Overrides and Surface Finish
Using Overrides and Surface Finish to Create More Accurate OpenRocket Simulations
As you develop your design, OpenRocket estimates the rocket's stability and performance characteristics based on entered and known information about the components you choose. As you accumulate parts, and build and test your rocket, you will obtain more accurate information than what OpenRocket estimated. With this knowledge, you can override the initial estimates and create more accurate flight simulations.
On this page, how OpenRocket looks at the different overrides will be explained in more detail, and practical examples of their use will be given.
Overrides
OpenRocket allows you to override three values that are determinative of a rocket’s flight characteristics: mass, center of gravity (CG), and coefficient of drag (CD). All three of these values can be overridden at the stage level, the component group level, or the single component level. In addition, while not being an override in its own right, OpenRocket allows you to adjust the surface roughness of components so that drag estimates more accurately reflect actual flight data.
Override Mass
The database of parts included with OpenRocket has estimates for component masses. These estimates come from a variety of sources, ranging from data provided by the manufacturer and measured values to calculations based on the volume of the part and the "standard" density of the material it is made from. The accuracy of these sources varies, but nothing is more accurate than putting the actual part on a scale and weighing it. Once a part's measured mass is known, the mass override can be used to change the mass value, more closely reflecting the part's mass, and thereby the rocket's actual mass.
Override Center of Gravity
OpenRocket also estimates each component's center of gravity based on its materials and geometry, and calculates the overall center of gravity of each stage, and the entire rocket based on that. However, this really can't account for the weight of things like glue and paint, which can be significant and can move the center of gravity forward or back. Once you've got some components assembled, you can measure the CG (using any of the time-honored methods, like hanging from a string or balancing on a knife edge) and override this, too.
Override Coefficient of Drag
The drag on a rocket in flight is a function of the rocket's velocity, the density of the atmosphere, the frontal area of the rocket, and a number called the coefficient of drag. This, in turn, is a result of the airflow over the rocket, and is estimated based on the rocket's geometry and surface finish. Once you've actually flown the rocket and compared your simulation to the actual flight, you can adjust the rocket's coefficient of drag to better match the flight data.
One caution, the coefficient of drag does not remain constant through the flight, especially if the rocket gets near the speed of sound. At low speeds the friction on the rocket's body dominates the coefficient of drag, but as the speed increases the effect of the pressure drag (the drag caused by the pressure of the front of the rocket hitting air) and the base drag (caused by the low pressure area left behind the rocket) becomes progressively more significant. Adjusting the coefficient of drag based on a flight with a small motor, and using this result to estimate behavior with a larger motor can give misleading results. How to use drag coefficient overrides to "adjust" a calculated drag coefficient will be discussed later.
Override for All Subcomponents
OpenRocket designs are structured as a tree: each component (except the rocket itself) has a parent component and may have children (subcomponents). For instance, a single stage rocket will have (at least) a nose cone and a body tube as subcomponents. If fins are attached to the body tube, the fin set is a subcomponent of the body tube. When applying overrides to a component, you can specify whether to apply the override to all of its subcomponents as well. If you don't override the subcomponents, then the override applied to the parent component is combined with that of all subcomponents. For instance, suppose a body tube has a calculated mass of 100 grams and its fins have a calculated mass of 50 grams, the combination of body tube and fin set will have a mass of 150 grams. If you override the body tube mass, set it to 110 grams, and don't override the subcomponents, the total mass is now 160 grams.
If you do override the subcomponents, then the set mass of the parent replaces the mass of the entire subtree. So, using the same body tube and fin set as before, if you set the mass override of the body tube to 170 grams, and override the subcomponents, the total mass is 170 grams; the fins contribute no additional mass to the assembly.
But, why would you want to do this? Ultimately, the goal is to match the simulated mass to the measured mass after the rocket is finished, taking into account such things as adhesives and any exterior finish (paint and decals). In the case described above, the mass override is for the entire assembly -- the idea is that if you actually glue the fins to the body tube and weigh the result at 170 grams, that's what you want the finished mass of the assembly to be.
Using Overrides to Produce a More Accurate Simulation
Let's look at how you can apply overrides at different points in the design, construction, and simulation of the rocket to improve the accuracy of your simulations.
When Designing Your Rocket
You start designing your rocket by selecting different components, parts that have associated mass values and calculated values for the center of gravity and coefficient of drag. As you add parts, OpenRocket continuously updates to combined values, calculating an estimated mass, center of gravity, and coefficient of drag for the entire rocket. Then, based on your mass and performance goals, you select a motor. When you have, it is time to make sure the center of gravity is forward of the center of pressure by a comfortable stability margin; assuming it is, you can order your parts and start putting the rocket together.
When You Have Your Parts
Once you have your parts -- the nose cone, body tube, fin stock, and whatever recovery devices and other hardware you're using -- you can weigh each component separately. You're likely to find that the parts weigh something close to the calculated values, but are not exactly right. So, this (the individual part) is the lowest level on which you can override a mass. After overriding the mass of each part, you can check again to be certain that the center of gravity is still forward of the center of pressure by a comfortable stability margin, and modify your design as needed.
As You Build Your Rocket
When assembling your rocket, as you complete subassemblies, you can weigh these subassemblies and measure their centers of gravity, applying mass and center of gravity overrides at the second (subassembly) level. When applying an override to a subassembly, you want to check the "Override for all Subcomponents" so that you don't count a subcomponent within the assembly twice.
Finally, when your rocket is complete, you can again weigh and measure the center of gravity of the entire rocket, and apply these overrides at the highest (stage) level. Once again, at this level you'll want to override subcomponents.
After Your First Test Flight
Once you've flown your rocket for the first time, you can compare your simulations to your actual flight data. Since each of your simulations was based on overridden measured mass and center of gravity values, the only thing you need to adjust at this point is drag. You can take two approaches to this, you can adjust the friction drag by changing the rocket's surface finish, or you can directly modify the drag coefficient. The process is much the same in both cases.
To change the surface finish, you select external components (parts of the rocket's actual airframe) and simply select a different surface finish. Then, rerun the simulation again, and compare the new difference between the actual flight results and your new simulation; you continue to adjust the surface finish until you are satisfied that the simulation results are as close as possible to the actual flight data. Even though only the airframe surface matters, for the first few adjustments, it is easier to select a new component finish on any part and then simply click the "Apply to All Components" button. After you have bracketed your desired simulation result, you can fine-tune the simulation by adjusting the surface finish for individual components.
To directly adjust the drag coefficient, you apply a drag coefficient override to the entire stage. In this case you don't want to override the subcomponents; what you're doing is applying a constant offset to the drag coefficient being calculated over the course of the flight.
After applying these modifications, you will have a more accurate simulation for future flights with different motors.