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Simulations are the core of what OpenRocket is built for. In a few clicks you can predict how high a rocket will fly, verify its stability off the launch rod, and dial in the right ejection delay — before you ever buy a motor.

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Why simulate?

Simulations are most valuable once you move beyond Estes A–C motors. As soon as you start choosing from the hundreds of mid- and high-power motors available, you need a way to answer two questions before every flight:
  • What altitude will this motor reach? You may need to stay below a waiver ceiling, avoid cloud cover, or keep the rocket in visual range.
  • What ejection delay should I use? Too short and the parachute opens while the rocket is still screaming upward — tearing a slot (a “zipper”) through the airframe. Too long and the rocket free-falls past apogee before deployment, risking a hard ground hit or structural damage.
OpenRocket answers both questions, and much more, in seconds.

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The Flight Simulations window

Open the Flight Simulations tab from the main window. The table lists every simulation in your rocket document. Each row shows:
  • Name — the simulation name
  • Configuration — the motor configuration used (usually named for the motor)
  • Calculated result columns: Max. altitude, Max. velocity, Max. acceleration, Time to apogee, Flight time, Ground hit velocity, Optimum delay
Result columns are blank until you run the simulation. A green circle with a check mark means the simulation ran successfully. A yellow exclamation mark means the simulation ran but produced at least one warning.
Simulation 3 in the A Simple Model Rocket example shows an exclamation mark because the motor delay is too short — the chute would deploy while the rocket is still moving fast, likely causing a zipper. Always check warnings before flying.
Common conditions that flag a simulation as failed or warned:
  • Too slow off launch rod — the rocket may be unstable at launch
  • Too short a delay — early parachute deployment, zipper or separation likely
  • Too long a delay — late parachute deployment, hard ground hit likely
  • Ground hit velocity too high — risk of damage or injury

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Running a simulation

1

Select one or more simulations

Click a simulation row to select it. Hold Shift or Ctrl/Cmd to select multiple rows.
2

Click Run simulations

Click the Run simulations button. OpenRocket calculates the flight and fills in all result columns. You can run a single simulation, a selection, or all of them at once.
3

Review the results

Check the result columns. Pay special attention to Optimum delay — this is the delay (in seconds) at which the ejection charge would fire at exactly apogee. Compare it to the delay printed on the motor you plan to use.

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Motors and configuration

Each simulation is tied to a motor configuration — a named set of motors assigned to your rocket’s motor mount tubes. To simulate a new motor, you need a new configuration.

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Creating a configuration

1

Open the Motors & Configuration tab

Click the Motors & Configuration tab in the rocket design window.
2

Create a new configuration

Click New Configuration. The configuration appears in the list. Use Rename Configuration, Copy Configuration, or Remove Configuration to manage your configurations.
3

Select a motor

In the Motor mounts table, click Select motor for the motor mount you want to populate. This opens the Motor Selection panel.

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Motor Selection panel

The Motor Selection panel lists every motor in OpenRocket’s built-in database. Use the filters to narrow the list:
Restricts the list to motors from specific manufacturers. This is especially useful for reloadable motors: if you own an AeroTech RMS casing, only AeroTech reloads will fit it.
Limits results to a range of impulse classes. No reason to display J–O motors when designing a small model rocket.
Filters by diameter and length. A motor larger than your motor mount tube cannot be used, though motors smaller than the maximum diameter are common — use an adapter to get the correct fit.
Prevents you from creating duplicate configurations for the same motor.
The columns in the motor list are sortable. Click any column header to sort by that field. The Show Details section at the bottom of the panel displays the selected motor’s thrust curve graph, average thrust, total impulse, burn time, and certification information.

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Setting the ejection charge delay

The Ejection charge delay field on the Motor Selection panel is one of the most important settings in OpenRocket. It accepts:
  • A value chosen from the drop-down list of standard off-the-shelf delays available for that motor
  • Any custom numeric value you type in seconds
Recommended workflow for adjustable delays:
1

Select a motor and an initial delay

Choose any available delay from the drop-down to start.
2

Run the simulation

Note the Optimum delay value in the results table.
3

Enter the optimum delay

Return to Select motor and type the optimum delay (or the closest value your delay-drilling tool can achieve) into the Ejection charge delay field.
4

Run again to confirm

Re-run the simulation. The warning about delay should be gone or significantly reduced.
Even for non-adjustable delays, check Optimum delay and choose the motor whose stock delay is closest to the simulated value.

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Getting a good simulation

OpenRocket simulates an ideal rocket in an ideal virtual world. Real flights differ because of:
  • Local air density (a function of temperature and barometric pressure at launch time)
  • Manufacturer tolerances in motor components
  • Mismatch between the simulated and actual launch angle
  • Wind speed and direction variation with altitude
  • Fin flutter, surface finish, and real-world component performance
  • Inaccuracies in the simulated model itself
Of these, model accuracy has the largest effect on prediction quality.

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Building an accurate digital model

Weigh every component

Use a digital kitchen scale. Weigh short items (shock cord, wadding) in multiples and divide. Glue, filler, and paint all add mass.

Measure thicknesses and lengths

Inexpensive digital calipers are sufficient. Record wall thickness, fin chord, span, and all tube lengths.

Choose correct materials

Select the closest matching material from the component’s material menu. If needed, create a Custom material rather than using a mass override, which may not correctly represent the center of mass.

Build the model before the rocket

The best approach is to design your OpenRocket model before assembly. That way you can weigh and measure each part as you build.

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Starting from a downloaded model

If you start from a downloaded .ork or .rkt file (OpenRocket can open both), verify that:
  • All component types and dimensions match your actual rocket
  • The total weight and center of mass match your physical rocket
  • Any mass overrides in the downloaded file are adjusted or removed — overrides are often added to make an imperfect model agree with reality, but they may distort the weight distribution

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Expectations on accuracy

A carefully built OpenRocket model can predict ejection delay and altitude well. That said, OpenRocket tends to predict slightly more altitude than a rocket achieves in practice. Treat simulation results as useful estimates, not guarantees.
If you track your flights with an altimeter or flight computer, log the predicted and actual altitudes. Over a series of flights you’ll develop a personal calibration factor for your rockets and motors.