Journey mode

Journey mode plans a trip between two places in the solar system and plays it. The page is journey.html of the demo; the arithmetic under it is the journey entry of this package, with no DOM and no dependency, so a server, a test or your own page can use it.

Two kinds of craft

A ballistic craft is a real rocket. It leaves on a Hohmann transfer: a burn from a parking orbit, a long coast along half an ellipse, and a braking burn at the other end. It can leave only on certain days, the launch windows, and the page finds them from the planets' real positions.

A craft that burns all the way is a ship of the SPX fleet. It accelerates to the halfway point, turns end for end, and burns the same acceleration to stop (flip and burn). The formulas are relativistic, so they hold for the interstellar trips of a later phase; in the solar system they reduce to 2 sqrt(d / a).

The arithmetic

import { circularSpeed, flipAndBurn, G0, GM_KM3_S2, hohmann, synodicPeriod } from "@johnmorrisdotca/uchusen-rockets/journey";

const AU = 149_597_870.7;
const mars = hohmann(GM_KM3_S2.sun!, AU, 1.52371034 * AU);
console.log((mars.seconds / 86_400).toFixed(1), "days,", mars.dv1.toFixed(2), "and", mars.dv2.toFixed(2), "km/s");
console.log(synodicPeriod(365.256, 686.98).toFixed(1), "days between windows");
console.log(circularSpeed(GM_KM3_S2.earth!, 6378.137 + 185).toFixed(2), "km/s in a 185 km orbit");

const ship = flipAndBurn(0.5 * AU * 1000, 0.015 * G0, 600);
console.log((ship.frameSeconds / 86_400).toFixed(1), "days at 0.015 g, half an AU");

258.9 days, 2.94 and 2.65 km/s
779.9 days between windows
7.79 km/s in a 185 km orbit
16.5 days at 0.015 g, half an AU

These numbers are checked against the ones NASA prints: about 259 days to Mars, 2.94 and 2.65 km/s, 779.9 days between windows.

Places

The core imports no ephemeris. You hand it a BodySource: the places, and where they are at a time. The demo makes one from wakusei-planets (demo/journey-bodies.ts), whose positions come from NASA JPL's approximate orbit elements. A toy source of circular orbits is enough to try the planner:

import { craftOf, planTrip, type BodySource } from "@johnmorrisdotca/uchusen-rockets/journey";

const AU = 149_597_870.7;
const day = 86_400_000;
const circle = (au: number, days: number) => (ms: number) => [au * Math.cos((2 * Math.PI * ms) / day / days), au * Math.sin((2 * Math.PI * ms) / day / days), 0] as const;
const source: BodySource = {
  stops: [
    { id: "earth", name: { en: "Earth", ja: "地球" }, kind: "planet", radiusKm: 6378.137, colour: "#4c88c4" },
    { id: "mars", name: { en: "Mars", ja: "火星" }, kind: "planet", radiusKm: 3396.19, colour: "#c76a3f" },
  ],
  positionAu: (id, ms) => (id === "earth" ? circle(1, 365.256)(ms) : circle(1.52371034, 686.98)(ms)),
  offsetKm: () => [0, 0, 0],
};

const trip = planTrip(source, craftOf("kaminari")!, "earth", "mars", Date.UTC(2026, 10, 4));
console.log(trip.verdict.line);
console.log(Math.round(trip.frameS / 86_400), "days,", Math.round(AU), "km to an AU");

Can do it: 0.52 per cent of the propellant, at 0.015 g.
37 days, 149597871 km to an AU

What a craft can do

planTrip never hides a craft. It returns a verdict in one plain line, and playable says whether Play is offered:

Every figure of a craft names its source, or is marked as an estimate and says what it is worked from. The page shows an estimate with "est.".

Playing a trip

A trip is a list of phases (preheat, ignition, ascent, burn, coast, flip, brake, arrival). A real coast takes months, so each phase maps its mission time onto screen time on its own: stateAt(trip, source, playbackSeconds) returns everything true at that moment, from nothing but the trip and the time, so any frame can be drawn on its own and a test needs no clock.

import { CRAFTS, duration } from "@johnmorrisdotca/uchusen-rockets/journey";

for (const craft of CRAFTS) console.log(craft.name, craft.drive);
console.log(duration(258.8 * 86_400));

Saturn V ballistic
Space Launch System ballistic
Falcon Heavy ballistic
ISV Kaminari, SPX-207 continuous
258 d 19 h

What the page draws

One 2D canvas for the stars, SVG for the map and the route, and the package's own line drawing of the craft turned on its side: no WebGL, so the page makes no context to let go. It asks for a frame only while a trip plays and the page is on screen, at most 30 a second. The address keeps the choices (?from=earth&to=mars&craft=falcon-heavy).

What is left after each burn

The side view shows only what is left. A rocket is drawn whole through the countdown and the ascent, then as the upper stage with its payload for the departure burn and the coast, then as the payload alone from the turn round: Saturn V as the S-IVB with the Apollo spacecraft, then the command and service module; the Space Launch System as the upper stage with Orion, then Orion; Falcon Heavy as its upper stage with the payload, then the payload in its fairing. Nothing is drawn new: each stage is the rockets' own parts (stageOf says which), closed up into one stack and fitted to the whole rocket, so the scale never changes. A rocket with boosters on its sides, such as Falcon Heavy at lift-off, is drawn from the front so that all of its cores show.

The trip's numbers have a Fuel row, which is the propellant used (or n/a, with the reason: for most rockets no source gives the engine's specific impulse), and a row of its own for the payload, "Payload to" the target.