understanding open rocket and its calculations

[UVU_Team_Rocket]

i am just curious, but how does open rocket calculate the drag force of the rocket when drag is dependent on Cd and Cd is dependent on drag force.
So mathematically, how is that done? Runge Kuta?

i am curious very much.

thanks

 

[thzero]

i am just curious, but how does open rocket calculate the drag force of the rocket when drag is dependent on Cd and Cd is dependent on drag force.
So mathematically, how is that done? Runge Kuta?

i am curious very much.

thanks

https://github.com/openrocket/openrocket

 

[UVU_Team_Rocket]

https://github.com/openrocket/openrocket

is there something in there specifically you are referencing too?

thanks for the link

 

[UVU_Team_Rocket]

is there something in there specifically you are referencing too?

thanks for the link

 

[thzero]

is there something in there specifically you are referencing too?

thanks for the link

Yup, its the source code. Everyone is free to examine it and figure out how it works. Or run the program and look around for a bit.

 

[neil_w]

how does open rocket calculate the drag force of the rocket when drag is dependent on Cd and Cd is dependent on drag force.

 

[kjhambrick]

i am just curious, but how does open rocket calculate the drag force of the rocket when drag is dependent on Cd and Cd is dependent on drag force.
So mathematically, how is that done? Runge Kuta?

i am curious very much.

thanks

As @thzero said, one can review the source code on GitHub.

But looking in OpenRocket at [Edit]->[Preferences]->[Simulation] the Sims are done via the 6-DoF Runge-Kutta-4 Method:


HTH

-- kjh

EDIT: I knew this was online somewhere but I couldn't find it.

Sampo Niskanen's 2013 Masters Thesis Paper is here: Development of an Open Source model rocket simulation software (Master’s thesis)

 

[JoePfeiffer]

It isn't so much that Cd and drag are defined in terms of each other as there is a mathematical relationship between them. You can measure drag in a wind tunnel and use that relationship to calculate Cd, or you can estimate Cd and use it to calculate drag.

In our case, we use the geometry and surface finish of the rocket to estimate Cd, and then calculate drag from that. People have already pointed you to Justilla's (Niskanen's) thesis and the source code; two other good places to look are Barrowman's dissertation and "Fluid-Dynamic Drag" by Hoerner.

One thing to note is that Runge-Kutta is the numerical integration technique that's used to calculate the rocket trajectory, it isn't part of the drag calculation.

 

[UVU_Team_Rocket]

It isn't so much that Cd and drag are defined in terms of each other as there is a mathematical relationship between them. You can measure drag in a wind tunnel and use that relationship to calculate Cd, or you can estimate Cd and use it to calculate drag.

In our case, we use the geometry and surface finish of the rocket to estimate Cd, and then calculate drag from that. People have already pointed you to Justilla's (Niskanen's) thesis and the source code; two other good places to look are Barrowman's dissertation and "Fluid-Dynamic Drag" by Hoerner.

One thing to note is that Runge-Kutta is the numerical integration technique that's used to calculate the rocket trajectory, it isn't part of the drag calculation.

ok, so how did you guys estimate the cd based on that information? i have used wind tunnels to get the drag force and then solve for Cd. I have done that method, but im curious how you guys did it wihtout using a wind tunnel haha

thank you for the information

 

[UVU_Team_Rocket]

Yup, its the source code. Everyone is free to examine it and figure out how it works. Or run the program and look around for a bit.

thank you i am just curious of how it was done. Hopefully i will figure it out.

 

[thzero]

thank you i am just curious of how it was done. Hopefully i will figure it out.

Code and thesis

 

[JoePfeiffer]

ok, so how did you guys estimate the cd based on that information? i have used wind tunnels to get the drag force and then solve for Cd. I have done that method, but im curious how you guys did it wihtout using a wind tunnel haha

thank you for the information

The most direct route is probably reading the thesis, second is the code.

 

[Alan15578]

ok, so how did you guys estimate the cd based on that information? i have used wind tunnels to get the drag force and then solve for Cd. I have done that method, but im curious how you guys did it wihtout using a wind tunnel haha

thank you for the information

Estes TR-11 is probably the best place to start in the model rocket literature.

