terminal-space-program

README

terminal-space-program

Terminal-native orbital-mechanics rocket simulator. A take on Kerbal Space Program that lives in your terminal, distributed as a single static Go binary.

┌───────────────────────────────────────────────────────────┐
│ terminal-space-program — Sol         [Menu]  [Missions]   │
│ ┌─────────────────────────────────────┐ ┌───────────────┐ │
│ │    · ·                              │ │ CLOCK         │ │
│ │  ·     ·                            │ │   T+2026-04-29│ │
│ │ ·   ⊙   ·       · ⊕ ·               │ │   warp: 100x  │ │
│ │  ·     ·                            │ │               │ │
│ │    · ·                              │ │ VESSEL  PROP  │ │
│ │                                     │ │ S-IVB-1 fuel  │ │
│ │                              view:  │ │ alt 500 26000 │ │
│ └────────────────────────────── orbit ┘ │ v 7.6   Δv 6k │ │
│ [q] quit [s] system [m] burn [?] help   └───────────────┘ │
└───────────────────────────────────────────────────────────┘

Install

Latest release: v0.8.5.

# Linux x86_64
curl -L https://github.com/jasonfen/terminal-space-program/releases/latest/download/terminal-space-program-linux-amd64.tar.gz | tar xz
./terminal-space-program

Replace linux-amd64 with linux-arm64, darwin-amd64, darwin-arm64, or windows-amd64 (use the .zip variant on Windows).

No Go toolchain, no libc dance. CGO_ENABLED=0 static binaries.

Build from source

git clone https://github.com/jasonfen/terminal-space-program
cd terminal-space-program
go build ./cmd/terminal-space-program
./terminal-space-program

Requires Go 1.24+ (bubbletea dependency chain).

Quick tour

You spawn as S-IVB-1 in a 500 km circular prograde LEO. The left panel is the orbit canvas — Sun (or whichever body you focus on) at the center, planets on their actual orbits, your spacecraft as a small chevron oriented along velocity. The right HUD shows clock, focus, vessel + propellant state, attitude, planned nodes, projected post-burn orbit, frame transitions, system info, and a Hohmann preview to the cursor-selected body. Time-warp with . / , to watch planets move; pause with 0 or space.

To make something happen:

1. Press ←/→ (or click) to scroll the cursor through bodies. Pick Mars.
2. Press H to plant a Hohmann transfer — two finite-burn nodes appear (geocentric departure + Mars-frame arrival), each color-coded with its predicted post-burn orbit on the canvas, listed in the HUD with Δv and time-to-fire.
3. Time-warp forward. The departure node fires at its trigger time; warp clamps to ≤10× during the burn so the integrator keeps temporal resolution. Your trajectory unrolls past Earth's SOI, the predictor switches frames, and the curve bends sunward as it should.
4. The arrival node fires near Mars and drops you into a low capture orbit. For phasing-aware launch windows, use P (porkchop) instead of H.

For burns by hand, press m to open the planner. Pick a mode (prograde / retrograde / normal± / radial±), choose when it fires (absolute T+, or next peri / next apo / next AN / next DN), set Δv, and pick a throttle. Burn duration is derived from Δv via the rocket equation — no separate field. The PROJECTED ORBIT block previews apo / peri / AN / inclination of the result live as you edit. Commit with Enter; the integrator switches from Verlet (free flight) to RK4 (thrust) and ticks the burn across multiple frames with mass loss tracked from the rocket equation.

For real-time stick (KSP-style), throttle up with z, attitude with w/s/a/d/q/e, then engage with b.

Keybindings

The in-game ? overlay is the source of truth; this table mirrors it.

