Every committed edit recomputes mass, balance, aerodynamics and stability, so preparing to simulate is mostly a final review of numbers you've been watching throughout the build.
Materials and estimated properties#
Each part's mass comes from its geometry and material: volume times density for structure, fabric and cord densities for parachutes, direct weights for payloads and motors. The estimated part properties feed the rocket-level values used by the simulation: total mass, CG, CP, drag and inertia.
- Mass
Rolled up through the tree, so you can read the mass of a bay, a stage or the whole vehicle at a glance.
- Center of gravity (CG)
The mass-weighted balance point, recomputed live as you move hardware around.
- Center of pressure (CP)
Computed with the Barrowman method from the nose, body and fin geometry — the same approach used across hobby rocketry.
- Drag & inertia
A drag coefficient (CD) assembled from body, fin and hardware contributions, plus full moment-of-inertia tensors for 6-DOF simulation.
Overrides#
Estimated values are just defaults. If you've put the actual part on a scale, use that number:
- Select the part. Its details panel has a Mass section, a Center of Gravity section and, on external parts, a Drag Coefficient section, each showing its estimated value.
- Flip the section's Override switch. The field starts from the estimated value, so you adjust from the model's best estimate rather than a blank box. The estimate stays visible for comparison.
- Enter the measured value. The design recomputes immediately, and the tree marks a mass-overridden part with a small dot.
Flip Include children after you've weighed an assembled section. The measured value covers the part and everything nested inside it, which is useful when fine-tuning the final configuration without overriding every component separately. Motor mass is never absorbed; it is added on top automatically, so weigh the section without motors. Parts inside the overridden section show overridden by with the ancestor's name, so you can see where the measured value comes from.
The CG override has one extra trick: the pencil button beside the field enters an edit mode where you drag the CG marker along the rocket on the canvas.
Stability and thrust#
The stability readout tracks the margin between CG and CP in calibers (body diameters). ZenRockets flags a margin under 1 caliber as marginal, and over 6 as overstable — enough that the rocket weathercocks into the wind. Moving internal parts changes the rocket's CG, which changes the stability margin. The readout updates immediately, so you can see the effect of each placement change.
First-stage thrust is checked against liftoff weight. If it is too low, validation reports an error and blocks the simulation.
Validation status#
Validation updates as you edit the rocket. The status button appears in the workspace corner and beside Run in Simulate mode. Green means there are no blocking issues, amber means there are warnings, and red means errors are blocking the simulation. The number shows how many issues were found. Open the list for more detail and suggested fixes.
Issues come in three flavors:
- Errors block simulation — structural impossibilities like a stage without a motor, no airframe, zero mass or length, or average thrust below liftoff weight.
- Warnings do not block simulation. They flag conditions such as a missing recovery device, deployment logic that cannot trigger, no fins, stability outside the 1–6 caliber range, or gaps, overlaps and diameter changes in the airframe.
- Flight alerts appear only after a run, flagging what the simulation itself saw: a deployment at dangerous speed, a charge firing under thrust, a hard landing.
Once the errors are cleared and the warnings reviewed, switch to Simulate mode and run the simulation. Validation only checks whether the model is internally consistent. Simulation results are for educational purposes only and do not certify a real rocket as flightworthy. See Safety.