Rocksim vs RASAero Cp

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KevinT

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Working on my L3. My rocket is 130" long, 5.5" diameter with 3 fins. When I run the design in Rocksim it calculates a Cp of 100.4" using the Rocksim Stability Equations, and in RASAero it calculates 95.4" using the Roger's modified Barrowman equations. This is roughly a caliber in difference. The motor is an AT M1939W . Does anyone have any recommendations on which is a better predictor of Cp.
 
Can you post the RockSim and RASAero files so that we can see what your design looks like. Both programs are good predictors of Cp.
 
I find modified Barrowman to be pretty conservative, and true CP to usually be a little aft of what RockSim thinks it is.
 
OpenRocket (Barrowman, I think) = most conservative, CP more forward
RASAeroII (Modified Barrowman) = in the middle
RockSim (RockSim method) = most liberal, CP more aft
 
OpenRocket (Barrowman, I think) = most conservative, CP more forward
RASAeroII (Modified Barrowman) = in the middle
RockSim (RockSim method) = most liberal, CP more aft

That how I see it to. My dilemma is if I use rocksim and have the static margin of 1.0. It would be essentially 0 using rasaero. However, If I use rasareo at 1.0 then rocksim would be 2.0. This would probably be a good choice to dial in my Cg and Cp to match up with a static margin of 1.0 using rasareo.
 
Here are my files and a picture of my rocket in it's current state of completion (nothing is glued and the fins are paper templates)
 

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Here are my files and a picture of my rocket in it's current state of completion (nothing is glued and the fins are paper templates)

Thanks for the files. Sharp looking rocket! Will fin flutter be a concern?
 
I'd stick with RASAero. Some people over the years have had trouble with the Rocksim modified method. I dug into its derivation once and came away unconvinced. A little extra conservatism is not going to hurt.
 
I'd stick with RASAero. Some people over the years have had trouble with the Rocksim modified method. I dug into its derivation once and came away unconvinced. A little extra conservatism is not going to hurt.

I totally agree. Thanks for the input. If I make an error somewhere, I would rather fall on the side of being over stable for my L3.
 
You used the CG and Mass Overide in Rocksim - so I assume you packed the rocket with assorted gear to get a reasonable weight representation? RASAero has you right at the line on the Stability, and it gives you the warning. But if you look at the CG graph during the flight, it moves forward right quick because the motor is loosing mass right quick. RASAero will keep you safe for sure.
 
You used the CG and Mass Overide in Rocksim - so I assume you packed the rocket with assorted gear to get a reasonable weight representation? RASAero has you right at the line on the Stability, and it gives you the warning. But if you look at the CG graph during the flight, it moves forward right quick because the motor is loosing mass right quick. RASAero will keep you safe for sure.

Thanks for the input. Correct. The rocket was packet with all recovery components, deployment electronics, mass to represent the motor and mass to represent the fins was added to the rocket. I then used a rope to find the Cg and all components were weighed to get close to final weight. I used these in the mass override.

Now I'm at the point of fine tuning the final fin size to properly position the Cp.
 
Correction, for the mass override is did not include the motor mass in rocksim because that gets added by rocksim. The final rocksim rocket weight and Cg basically matched what I was getting with my rope test. That final Cg data and weight was what I put into rasaero.
 
The big thing people overlook is that CG and CP are Dynamic in nature. Motor burn changes CG, speed and Angle of Attack change CP (mach transition, etc.),etc,etc. Looking at a single point "static stability" kicked out by any program only gives you a very limited bit of information.

ALL Flights (A-motors thru moon-shots) are highly dynamic. Look at the stability curves, and learn to understand what they are showing you. You'll get more insights into overall flight performance.

Mike.
 
The big thing people overlook is that CG and CP are Dynamic in nature. Motor burn changes CG, speed and Angle of Attack change CP (mach transition, etc.),etc,etc. Looking at a single point "static stability" kicked out by any program only gives you a very limited bit of information.

ALL Flights (A-motors thru moon-shots) are highly dynamic. Look at the stability curves, and learn to understand what they are showing you. You'll get more insights into overall flight performance.

Mike.

I agree. Thank you for the feedback.
 
RASERO is really intended for mach flights. Rocksim in my experience and opinion is for flights less then mach. I am a old school flyer and cheap. I dont need all the bells and whistles that rocksim offers plus i dont want to pay for it. I believe RASERO is the most accurate software I have used.

If your flying low, mid, high power below mach rocksim is fine for you. but when flying low, mid, and high power at mach or faster Rasero is your best bet for accuracy.
 
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