 

[kjhambrick]

Estes TR-11 is probably the best place to start in the model rocket literature.

Thanks @Alan15578 !

I had forgotten Estes TR-11 - "Aerodynamic Drag of Model Rockets".

I did a quick google search but I didn't find it online.

Attached is a copy I've had for years-n-years ( don't recall where I got it ).

And rereading @UVU_Team_Rocket 's original question:

<<snip>> i am just curious, but how does open rocket calculate the drag force of the rocket when drag is dependent on Cd and Cd is dependent on drag force.

What @JoePfeiffer said VERY is important ( I missed it in your original question )

JoePfeiffer said:
It isn't so much that Cd and drag are defined in terms of each other as there is a mathematical relationship between them. You can measure drag in a wind tunnel and use that relationship to calculate Cd, or you can estimate Cd and use it to calculate drag.

Anyhow ... as @Alan15578 said, Estes TR-11 is a great place to get started -- it describes how to estimate CD from the wetted components of your rocket.

OTOH, if you want to know specifically about Open Rocket, check out Sampo's Masters Thesis and the OpenRocket Source Code.

HTH

-- kjh( EDIT: @Alan15578 keeps sending me on 'wild google chases' )

 

[UVU_Team_Rocket]

Thanks @Alan15578 !

I had forgotten Estes TR-11 - "Aerodynamic Drag of Model Rockets".

I did a quick google search but I didn't find it online.

Attached is a copy I've had for years-n-years ( don't recall where I got it ).

And rereading @UVU_Team_Rocket 's original question:


What @JoePfeiffer said VERY is important ( I missed it in your original question )


Anyhow ... as @Alan15578 said, Estes TR-11 is a great place to get started -- it describes how to estimate CD from the wetted components of your rocket.

OTOH, if you want to know specifically about Open Rocket, check out Sampo's Masters Thesis and the OpenRocket Source Code.

HTH

-- kjh( EDIT: @Alan15578 keeps sending me on 'wild google chases' )

will doi, i will check out the thesis and the other article you posted. Its the estimation part i am curious about.
i do not know how to program so looking at source code is going to be a nightmare for me haha. But i will still check it out and see what it looks like at least.

thanks for all this information. Let me at least read the papers and then i can start asking questions directly based on what i have read.

 

[Art Upton]

Estes TR-11 is probably the best place to start in the model rocket literature.

That publication is what got me to buy a RPN programable calculator back in the early/mid 70s

 

[UVU_Team_Rocket]

Thanks @Alan15578 !

I had forgotten Estes TR-11 - "Aerodynamic Drag of Model Rockets".

I did a quick google search but I didn't find it online.

Attached is a copy I've had for years-n-years ( don't recall where I got it ).

And rereading @UVU_Team_Rocket 's original question:


What @JoePfeiffer said VERY is important ( I missed it in your original question )


Anyhow ... as @Alan15578 said, Estes TR-11 is a great place to get started -- it describes how to estimate CD from the wetted components of your rocket.

OTOH, if you want to know specifically about Open Rocket, check out Sampo's Masters Thesis and the OpenRocket Source Code.

HTH

-- kjh( EDIT: @Alan15578 keeps sending me on 'wild google chases' )

ok, been reading the estes article. They did get values from empirical data from wind tunnels according to their graphs. Is there a specific page or paragraph you had in mind about finding the Cd from wetted components?

thanks

EDIT: I think i found it. Its in the appendix

 

[kjhambrick]

ok, been reading the estes article. They did get values from empirical data from wind tunnels according to their graphs.

@UVU_Team_Rocket --

Yes, the empirical data are a result of years -n- years of subsonic aerodynamic research and development.

Check Appendix II -- Dr. Gregorek's suggested reading list.

Is there a specific page or paragraph you had in mind about finding the Cd from wetted components?

Hmmm ...

If one wants to understand model rocket drag, I would say that one needs to read and understand ALL of it

But if you want the punch line, maybe read the section: COMBINING DRAG COEFFICIENTS starting at the bottom of page 26 ( PDF page 30 ).

