Thrusterz

An orbital mechanics flight game. Pilot a rocket in real time — a fuel-limited main engine and side thrusters for rotation — under real Newtonian gravity, and hit your destination. World 1 builds up from a single launch to interplanetary transfers, gravity assists and multi-moon tours; World 2 adds rockets that come apart: staging, payloads and the debris they leave behind.

Play

No build step. Open index.html in a browser, or serve the folder:

npm start            # http://localhost:8080

A first visit (no stars, never been past it) opens on a landing page: what the game is, Play now straight into Training's first mission, a way to skip Training, and a restore code for pilots on a new device. The pilot name is asked after the first mission you win, when there are stars worth saving. ?landing shows the landing page again. The page's text is plain HTML in index.html, so search engines and link previews can read it.

The menu shows one world per tab. World 1 is open from the start; each later world opens once the world before it has earned enough stars (World 2 needs 50 of World 1's 90). Every mission inside an open world is playable, and the first one you haven't cleared is marked Next up. ?unlock opens every world. ?level=N jumps straight to mission N, counting across all worlds in menu order.

Controls

Key	Action
Space (hold)	Main engine
A D / ← →	Side thrusters (rotate)
W S	Throttle up / down
G	Gyro assist on/off (auto-stops spin, but caps the run at ★★)
E	Drop cargo, or drop the spent stage and fly the next one
, .	Time warp down / up (warp drops to 1× while thrusters fire)
V	Cycle reference frame (auto / inertial / each body)
F / O	Follow ship / overview · mouse wheel zoom, drag pan
[ ]	Shorter / longer trajectory prediction
R · Esc	Restart · pause

Touch devices get on-screen controls: rotate on the left thumb, BURN on the right, warp / frame / gyro in the middle. Pinch to zoom. Tilt to steer (on by default, with a one-time note that Settings can switch it off; iOS asks for motion access on the first tap) fires the side thrusters when you tip the phone left or right like a steering wheel, exactly as if ⟲ / ⟳ were held (past 12°, letting go below 7°; lying flat counts as level). The buttons keep working alongside it and win if they disagree, and light up while tilt is firing them.

On phones the game plays in landscape and fullscreen: a portrait phone gets a "rotate your phone" screen, and leaving fullscreen, rotating back or switching apps pauses the mission. iPhone Safari has no fullscreen API for pages, so there it's landscape-only; Add to Home Screen launches it fullscreen (via the web app manifest).

New players get big contextual prompts ("Press and hold SPACE to launch", "Still spinning! Tap D to stop", warp, reference frame). Each disappears once you do it, and stops appearing after you've mastered it.

Mechanics

• Main engine — constant thrust along the ship's nose. The tank follows the rocket equation (Δv = vₑ · ln(m₀/m)), so the ship accelerates harder as it gets lighter. The Δv gauge shows what's left.
• Side thrusters (RCS) — torque, not direct steering: they change your spin rate, and you keep spinning until you counter-fire. A quick tap is a fixed small nudge (2°/s); holding ramps up. Holding the opposite key brakes the spin to exactly zero and stops there. The HUD shows your spin rate. RCS propellant is limited (60 s of firing by default, which no mission comes close to; the test levels Spin Burn, Asteroid Run and Sentry Field set real budgets, and Fixed Heading has none at all). Gyro assist (G) damps the spin for you, but switching it on caps that run at ★★.
• Trajectory prediction — the dashed line is your coast path. The magenta marker and ghost show the closest approach to your current objective and where the target will be at that moment.
• Objectives — impact a body (optionally inside a landing zone on its spinning surface), reach a zone, escape past a distance, enter an orbit band, park at a point such as L4, or rendezvous (distance and relative speed). Missions can chain objectives.
• Orbit goals check the whole orbit, not where you are right now: the orbit's lowest and highest points (two-body orbit about the target) must both sit inside the band, in the required direction, confirmed for a few seconds with the engine off. Sweeping through the band doesn't count. Parking goals (L4/L5) also require matching the point's speed.
• Stars — 3★ for finishing under the mission's par Δv without gyro assist, 2★ under 1.4× par.
• Score — up to 6,500 per mission for finishing, fuel efficiency against par, speed, and flying without gyro assist. Personal bests are saved locally.
• Stages (World 2) — rockets are stacks of stages, each with its own tank and engine, and each stage's Δv counts everything above it. E / DEPLOY drops the spent stage and lights the next one.
• Cargo (World 2) — engineless payloads (pods, relays, probes, satellites). E / DROP releases the next one with your exact position and velocity, so your dashed path at that moment is exactly where it goes; the readout says DROP NOW → IMPACT when it would land on target. Cargo can have its own objectives: land in a zone, hold an orbit, park at a Lagrange point, reach a station, or spread out into a constellation.
• Debris (World 2) — dropped stages keep flying. Their fate is computed when they're dropped: burn up on reentry (fine), drift clear, or threaten a protected object. A stage left on a bound orbit that crosses a protected object's orbit fails the mission at once, since it would hit eventually; the danger band and crossing points are shown.

