Over-optimistic RASAero sims?

[LithosphereRocketry]

So I'm thinking about upgrading my current 18mm min-diam rocket to a more efficient airframe. I simmed it in OR and got ~3000 feet on an AT D10-7, in the ballpark of what I would expect. But when I re-did it in RASAero, it said around 4000-4500 feet, which I would think would be WAY more than reality, given that the D altitude record w/ altimeter confirmation is only 2500 feet. (Non altimeter is around 4000 feet; however, I'm not sure quite how much I trust that.)

So what am I missing? I've gotten the impression that RASAero is to be trusted, but I'm kinda skeptical about this sim.

Here it is, in a .zip to allow me to upload it. View attachment 18mm_Performance_2.zip

The boattail is meant to simulate the aerodynamics of the 1/4" of motor hanging out the back.

Also, do people have any experience with standard vs. Rogers Modified Barrowman sims? The Rogers version gave me 4k instead of 4500 but more stability.

 

[Buckeye]

Did you input the mass as rocket plus motor? RASAero II requires that.

 

[Chuck Rogers]

The boattail is meant to simulate the aerodynamics of the 1/4" of motor hanging out the back.

1) Remove the boattail. Once there is a step from the rocket body diameter to the motor diameter, or the motor nozzle diameter, then it is all base area.

2) As Buckeye noted, in RASAero II you input the lift-off weight. Overall for a simulation it's important to have a realistic lift-off weight.

3) If the rocket is not tower launched, did you add a rail guide or launch lug?


Chuck Rogers
Rogers Aeroscience

 

[LithosphereRocketry]

Did you input the mass as rocket plus motor? RASAero II requires that.

Yes.

It's pretty close to optimal weight so the sim would adjust it to match anyway.

By the way, I'm simming it with no LL because I'm planning on 3D printing some fly-away lugs for it.

 

[AlphaHybrids]

Yes.

It's pretty close to optimal weight so the sim would adjust it to match anyway.

By the way, I'm simming it with no LL because I'm planning on 3D printing some fly-away lugs for it.

For 18mm, I'd make a quick tower out of some PVC pipe. Will be much more consistent than fly away lugs and you won't have any of the initial drag.

Edward

 

[LithosphereRocketry]

1) Remove the boattail. Once there is a step from the rocket body diameter to the motor diameter, or the motor nozzle diameter, then it is all base area.

2) As Buckeye noted, in RASAero II you input the lift-off weight. Overall for a simulation it's important to have a realistic lift-off weight.

3) If the rocket is not tower launched, did you add a rail guide or launch lug?


Chuck Rogers
Rogers Aeroscience

Sorry I missed your post earlier. Removing the boat tail didn't change much. I'm planning on fly-away lugs or a tower. Adding the lug didn't get within a reasonable range anyway.

By the way, three unrelated RASAero questions-
-Is there any way to change RASAero's "home" folder for files, etc.? It's kinda annoying to have to navigate to where I keep all of my rocket stuff every time I want to open a file.
-Why is nozzle size a parameter for the simulation? What would it change?
-Is there any way to ask for the drag force on a fin at a given angle of attack and speed? I've been messing around in OR and RAS and haven't found a good way to do that.

Thanks for all the help! The great thing about small communities is being able to ask a question... and get an answer on the same day... from the person who actually develops what you're asking about.

 

[AlphaHybrids]

Sorry I missed your post earlier. Removing the boat tail didn't change much. I'm planning on fly-away lugs or a tower. Adding the lug didn't get within a reasonable range anyway.

By the way, three unrelated RASAero questions-
-Is there any way to change RASAero's "home" folder for files, etc.? It's kinda annoying to have to navigate to where I keep all of my rocket stuff every time I want to open a file.

Create a shortcut to that folder and place it on your desktop. One click and you are there.

Edward

 

[Buckeye]

-Is there any way to ask for the drag force on a fin at a given angle of attack and speed? I've been messing around in OR and RAS and haven't found a good way to do that.

-Why is nozzle size a parameter for the simulation? What would it change?

In OR, Tools>Component Analysis lets you vary Mach Number and shows the CD breakdown for the fin set. Drag Force = 0.5*rho*CD*Aref*V^2. I guess you could divide this value by the number of fins to give force on one fin. At an angle of attack the flow is non-symmetric, so you probably need Computational Fluid Dynamics simulation to see each fin.

Nozzle size has something to do with computing the base drag differences when the motor is burning vs. coasting after burnout.

 

[Buckeye]

I tried to run your simulation, but rasp.eng in my RASAero installation doesn't contain AT D10. There is AP D10 (Apogee). Did you add the AT motor to your engine database, and is that the exact same motor profile found in OpenRocket? Most simulation discrepancies, beyond human error, usually boil down to bad motor data.