Global

Key	Action
Esc	Back / open splash menu (save / load / quit) on home view
Ctrl+C	Quit immediately
?	Toggle help overlay
i	Body info screen
Tab	Switch system (Sol → Alpha Cen → TRAPPIST-1 → Kepler-452)
0, Space	Pause / resume sim
. / ,	Warp up / down (1× → 100000×; clamped to ≤10× during a burn)

Orbit view

Key	Action
→ / l	Cursor: next body
← / h	Cursor: previous body
+ / -	Zoom in / out
f / F	Cycle camera focus forward / backward (system → bodies → craft)
g	Reset camera focus to system
v	Cycle view (top → right → bottom → left → orbit-flat)
n	Plan a default node (T+5 min, prograde, 200 m/s, finite-sized)
N	Clear all planned nodes
H	Auto-plant Hohmann transfer to selected body (intra-primary for moons of the craft's parent; moon → parent escape via bound transfer ellipse)
I	Plant inclination match — rotates the orbital plane to the selected body's inclination (or 0° equatorial when no body is selected)
P	Porkchop plot for selected body; Enter on a cell plants that Lambert transfer. Inter-primary only — moon targets show a banner redirecting to H
R	Refine plan — re-Lambert from live state, plant mid-course correction + update arrival
m	Open maneuver planner
S / L	Save / load game ($XDG_STATE_HOME/terminal-space-program/save.json)

Manual flight

Key	Action
z / x	Throttle full / cut
Z / X	Throttle +10 % / -10 %
w / s	Attitude prograde / retrograde
a / d	Attitude normal+ / normal-
q / e	Attitude radial+ / radial-
b	Engage / cut manual burn (main engine, throttle > 0)
r	Engine: main / RCS (v0.8.0+)

Attitude keys orient only in main mode — pressing b is what fires the engine. In RCS mode the attitude keys also fire one 0.1 m/s monoprop pulse per keypress (held keys produce a sustained pulse train at the terminal's key-repeat rate). The HUD's ATTITUDE block shows the armed engine; the PROPELLANT block shows monoprop level and remaining RCS Δv.

Mouse

Click-only. No drag, no wheel-zoom.

Click target	Action
[Menu] (top-right)	Save / load / quit confirmation menu
[Missions] (top-right)	Mission list with status glyphs (✓/✗/·)
Body	Focus body (same as cursoring onto it with ← / →)
Vessel	Focus craft
Planted node	Open planner pre-loaded for that node (edit-replace, fire time preserved)
Empty canvas	Open planner with a new node staged at the orbit point nearest the click
HUD panel	Open body info
Porkchop cell	Move cursor to that cell (then Enter to plant)

Maneuver planner (m)

Key	Action
Tab / Shift+Tab	Cycle field focus (mode → fire-at → Δv → throttle)
← / →	Cycle the focused cycle field (mode or fire-at trigger)
digits / backspace	Edit Δv or throttle
Enter	Commit burn
Esc	Cancel and back to orbit view

Δv drives both delivered Δv and burn duration via the rocket equation t = (m₀/ṁ)·(1 − exp(−Δv/(Isp·g₀))). Zero-thrust craft fall back to impulsive automatically. The fire-at field selects absolute T+ or one of next peri / next apo / next AN / next DN; event-relative nodes resolve their trigger lazily on the first tick after plant. The throttle field is per-node (not the live craft throttle) so adjusting throttle during a coast doesn't slow an in-flight planted burn. PROJECTED ORBIT readouts apo / peri / AN / inclination of the resulting orbit live as you edit.

Porkchop plot (P)

Key	Action
← / →	Departure-day cursor
↑ / ↓	Time-of-flight cursor
Enter	Plant Lambert transfer for selected cell
Click cell	Move cursor to that cell (then Enter to plant)
Esc	Back to orbit view

Cursor opens snapped to the minimum-Δv cell. · glyphs mark cells where Lambert didn't converge — Enter on those is a no-op.