Missions

World 1 · Flight School

#	Mission	Teaches
1	Liftoff	Main engine
2	Point and Burn	Side thrusters · rotation
3	Full Stop	Flip and burn · time warp
4	Circularize	Prograde burns · camera
5	Reach Orbit	Launching into orbit · gravity turn
6	Deorbit	Retrograde burns
7	Landing Zone	Timing a deorbit burn
8	Turn and Burn	Two-burn Hohmann transfer
9	Skimmer	Lowering an orbit
10	Wrong Way	Orbit direction · bi-elliptic transfer
11	Moonshot	Intercepting a moving target · phasing
12	Free Return	Free-return trajectory
13	Cycler	Repeating trajectories
14	Docking	Phasing · rendezvous
15	Constellation	Phasing within an orbit
16	Rescue	Rendezvous on an eccentric orbit
17	Lunar Capture	Capture burns · reference frames
18	Moon Landing	Landing on a moving, spinning moon
19	Moon to Moon	Leaving a moon · nested orbits
20	Lagrange Point	Lagrange points · co-orbital rendezvous
21	Trojan Swap	Drifting with orbital period
22	Event Horizon	Extreme Oberth effect
23	Slingshot	Gravity assists
24	Twin Suns	Three-body chaos
25	Interplanetary	Escape burns · transfer windows
26	Sundiver	Why the Sun is hard to reach
27	Asteroid Belt

World 2 · Payloads

#	Mission	Teaches
1	Launch a Satellite	Payload deploy
2	Burn It Dry	Use every stage fully
3	Against the Spin	Launch direction · retrograde orbit
4	Stationary	Synchronous orbit
5	Moon Probe	Transfer stage + capture
6	Homecoming	Return stage · targeted landing
7	Release Point	Passive drop · release timing
8	Impactor	Drop, then dodge
9	Relay Drop	Place cargo on an orbit
10	Moon Mail	Drop on a flyby
11	Twin Probes	Two drops, two targets
12	Constellation	Phasing orbits
13	Three Stages	Multi-stage rockets
14	Kick Stage	Apogee kick · long, weak burns
15	Escape Velocity	Escape velocity
16	Solar Probe	Stacks for big Δv
17	Outer Planet	Interplanetary orbiter
18	Splashdown	Range safety
19	Clear the Station	Debris hazards
20	Busy Orbit	Where your booster ends up
21	Leave No Junk	Deorbiting the upper stage
22	Fuel Depot	Pickups on the way up
23	Relay Pair	Lagrange drops
24	Junkyard	Planning every drop
25	Lander and Orbiter	Targeted drop + capture
26	Deep Impact	Impactor + flyby
27	Voyager	Gravity assist with a stack
28

The reference frame starts on Auto: it follows the body whose sphere of influence you're in, or the star you're orbiting when you're in none.

World 2 flies through a green nebula with distant planets in the background, World 3 through an ember-red one, so you can always tell the worlds apart. Experimental levels (the menu's last tab) try out new mechanics that may or may not grow into a world, one level each: fuel pickups (Fuel Run), atmospheres and aerobraking (Aerobrake), wormholes (Shortcut), tethers (Swing), solar sails (Sunward Sail), event horizons and strong gravity (Whirl), radiation belts (Radiation Belt), magnetospheres and storms (Storm Warning, The Slot, Storm Front), droppable shields (Heavy Water), refuelling depots (Top Up), soft landings (Touchdown) and ion engines (Long Burn). World 3's missions are built in tools/w3/ (levels, proofs and naive plans that must lose, one file per chapter) and written into js/levels.js with npm run build:w3. See docs/ROADMAP.md for future worlds, the leaderboard design and the level-maker plan.

