How to "back-sim" in Open Rocket for CD and adjusting to true altitude. Also CP / Caliber movement with speed.

[Art Upton]

I originally Simed this rocket in RockSim and RASaero on a long ago removed computer. I loaded the backup rtk file last week and was trying to figure out the proper way to back-sim to get the sim to show the altitude that the rocket reached at least. Trying to get both that and speed to match is probably asking too much.

I know you can change the CD sort of by changing paint finish, but doing that to get altitude results in the mach number going higher then.

The OpenRocket Sim of this 4" rocket in my Avatar starts out with a Caliber of .865.
However when graphed with the launch, and watching CP vs Mach number you can see it change below.
Caliber is in Blue on the plot, CP location in yellow. And yes it flew straight and true.

When I built this I was using RockSim and RASaero on a different computer long removed. Each element of the build tree was weigh and over-ridden just for that sub-assembly. Trying to make it match what the two ARTS-1 flight computers produced, ARTS reported 17, 415.

This rocket can also be flown Min Diameter. The motor mount is not glued in, it is only screwed in at the centering ring with three screws, one each in-between the fins. Think of it simply as a motor adapter.

Edit: Yes the Sim is using the launch site data for Black Rock. When set to my location it does not go as high due to air density difference

 

[kjhambrick]

Art --

Yes, a 'back-sim' would be a kool feature to have

But like you, the only way I've found in OR is to mess with the surface finish or masses so I can't help with your Q.

But I have a Q ...

If I am reading correctly, you've got a difference of -1067 ft between sim'd apogee ( 16,348 ft ) and actual apogee ( 17,415 ft ) which is about 6% less than actual.

What motor are you flying in the OR sim ?

I've noticed that some of the thrust curves from the olden days have changed over the years.

Could there be differences in the thrust curve data for the motor you flew and the OR thrust curve data, from ThrustCurve.org ?

-- kjh

P.S. That is a beautiful launch picture -- looks like about an 8-foot flame !

 

[neil_w]

You can override the drag coefficient of the entire rocket. That’s the easiest way, although it’s still trial and error to dial in the exact value.

 

[Art Upton]

Art --

Yes, a 'back-sim' would be a kool feature to have

But like you, the only way I've found in OR is to mess with the surface finish or masses so I can't help with your Q.

But I have a Q ...

If I am reading correctly, you've got a difference of -1067 ft between sim'd apogee ( 16,348 ft ) and actual apogee ( 17,415 ft ) which is about 6% less than actual.

What motor are you flying in the OR sim ?

I've noticed that some of the thrust curves from the olden days have changed over the years.

Could there be differences in the thrust curve data for the motor you flew and the OR thrust curve data, from ThrustCurve.org ?

-- kjh

P.S. That is a beautiful launch picture -- looks like about an 8-foot flame !

It is the CTI-M1400 and back in the day I was able to back sim the CD in RS; as it had similar difference.

There is a "bump" in the body at the Fiberglass Fincan that slips over the Body tube. Cutis used to make those now and then back then.

Thanks on the PIC, it is my favorite.

 

[Art Upton]

You can override the drag coefficient of the entire rocket. That’s the easiest way, although it’s still trial and error to dial in the exact value.

Other then the few "Finish Choices" where is the rocket CD override, perhaps it is staring me in the face and I don't see it?

 

[Art Upton]

Other then the few "Finish Choices" where is the rocket CD override, perhaps it is staring me in the face and I don't see it?

Duh, it was staring at me I found it

 

[Art Upton]

Thank you @neil_w !

 

[OzHybrid]

I originally Simed this rocket in RockSim and RASaero on a long ago removed computer. I loaded the backup rtk file last week and was trying to figure out the proper way to back-sim to get the sim to show the altitude that the rocket reached at least. Trying to get both that and speed to match is probably asking too much.

I know you can change the CD sort of by changing paint finish, but doing that to get altitude results in the mach number going higher then.

The OpenRocket Sim of this 4" rocket in my Avatar starts out with a Caliber of .865.
However when graphed with the launch, and watching CP vs Mach number you can see it change below.
Caliber is in Blue on the plot, CP location in yellow. And yes it flew straight and true.

When I built this I was using RockSim and RASaero on a different computer long removed. Each element of the build tree was weigh and over-ridden just for that sub-assembly. Trying to make it match what the two ARTS-1 flight computers produced, ARTS reported 17, 415.

This rocket can also be flown Min Diameter. The motor mount is not glued in, it is only screwed in at the centering ring with three screws, one each in-between the fins. Think of it simply as a motor adapter.

Edit: Yes the Sim is using the launch site data for Black Rock. When set to my location it does not go as high due to air density difference

View attachment 649510
View attachment 649511
View attachment 649512
View attachment 649519

That colour scheme stands out nicely, good choice.

 

[Art Upton]

Since 1975/76

 

[OzHybrid]

Since 1975/76

Wow, a colour that can travel through time. Have you got a 6" dia avbay inside a 3"dia rocket too?