Can somebody explain the part-info numbers for intake air?

[weregamer]

I am completely baffled by what I see. A Goliath claims to require HUNDREDS of intake air, but in flight it's 100% satisfied at about Mach 1 with only about 40.

I see in many places that different intakes lose efficacy at higher speeds, which makes sense. But why does it seem that an engine that says it needs 7 intake air runs just fine with an intake that gives 2? (Or 0.5?)

 

[Dark Lion]

I honestly don't know this for sure, but intuitively, the units of air must be volume. This is my best guess because it would also answer your question here:

13 minutes ago, weregamer said:

But why does it seem that an engine that says it needs 7 intake air runs just fine with an intake that gives 2? (Or 0.5?)

Consider your speed at any given time will increase and decrease the amount of air that can be taken into your chosen intakes. If the intake provides 2 units of air, surely that number must correspond with the volume of air while the craft sits still. However, if you increase your speed, a greater volume of air flows through at a time. The faster you're moving, the more intake air is captured for combustion.

Someone else may come along with a more scientific answer though. Again, I'm just guessing about the units being volume.

[weregamer]

It's clear that it refers to a volumetric unit, and as I said there's lots of evidence, even within the game, that the amount that comes in an intake varies with speed.

My point is, the numbers for the engines don't even seem to be related to the numbers for intakes. Intakes have tiny numbers - 0.2, 1, 2, all the way up to a huge five. But engines have numbers almost an order of magnitude higher - 5, 7, and in the case of the Goliath, 136!

I can also imagine that maybe the engines are reporting maximum needs at some velocity (mach number), but they don't say what that velocity is, and again the Goliath *loses* power as it speeds up, so its low-speed intake should not be ten or more times lower than its maximum.

Since so many people build successful jets, and I've even seen some nice analyses of intake performance at various speeds, I'm just baffled why there's no corresponding explanation or analysis for engines and how the numbers relate.

[FleshJeb]

I THINK it's [Area * (craft speed + intake base speed) ] * (Mach Curve) = (Volume / second). The second number is stored volume, which is probably just a buffer to smooth things out.

Jet engines burn a certain ratio of air to fuel, and the amount of fuel they're able to burn per second varies with speed. So, once they pass their peak performance speed, they burn less fuel and require less air.

This is why it's really easy to flame out the Wheesley on the runway. It has a huge intake air ratio, and is at peak fuel usage at zero speed.

[Vanamonde]

Nuts-and-bolts kind of question moved to the nuts-and-bolts kind of subforum, where @bewing will probably come by soon to enlighten us all. 

[Rocket In My Pocket]

On an interesting side note since the Goliath was mentioned specifically; it has a built in air intake, which is mostly sufficient to operate itself. Something I didn't realize myself till fairly recently.

https://wiki.kerbalspaceprogram.com/wiki/J-90_"Goliath"_Turbofan_Engine

"It is the only jet engine with large radial size, and built in air intake."

Intake speed: 30

Intake area: 0.03 m2

Maximum intake air: 3.4 "Air units"

Edited October 4, 2018 by Rocket In My Pocket

[bewing]

FleshJeb is right, but I'm a practical guy and don't go in for all this theoretical stuff. Look, the deal is that the devs are trying to make your life hard, all right? They build these parts so that they always have limitations that you have to deal with, and engineer around.

The first basic point about "air" is that engines and intakes are always throttled at all points in their airspeed/thrust curves. The maximum amount of air that an engine needs is based on its maximum thrust -- which can never actually be achieved, because the engine is always limited by its thrust curves (so it's a worthless number). Additionally, you always have to keep in mind that at most one air intake can ever be assigned to a single engine at any one time.

At zero speed, an engine/intake combo is almost always throttled by the intake air requirement. So the important number at a standstill is the effective intake speed -- which corresponds to "how good is the turbofan at sucking in air from the surroundings when standing still". If you plan to do a lot of low-speed taxiing, pay attention to this number. If you have an intake that's got a bad intake speed, then you need to keep your throttle low while taxiing, or you will flame out. This can be problematic if you are driving around the countryside, and trying to go up hills with jet power.

In flight at low altitude, you have so much air getting crammed into your intake that it will always provide more air than any engine will need (because the engines are always limited by their thrust curves). So you don't have to sweat it in this case.

At high altitudes you have to fly fast. At high speeds the various air intakes choke, and the engines also choke because of their respective airspeed/thrust curves. Each of them chokes in different ways and at different speeds -- except for the shock cone intake, which never chokes. In some rare intake/engine combos the intake will choke first. But usually it's the engine that flames out while the intake can still provide enough. All you need to do at high speed is open the context menu of the engine and see if the "Prop. Requirement" is 100% or not. If it's not, then the engine is being limited by the intake. Then you have to decide if you are already flying fast enough to suit your tastes, and whether you actually care about going a few m/s faster. None of the numbers that you see in the part menu in the SPH tell you what the performance curves of the intakes/engines look like. You either have to look them up here online, or you have to learn by doing.

Goliath engines are thrust-limited so they can't go faster than mach 1. Rapiers go really fast. And each of the engines has a sort-of-matching air intake that can provide enough air through that engine's flight envelope.

So when it comes to "air" and the stats you see on the air intakes, the "effective base speed"/"effective air speed" is the only number that gives you any immediate useful feedback. On the engine, look at the Prop Requirement. If you want to know about high speed intake performance, do a search here for some of the graphs that other players have created.

 

[weregamer]

Thank you so much, that clarifies pretty much all the stuff that had me confused.

5 hours ago, bewing said:

Additionally, you always have to keep in mind that at most one air intake can ever be assigned to a single engine at any one time.

Follow-up: Is this assignment dynamic or static? If I have one engine but two intakes - one that has a better low-speed volume and one that does better at high speed - will it switch over as I speed up?

[bewing]

Dynamic. Yes, it will switch.

 

[weregamer]

Bwah hah hah hah, now I have the POWER!

Thanks a bunch.

[swjr-swis]

8 hours ago, bewing said:

Additionally, you always have to keep in mind that at most one air intake can ever be assigned to a single engine at any one time.

Sorry, but this statement is not correct. Engines can and will draw air from multiple intakes to fulfill their requirement.

It's quite easy to demonstrate in-game: occlude a Goliath by attaching its front node, so its own intake is no longer working. Add a bunch of smaller intakes (say, 4x circular ones at the end of an inverted quad-coupler). Fire up the engine at full throttle, and then close/open one or more intakes at will to see quite clearly that there will absolutely be multiple intakes feeding into the Goliath - its max output thrust and the time it takes for the engine to choke will vary linearly according to the nr of intakes left open.

 

Spoiler

4 intakes open, max thrust 87.3 kN

3 intakes open, max thrust 65.4 kN

2 intakes open, max thrust 43.6 kN

1 intake open, max thrust 21.8 kN