▶ Live demo — apps.charliekrug.com/escape-velocity
An orbital mechanics simulator you play in one drag. Pull a satellite off a tiny planet, let go, and watch real two-body gravity decide what happens next: it arcs back and crashes in a burst of debris, snaps into a stable glowing orbit, or breaks past escape velocity and flies off the screen.
Most gravity toys on the web are one of two things: a canned animation that just loops, or a full N-body sandbox you have to learn before anything happens. Apogee is neither. One planet, one satellite, one drag. The trajectory you see is integrated frame by frame from real inverse-square gravity, so every launch is an actual orbital-mechanics outcome rather than a scripted one. No login, no install, no tutorial.
Click-drag away from the planet and release. Where the drag vector lands sets the launch velocity, and the physics takes it from there:
- Crash: too slow, and the ellipse dips into the planet. The satellite arcs back and scatters into debris.
- Orbit: hit the stable range and it closes into a repeating loop. Hold it for two full periods and the orbit locks, with a glowing confirmation ring and a running orbit count.
- Escape: pull past the local escape velocity and it leaves on a hyperbolic path, clearing the boundary and flying off for good.
- Real two-body gravity, integrated with semi-implicit (symplectic) Euler so a stable orbit stays stable for as long as you watch it instead of spiraling from numerical drift.
- Live HUD reading velocity, altitude, apoapsis, periapsis, orbital period, and a lifetime
orbit count kept in
localStorageacross reloads. - Adjustable gravity on a slider, locked mid-flight so it never rewrites a trajectory that is already in progress.
- Synthesized sound for drag-charge, launch, crash, orbit-lock, and clicks, generated in code with WebAudio (no audio files), behind a mute toggle that persists.
- Mouse and touch launch with a live drag indicator, plus a debris burst, screen shake, and an outcome overlay showing the run's stats.
- Accessible and responsive at 390 / 768 / 1440 px, keyboard-operable with visible focus,
and honoring
prefers-reduced-motion.
Vanilla JavaScript (ES modules) and the Canvas 2D API. No framework, no bundler, no build step.
The whole thing ships as static files: index.html, src/, and styles/. The physics, input,
HUD, particle, storage, and audio modules are covered by Vitest unit tests
plus fast-check property tests.
npm install
npm run dev # serve the static site (defaults to http://localhost:8000)Then open the served URL and drag from the planet. There is no build step; the source runs as-is.
npm test # full unit + property suite (physics, input, hud, particles, stats, audio, storage)
npm run test:coverage # same, with a v8 coverage reportdocs/VISION.md: the problem, the audience, and what "done" meansdocs/DESIGN.md: visual direction, tokens, and the game-feel plandocs/ARCHITECTURE.md: module map, data flow, and how to run and testdocs/BACKLOG.md: the story breakdown with acceptance criteria
MIT. See LICENSE.
More of Charlie's projects → apps.charliekrug.com