Zephyr Stability (Openrocket vs. RockSim)

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Colton Acosta

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Hi Everyone,

I am currently working toward an L1 certification using the Zephyr kit from Apogee. I am basically done with construction, but when I started looking at the OpenRocket model to determine my delay for motor ejection I noticed a really disconcerting discrepancy between the OpenRocket and Rocksim models. Using the .rkt file provided directly from Apogee and recommended motors (ex. H100, I140) OpenRocket gives me static stability margins ranging between 0.45-0.6 (marginal), whereas Rocksim gives me values above 1.0 (stable) due to the CP being calculated ~2" further back than in OpenRocket. Changing the Rocksim CP calculation method to the classical Barrowman equations gives the same results as OpenRocket. I tried changing the fins in OpenRocket from Freeform to trapezoidal as I read that RockSim is better with more complex fin geometries, but I didn't see a significant change in CP. I am under the impression that the Zephyr should be a very stable and a safe L1 cert rocket, so I am unsure what to trust here. I've read that for shorter, stubby rockets marginal stability can be acceptable but I have no experience here or intuition for the numbers. Every rocket I've seen fly has had a static margin of at least 1.5. Has anyone else run into this before? Am I missing something?

Colton
 

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Hi Everyone,

I am currently working toward an L1 certification using the Zephyr kit from Apogee. I am basically done with construction, but when I started looking at the OpenRocket model to determine my delay for motor ejection I noticed a really disconcerting discrepancy between the OpenRocket and Rocksim models. Using the .rkt file provided directly from Apogee and recommended motors (ex. H100, I140) OpenRocket gives me static stability margins ranging between 0.45-0.6 (marginal), whereas Rocksim gives me values above 1.0 (stable) due to the CP being calculated ~2" further back than in OpenRocket. Changing the Rocksim CP calculation method to the classical Barrowman equations gives the same results as OpenRocket. I tried changing the fins in OpenRocket from Freeform to trapezoidal as I read that RockSim is better with more complex fin geometries, but I didn't see a significant change in CP. I am under the impression that the Zephyr should be a very stable and a safe L1 cert rocket, so I am unsure what to trust here. I've read that for shorter, stubby rockets marginal stability can be acceptable but I have no experience here or intuition for the numbers. Every rocket I've seen fly has had a static margin of at least 1.5. Has anyone else run into this before? Am I missing something?

Colton
RockSim files don’t always port over to OR perfectly - I’d dig into any and all the mass overrides, there’s probably a value that’s out of whack. It’s a bit suspect that the Zephyr design shows marginal stability with as common a cert motor as an H100.
 
RockSim files don’t always port over to OR perfectly - I’d dig into any and all the mass overrides, there’s probably a value that’s out of whack. It’s a bit suspect that the Zephyr design shows marginal stability with as common a cert motor as an H100.
Yeah I was thinking'd I try and recreate the model in OpenRocket anyway.
 
Apogee web site states CP at 114cm. OR sim reports CP at 101cm.

However, I have seen a number of Zephyr's fly L1 cert with H283 motors. No stability issues in flight.
Built it by the instruction and it will fly fine.

It is a simple rocket so recreating it in OR will not take much time.
This will also allow you to check each part for weight and sizes.
 
I sure hope it's stable when built from the kit -- that's what my wife is building for her L1!

Just applied the "base drag hack" to the design (no other changes), and configured with an H100 motor I get a stability margin of 1.54 cal. I wonder if RockSim's "enhanced Barrowman" includes base drag? That's my personal #1 priority after this release is out.
 
Just applied the "base drag hack" to the design (no other changes), and configured with an H100 motor I get a stability margin of 1.54 cal. I wonder if RockSim's "enhanced Barrowman" includes base drag? That's my personal #1 priority after this release is out.
Hmm, interesting thought. Although if Rocksim is incorporating the hack and they haven't told anyone about it, that means there are folks out there doing a double-hack. We should be able to figure if that's what it's doing, will get on that. I suspect there's more to their "enhanced calculations" than that, but who knows.
 
I sure hope it's stable when built from the kit -- that's what my wife is building for her L1!

Just applied the "base drag hack" to the design (no other changes), and configured with an H100 motor I get a stability margin of 1.54 cal. I wonder if RockSim's "enhanced Barrowman" includes base drag? That's my personal #1 priority after this release is out.
Interesting. Where can I learn how to do the base drag hack? Seems like it was in some newsletter and a google search isn't pulling up too many helpful results.
 
I wrote an email to Tom Vanmillagan about this same discrepancy. His reply was that the Vephr is very stable without giving explanation about the
.rkt file. Anyway we have had multiple flights with all sorts of motors with no stability issues at all. Recreatomg a file from scratch is a good idea.
 
I have flown the Zephyr many times. It's my test bed for trying new things. Here it is on an H100, an I240-DM, and I236 when I added a dual deployment to it. It has always flown straight as an arrow and recovered it without any issues.
 

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I noticed the same thing when I did my L1 with the Zephyr. RockSim has a propriety CP formula set as default. I too opened the file in OR and became concerned. However, I can assure you the Zephyr is in fact very stable. I flew H, I and J up to a J425 with no issue. Add some ballast to your nosecone for peace of mind.
 
I built my Zephyr stock with the exception of adding a combination baffle plate/ shock cord eyebolt mount. The baffle plate was mounted right at the CG point. I got my L1 with a H100 and had a second flight on an I140 with no issues.
Zephyr baffle.jpg LV1 3.jpg
 
a little nose cone weight is a good thing


I modelled my Zephyr in RockSim as well. I was concerned about larger motors needing nose weight but I need not have worried. It has 1+ calibre stability on all the 1 to 6 grain motors I sim'd. That said, my file was based on my actual rocket where I chamfered the leading fin edges and lost several grams of weight from the square edges which moved the CG marginally forward.

I also added a thin-wall coupler ahead of the other heavy wall coupler due to an incident I had where the chute failed to 100% open and it came down harder than I would have liked. It left the main tube a little soft so the addition of the coupler reinforced everything and moved the CG forward.
 
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I sure hope it's stable when built from the kit -- that's what my wife is building for her L1!

Just applied the "base drag hack" to the design (no other changes), and configured with an H100 motor I get a stability margin of 1.54 cal. I wonder if RockSim's "enhanced Barrowman" includes base drag? That's my personal #1 priority after this release is out.
OMG, so is mine! She just got it yesterday.
 
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