Zephyr build - Open Rocket concerns

[JoePfeiffer]

All right, I re-read the Levison articles in the Peak of Flight. His motivation for mimicking a flat plate CP in RockSim was to make the Estes Fat Boy stability margin above 1.0 caliber so that it jived with the rules of thumb. Without the imaginary cone, the margin was 0.6.

The Fat Boy was still stable with a margin greater than zero. 1.0 caliber is not a hard law of aerodynamics. It is just a slop factor.

This base drag hack now seems like a silly waste of time in any software. It is just easier to remember that smaller stability margins are OK on stubby rockets.

I interpret the article a little differently than you do. Yes, it was considering short stubby rockets that got him interested in the problem. But he isn't inventing mythical structures to make himself feel better about the rocket's stability, he's finding a way, within the constraints of the software, to consider an aerodynamic effect that can't be captured directly by that software in order to get a more accurate estimate of the CP.

 

[Buckeye]

I interpret the article a little differently than you do. Yes, it was considering short stubby rockets that got him interested in the problem. But he isn't inventing mythical structures to make himself feel better about the rocket's stability, he's finding a way, within the constraints of the software, to consider an aerodynamic effect that can't be captured directly by that software in order to get a more accurate estimate of the CP.

Where does it say the CP is now more accurate? There is mention of wind tunnel tests for spools and flat plates, but no wind tunnel or CFD evidence of a Fat Boy with a margin of 1.14. Instead, just "feel good" affirmations:

Applying this correction to the Estes Fatboy launched on a C6-5 motor (see figure 3, note stability margin of 0.67) moves the CP aft to give a stability margin of 1.14 (see fi gure 3.) This means the Estes Fatboy design should fly stable without any added nose weight on a C6-5 motor (as everyone knows it will)!

It is interesting to note that the base vortex correction brings the stability margin of this design to just over one where the rules of thumb for a stable design dictate that it should be.

I was able to bring the calculated stability margin up to 1.14, which is by rule of thumb indicates a stable design in contrast to the stability margin for the unmodified simulation of 0.67.

Nothing wrong with a margin of 0.67. That's still stable.

I can see applying the base drag trick to a Sputnik, because the Sputnik doesn't look like a rocket in the Barrowman assumptions.

I really need to read the Barrowman and Galejs papers and see how the base of the rocket is treated.

...and someday a hobby-level CFD program will put this all to rest!

 

[Buckeye]

That guide has a 2003 copyright, and Levison's article wasn't published until 2006. So the fact the guide doesn't mention it says nothing about whether RS is using it now.

Good point.

However, issue #238 was printed in 2009, and it didn't mention adding Levison's base drag to the Rocksim method.

In the preface to Levison's articles, TVM writes that users are at their own risk.

If an accurate stubby-rocket simulator was added to Rocksim somewhere around 2006, I think it would have been advertised. I don't recall Apogee touting it.

so, my hunch is that the base drag hack is not included in the Rocksim method.

 

[JoePfeiffer]

Where does it say the CP is now more accurate? There is mention of wind tunnel tests for spools and flat plates, but no wind tunnel or CFD evidence of a Fat Boy with a margin of 1.14. Instead, just "feel good" affirmations:

<snip>
There's lots and lots of papers in the professional literature (primarily old papers) that also don't validate their results. They simply solve aerodynamic problems using "Newtonian aerodynamics", and present those theoretical results. That's what Levison does, noting that base drag effects aren't accounted for in Barrowman's equations and considering what their contribution should be.

I can see applying the base drag trick to a Sputnik, because the Sputnik doesn't look like a rocket in the Barrowman assumptions.

And Sputnik is a perfect example of a place it wouldn't be appropriate -- it doesn't have a sawed-off base, it's a sphere. I don't know anything about them, but apparently (based on comments here on TRF) there are formulas describing how sticks like on fireworks provide stability. That would be the direction to go simming a Sputnik.

I really need to read the Barrowman and Galejs papers and see how the base of the rocket is treated.

It isn't. That's why we're having this discussion.

...and someday a hobby-level CFD program will put this all to rest!

The tools to do that are already out there. I've made a couple of false starts on learning enough about CFD to do some of that, but got distracted. And don't forget, CFD models ought to be validated, too.

 

[Buckeye]

<snip>
There's lots and lots of papers in the professional literature (primarily old papers) that also don't validate their results. They simply solve aerodynamic problems using "Newtonian aerodynamics", and present those theoretical results. That's what Levison does, noting that base drag effects aren't accounted for in Barrowman's equations and considering what their contribution should be.

And Sputnik is a perfect example of a place it wouldn't be appropriate -- it doesn't have a sawed-off base, it's a sphere. I don't know anything about them, but apparently (based on comments here on TRF) there are formulas describing how sticks like on fireworks provide stability. That would be the direction to go simming a Sputnik.

It isn't. That's why we're having this discussion.

The tools to do that are already out there. I've made a couple of false starts on learning enough about CFD to do some of that, but got distracted. And don't forget, CFD models ought to be validated, too.

Are we reading the same papers? I am looking at these:

https://www.apogeerockets.com/education/downloads/Newsletter154.pdfhttps://www.apogeerockets.com/education/downloads/Newsletter158.pdf
Maybe he was thinking it or alluding to it, but Levison never outright says that Barrowman is missing the all-important base drag and that is why he needs to add it back in. To me, the gist of the papers is to move simulated stability margin above 1.0 on proven designs.

If I have the dimensions correct, 0.67 Barrowman stability margin on a Fat Boy equates to 13% of the rocket length. That falls in line with the 8%-15% that some like to use for good stability, rather than 1 caliber.

Which CFD tools are you referring to? Yes, you can roll your own with OpenFoam or the simulators that are included in commercial CAD packages like Solidworks. If you have $$$ or access at the workplace, you use StarCCM+ or similar professional grade software. I was thinking more along the lines of what the AeroRocket guy was offering for 2D rocket shapes but modernized for any arbitrary 3D rocket geometry and faster computing by hobbyists, maybe with GPUs.