open rocket and drag

[Lt72884]

I know i have been asking alot of questions, but i have hit that stage in school where book work and theory can only take you so far. I am studying fluid dynamics and drag at the moment in lab and class. All the work we have done for assignments, the numbers came from charts based on wind tunnel experiments over the last 50 years or so. I just finished numerical methods class and had no idea that runge kuta was a thing or those types of techniques existed.
Here is my question, in open rocket when i do a "component analysis" and it shows me my drag coefficients. Are those Cd's in open rocket calculated from a set of drag force data that came from wind tunnel tests? or does open rocket calculate all of the drag values?
reason i ask, alot of wind tunnel data say that for a model rocket, use 0.75, however, for my LOCIV rocket, my Cd is 0.3 according to the analysis

thanks for all the help

 

[waltr]

I am sure that OR does the calculation (Barrowman Equations).

I do get different Cd values (Total Cd) on different rocket designs.
0.632 for my LaserLOC 223
0.869 for extended Loc Goblin.

0.3 seems a bit too low for a Loc IV.
Surface finish WILL have a effect.

 

[mikec]

OR is open source so you can just go look and see how it works. See src/net/sf/openrocket/aerodynamics/BarrowmanCalculator.java And Barrowman's thesis http://mae-nas.eng.usu.edu/MAE_5900_Web/5900/USLI_2010/Flight_Mechanics/Barrowman.pdf has plenty of stuff about comparison with experimental data. This is, as waltr says, extremely dependent on surface finish and fin cross-section, and OR has only limited ability to define the latter (you can say "airfoil" but not what that means, at least as of 15.04.)

That said, the developers of RASAero have done a lot more work with trying to model and match actual data for more extreme flights. http://rasaero.com/

Keep in mind that Cd is velocity-dependent, also. You can see this in OR by moving the mach number slider in component analysis. I've found that it does a pretty credible job with flights up to at least mach 2.5 or so, I've not gone faster myself.

 

[Lt72884]

OR is open source so you can just go look and see how it works. See src/net/sf/openrocket/aerodynamics/BarrowmanCalculator.java And Barrowman's thesis http://mae-nas.eng.usu.edu/MAE_5900_Web/5900/USLI_2010/Flight_Mechanics/Barrowman.pdf has plenty of stuff about comparison with experimental data. This is, as waltr says, extremely dependent on surface finish and fin cross-section, and OR has only limited ability to define the latter (you can say "airfoil" but not what that means, at least as of 15.04.)

That said, the developers of RASAero have done a lot more work with trying to model and match actual data for more extreme flights. http://rasaero.com/

Keep in mind that Cd is velocity-dependent, also. You can see this in OR by moving the mach number slider in component analysis. I've found that it does a pretty credible job with flights up to at least mach 2.5 or so, I've not gone faster myself.

so does OR use the standard 0.75 cd as the starting point and initial guess then iterate till it gets a correct value?
Thanks for the links.

 

[Lt72884]

I am sure that OR does the calculation (Barrowman Equations).

I do get different Cd values (Total Cd) on different rocket designs.
0.632 for my LaserLOC 223
0.869 for extended Loc Goblin.

0.3 seems a bit too low for a Loc IV.
Surface finish WILL have a effect.

ok, so OR must use some sort of intial value as its first guess in order to solve the actual Cd, since Cd is based on the drag force determined from wind tunnel study.

 

[mikec]

AFAIK, the Barrowman equations are closed-form approximations of some kind of combo of theory and empirical data, they're not iterative and they don't use an "initial guess" for anything. But you could read Barrowman's original description linked above if it actually mattered to you.