Physics

js/physics.js — G = 1, time in seconds at 1× warp.

• Celestial bodies ride analytic Kepler rails (circular or eccentric, solved with Kepler's equation), so the predictor and the actual flight agree exactly.
• Barycentric reflex: a massive moon and its planet both orbit their shared barycenter, so binary stars fall out of the same rule.
• Rails can't respond to each other the way true n-body bodies do, so the ship's gravity is computed relative to its host body (innermost sphere of influence): host acceleration + host pull + tidal pull from everything else. This is what the full n-body problem gives, and it keeps parking orbits stable instead of drifting from rails artifacts.
• Ship integration is RK4 — fixed 1/120 s steps while engines fire, adaptive steps while coasting (shorter near bodies, never tunnelling through them).

Layout

index.html          page shell, HUD and menus
css/style.css
js/physics.js       rails, gravity, integrator, coast prediction
js/flight.js        Mission: ship, stages, cargo, debris, objectives
js/levels.js        mission definitions
js/game.js          rendering, input, HUD, menus, progress (localStorage)
fonts/              bundled fonts (offline; OFL)
tools/test-physics.js   physics sanity tests            (npm test)
tools/browser-check.js  headless Chromium: every mission under random input,
                        two-finger taps, corrupted saves (npm run test:browser)
tools/brand/            icon + share card sources; render.js regenerates img/
tools/check-levels.js   random burn search proving World 1 missions solvable
                        within their fuel budget         (npm run check-levels)
tools/autopilot.js      scripted autopilot (launch, burns, drops, deploys); with
                        opts.turn it turns with the real side thrusters and
                        reports the RCS seconds used
tools/proofs.js         a winning flight plan for every World 2 mission and the
                        test levels; npm test flies them all
tools/check-scripted.js re-tunes a proof's parameters    (npm run check-scripted [id] [iters] [opt])
tools/rcs-report.js     RCS seconds each proof needs with honest turning
                                                         (npm run rcs-report [id])

physics.js, flight.js and levels.js have no DOM dependencies and run under Node, which is how the tools use them.

Online: accounts, cloud save, leaderboards

A pilot account is created the first time it's needed (no sign-up). Progress syncs to the server and merges both ways, keeping the best of each; a restore code or Sign in with Apple brings it back on another device. Every win is submitted to per-mission leaderboards (top score and fastest): the game records each flight's control inputs tick by tick (js/replay.js) and the server re-flies them with the same simulation before ranking, using deterministic math (js/dmath.js) so a run replays identically on any JavaScript engine. Offline, everything still works and runs wait in a queue.

api/                Vercel Functions: player, restore, apple, progress, runs, board
api/_lib/           routes, Upstash Redis store (in-memory locally)
js/online.js        client: account, sync, run queue, Apple sign-in
js/bridge.js        talks to the native app shell (mobile/App.js)
js/config.js        API address for the app
npm run dev         game + API on http://localhost:8080

iOS app (Expo)

mobile/ is an Expo app that runs the game full screen in a WebView, built and signed in the cloud with EAS (no Mac needed). The game is packed into one file (npm run build:mobile), and js/bridge.js connects it to native storage, Sign in with Apple and haptics. Release guide: docs/APP_STORE.md.

Stability

The game loop survives a failing frame, physics and path prediction run on per-frame time budgets (time warp steps down if a device can't keep up), numerical blow-ups end the mission cleanly, and saved progress is validated on load. Add ?debug to the URL for an fps / script-time readout. Both test suites run on every push (.github/workflows/test.yml). For the App Store release steps see docs/APP_STORE.md.

Roadmap

Next up (details in docs/ROADMAP.md):

• Control switching between craft (booster landing, lander and orbiter)
• Secondary (translation) thrusters for fine docking
• Autopilot holds (prograde / retrograde) as an unlockable upgrade