At night · aeronautic

countermeasures.tsx

Starting…

import { useAnimations, useGLTF } from "@react-three/drei";
import { useFrame } from "@react-three/fiber";
import {
  Countermeasures,
  type CountermeasuresHandle,
} from "@aeronautic/countermeasures/react";
import {
  burn_time,
  FLARE,
  flare_at_pressure,
  flare_light,
  grain_at,
  terminal_speed,
} from "@aeronautic/countermeasures/physics";
import {
  AIR_GAS_CONSTANT,
  exposure_ev100,
  standard_atmosphere,
} from "@aeronautic/core";
import {
  type FC,
  type RefObject,
  useEffect,
  useMemo,
  useRef,
  useState,
} from "react";
import { type Group, LoopOnce } from "three";
import { base_path } from "@/lib/shared";
import { Aim, Stage } from "./stage";

const MODEL = `${base_path}/fighter.glb`;

// A full moon lights the ground to about 0.25 lux at most; a hemisphere light
// gives an upward face its intensity in lux. The flares light the rest. It
// lights the smoke from overhead too, as the sun would
const MOON_LUX = 0.25;
const MOON_UP = [0, 1, 0] as const;

// The MJU-7's flame when it lights, still
const mju7 = FLARE.mju7;
const lit = flare_light(mju7, grain_at(mju7, 0));

/**
 * The standard atmosphere at an altitude, and the flare in it: how long it
 * burns, slower in thinner air, and how fast it falls once slowed.
 * @param altitude_m How high
 * @returns The air's density, the burn and the fall
 */
const flare_at = (altitude_m: number) => {
  const air = standard_atmosphere(altitude_m);
  const density = air.pressurePa / (AIR_GAS_CONSTANT * air.temperatureK);

  return {
    density,
    burn_s: burn_time(flare_at_pressure(mju7, air.pressurePa)),
    falling_m_s: terminal_speed(mju7, density),
  };
};

// Close under the tail, or off to the side, far enough to see a program's
// flares fall away behind: at 250 m/s each burns out half a kilometre back
// and forty metres down
const VIEWS = {
  close: {
    label: "close, under the tail",
    camera: [-42, 4, -14],
    target: [0, -6, 28],
  },
  side: {
    label: "off the side, 650 m",
    camera: [-650, -20, 260],
    target: [0, -20, 260],
  },
} as const;

type View = keyof typeof VIEWS;

// A fighter low down, slower, and holding still for the flares to fall
const AIRSPEEDS = {
  250: "250 m/s",
  150: "150 m/s",
  0: "0 m/s, hovering",
} as const;

type Airspeed = keyof typeof AIRSPEEDS;

export type Settings = {
  view: View;
  airspeed: Airspeed;
  salvo: number;
  burst: number;
  visibility_m: number;
  ev100: number;
  altitude_m: number;
};

// Two flares a burst, four bursts, a clear night, a kilometre up
export const START_SETTINGS: Settings = {
  altitude_m: 1000,
  view: "close",
  airspeed: 250,
  salvo: 4,
  burst: 2,
  visibility_m: 23_000,
  ev100: 8,
};

/**
 * The fighter at night, its two dispensers under its tail.
 * @param props The settings, how many times it has fired, and where to say
 *   what the flares are doing
 * @returns The model and its countermeasures
 */
const FlaringFighter: FC<{
  settings: Settings;
  fired: number;
  chaffed: number;
  readout: RefObject<HTMLSpanElement | null>;
}> = ({ settings, fired, chaffed, readout }) => {
  const { scene, animations } = useGLTF(MODEL);
  const body = useRef<Group>(null);
  const flares = useRef<CountermeasuresHandle>(null);
  const { actions } = useAnimations(animations, body);

  useEffect(() => {
    // The afterburner cones and the pylons are meant hidden, flagged so in
    // their extras, which three hands over as userData
    scene.traverse((node) => {
      if (node.userData.visible_default === false) node.visible = false;
    });

