Use the case to set the motor's overall envelope. Choose the propellant and fit the grain stack inside it, then size the nozzle. If the motor will have an ejection charge, add its delay options too. ZenRockets checks the geometry while you work and uses it to simulate the thrust curve.
The case#
The case is defined by the numbers on your drawings: motor diameter and length, wall thickness, a liner thickness for the thermal barrier, forward closure length, and the case's dry mass. Optional thrust-ring dimensions capture retention hardware. The case CG can be overridden with a measured value — type it in, or use the pencil button to drag the marker directly on the schematic.
Propellant and grains#
The built-in library includes sugar propellants, composites and BP. Each propellant includes the density, thermochemistry and burn-rate data needed for simulation. You can also record your own formulations in the account catalog; they are not yet available in the motor picker.
Then shape the grains. Five core geometries cover the standard options:
- BATES
The workhorse — a centered circular core, with optional inhibited ends.
- Finocyl
A cylindrical core with radial fin slots (up to 64), for high initial surface area and a flatter curve.
- Moon burner
A core offset from center, trading progressivity for a long burn.
- C-slot
A slot cut to the wall, for strongly regressive profiles.
- End burner
No core — burns face-first from one end. Low thrust, maximum burn time.
Set the grain count, per-grain length and spacing, and the whole stack is checked against the chamber. Propellant mass is computed from the geometry and density, or overridden with the weighed value from your cast.
The nozzle#
Set the nozzle's throat, inlet and exit diameters, its overall length and the throat position. The efficiency factor defaults to 0.85 and adjusts the nozzle performance used in the simulation.
Ejection#
The ejection settings determine which delay options, if any, are available when the motor is installed in a rocket:
- None — a plugged motor, no charge.
- Fixed delays — a discrete set (say, 6, 8, 10 seconds), like drilled single-use delays; a zero entry models a booster charge.
- Adjustable — a continuous min–max range, like a reloadable delay system.
When the motor is installed, the builder shows these delay options. During simulation, the charge fires at the selected delay and triggers any recovery or staging event tied to it.
The motor editor runs two kinds of validation. Geometry checks run as you edit, while ballistic checks run each time you simulate the motor. Open Design Issues to see what was flagged, how serious it is and how to mitigate it.