Kerbal Gravity Program (KGRP)

KGRP is a mission planner for Kerbal Space Program that runs outside the game. You define a spacecraft, stack events (coast, maneuver, burn, engine toggle, science, and more) on a timeline, and KGRP simulates them with real orbital mechanics. Plan in KGRP, then fly the plan in KSP.

It is built for KSP players who want to work out transfers, delta-v budgets, and timelines before launching — and it is flexible enough that you can model arbitrary spacecraft and solar systems (KSP stock, OPM, or your own).

Status: stable (v1.0.0). The core is solid; the math is checked and the planner flies end to end.

Demo

Watch KGRP plan and fly a mission right in the terminal:

On sites that allow raw HTML, embed the player directly:

<script id="asciinema-player-fBPRPZsKrolGCy2I" src="https://asciinema.org/a/fBPRPZsKrolGCy2I.js" async></script>

Features

• Round-trip mission planner — define a vessel, schedule a timeline of events, and step the sim forward with advance.
• go transfer planner — pick a start and end orbit and KGRP computes the burns, coast phases, and total delta-v for you.
• Editable orbit presets — low orbit, synchronous, escape, custom orbits.
• Full event timeline — coast, propagate, rkf45, maneuver, burn, engine control, attitude, rcs, stage, resource transfer, dock, undock, reference-body change, surface (land/launch), science, checkpoint.
• 3D renderer — a PyVista/Qt window to watch the mission unfold.
• Save/load — systems, tickets, and whole missions as JSON.

Installation

Requires Python 3.10+.

# From the terminal (using pip):
pip install kgrp

# Or using uv
uv tool install kgrp

# This installs the `kgrp` command (entry point -> CLI_use.CLI:main).

Verify it works:

kgrp

You should see the KGRP prompt (krgp >). Type help and press Enter.

Quickstart

KGRP is a REPL. The whole loop is five commands, typed at the krgp > prompt (launch with kgrp):

load system planets_ksp.json   # or: new system  (build your own)
new ticket                     # wizard creates a vessel + first ticket
go                             # pick two orbits; KGRP plans the burns
advance <end-ut>               # run the timeline (UT is printed by go)
render                         # optional 3D view

help lists every command; help <command> (e.g. help go) explains one. That is it: load a system -> new ticket -> plan with go -> advance the timeline -> render.

Workflow example

1. A local transfer (Kerbin -> Mun)

krgp > load system planets_ksp.json
krgp > new ticket
# ... answer the wizard prompts (name, body = Kerbin, defaults are fine) ...
krgp > go current moon:3a
Transfer plan:
  Current (~800,000m circular, 0.0 deg)  ->  Mun orbit (12,000,000m circular)  (about Kerbin)
  Burns: 2   Total delta-v: 1,126.6 m/s   Duration: 27,066.6s
    burn   @ 0.0s  dv=775.9 m/s  Raise apoapsis to transfer
    coast  0.0s -> 27,066.6s  Coast to apoapsis
    burn   @ 27,066.6s  dv=350.7 m/s  Circularize / match target
krgp > advance 27067
# vessel is now on the transfer; advance further to reach the Mun
krgp > render

go takes <start-orbit> <end-orbit> plus optional key=value overrides (peri_alt=, apo_alt=, incl=, arg_p=, lan=). Or run go with no arguments to pick both orbits from a menu.

2. An interplanetary transfer (Kerbin -> Duna)

Use go -i <target> (alias go_i <target>). It plans the escape burn, the heliocentric leg, and the arrival burn in one shot:

krgp > go_i duna
Interplanetary transfer plan (Kerbin -> Duna):
  Escape from Kerbin -> SOI exit -> Kerbol frame
  Total delta-v: 2,583.5 m/s   Burns: 3
    [Kerbin] escape burn  dv=870.3 m/s
    coast to SOI exit @ 196,411s
    refbody -> Kerbol
    burn   @ 1,994,868.8s  dv=946.1 m/s  Raise apoapsis to transfer
    coast  1,994,868.8s -> 8,719,637.1s
    burn   @ 8,719,637.1s  dv=767.2 m/s  Circularize / match target
krgp > advance 8719637    # or 'advance 8.7M' if your fingers object
krgp > render

Override the arrival orbit with the same key=value overrides, e.g. go_i duna peri_alt=200000 apo_alt=200000. go -i / go_i work from any body that orbits a star (stock or OPM).

3. Save the mission and come back

krgp > save mission duna_trip.json
krgp > exit
# later:
kgrp
krgp > load mission duna_trip.json

save mission writes one file with the system, ticket, and vessels; load mission brings it all back. You can also save system / load system and save ticket / load ticket separately.

Command overview

Type help for the full list, and help <command> (e.g. help go, help add) for details. The headline commands:

Time input is human-friendly

You rarely need raw seconds:

advance 2d 5h          # a duration, added to the current UT
advance 21600          # or a raw UT in seconds
time "Year 2, Day 100" # a calendar timestamp
go current preset:molniya incl=30   # per-use orbit overrides

Valid time units: y (year), mo (month), d (day), h (hour), m (minute), s (second). Words like days/hours work too.

Tips for new players

• Start from a preset, not a blank page. new ticket hands you a vessel with an engine and fuel already attached — you do not start with a paperweight.
• Let go do the math. Instead of hand-computing burns, run go and read the delta-v budget it prints.
• Use advance to walk time, one coast/burn at a time. time jumps the cursor without simulating (handy for inspection).
• Save your work. save mission writes a single file with the system, ticket, and vessels; load mission brings it all back.
• Stuck? help and help <command> are always available, and system validate will tell you if your system has problems.

Project layout

• CLI_use/ — the REPL, command parsing, completion, and the go planner.
• basic_systems/ — physics: bodies, orbits, integrators, and the renderer.
• planets_ksp.json / planets_ksp_opm.json — stock and OPM system data.
• planets.json - Actual Solar System data
• tests/ — pytest suite (python -m pytest).

Running the tests

python -m pytest

Credits

Built by Gautham, powered by an unreasonable amount of Kerbal optimism.

• Jebediah Kerman — chief test pilot. Assumes every plan works until physically proven otherwise (and sometimes after).
• Bill & Bob — held the ladder. Reviewed nothing.
• The math — probably fine. Mostly. We checked it, like, twice.
• The deep-space navigation charts — still don't list your spelling mistakes, but they do list Duna now.
• You — for reading this far instead of just launching and hoping.

KGRP plans the mission. Flying it is still gloriously your problem.

License

GNU GPLv3 — see the LICENSE file. Go forth and transfer.