Third-Party Compatibility

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Preface

OpenRocket is one of the best flight simulation software packages for model rockets, from low to high power. Even so, there are times when structural strength and aerodynamic predictions should be independently verified, such as when extremely high speeds or velocities are anticipated.

Fin Flutter: Very high speeds and velocities can generate oscillations (rapid back and forth movements) within a component that compromise its structural integrity. The component most susceptible is the fins, a phenomenon common called "fin flutter." At its extreme limit, excessive fin flutter can result in one or more fins being literally ripped off the body tube near the fin root edge (called divergence), even if the fins are through-the-wall (the fin materially itself is tearing). OpenRocket does not analyze fin flutter or divergence velocity, but, if there is such a concern, AeroFinSim performs this type of analysis, and more.

Mach and Above Mach Flights: During flights that exceed the speed of sound, a change in the drag coefficient of the rocket can significantly affect altitude. Overall, the drag coefficient estimations used by OpenRocket match the experimental results with reasonable accuracy, and the results of actual test flights. But, at higher supersonic speeds the OpenRocket simulation produces less reliable results, producing a too high drag coefficient. If a flight requires a high level of accuracy, RASAero uses flight projection equations equivalent to professional engineering aerodynamic analysis methods.

Third-Party Software

AeroFinSim

Overview

the function of AeroFinSim can be divided into three types of analayses, structural stress, aeroelasticity prediction of fin flutter and divergence velocity, and stability of spin stabilized rockets that use canted fins to achieve rocket rotation.

The structural stress portion of the program determines how strong the fins are by determining the aerodynamic fin loads. In its structural analysis mode, AeroFinSim looks at the material of the fins, how long the fins are, their span, how thick they are, the size of the fillets, how they are attached to the rocket (through-the-wall or butt-joint), and what type of glue is used. Using this information FinSim computes the maximum allowable bending force the fin can handle without causing fin separation. Then, using the maximum angle-of-attack the rocket will attain, AeroFinSim computes aerodynamic loading based on the geometry of the fins in terms of lift and drag, displaying the highest speed that can be tolerated before the fins will shred or separate from the rocket.

The aeroelasticity portion of the program predicts flutter and divergence velocity for up to six sets of fins. Fin flutter and divergence are vibrations of the fin caused by the coupling of free flight aerodynamic forces with lightly damped structural modes of vibration, that can range from a slight buzzing sound to instances where the oscillations are so severe the fins are stripped off the rocket. Fin flutter and divergence also create excess drag, causing the rocket to lose altitude and flight speed. AeroFinSim predicts when flutter occurs, so you can either strengthen the fins or choose a different rocket motor that limits the speed of the model.

The spin stabilization portion of the program analyzes rockets that use canted fins to achieve rocket rotation. AeroFinSim computes the center of pressure location of a spin stabilized rocket by applying the principals of gyroscopic motion. The SpinSim routine computes precession angle, added moment coefficient due to spin stabilization, total pitch moment coefficient with spin stabilization, rotary speed, precession speed and total drag coefficient (Cd) due to spin stabilization.

Obtaining AeroFinSim

The AeroFinSim application was developed by John Cipolla, the Chief Aerodynamicist at AeroRocket Engineering, and may be obtained from AeroRocket Engineering by asking download permission and instructions through an email request to John Cipolla.This request should contain your name, and your intended use of the application (one or more of the program portions for use only within model rocketry). Because of the nature of the application and its potential for misuse, John Cipolla reserves the right to grant or deny any such request, at his sole discretion.

Compatibility

AeroFinSim does not save, open, or import or export design data. The fin data points and material selections must be manually entered each time the AeroFinSim application is opened.

RASAero

Overview

RASAero is a combined aerodynamic analysis and flight simulation software package for model rockets and high power rockets, amateur rockets, and sounding rockets. The RASAero aerodynamic prediction methods are the most accurate available for model, high power, and amateur rockets, and are of equivalent accuracy to professional engineering method aerodynamic analysis codes used for missiles, sounding rockets, and space launch vehicles.

RASAero can also be used for predicting aerodynamic coefficients for use in other flight simulation programs for orbital rockets.

The RASAero software is free software and can be downloaded and installed for free. The RASAero software is not open-source software.

Obtaining RASAero

The RASAero software was developed by Charles E. (Chuck) Rogers and David (Coop) Cooper. The software can be downloaded for free from the Rogers Aeroscience website at: RASAero.com

Compatibility

RASAero imports the Outer Mold Line (OML) of the rocket from the design file, not any of the internal components, and RASAero does not handle ring fins, or strap-on boosters. Simply put, the OML consists of the outer lines of the rocket including the nose cone, body tubes, and fins, no internal components. However, RASAero also analyses nose cone tip radius, launch shoes, 6 additional fin airfoil section types, and protuberances such as camera shrouds, that OpenRocket does not feature.

OpenRocket does not export a design file directly into the RASAero file format. However, OpenRocket does export a design file into the Rocksim file format, which RASAero directly imports. At this time, to ensure that the process of importing such a file into RASAero is successful, it is recommended that the following steps be followed:

1. Save a copy of the design file using a different name; this file will be stripped of all incompatible components.
2. Delete all components except those necessary to create the Outer Mold Line (OML); leave only the rocket nose cone, body tubes, transitions, launch lugs, and fins (without fin tabs).
3. Save the modified design file.
4. Open the modified design file in OpenRocket 15.03.
5. Save the modified design as a RockSim (.rkt) file.
6. Open RASAero and import the previously saved RockSim (.rkt) design file.
7. After importing the RockSim (.rkt) design file into RASAero, the fin airfoil inputs should be checked. The rocket may have used an airfoil that is included in RASAero, but not included in OpenRocket or RockSim, and the airfoil may have been approximated by another airfoil in the OpenRocket or RockSim inputs.
8. After importing the RockSim (.rkt) design file into RASAero, the rail guide, launch shoe, and launch lug inputs should be checked. Some of the rail guide, launch shoe, and launch lug configurations are inputted differently in RASAero compared to OpenRocket and RockSim.

OpenRocket file export compatibility with RASAero, to the extent possible, is expected in the future.

Please Note: Although RASAero is the most accurate aerodynamic analysis software for use in model rocketry, it is not without limitations which can prevent the correct importation of a design file. By way of example, RASAero fin shapes are currently limited to straight-line fin leading and trailing edges and straight-line fin tip chords parallel to the rocket body, and fins featuring a convex angle or other non-standard profile will not import correctly. Complex fin shapes have to be approximated by straight-line segments with the fin tip chord parallel to the rocket body.

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