I did run the AP D10 simulation and got same as you reported ~4000 ft.

Care to post your OR file as well?

 

[dhbarr]

https://www.google.com/url?q=https://www.nar.org/SandT/pdf/Aerotech/D10.pdf

 

[Buckeye]

https://www.google.com/url?q=https://www.nar.org/SandT/pdf/Aerotech/D10.pdf

The question is not where to find the motor file, but if the file is consistent in the OP's RA and OR simulations.

By the way, the .eng file in the link above says it is an Apogee motor, not Aerotech. So which is it?

https://www.thrustcurve.org/simfilesearch.jsp?id=1659

 

[dhbarr]

The question is not where to find the motor file, but if the file is consistent in the OP's RA and OR simulations.

By the way, the .eng file in the link above says it is an Apogee motor, not Aerotech. So which is it?

https://www.thrustcurve.org/simfilesearch.jsp?id=1659

That was my point. AT makes the D10, which sometimes has an AP badge on it.

 

[Chuck Rogers]

-Is there any way to change RASAero's "home" folder for files, etc.? It's kinda annoying to have to navigate to where I keep all of my rocket stuff every time I want to open a file.

For rocket .CDX1 files, unfortunately No. But as others have noted, there are workarounds.

-Why is nozzle size a parameter for the simulation? What would it change?

It's used to calculate the reduction in the body base drag with the rocket motor thrusting (power-on). RASAero II calculates power-on and power-off Drag Coefficient (CD). You can enter a nozzle exit diameter of zero, and the power-on CD will then be the same as the power-off CD.

-Is there any way to ask for the drag force on a fin at a given angle of attack and speed? I've been messing around in OR and RAS and haven't found a good way to do that.

You can use the RASAero II Run Test feature (see Pages 68-74 in the RASAero II Users Manual) to print out Axial Force Coefficient (CA) and Normal Force Coefficient (CN) for an angle of attack up to 15 deg. Just create a copy of the rocket .CDX1 file with No Fins, and then run the rocket with and without Fins using Run Test. You can then calculate the Delta CA and Delta CN due to the Fins by looking at the difference between the Fins and No Fins CA and CN. You'll have to calculate the Axial Force and Normal Force on the Fins from the Delta CA and Delta CN, RASAero II does not do this directly.

What you do is pick a point on the flight simulation (Mach Number and Altitude), and get the CD and the Drag force. Divide the Drag Force by the CD and you'll get a number (dynamic pressure times the rocket frontal area). Pick the same Mach Number point on the Run Test output and get the Delta CA. Multiply the Delta CA by the number (dynamic pressure times the rocket frontal area) and you'll get the Axial Force on the Fin in lbs.

Same thing for the Normal Force on the Fin (actually the force you'll be most concerned about). Pick a point on the flight simulation, and get the CL and the Lift force. Divide the Lift force by the CL and you'll get a number (dynamic pressure times the rocket frontal area). Pick the same Mach Number point on the Run Test output and get the Delta CN. Multiply the Delta CN by the number (dynamic pressure times the rocket frontal area) and you'll get the Normal Force on the Fin.

Note that this will be the Axial Force on all of the Fins, and the Normal Force in one plane on all of the Fins. For Four Fins, the Normal Force would be divided by two, and that would be the Normal Force on each Fin.

Thanks for all the help! The great thing about small communities is being able to ask a question... and get an answer on the same day... from the person who actually develops what you're asking about.

Your welcome!


Chuck Rogers
Rogers Aeroscience

 

[Salvage-1]

If i remember correctly nozzle exit diameter also effects motor efficiency.
You could have exactly the same grains, yet two different nozzle exit diameters can determine if it is a F or G for example
So, if you are inputting the nozzle info incorrectly, you could be 'faking' a higher efficiency motor.

 

[Buckeye]

If i remember correctly nozzle exit diameter also effects motor efficiency.
You could have exactly the same grains, yet two different nozzle exit diameters can determine if it is a F or G for example
So, if you are inputting the nozzle info incorrectly, you could be 'faking' a higher efficiency motor.

Motors in the simulators are defined by the given thrust curve (.eng or .rse)

 

[Salvage-1]

Motors in the simulators are defined by the given thrust curve (.eng or .rse)

Go do a couple of sims and look at the difference in altitudes if you put in the incorrect nozzle exit diameter. Thats why I said 'faking'.

edit
although this may be more of an issue with larger motors.

 

[Buckeye]

You are faking the rocket aerodynamics, not the motor characteristics.

 

[Salvage-1]

You are faking the rocket aerodynamics, not the motor characteristics.

yup, sorry, faking the aerodynamics to act like more oomph, not braining well today!