    // It's modelled on its wheels: fold them away
    const retract = actions.ANIM_Gear_Retract;

    if (!retract) return;

    retract.setLoop(LoopOnce, 1);
    retract.clampWhenFinished = true;
    retract.play();
  }, [scene, actions]);

  const exposure = exposure_ev100(settings.ev100);
  const look = useMemo(
    () => ({
      exposure,
      sunDirection: MOON_UP,
      sunColor: [1, 1, 1] as const,
      sunIntensity: MOON_LUX * exposure,
    }),
    [exposure],
  );

  const { density, burn_s, falling_m_s } = useMemo(
    () => flare_at(settings.altitude_m),
    [settings.altitude_m],
  );

  // As it leaves: the airspeed, and the cartridge's throw square to it
  const thrown_cd = useMemo(
    () =>
      flare_light(
        mju7,
        grain_at(mju7, 0),
        undefined,
        undefined,
        Math.hypot(settings.airspeed, mju7.ejectionMPerS),
        density,
      ).intensityCd,
    [settings.airspeed, density],
  );
  const program = useMemo(
    () => ({ salvo: settings.salvo, burst: settings.burst }),
    [settings.salvo, settings.burst],
  );

  // The same program, of chaff: a cloud a cartridge
  const chaffed_once = useRef(chaffed);

  useEffect(() => {
    if (chaffed === chaffed_once.current) return;

    chaffed_once.current = chaffed;
    flares.current?.fire({ ...program, payload: "chaff" });
  }, [chaffed, program]);

  useFrame(() => {
    const current = flares.current;

    if (!current || !readout.current) return;

    readout.current.textContent =
      `${current.burning} burning, ${current.pending} to come · ` +
      `each ${(lit.intensityCd / 1000).toFixed(0)} kcd still, ` +
      `${(thrown_cd / 1000).toFixed(0)} kcd at the grain as thrown, ` +
      `lit after ${mju7.ignitionDelayS} s for ${burn_s.toFixed(1)} s · ` +
      `${current.smoking} smoke trails · falling at ` +
      `${falling_m_s.toFixed(0)} m/s once slowed · ` +
      `${current.chaffClouds} chaff clouds, sinking at ` +
      `${current.chaffFallMPerS.toFixed(2)} m/s`;
  });

  return (
    <>
      <group ref={body}>
        <primitive object={scene} />
        {/*
          On the model's origin, which flies -Z, the dispensers where the
          defaults put them, under the tail
        */}
        <Countermeasures
          ref={flares}
          program={program}
          flight={{ airspeedMPerS: settings.airspeed }}
          air={{
            altitudeM: settings.altitude_m,
            visibilityM: settings.visibility_m,
          }}
          look={look}
          fire={fired}
        />
      </group>
      <hemisphereLight args={["#ffffff", "#000000", MOON_LUX * exposure]} />
    </>
  );
};

/**
 * The flare and chaff buttons, the view, the program, the airspeed, the haze and the
 * exposure, and the altitude if asked for.
 * @param props The settings, how to change them, how to fire, and whether
 *   the altitude is shown
 * @returns The controls
 */
const CountermeasuresControls: FC<{
  settings: Settings;
  change: (next: Partial<Settings>) => void;
  fire: () => void;
  chaff: () => void;
  altitude: boolean;
}> = ({ settings, change, fire, chaff, altitude }) => (
  <div className="example-controls">
    <button type="button" onClick={fire}>
      Flares
    </button>
    <button type="button" onClick={chaff}>
      Chaff
    </button>
    <label>
      view
      <select
        value={settings.view}
        onChange={(event) => change({ view: event.target.value as View })}
      >
        {Object.entries(VIEWS).map(([value, { label }]) => (
          <option key={value} value={value}>
            {label}
          </option>
        ))}
      </select>
    </label>
    <label>
      airspeed
      <select
        value={settings.airspeed}
        onChange={(event) =>
          change({ airspeed: Number(event.target.value) as Airspeed })
        }
      >
        {Object.entries(AIRSPEEDS).map(([value, label]) => (
          <option key={value} value={value}>
            {label}
          </option>
        ))}
      </select>
    </label>
    {altitude && (
      <label>
        altitude {(settings.altitude_m / 1000).toFixed(1)} km
        <input
          type="range"
          min={0}
          max={15_000}
          step={500}
          value={settings.altitude_m}
          onChange={(event) =>
            change({ altitude_m: Number(event.target.value) })
          }
        />
      </label>
    )}
    <label>
      burst {settings.burst}
      <input
        type="range"
        min={1}
        max={4}
        step={1}
        value={settings.burst}
        onChange={(event) => change({ burst: Number(event.target.value) })}
      />
    </label>
    <label>
      salvo {settings.salvo}
      <input
        type="range"
        min={1}
        max={8}
        step={1}
        value={settings.salvo}
        onChange={(event) => change({ salvo: Number(event.target.value) })}
      />
    </label>
    <label>
      visibility {(settings.visibility_m / 1000).toFixed(1)} km
      <input
        type="range"
        min={Math.log(500)}
        max={Math.log(50_000)}
        step={0.01}
        value={Math.log(settings.visibility_m)}
        onChange={(event) =>
          change({
            visibility_m: Math.round(Math.exp(Number(event.target.value))),
          })
        }
      />
    </label>
    <label>
      exposure EV {settings.ev100.toFixed(1)}
      <input
        type="range"
        min={2}
        max={14}
        step={0.1}
        value={settings.ev100}
        onChange={(event) => change({ ev100: Number(event.target.value) })}
      />
    </label>
  </div>
);

