Most HPR flyers discover OpenRocket first. It has been the community standard since 2008 — free, desktop-based, rigorously validated against thousands of flights, and backed by a large active forum. If you’re reading this, you probably already have it installed.
So where does Project APEX fit? The short answer is: alongside OpenRocket, not instead of it. The two tools are built for different moments in the HPR workflow, and understanding that distinction makes both more useful.
What OpenRocket does best
OpenRocket is a full rocket design environment. You build your rocket geometry component by component — body tubes, transitions, nose cones, fins, motor mounts, parachutes, and more — and it calculates CP, CG, and simulated flight from that detailed model. It handles multi-stage rockets, clustered motors, rail buttons, and complex fin geometries. Its simulation engine has been validated against a huge body of real flights, and the community has been refining it for nearly two decades.
If you are designing a new rocket from scratch, need to model every component accurately, or are documenting a build for a certification flight, OpenRocket is the right tool for that job. It is also entirely offline — no internet connection required once it is installed, which matters at remote sites.
What Project APEX does differently
APEX takes a deliberately different approach. Rather than building a full component model, you enter the key parameters that drive flight performance: airframe diameter and length, nose cone shape and length, fin geometry, launch mass, and CG position. From those inputs, the physics engine — Barrowman equations with Prandtl-Glauert compressibility corrections, a seven-component drag model, and a fourth-order Runge-Kutta integrator — runs a complete flight simulation in under a second.
That speed is intentional. Changing a single parameter and re-running takes seconds, which makes the parameter sweep genuinely useful: select any input, set a minimum, maximum, and step count, click Run Sweep, and APEX runs a full simulation at every step. The result is a complete chart of how apogee, max velocity, max Mach, and stability margin respond across the entire range — in one click, without touching a spreadsheet.
The most distinctive feature, though, is flight data overlay. After a flight, load your Featherweight Blue Raven altimeter CSV directly into APEX and compare the measured altitude and acceleration traces against the simulation side by side. The Aerodynamics tab back-calculates your actual Cd from the flight data and plots it against the model curve — a direct validation of how well the physics predicted reality. If the curves do not align, a Cd Scale Factor lets you calibrate the model to match your specific airframe, and that calibrated configuration saves for future flights on the same rocket.
APEX also runs entirely in a browser. No install, no Java runtime, no operating system dependencies. Open the page and the simulator loads.
Where each tool earns its place in a real workflow
The clearest way to understand the split is to follow a flight from design to post-flight analysis.
Design phase — OpenRocket. Model every component accurately, generate a CP/CG diagram, check stability margin before you order material. This is what OpenRocket was built for.
Pre-flight optimisation — APEX. Once the geometry is settled, load the key parameters into APEX and run a parameter sweep on fin semi-span, nose cone shape, or launch mass. The sweep shows you the full performance landscape instead of a single data point, and comparing motor options is a matter of loading each thrust curve and re-running.
At the range — APEX. No laptop needed. Mobile mode is in active development — the core simulation loop will be fully functional on a phone, built for exactly the scenario of a quick stability check before a motor swap.
Post-flight analysis — APEX. Load the altimeter CSV, overlay it on the simulation, calibrate your drag model to match. The next pre-flight prediction for the same airframe will be meaningfully more accurate as a result.
Which one should you use?
Both. OpenRocket for design and certification documentation. APEX for optimisation, pre-flight sanity checks, and post-flight model validation.
The flyers who get the most out of APEX typically already have OpenRocket in their workflow — APEX adds a layer of real-data validation that a design environment alone was never built to provide. The two tools cover the full lifecycle of an HPR flight, and each is stronger for the moments it was designed around.
Project APEX runs in any browser with no install. If you have a rocket with geometry you want to check, you can have a simulation running in under five minutes.
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Project APEX is developed by 434 Aerospace. Questions or feature requests — [info@apexrocketsim.com]