That section calculates the zero-lift drag coefficient of a rocket by combining the individual coefficients of drag of each wetted component.

But it may not make much sense unless one understands the concepts developed in pages 1-26.

At least a print out of the List of Symbols in Appendix III

EDIT: I think i found it. Its in the appendix

I believe Appendix I starting on page 47 is a deeper dive into applying the concepts developed in the paper to a different rocket with arbitrary properties.

Appendix II on page 50 is a bibliography.

Appendix III on page 51 is a List of Symbols which is very handy to have when reading pages 1-50.

Final note: the specifics presented in the paper are for subsonic model rockets.

Transsonic aerodynamics are a whole 'nuther animal ...

HTH.

-- kjh

 

[UVU_Team_Rocket]

@UVU_Team_Rocket --

Yes, the empirical data are a result of years -n- years of subsonic aerodynamic research and development.

Check Appendix II -- Dr. Gregorek's suggested reading list.


Hmmm ...

If one wants to understand model rocket drag, I would say that one needs to read and understand ALL of it

But if you want the punch line, maybe read the section: COMBINING DRAG COEFFICIENTS starting at the bottom of page 26 ( PDF page 30 ).

That section calculates the zero-lift drag coefficient of a rocket by combining the individual coefficients of drag of each wetted component.

But it may not make much sense unless one understands the concepts developed in pages 1-26.

At least a print out of the List of Symbols in Appendix III


I believe Appendix I starting on page 47 is a deeper dive into applying the concepts developed in the paper to a different rocket with arbitrary properties.

Appendix II on page 50 is a bibliography.

Appendix III on page 51 is a List of Symbols which is very handy to have when reading pages 1-50.

Final note: the specifics presented in the paper are for subsonic model rockets.

Transsonic aerodynamics are a whole 'nuther animal ...

HTH.

-- kjh

i read the section in the appendix about skin friction and using the Reynolds number. That made sense to me. I recognize those charts from my fluid dynamics class a couple years ago.

I then went back and toyed around with open rocket and noticed that OR has a list of surface roughness values to choose from which then indicated to me that OR is using the same technique as the estes paper from a set of values. Going through the source code, which i did not find any calculations because i have no idea how to browse the code or what im looking for, i did find a pdf image of surface roughness vs Reynolds number.

I pulled a double yesterday so i will re-read what i read and wee what i can understand. I do agree to read all of it. Eventually i will do that this week. I will take it section by section. Now i have to ask a professor how these Cf vs reynolds number graphs (figure A-1 Pg 48) were originally created. Im guessing again, years n years of testing as you mentioned?

second, how the heck do i browse/view this source code??? i have no idea what im looking at or for hahaha. How do i see the actual code for calculating the Cd and where its getting all the data it needs?

i really appreciate all of your help and the others help as well with this.

 

[Buckeye]

second, how the heck do i browse/view this source code??? i have no idea what im looking at or for hahaha. How do i see the actual code for calculating the Cd and where its getting all the data it needs?

You really don't need to read the source code. TR-11, the thesis, and playing around in OpenRocket should help you understand the aero drag models.

Hint: When somebody asks what OR is doing, a common response is to say "go look at the source code" because it is freely available. It is a veiled way of saying RTFM. However, I think OR code questions should get more leeway here, because TRF is the official help site for the software.

 

[UVU_Team_Rocket]

You really don't need to read the source code. TR-11, the thesis, and playing around in OpenRocket should help you understand the aero drag models.

Hint: When somebody asks what OR is doing, a common response is to say "go look at the source code" because it is freely available. It is a veiled way of saying RTFM. However, I think OR code questions should get more leeway here, because TRF is the official help site for the software.

ok, that makes sense. I finished most of the TR11 paper today and some sections of the thesis. Years and Years of testing to make those Cf vs Reynolds numbers charts as well as roughness charts.

