Editing Overrides and Surface Finish (section)

== 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 (C<sub><small>D</small></sub>). 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 that single stage as the child of the rocket, and the stage 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.