/**
 * The docs' fighter letting flares go at night.
 * @param props Where to start from, and whether the altitude can change
 * @returns The example
 */
export const CountermeasuresExample: FC<{
  start?: Partial<Settings>;
  altitude?: boolean;
}> = ({ start, altitude = false }) => {
  const [settings, set_settings] = useState<Settings>({
    ...START_SETTINGS,
    ...start,
  });
  const [fired, set_fired] = useState(0);
  const [chaffed, set_chaffed] = useState(0);
  const readout = useRef<HTMLSpanElement>(null);

  const change = (next: Partial<Settings>) =>
    set_settings((previous) => ({ ...previous, ...next }));

  return (
    <Stage
      camera={[-42, 4, -14]}
      target={[0, -6, 28]}
      fov={45}
      floor={null}
      sky={false}
      overlay={
        <>
          <CountermeasuresControls
            settings={settings}
            change={change}
            fire={() => set_fired((count) => count + 1)}
            chaff={() => set_chaffed((count) => count + 1)}
            altitude={altitude}
          />
          <span ref={readout} className="example-note" />
        </>
      }
    >
      <Aim
        camera={[...VIEWS[settings.view].camera]}
        target={[...VIEWS[settings.view].target]}
      />
      <FlaringFighter
        settings={settings}
        fired={fired}
        chaffed={chaffed}
        readout={readout}
      />
    </Stage>
  );
};

export default CountermeasuresExample;

The fighter flies at 250 m/s, a kilometre up, on a clear night. Fire, and its two dispensers let the program go, in turn. Nothing about the flares is drawn to a size or a timer; the airstream's toll is an estimate, and says so.

Each flame trails what the airstream sweeps off it: dimmer at the grain and streaked out behind as it leaves at 250 m/s, gathering back into a ball as it slows, its trail reddening as it cools. Set the airspeed to 0 and the flare is a ball, falling.
Each lights a moment after it leaves, 0.1 s, an estimate.
Each leaves its smoke, lit by its flame where it has just passed and by the moon, faint, once the flames are out.
Chaff lets the same program go as dipoles: each a pale puff where it was let go, sinking at a fifth of a metre a second. At night only the flares show it: fire both.
Look from the side, 650 m off, to see a program fall away behind: each burns out half a kilometre back and forty metres down.
Each flare is thrown down at 30 m/s, and slowed by its own drag at hundreds of metres a second squared: within a second it is far behind, falling.
Each burns about three seconds, brightest as it lights, fading as its grain shrinks, and lights the airframe as it goes.
Slow down and the flares stay closer; at rest they only fall, at the few tens of metres a second their drag allows.
Change the program: more flares a burst, or more bursts.
Lower the visibility and the far flares dim and redden.
Change the exposure and the glare round each flame grows or shrinks, as the eye's scatter carries more or less of it over what the screen shows.

The page's bloom is the docs' own, added to everything. The flames' glare is the eye's, and would be drawn without it.