Overrides and surface finish

Use mass, CG, and drag coefficient overrides together with surface finish settings to produce more accurate OpenRocket simulations.

As you develop a design, OpenRocket estimates stability and performance from component geometry and material data. As you acquire real parts, build the rocket, and collect flight data, you can replace these estimates with measured values. This page explains how to use mass, center of gravity (CG), and coefficient of drag (C_D) overrides, and how surface finish settings affect drag.

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Overrides

OpenRocket lets you override three values that directly determine flight characteristics:

Mass

Replace the calculated mass of a component or assembly with a measured value from a scale.

Center of gravity

Replace the calculated CG position with one measured by balancing the assembled rocket on a knife edge or hanging it from a string.

Coefficient of drag (C_D)

Adjust the drag coefficient after comparing a completed simulation to real flight data.

All three overrides can be applied at three levels:

• Single component — the lowest level; overrides only the one component
• Subassembly (component group) — overrides the parent component and optionally its children
• Stage — the highest level; overrides the entire stage

Override for all subcomponents

OpenRocket’s component tree is hierarchical: each component may have child subcomponents. When you apply an override, you can choose whether to apply it to all subcomponents as well.

Without subcomponent override: The override value is added to the values calculated for all subcomponents. For example, if a body tube has a calculated mass of 100 g and its fins have a calculated mass of 50 g, overriding the body tube to 110 g results in a total of 160 g.

With subcomponent override: The override value replaces the mass of the entire subtree. Overriding the body tube to 170 g with subcomponent override enabled means the total assembly mass is 170 g — the fins contribute no additional mass.

Use the subcomponent override when you have weighed a completed subassembly (including glue, paint, and hardware). The override should then represent the total measured mass of that assembly, and subcomponents should be overridden so they are not counted twice.

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Using overrides throughout the build process

When designing your rocket

During the design phase, OpenRocket calculates estimated mass, CG, and C_D from component geometry and material density. Use these estimates to select a motor and verify that the CG is forward of the CP by a comfortable stability margin before ordering parts.

When you have your parts

Once you have the physical components, weigh each one individually. Override the mass of each component at the single component level with the measured value. Recheck the CG-to-CP margin after applying these overrides.

As you build your rocket

As subassemblies are completed, weigh each one and measure its CG. Apply mass and CG overrides at the subassembly level, checking Override for all subcomponents so that the subcomponent masses are not double-counted.

When the rocket is fully assembled, weigh it and measure its CG one final time. Apply these overrides at the stage level with Override for all subcomponents enabled.

Apply a stage-level mass and CG override on every rocket you build. This gives the most accurate starting point for simulation before the first flight.

After your first test flight

After the first flight, compare your simulation results to actual flight data (altitude, velocity profile, and so on from an altimeter or on-board logger). Because mass and CG are already overridden with measured values, the main remaining variable is drag. Adjust drag in two ways:

1. Change the surface finish — adjusts friction drag across the airframe without overriding any C_D values.
2. Apply a stage-level C_D override — nudges the overall drag coefficient up or down while preserving the per-component C_D calculations.

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Mass and CG override details

Stability margin and rotational inertia

The recommended stability margin is:

• Not less than 1.0 caliber for subsonic flights
• Not less than 2.0 calibers for transonic and supersonic flights

When you override the mass and CG of subcomponents, the rocket’s rotational inertia (moment of inertia) changes. A poorly placed mass override can make a rocket simulate as stable when it is not, or as unstable when it is, particularly near the minimum stability margin.

Beginning with OpenRocket 22.02, you can override a stage’s mass or CG without overriding subcomponents. This adds the override value as a delta on top of the calculated values, preserving the existing distribution of mass and its contribution to rotational inertia.

Matching measured mass and CG

The recommended approach to minimize impact on rotational inertia is:

1. Compute the difference between the measured and calculated mass (or CG).
2. Enter that difference as the override value on the stage or subassembly component.
3. Leave Override for all subcomponents unchecked.

This adds the discrepancy (from glue, paint, decals, and so on) without redistributing the component masses that OpenRocket uses for inertia calculations.

Adjustable weight systems

You can use the stage-level mass and CG overrides to simulate an adjustable nose weight that compensates for different motor sizes.

Simplified approach

1. Add a mass component for ballast

   Place a Mass component inside the airframe. Position it relative to the Tip of the nose cone, entering the measured distance from the nose tip to the rocket’s actual CG (flight-ready, without motor).

2. Set the stage CG override

   In the Stage configuration pane, enter the distance from the nose tip to the center of your adjustable weight as the Override center of gravity value.

3. Tune mass for your motor

   Select your motor configuration. On the Stage configuration pane, adjust the Override mass value until the stability margin between CG and CP is comfortable. Then add that exact amount of weight to the physical adjustable weight system in the rocket.

Before heading to the range, install the motor (without igniter) and verify the stability margin physically — mark the CG in blue and the CP in red on the airframe.

Precision approach

1. Override each component

   Weigh every individual part. Enter the measured mass override for each component at the single-component level.

2. Override each subassembly

   Using the component tree as a guide, identify each parent component. As each subassembly is completed during construction, weigh it and enter its mass as an override on the parent’s Override tab, checking Override mass and CG of all subcomponents. This accounts for adhesive, fasteners, and other build materials.

3. Apply stage-level overrides

   Follow the simplified approach steps to set the stage-level mass and CG overrides and tune for each motor configuration.

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Surface finish settings

Surface finish (roughness) determines how air flows over the airframe. A smoother surface produces less friction drag; a rougher surface produces more. OpenRocket offers five surface finish options, in order from highest drag to lowest:

Finish	Drag contribution
Rough	Highest
Unfinished	High
Regular paint	Medium
Smooth paint	Low
Polished	Lowest

Each component has its own surface finish setting, accessible on its Appearance tab (under General > Component finish in the configuration window). You can apply a single finish to every component at once by clicking Set for all on any component’s Appearance tab.

Set for all only applies to components that exist at the time it is used. Components added afterward will use the default finish.

Changing the surface finish adjusts the overall C_D by modifying friction drag across all exterior components, without overriding any individual component’s drag coefficient. This preserves OpenRocket’s ability to calculate natural C_D variations caused by velocity and atmospheric density changes during flight.

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Coefficient of drag (C_D) overrides

How the stage C_D override works

The stage C_D override behaves the same way as the mass and CG overrides:

• With subcomponent override enabled: Your value replaces the aggregate C_D of all components.
• Without subcomponent override: Your value is added to the calculated C_D of all components.

Recommended workflow

1. Bracket with surface finish

   After your first flight, change the surface finish setting (applying to all components) to make the simulated flight closer to the actual result. Try the two finish settings that bracket your target and identify which simulation result is lower.

2. Set the closer finish

   Set the surface finish to the option from the two bracketing simulations that produced the lower result.

3. Fine-tune with stage C_D override

   If additional precision is needed, apply a small C_D offset at the Stage level. Leave Override for all subcomponents unchecked. Adjust the value incrementally and rerun the simulation until the results match actual flight data.

Overriding C_D at the component or subassembly level is not recommended. The drag coefficient changes with velocity and atmospheric density throughout the flight, especially near the speed of sound. A C_D calibrated from a flight with a small motor may give misleading results when used with a larger motor. Use the stage-level offset approach instead.

Applying this method at the stage level without overriding subcomponents retains per-component C_D calculations and the simulation’s ability to model natural drag variation throughout the flight envelope.