How orhelper works¶

A few ideas explain nearly everything about orhelper. Read this page once and the rest will make sense.

OpenRocket runs inside your Python process¶

OpenRocket is a Java program. orhelper uses JPype to start a Java virtual machine (JVM) inside your Python process and to load OpenRocket's classes into it. Nothing is translated or re-implemented: when you run a simulation, it is OpenRocket's own simulation engine that runs.

That has three consequences.

1. Start OpenRocket once, in a `with` block¶

OpenRocketInstance starts the JVM when you enter the with block and shuts it down when you leave it. Everything that uses OpenRocket goes inside:

with orhelper.OpenRocketInstance() as instance:
    helper = orhelper.Helper(instance)
    ...   # load, simulate, read results

A JVM can be started only once per Python process, even after it was shut down. A second with block raises JVMAlreadyStartedError. Do all the OpenRocket work in one block, and copy the results you need (NumPy arrays and numbers) out of it. To run several OpenRocket versions, or to run in parallel, use several processes: see Notebooks and parallel runs.

2. Most objects are Java objects¶

Helper has the Python-friendly operations: loading and saving files, running simulations, and reading results as NumPy arrays. Everything else you get back is a Java object:

document = helper.load_doc("my_rocket.ork")   # Java: OpenRocketDocument
rocket = document.getRocket()                 # Java: Rocket
simulation = document.getSimulation(0)        # Java: Simulation
options = simulation.getOptions()             # Java: SimulationOptions
options.setWindSpeedAverage(5.0)              # a Java method call

You call their Java methods (getFoo(), setFoo(value)) directly. Python numbers, strings and booleans are converted automatically, and Java numbers come back as Python numbers.

3. Units are SI, and angles are in radians¶

OpenRocket works in SI units internally, and so do the Java methods and the arrays that orhelper returns: metres, kilograms, seconds, newtons, and radians for angles (so a 45 degree launch rod angle is math.radians(45)). OpenRocket's user interface shows nicer units such as grams, but those don't reach your script. The flight data reference lists the unit of every variable.

A few exceptions

A few values do depend on the OpenRocket version. Latitude and longitude in the results are in radians in OpenRocket 22.02 and 23.09, and in degrees in 24.12 and later. See OpenRocket versions.

Finding out what a Java object can do¶

You can use dir() in a Python session to list the methods of a Java object:

print([name for name in dir(options) if name.startswith("set")])

OpenRocket's own source code is the full documentation: browse the info.openrocket.core package on GitHub. In OpenRocket 22.02 and 23.09 the same classes are in the package net.sf.openrocket.

To write code that works with all versions, reach the classes through the instance rather than by their package name:

Coordinate = instance.openrocket_core.util.Coordinate   # info.openrocket.core, or net.sf.openrocket

The pieces¶

You use	For
`OpenRocketInstance`	starting and stopping OpenRocket, choosing which one
`Helper`	loading and saving files, running simulations, reading results
`FlightDataType`	naming the variables you want from a simulation
`FlightEvent`	the events (burnout, apogee, ...) a flight consists of
`AbstractSimulationListener`	running your own Python code during a simulation
the Java objects	everything else: rocket components, simulation settings, ...