Features

• Two-body patched-conic propagation with SOI-aware state transitions. Symplectic Verlet for free flight, RK4 for active burns. Stumpff-universal-variables Lambert solver (Curtis Algorithm 5.2) with an explicit prograde/retrograde flag.
• Auto-plant transfers. H plants Hohmann (heliocentric, intra-primary, or moon-escape — the planner picks based on craft + target frames). P shows a porkchop heatmap; Enter on a cell plants the Lambert-derived transfer. R re-runs Lambert from live state to refine the planted arrival.
• Burn scheduling. Maneuver nodes fire at absolute T+ or on the next periapsis / apoapsis / AN / DN crossing — the planner resolves event-relative nodes lazily against the live orbit. Inclination-change planner (I) plants a single normal-burn to rotate the orbital plane.
• Per-node throttle. The maneuver form's throttle field is captured at fire-time onto ActiveBurn, so adjusting the live throttle knob mid-coast doesn't slow a planted burn.
• Manual flight. Real-time stick layered on top of the planted-node planner. Throttle, six attitude modes, and an explicit b engage so accidental attitude-key presses can't fire the engine.
• RCS / monopropellant mode (v0.8.0+). r toggles between the main engine and a precision-maneuver monoprop thruster pool. In RCS mode each attitude keypress fires a fixed 0.1 m/s pulse off the monoprop budget (~30 m/s on the default S-IVB-1) — the proximity-ops thruster v0.8.3 docking will lean on. Each pulse drops a fading puff marker on the canvas (placeholder visual; v0.8.2 replaces with per-thruster glyphs).
• Multi-craft slate (v0.8.1+). n opens the spawn form (loadout / parent body / altitude / direction); [/] cycles which craft the player is flying. Each craft owns its own planted nodes, in-flight burn, attitude, and engine state — burns fire on the craft they were planted for regardless of which craft you're currently flying. The HUD's BURNS and NODES blocks list every craft's state simultaneously; clicking a node row opens the maneuver planner for edit-replace. Title bar shows CRAFT N/M chip when more than one craft is loaded. Save schema bumped v4 → v5 to nest per-craft state.
• Craft types (v0.8.2+). Four loadouts in the launch catalog: S-IVB-1 (yellow ▲, J-2 third stage), ICPS (blue ◆, RL-10 low-TWR), RCS-tug (pink ●, pure monoprop, no main engine), Lander (mint ▼, throttleable descent stage). Each carries propulsion + visual differentiation; the orbit canvas renders every craft with its loadout glyph + color so they read distinctly even at small zoom.
• Capture preview (v0.8.2+). Plant a Hohmann to another body and the HUD's CAPTURE PREVIEW block shows what you'll arrive with — relative approach speed and predicted prograde / retrograde direction (a prograde Hohmann to Luna naturally captures retrograde, ~110° around Luna; the preview surfaces this before fire so you're not surprised). Inclination match also works from equatorial source orbits.
• Atmospheric drag (v0.8.4+). Earth + Mars carry exponential atmospheres (ρ(h) = ρ_0 · exp(-h/H)) with co-rotating velocity reference, wired through both the live integrator (drag-aware Verlet) and the predictor — so projected orbits decay realistically on atmospheric-skim trajectories. Below the cutoff altitude the Kepler warp-lock retreats to Verlet sub-stepping; surface-impact craft clamp to the body and stop. A faint haze ring renders at cutoff + scale-height around bodies with atmospheres.
• Docking + undocking (v0.8.3+). Two craft within 50 m and below 0.1 m/s relative velocity sharing a primary frame fuse on the next tick — composite at the mass-weighted centroid with momentum-conserving velocity, summed fuel + monoprop pools, inheriting the active partner's identity (name, glyph, color, planted nodes). U undocks back into the original components, sharing the composite's pools proportionally. HUD's RENDEZVOUS block surfaces live range + relative velocity to the nearest co-orbiting craft so the player can RCS-null the residuals; spawn form's POSITION = alongside active drops a new craft inside the docking gate for testing. Engine-firing flame visual + per-thruster RCS puff visual replace the v0.8.0 placeholder dot.
• Predicted post-burn orbit. PROJECTED ORBIT block on both the orbit screen and m form chains every planted node, frame- rebases per node (so a Hohmann arrival reads in the destination frame), and shows apo / peri / AN / inclination of the resulting orbit live.
• Frame transitions. The HUD surfaces upcoming SOI / frame changes implied by the planted-node chain (e.g. the zero-Δv arrival marker from a moon-escape).
• Body hierarchy + moons. Earth + Luna, Mars + Phobos / Deimos, the four Galilean moons, Titan, Enceladus. Recursive BodyPosition / bodyInertialVelocity so SOI math walks the hierarchy correctly.
• Per-pixel body textures (v0.8.5+). Sun (limb-darkened solar disk + sunspots + corona halo), Earth (polygon-rasterised 144×72 continental mask with biome-shaded land + deserts + ice + atmospheric blue-marble limb), Moon (near-side maria + far-side Orientale / Moscoviense / Ingenii / South Pole-Aitken basin + polar craters), Mars (Syrtis Major / Solis Lacus / polar caps), Jupiter (10-band zone/belt + Great Red Spot), Saturn (banded cloud + polar hexagon + four-band ring system C / B / Cassini Division / A / F), the four Galileans (Io paterae, Europa lineae, Ganymede regiones, Callisto crater rays), Uranus (subtle pole-on banding from its 98° tilt), Neptune (banded blue + Great Dark Spot). Render at body radii ≥ 12 px; below that bodies fall back to a colored disk.
• Sim-time planet rotation (v0.8.5+). Body axes spin at sidereal-rotation rates; tidally-locked moons (Luna, Phobos, Deimos, Galileans, Titan, Enceladus) keep the same face pointed at their parent regardless of orbit phase. View-aware projection combined with axial tilts (Earth 23°, Mars 25°, Saturn 27°, Uranus 98° — the iconic roller, etc.) means top / right / left / bottom views show genuinely different geometry per body — top view of Earth reveals the Arctic, side views show the equator with surface features drifting east at the body's rotation rate. Rotation rate caps above warp 10000× so the disk stays watchable at extreme warp.
• Tilted ring system (v0.8.5+). Saturn's rings render in the body's equatorial plane and foreshorten correctly per camera view: ~89% aspect from top, ~45% aspect from side, edge-on flat perpendicular to the tilt direction.
• Missions. Three predicate kinds (circularize / orbit_insertion / soi_flyby) with sticky pass/fail state, embedded starter catalog (1000 km LEO circularize, Luna orbit insertion, Mars SOI flyby). Reachable via the [Missions] title-bar button.
• Persistence. Save / load to JSON at $XDG_STATE_HOME/terminal-space-program/save.json. Schema v5 round-trips clock, focus, the entire craft slate (each craft's RCS pool, planted nodes with per-node throttle, in-flight burn, attitude, engine mode), and missions. Pre-v5 saves auto-migrate (singular Craft → 1-entry slice).
• Modding. Custom systems via JSON overlay, per-body color via theme.json (see Custom systems and Theming below).
• Multi-system viewing. Sol, Alpha Centauri, TRAPPIST-1, and Kepler-452. The craft is locked to Sol today; switching systems just changes the camera.

