

[mtnmanak]

A pod is not modeled as a tube fin. Make a dimensionless pod, add a tube fin to it, and add a fin with an offset to the outside of the tube fin.

It appears to me that you can't add a tube fin to a pod...:

The only components I can add are "nosecone", "body tube" and "transition"

 

[jmasterj]

It appears to me that you can't add a tube fin to a pod...:

The only components I can add are "nosecone", "body tube" and "transition"

View attachment 599673

Sorry, I would add a dimensionless body tube to the pod, then the tube fin and regular fin. Forgot the pod isn't an actual object.

 

[mtnmanak]

Sorry, I would add a dimensionless body tube to the pod, then the tube fin and regular fin. Forgot the pod isn't an actual object.

Got it - Closer!

When I add the "phantom tube" and a tube fin, it adjusts the CP accordingly (same as if I just added the tube fin to the main body):



But, when I add the straight fin using the same method, it doesn't affect the CG:

 

[mtnmanak]

If all else fails, I guess I could just use the modeling with the tube fins and know that I have a fudge factor (in my favor) for CP with the straight fins. It wouldn't be great, but I should be able to at least make an educated guess about stability and know that my speeds and altitudes would be significantly lower than what the sim would show. Would get me in the ballpark enough to launch it, I think.

 

[jmasterj]

Interesting. I don't have any ideas about the CG (I assume that override makes them more than weightless).

 

[mtnmanak]

Burning out for tonight, will iterate on it some more tomorrow and see what it looks like when I try to model the whole thing this way.

@neil_w - what would be really cool (and make this use-case a lot easier) would be if you could add straight fins to the tube fins. I have a couple other rocket designs that would benefit from that feature.

Note: Rocksim has the same issues with pods. I tried it there and got the same results.

 

[neil_w]

For starters, please upgrade to the release version of 22.02.

https://openrocket.info/downloads.html?vers=22.02

 

[mtnmanak]

For starters, please upgrade to the release version of 22.02.

https://openrocket.info/downloads.html?vers=22.02

Thanks! Didn't realize I was running an old version.

I am still getting a result where the straight fin is not being counted in the CP, but here is an updated ORK file saved from the latest version:

 

[JoePfeiffer]

The single fin is affecting CP in yaw, but not pitch. OR reports worst case CP in the summary window and rocket view, so it looks like nothing is happening.

 

[neil_w]

(The latest file still crashes OR on both my machines... anyone else getting this?)

Anyway, a few relevant notes:
1) Body tubes are not tube fins. OR expects there to be a nose cone at the front of the rocket, and for no air to flow through the inside. The release version of OR will throw a warning if you try to have an open-ended body tube. Unfortunately, it's not clear that everyone understands the warning; we'll probably need to work on that.

2) In the current version of OR, a pod is like an attachment point for a new rocket (or, perhaps more accurately, a "stage"). That always starts with the body components (nose cone, body tube, transition), and then other things attach from there. This is not always super-convenient, and the Viper example of open-tube pods with attached fins is probably the worst case scenario for convenience. We will increase the flexibility of pods in the future, but for now we just have to live with what's there.

3) As Joe mentioned above, much like in the real world, you don't get stability from one fin. Always evaluate your CP after you've added the full set. The same effect can be observed is you forget pods and simply add a single fin to a body tube. CP does not move.

 

[mtnmanak]

Thanks for all the suggestions!

Adding the side fins did not affect the CP at all, but the tube fins should get me in the ballpark. I know the CP is still behind the CG and the fins will only push it back further. I will just have to do some Kentucky windage and guestimate how much the drag of the fins would affect a simulated launch (assuming the sim is not calculating the straight fins into the launch estimations). Probably will take whatever the sim gives me as a safe motor for launch speed and that gets me to about 1000 feet and use a motor roughly double the power and burn rate for the first launch. Should at least get it in the air safely, then I can take the empirical evidence and adjust the sim results accordingly.

 

[jmasterj]

Thanks for all the suggestions!

Adding the side fins did not affect the CP at all, but the tube fins should get me in the ballpark. I know the CP is still behind the CG and the fins will only push it back further. I will just have to do some Kentucky windage and guestimate how much the drag of the fins would affect a simulated launch (assuming the sim is not calculating the straight fins into the launch estimations). Probably will take whatever the sim gives me as a safe motor for launch speed and that gets me to about 1000 feet and use a motor roughly double the power and burn rate for the first launch. Should at least get it in the air safely, then I can take the empirical evidence and adjust the sim results accordingly.

View attachment 599766

Could you attach the regular fins as a set (with offset) to the main body tube, instead of as single fins to the pods? Might get you the correct evaluation of the fins set together.

 

[mtnmanak]

Could you attach the regular fins as a set (with offset) to the main body tube, instead of as single fins to the pods? Might get you the correct evaluation of the fins set together.

I did try doing that - is there actually a way to offset the fins outward from the body tube? I couldn't find any setting that would let me do that without using a pod.

 

[neil_w]

I did try doing that - is there actually a way to offset the fins outward from the body tube? I couldn't find any setting that would let me do that without using a pod.

No, there is no way to do that, at least not in a kosher way. I am still bothered that you're not seeing the effects of the fins, and by the fact that I can't open your file.

But in the meantime you *could* try something silly like this and see what happens. Sorry for the crude drawing.


Signed,
Your resident OR abuser

 

[mtnmanak]

No, there is no way to do that, at least not in a kosher way. I am still bothered that you're not seeing the effects of the fins, and by the fact that I can't open your file.

