Open Rocket 22.02.Beta.05 and LOC Precision T-Loc Simulation

[gsitexas]

First of all, a really special "Thank You" to the developers and everyone involved in the development of Open Rocket 22.02. Beta.05 software, it is really awesome, and all your hard work to bring this software to the community is very much appreciated!

I used this new version of Open Rocket software to successfully model the LOC Precision 2.63" T-Loc rocket using the kit that I received last week (see attached file).

FYI, occasionally when I open the attached file the Parachute shows to be of extreme length for some reason. If this happens, please just reset the Packed Length to 3.5" and the model will correct itself as defined.

My question pertains to how to "most" accurately model a short rocket with high drag fin can components.

As you can see in the attached model file, I used a conical nose cone having a base diameter the same diameter as the T-Loc body tube (2.63") and a length of 8.2582" (Pi times the 2.63" diameter). The mass of the "phantom" nose cone was set to zero in the component override section.

Simulations with the T-Loc modeled as described above appears to function accurately and provides Estimated Altitude results that appear quite close to those published by LOC Precision and those obtained using Thrustcurve.org. The simulations also confirmed that I need to add ~ 1 oz of nose weight to achieve stable flight for all the motor options that I anticipate flying with this rocket.

My question pertains to the Cp location as defined by LOC Precision at 19" +/- 0.25" from the tip of the nose cone versus the Cp location defined by this Open Rocket model simulation located at 19.441" from the nose cone tip.

Please note my model does include an added third centering ring located at the top of the fin leading edge and an Aero Pack 29mm motor retainer, however, deletion of either or both of these items only results changes to Cg location, no change noted to the Cp location.

To obtain, the most accurate simulation results, is it recommended that I shorten the length of the "phantom" cone to reposition the simulation Cp location to the 19" for nose cone tip location published by the kit vendor or should I leave the simulation model as currently defined? FYI, it requires reducing the "phantom" cone length from 8.2582" to 5.5" to move the Cp location from 19.441" to 19.002".

Thank you in advance for your input and recommendations!

Best regards,

Keith

 

[thzero]

I'd leave it as is.
Removing adding internal things like centering only affect Cg not Cp. Cp is based on fin shape, location, body tube, etc. Unless the vendor has accounted for that all at best the vendor is an approximate measurement.

 

[Buckeye]

I like that LOC puts some bounds on CP, but at the same time it implies a level of precision that doesn't exist. 19.441 vs. 19.25 is splitting hairs. Don't worry about it.

People think the altitudes and CP that the vendor prints on the face card are gospel. Does LOC perform wind tunnel testing, CFD, and rigorous flight measurements with various motors? I doubt it. They use the same simple simulation tools as the rest of us.

Thrustcurve performs simplified, nominal trajectory analysis with a constant Cd taken from typical 3FNC performance. If OR simulations with fins-on-fins (or pods, or tubes) gives the same altitude result, then I would be suspicious. At least I would skip the effort of designing the rocket in OR and just use the easy Thrustcurve model.