Custom systems

Drop additional system JSON files into $XDG_CONFIG_HOME/terminal-space-program/systems/ (or ~/.config/terminal-space-program/systems/ if XDG_CONFIG_HOME is unset) and they'll merge with the built-in catalog at startup. User files win on systemName match — e.g. dropping a sol.json there replaces the embedded Sol entirely. Otherwise they append. The body-info screen (i) shows source: embedded | user so you can tell which catalog a body came from.

Schema mirrors the embedded files in internal/bodies/systems/*.json. Malformed user files print a warning to stderr at startup and are skipped — embedded systems always load. Save files carry a body_catalog_hash field, so a save taken on the embedded catalog rejects on first load after you add a custom system; that's by design (otherwise body references across saves could drift silently).

Theming

Drop a theme.json at $XDG_CONFIG_HOME/terminal-space-program/theme.json (or ~/.config/terminal-space-program/theme.json) to recolor either the UI tier or specific bodies:

{
  "ui":     {"alert": "#ff5f5f", "warning": "#ffaf00"},
  "bodies": {"earth": "#3b82f6", "mars":    "#dc2626"}
}

Both blocks are optional. UI keys match the lower-cased name of the package-level Color* var (e.g. alert, warning, plannednode, trajectory, currentorbit, craftmarker, foreignsoi, dim). Body keys match each body's id from systems/*.json. A body override wins over that body's per-body color field; UI overrides mutate the global tier colors at startup. Malformed theme.json prints a warning to stderr and falls back to defaults.