 

[Lt72884]

AFAIK, the Barrowman equations are closed-form approximations of some kind of combo of theory and empirical data, they're not iterative and they don't use an "initial guess" for anything. But you could read Barrowman's original description linked above if it actually mattered to you.

i have been reading that the last half hour or so. its a tough read haha. its like reading my fluids book all over again.

just found this book as well

https://ia600707.us.archive.org/13/...65/Fluid-dynamic_drag__Hoerner__1965_text.pdf

 

[bill_s]

In some cases in rocketry, Cd is not normal. For example RASP basic altitude and speed software asks you diameter and Cd but nothing about the fins. To be correct that has to include fin drag and a lot of other stuff not directly related to cross section area of the airframe. That's why high numbers such as .75 or higher are used. The actual Cd might be more like .35 for the entire cross section. That appears to be what Open Rocket feeds back to you, as at the bottom it gives "Reference length" equal to the diameter of the rocket and "Reference area" based on that diameter. You can then stick that back into basic programs like RASP. However when I used RASP with unusual designs I'd calculate the entire cross section and an equivalent diameter and feed it a Cd without so many fudge factors attached.

Certainly when you see drag coefficient for a car it includes the whole thing and they're not way slippier than a roclet.

 

[Lt72884]

In some cases in rocketry, Cd is not normal. For example RASP basic altitude and speed software asks you diameter and Cd but nothing about the fins. To be correct that has to include fin drag and a lot of other stuff not directly related to cross section area of the airframe. That's why high numbers such as .75 or higher are used. The actual Cd might be more like .35 for the entire cross section. That appears to be what Open Rocket feeds back to you, as at the bottom it gives "Reference length" equal to the diameter of the rocket and "Reference area" based on that diameter. You can then stick that back into basic programs like RASP. However when I used RASP with unusual designs I'd calculate the entire cross section and an equivalent diameter and feed it a Cd without so many fudge factors attached.

Certainly when you see drag coefficient for a car it includes the whole thing and they're not way slippier than a roclet.

Very interesting information. I am finding out many new things than i did yesterday haha. After using the wind tunnel at school, using excel to solve for the CD like i did, im learning ALOT!!!

 

[shockie]

i have been reading that the last half hour or so. its a tough read haha. its like reading my fluids book all over again.

just found this book as well

https://ia600707.us.archive.org/13/...65/Fluid-dynamic_drag__Hoerner__1965_text.pdf

I have that .pdf. great book.

 

[shockie]

Subsonic cd varies very little

 

[Lt72884]

Subsonic cd varies very little

those graphs look pretty similar to the ones i did with our wind tunnel comparing Cd to Reynolds number

 

[Buckeye]

In some cases in rocketry, Cd is not normal. For example RASP basic altitude and speed software asks you diameter and Cd but nothing about the fins. To be correct that has to include fin drag and a lot of other stuff not directly related to cross section area of the airframe. That's why high numbers such as .75 or higher are used. The actual Cd might be more like .35 for the entire cross section. That appears to be what Open Rocket feeds back to you, as at the bottom it gives "Reference length" equal to the diameter of the rocket and "Reference area" based on that diameter. You can then stick that back into basic programs like RASP. However when I used RASP with unusual designs I'd calculate the entire cross section and an equivalent diameter and feed it a Cd without so many fudge factors attached.

Certainly when you see drag coefficient for a car it includes the whole thing and they're not way slippier than a roclet.

The choice of reference area doesn't matter, as long as it is consistent. The total drag force is proportional to CdA. The A's cancel out. Cd and A are just fudge factors to make the math agree with observations.

Using the standard conventions for area in the automotive and aerospace industries, indeed a car "looks" way slipperier than a rocket, if you just compare shape efficiency. Cd car = 0.3, Cd rocket = 0.7

 

[bill_s]

The choice of reference area doesn't matter, as long as it is consistent. The total drag force is proportional to CdA. The A's cancel out. Cd and A are just fudge factors to make the math agree with observations.

Using the standard conventions for area in the automotive and aerospace industries, indeed a car "looks" way slipperier than a rocket, if you just compare shape efficiency. Cd car = 0.3, Cd rocket = 0.7

I consider the fins part of the shape. Rockets vary especially mine, and I used to actually use RASP, so switched to calculating equivalent diameter and used Cd .35 with good results. .7 would include a factor for how much bigger the whole thing is than what is measured. That's fine for comparing similar rockets but becomes useless to actually compare the 2 vehicles.