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I've just started building planes and am confused about the way air intake works. For instance, the J-90 "Goliath" Turbofan Engine is specified to consume a whopping 132.272 Air/sec while supplying Intake Air: 3.4 (presumably this is per second as well?).

How would I supply such an engine, let alone two?

The largest supplier of air that I can find seems to be the Engine Nacelle with Intake Air: 5.0 (similarly the Engine Pre-Cooler that takes in the same amount but is more expensive and lighter). That means, if I want to supply one Goliath, I'd need about 26 Nacelles, or 65 Radial Air Intakes (which claim to provide`Intake Air: 2.0). Presumably this is the maximum intake ASL, which supposedly drops higher in atmo (and probably depends on speed). So this doesn't sound like the correct way to determine air requirements.

So: How do I translate the Intake Air figure I see in the Item description to the usable amount of air/sec that I have available for my engines?

First, differentiate between intake air use/production, and intake air storage. It's a resource similar to electric energy or monopropellant. It's scooped by the intakes, stored in "tanks" provided by the intakes and drawn by engines from these "tanks". And since you're not supposed to be able to take a good supply of intake air to the orbit, the "tanks" provided are of puny size. Engine Nacelle can store 5 units of intake air, but it can scoop much, much more than that per second.

I can't give you a precise answer how to find out the rates, just - don't overthink it and don't worry. In most cases 1 intake (any) per engine is aplenty, 1 per 2 engines is sufficient. The primary practical difference is in aerodynamic properties, that is how much drag it causes - e.g. circular intakes are bad for supersonic flight.

Thing is, the amount of intake air changes with atmospheric density, and that changes with altitude - exponentially (it also depends on airspeed and the intake area facing "into the wind" so if you mount your intakes backwards - or your plane turns tail-forward - you may face air starvation). At altitudes up to 20km you'll have to fail pretty hard to starve your engine of air. Above that - around 24-26km - there's an altitude threshold at which air becomes too thin and your engines are starved of air. Note the exponential nature of pressure makes the threshold hit hard; you can do very little about shifting it. You may push maybe a kilometer up through use of plenty intakes, but it's not really worth it (the extra drag from the added intakes will contribute more negatively than the extra flight altitude will benefit you).

There's a plenty of other "hidden" characteristics of the jet engine, like airspeed-thrust curve, pressure-thrust curve, etc. Goliaths are, for example, subsonic engines which really lose power at higher speeds, and won't allow for extreme altitudes either. And they have intakes big and efficient enough that it's very hard to starve them of air - you'll run out of thrust long before you reach altitude where their intakes can't keep up with the consumption.