Its crazy how 47% or so of the drag is from the fins. I mean i knew a lot came from the fins, just didnt know it was that much.
now i want to build a wind tunnel taht can push 500mph wind speeds. The one i used at school, its max was 41 mph and when i tested a model rocket in it, its Cd was 0.009. My values were terrible because the speed was not fast enough to produce any real results.

thanks for all the help

 

[UVU_Team_Rocket]

View attachment 616003

very sorry for the LATE reply to this, but this is exactly how i feel in my fluids class haha. Sometimes the only solution is from actual testing and not math.

 

[Buckeye]

If your university offers a CFD software application class, take it. As a grad student, take CFD theory. CFD is more useful and available than wind tunnels.

 

[NTP2]

If your university offers a CFD software application class, take it. As a grad student, take CFD theory. CFD is more useful and available than wind tunnels.

+1 it’s easy to get a computer it’s hard to get a wind tunnel.

 

[thzero]

i read the section in the appendix about skin friction and using the Reynolds number. That made sense to me. I recognize those charts from my fluid dynamics class a couple years ago.

I then went back and toyed around with open rocket and noticed that OR has a list of surface roughness values to choose from which then indicated to me that OR is using the same technique as the estes paper from a set of values. Going through the source code, which i did not find any calculations because i have no idea how to browse the code or what im looking for, i did find a pdf image of surface roughness vs Reynolds number.

I pulled a double yesterday so i will re-read what i read and wee what i can understand. I do agree to read all of it. Eventually i will do that this week. I will take it section by section. Now i have to ask a professor how these Cf vs reynolds number graphs (figure A-1 Pg 48) were originally created. Im guessing again, years n years of testing as you mentioned?

second, how the heck do i browse/view this source code??? i have no idea what im looking at or for hahaha. How do i see the actual code for calculating the Cd and where its getting all the data it needs?

i really appreciate all of your help and the others help as well with this.

Well OR is Java, so thats pretty useful language to learn, I guess.

This is the core of the backend side of the application (where all the computation, etc, etc. is).
https://github.com/openrocket/openrocket/tree/unstable/core/src/net/sf/openrocket
This is the core location where I think Joe P lives... er I mean its where most of the code around the simulation should be
https://github.com/openrocket/openrocket/tree/unstable/core/src/net/sf/openrocket/simulation

 

[UVU_Team_Rocket]

+1 it’s easy to get a computer it’s hard to get a wind tunnel.

it is but the data has to come from somewhere. Thats the thing with Cd and Drag, they both depend on each other. And if i want to estimate it based on surface finish, i still have to use empirical data that was collected over years and years of testing. CFD on a pc still uses the estimates from data tables that the TR-11 report has listed, or it will do what i did. I created an excel sheet to calculate Cd and drag based off of sensor data i had collected from 5 or 6 users from the forum Lots of back substitution and numerical methods to solve it.
CFD is awesome though. I do not know how to program so it makes it very difficult.
EDIT: re-read my post, i promise im not being rude or mean, tired yes, but not mean. I am grateful fro all the information i have been given

 

[UVU_Team_Rocket]

Well OR is Java, so thats pretty useful language to learn, I guess.

This is the core of the backend side of the application (where all the computation, etc, etc. is).
https://github.com/openrocket/openrocket/tree/unstable/core/src/net/sf/openrocket
This is the core location where I think Joe P lives... er I mean its where most of the code around the simulation should be
https://github.com/openrocket/openrocket/tree/unstable/core/src/net/sf/openrocket/simulation

awesome, thanks for this. I just went and looked at the calculations. This gives me an idea for some practice projects this holiday break.

i also found this

https://ntrs.nasa.gov/api/citations/20010047838/downloads/20010047838.pdf

 

[UVU_Team_Rocket]

If your university offers a CFD software application class, take it. As a grad student, take CFD theory. CFD is more useful and available than wind tunnels.

that is true, and wind tunnels are still cool. I like to see the physical application of things. But it might be boring to see a rocket in a wind tunnel haha.

 

[Buckeye]

But it might be boring to see a rocket in a wind tunnel haha.

It is very boring to watch, unless some flow visualization is going on. However, CFD is better for that, too!

 

[Rschub]

Umm, intersection drag? I am guessing from the back and forth that this model assumes totally attached flow.