But in the meantime you *could* try something silly like this and see what happens. Sorry for the crude drawing.
View attachment 599776

Signed,
Your resident OR abuser

Good suggestion! Just tried it and it didn't do anything:



Wondering if this isn't a pod issue but an issue with combining tube fins and straight fins? The screenshot above shows how it looks using no pods on the top - it is just a normal tube fin with the crazy straight fin above it, attached to the main body tube.

 

[neil_w]

I'm a little perplexed, and the fact that I can't open your file to see is maddening.

 

[JoePfeiffer]

I'm not seeing any of the anomalies either of you are reporting. I downloaded the .ork file from post #45, and opened it successfully. As posted, CP is displayed as being at 33.9in. When I delete the rudder, it goes to 33.263 (which is close to the same location, but not the same). When I put the rudder back and delete either of the two wings it goes to 33.507 -- again close, but not identical.

I am surprised the fins don't have more of an effect than they do, but they are having some effect.

 

[mtnmanak]

I'm not seeing any of the anomalies either of you are reporting. I downloaded the .ork file from post #45, and opened it successfully. As posted, CP is displayed as being at 33.9in. When I delete the rudder, it goes to 33.263 (which is close to the same location, but not the same). When I put the rudder back and delete either of the two wings it goes to 33.507 -- again close, but not identical.

I am surprised the fins don't have more of an effect than they do, but they are having some effect.

Thanks for trying it out!

I tried a sim out with a J1299 and it didn't work. Went to 146 feet at Mach 0.5. This may just turn out to be one of those modeling efforts for the visuals and will have to test flight performance in the real world based on thrust:weight ratios.

 

[JoePfeiffer]

Well, yes -- the simulation is showing the CG behind the CP when you put that motor in, so it's unstable. I'd go through the design and replace the materials you've got with the ones you're really planning to use (at least I hope you're not using cardboard fins with a J1299!), complete your design (as posted it had no parachutes and no motor mount), and see how it sims.

It seems pretty common for a rocket to have its CP near the front of the fins; you've got it near the front of the tube fins which is reasonable. If it's still unstable I'd add some nose weight since there's every chance it really is.

 

[thzero]

I love me some cardboard fins!

 

[Tractionengines]

LOC Precision lists the CP for the 2.6" T-LOC as 19" +/-.25" but OpenRocket is showing the CP at 16.26" from tip of Nose Cone.

A)
So I am wondering one of 2 things. Did I do something wrong with the POD's and the cross fins at the ends? ...or... Is this a rocket that needs the "Base Drag Hack" to add some shift to the CP location? The L/D ratio, doen't seem too short, but maybe that's the 3" shift in CP. [ Or the 19" is wrong, but a lot of people fly the T-LOC, so I doubt that. ]

B)
Somewhere my weight overrides are not adding up. The SIM shows no-motor weight as 1.22lbs, but the actual weight is 11.2oz (.7lb). [ I'll dig into this, but if you see something quick, let me know. ]

 

[neil_w]

LOC Precision lists the CP for the 2.6" T-LOC as 19" +/-.25" but OpenRocket is showing the CP at 16.26" from tip of Nose Cone.

A)
So I am wondering one of 2 things. Did I do something wrong with the POD's and the cross fins at the ends? ...or... Is this a rocket that needs the "Base Drag Hack" to add some shift to the CP location? The L/D ratio, doen't seem too short, but maybe that's the 3" shift in CP. [ Or the 19" is wrong, but a lot of people fly the T-LOC, so I doubt that. ]

B)
Somewhere my weight overrides are not adding up. The SIM shows no-motor weight as 1.22lbs, but the actual weight is 11.2oz (.7lb). [ I'll dig into this, but if you see something quick, let me know. ]

Hmm, I dunno, it's definitely getting a significant contribution from the fin tips (which are implemented correctly). Without them, CP jumps forward another inch. Those fins are pretty small.

That rocket is under 10:1, and is therefore a reasonable candidate for base drag CP correction. I don't know if that's the difference in this case though. Will do some investigating.

 

[Tractionengines]

That rocket is under 10:1, and is therefore a reasonable candidate for base drag CP correction. I don't know if that's the difference in this case though. Will do some investigating.

I found my weight error. Some of the overrides were still from the 4" version. (Must have forgot to do a save somewhere along the lines. Nose, and Body were way too heavy.)

Adding the Base Drag Hack did shift the CP a lot. If I did it right it moved the CP from 16.25" all the way back to 19.41".

I think I may loose some of the Base Drag due to the Estes Retainer projecting past the back of the Body Tube. So for my marking CP on the body, I'm thinking go with a location @ 17.5". This would be about 1/2 way from No Base Drag Adjustment, and Full Base Drag Adjustment.

 

[neil_w]

FYI this tutorial is now on the official OR Tutorials page: https://openrocket.info/tutorials/pods-and-boosters.html. It's cleaned up a bit and updated but basically the same. There are other useful (I hope) tutorials there too, and I'll be adding more over time.

 

[Rocketholic]

I knew there was a thread here for OR and I'm glad I looked! Those trap-fins (perpendicular trapezoidal fins) are killer!

I'm scratch building a 2.5" Nike Zeus and the CP is just too darn forward so I was thinking about using trap-fins but couldn't figure out how. They move the CP back which is helping immensely with stability. My CP came out exactly where it is, proportionally, with other manufactured Zeus kits but that stability...

Here's a quick view of the first-go at them:

 

[neil_w]

I'm surprised you're having stability problems with that design. Did you consider just adding nose weight instead?

The fin tips look good, but if you're going for something resembling scale then not so much.