Airframes

airframe is what the aircraft is, and everything about its trails that is not the day follows from it:

• massKg and liftToDrag set the thrust: in level flight the engines push against the drag, the weight over the lift-to-drag ratio. Loaded to n g, the drag polar of a jet cruising for range, parasite drag three times the drag due to lift, makes it (3 + n²) / 4 of that: a quarter less unloaded, the same pushed as pulled.
• fuelPerThrust turns that thrust into fuel burnt, and sets the engines' efficiency, how much of the fuel's heat pushes the aircraft.
• dryThrustN caps it: each engine's sea-level static thrust dry, at military power, in newtons, lapsing with the air's pressure at altitude, to 0.36 of it at 11 km and Mach 0.85. A 9 g pull asks for 21 times the level thrust; the default fighter's engines have about 2.3 dry.
• maxThrustN is the same with full reheat, what the burner adds to it. For an engine with no reheat, give both the same.
• spanM spaces the wake's vortex pair, which carries the trails down.
• engines are where the trails start, one each.

The defaults: a heavy fighterCopy Anchor Link

Twin low-bypass engines close together at the tail, at 20 t and a lift-to-drag of 9: about 1 g of fuel per metre per engine, a quarter of the fuel's heat pushing it. The two trails merge into one within a second, drawn together by the vortices on either side.

An airlinerCopy Anchor Link

<Contrails
  airframe={{
    massKg: 70_000,
    spanM: 34,
    liftToDrag: 17,
    // A high-bypass fan at cruise
    fuelPerThrust: 16e-6,
    // A CFM56-7B's 27 300 lbf, in newtons, and no reheat
    maxThrustN: 121_400,
    dryThrustN: 121_400,
    engines: [
      [-5.75, -1.5, 2],
      [5.75, -1.5, 2],
    ],
  }}
  flight={{ airspeedMPerS: 230 }}
/>

A third of its fuel's heat pushes it, so its threshold is about a degree warmer than the fighter's. The engines sit under the wing, far from the centreline, so the trails stay two for longer before they wrap round the vortices, and sink with the wake about a hundred metres.

ReheatCopy Anchor Link

flight.throttle runs as a Flight's does and the afterburner reads it: 0 at idle, 1 at military power, on to 1.1 at full reheat. Dry, the engines hold the flight, their thrust the drag, whatever the throttle. Past the detent the burner lights as the afterburner's does, its first zone with a thump and the rest staging in to the stop, on a core pushed to military power. Its thrust is then what the burner makes, and its fuel what the burner burns, at reheat's dearer consumption.

<Contrails flight={{ airspeedMPerS: 250, throttle: 1.1 }} />

Read from a shared Flight, the trail lights with the plume: the throttle that lights the afterburner lays its fuel's water too.

Six times the fuel lays six times the water and the soot to freeze it on, in a plume that mixed with six times the air: two and a half times as wide and as dense. Less of the heat pushes, so the threshold is a degree colder and the plume takes longer to cool to it. What is laid while the burner is lit stays as thick once it goes out, behind the aircraft where it was flown.

Engines that moveCopy Anchor Link

An engine can be an Object3D, a node of the model, whose position is read every frame: a thrust-vectoring nozzle, or a model whose engines are placed by its own animation.

What it burnsCopy Anchor Link

<Contrails fuel="hydrogen" />

Hydrogen makes seven times the water for each kilogram, and nearly three times the heat: its mixing line is twice as steep, so it trails in air seven degrees warmer than kerosene does. With no soot it seeds a hundred times fewer crystals, which grow ten times larger from the same air: a trail that dims the light half as much. Give { waterIndex, heatJPerKg, iceIndex } for any other fuel.