Version history

Cycle	Theme	One-liner
v0.1	Foundation	Heliocentric viewer + Verlet integrator + body catalog.
v0.2	Burns	Spacecraft + impulsive burns + finite-burn integrator.
v0.3	Transfers	Lambert solver, porkchop plot, auto-plant Hohmann transfers.
v0.4	Persistence	Save / load with versioned envelope; mid-course refinement.
v0.5	Moons + visuals	Body hierarchy + major moons (Luna, Phobos/Deimos, Galilean, Titan, Enceladus); per-body color, vessel trail, HUD polish.
v0.6	Planner UX + missions	Burn-at-next scheduler, projected-orbit HUD, finite-burn-aware iteration, moon → parent escape, click-only mouse + 5-way views, mission scaffold, multiplayer design spike.
v0.7	Modding + manual flight + textures	External system / theme overlays, manual-flight stick (throttle + attitude), inclination-change planner, retrograde Lambert flag, textured Earth/Moon/Mars/Jupiter, per-node throttle, SOI / frame-transition HUD.
v0.8	Multi-craft polish (in progress)	RCS / monopropellant precision thruster (v0.8.0). Multi-craft slate with per-craft burns + spawn keystroke + selector + save schema v4→v5 (v0.8.1). Craft types (4 loadouts with glyph/color visuals), full spawn form, clickable HUD nodes, Hohmann capture-preview, equatorial inclination match (v0.8.2). Docking + undocking, RENDEZVOUS HUD, alongside-spawn, engine-firing flame + per-thruster RCS puff visuals (v0.8.3). Atmospheric drag (Earth + Mars exponential atmospheres, drag-aware Verlet wired into live integrator + predictor, surface-clamp on impact, haze halo) (v0.8.4). Sim-time planet rotation, view-aware texture projection with per-body axial tilts, polygon-rasterised Earth grid, far-side / polar Moon detail, tilted Saturn rings with C / B / Cassini Division / A / F bands, textured Sun + Galileans + Uranus + Neptune, tidally-locked moons keeping their iconic face on the parent (v0.8.5).

Per-version detail: docs/state-of-game.md. v0.5 release notes: docs/v0.5-release-notes.md. v0.6 / v0.7 / v0.8 plans: docs/v0.6-plan.md, docs/v0.7-plan.md, docs/v0.8-plan.md.

Future plans

v0.8 — multi-craft polish. Slice breakdown in docs/v0.8-plan.md:

• v0.8.0 — RCS / monopropellant mode for sub-m/s precision burns shipped.
• v0.8.1 — multi-craft foundation (selector + save schema v4 → v5 + keystroke spawn + per-craft burn state) shipped.
• v0.8.2 — craft types (4 loadouts with glyph/color visuals), full spawn form, clickable HUD nodes, capture preview, equatorial inclination match shipped.
• v0.8.3 — docking + undocking + RENDEZVOUS HUD + alongside-spawn + engine-firing flame + per-thruster RCS puff visuals shipped.
• v0.8.4 — atmospheric drag (realistic Earth + Mars, drag-aware predictor, atmospheric haze rendering) shipped.
• v0.8.5 — sim-time planet rotation + view-aware projection + textured Sun / Saturn / Galileans / Uranus / Neptune + polygon-rasterised Earth + far-side / polar Moon detail + tilted Saturn ring system shipped.
• v0.8.6 — controls polish bag (multi-rev porkchop UI keys, IterateForTarget toggle in m form, throttle-change warp clamp).
• v0.8.7+ stretch — mission scripting / editor.

Deferred to v0.9+: multiplayer implementation, interstellar transfer, N-body perturbations, solar lighting + day/night terminator + eclipses (needs ANSI 24-bit canvas mixing research first), atmospheric heating / structural overstress, drag-to-edit nodes, theme-file hot-reload, race-detector CI.

Full backlog in docs/state-of-game.md.

Implementation plan

Full design doc: docs/plan.md. Summary:

• Phased physics progression (viewer → Verlet → impulsive burns → finite burns + RK4 → SOI-aware predictor + Lambert → auto-plant transfers).
• Bubble Tea root model with screen-level sub-models (orbit / bodyinfo / maneuver / help / missions / menu).
• GoReleaser single-workflow CI; release artifacts on tag push.

Credits

Architectural foundation lifted (with MIT attribution) from furan917/go-solar-system. See NOTICE.md for the full acknowledgments list.

License

MIT. See LICENSE.