Regarding Intakes

[fireblade274]

Will increasing the number of air intakes on a craft increase their thrust more then there being less intakes? Anyone have an opinion on how many intakes should you use per engine is optimal?

[steve_v]

No, provided you have enough IntakeAir to run your engines.

One intake per engine should be enough, but it's a balancing act between thrust, intake air, velocity and drag.

More intakes won't necessarily equate to more thrust at altitude unless intake air is the limiting factor - air breathing engines loose thrust as air density decreases anyway, and more intakes won't change this. They'll just add more drag, and more drag = less speed = less thrust (thrust is a function of air density and velocity for KSPs jets).

The part description for the engine should give you an idea of where thrust peaks with regard to velocity.

Once you get far enough beyond peak thrust velocity (or altitude) that you stop accelerating, it's time to think about switching to rockets - at this point intakes become dead weight anyway.

Working out exactly where that point is for a particular craft is a matter of trial and error.

Air intakes also work better as velocity increases, so reducing drag for moar speed is often better than adding more intakes.

What you want is just enough intake area to prevent your airbreathers from flaming out before you're finished with them.

Edited August 9, 2015 by steve_v

[selfish_meme]

Also use shock cone intakes they are the least draggy, even better than any nosecone you can put on

[steve_v]

Yeah, unless you use FAR... in which case nosecone and intake drag actually makes sense.

[Geschosskopf]

Will increasing the number of air intakes on a craft increase their thrust more then there being less intakes? Anyone have an opinion on how many intakes should you use per engine is optimal?

As Steve_v mentioned, no. He explained one thing but I'll explain another.

Jet engines consume 2 resources: LiquidFuel and IntakeAir. LF is in fuel tanks, IA is created by intakes. But under the hood, the only difference between them is their names--the game handles consumption the same for all resources. Thus, for the engine to run, the ship must have tanks of both, from which the engine draws LF and IA in the proper ratio to function, same as a rocket needing tanks of both LF and O.

So, when you read the stats of an intake, you'll see a rating for IntakeAir. This is actually the capacity of the "tank" of IA. This number is of no significance whatsoever because what really matters is the rate at which the intake creates IA to fill this "tank". If the intake makes IA faster than the engine consumes it, then the "tank" will remain full (as shown on the resource list). If the engine consumes more AI than the intakes can make, then the IA resource bar will go away and the engine will flame out.

The rate of IA creation is a complicated function of the intakes area (it's most important stat) and the speed and altitude of the plane. The bigger the area, the more IA you make regardless of other conditions. The higher you fly, the thinner the air so the less IA you make. The faster you go, the more air can be swept up in a given time by the intake, so the more IA you make.

Back in pre-1.0 days, that was about all there was to it. This let you use multiple intakes per engine, so that you always had enough intake area to keep the engine supplied with IA even at ridiculously high altitudes, and SSTOs could leave the atmosphere on jets alone, needing only a tiny bit of rocket to circularize. But since 1.0, jet engines have had a thrust vs. altitude curve applied to them. This makes it so they just flat lose all their thrust at about 25km without flaming out, even with beaucoup IA available. Because of this, there is no longer any point in using multiple intakes per engine. What's the point of having extra IA if the engine can't use it anyway?

[GoSlash27]

Also use shock cone intakes they are the least draggy, even better than any nosecone you can put on

You know... people keep telling me this, but I ain't so sure. Has anybody done a test to confirm this?

Best,

-Slashy

[SAI Peregrinus]

You know... people keep telling me this, but I ain't so sure. Has anybody done a test to confirm this?

Best,

-Slashy

Yes. It's easy enough to re-test (not at my KSP PC right now) by just setting up a rig that launches multiple rockets simultaneously, and changing only the nose cones + some ballast.

[GoSlash27]

Also use shock cone intakes they are the least draggy, even better than any nosecone you can put on

Thanks, I just set up a flying balance beam. The shock cone is definitely the least-draggy.

Best,

-Slashy

[Sharpy]

What's the point of having extra IA if the engine can't use it anyway?

or more precisely, can't transform it into useful thrust. More intakes will keep the engines running at higher altitudes, but the difference here is between "running" and "producing thrust". Unless you're building a Stratospheric ISRU Station that uses jet engines for producing electricity, there's simply no point to keep them running above some 24km. All they do then is consuming fuel.

[AaronLS]

Yes. It's easy enough to re-test (not at my KSP PC right now) by just setting up a rig that launches multiple rockets simultaneously, and changing only the nose cones + some ballast.

I've done tests like this with nose cones and it's straightforward, but when you introduce air breathing engines it becomes alot more complicated as some profiles only really shine on a lower angle of ascent where they have time to accelerate in thick air. Because of the way air breathing engines behave, I think it's alot harder to come up with a definitive test.

Personally I've found inline nacel intakes to be nice, because you can still put a nose cone on the end of the stack. I usually use the tapered tank that holds 80 liquid fuel with a small nose cone on the tip as that provides a very aerodynamic tip.

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air breathing engines loose thrust as air density decreases anyway, and more intakes won't change this.

This is a key point. Something else very non-intuitive is more air does not mean more thrust, at least for ram jets. I used to think that more velocity meant more air intake, which meant more thrust to the ram jets. I.e. an indirect relationship. This is not true. It seems ram jet thrust is directly related to velocity, and air intake affects it only when you don't have enough air to supply it's demand. The air demand itself is a function of speed+altitude.

I notice when taking off, according to Kerbal Engineer, I'm only using 20% of my air intake with ram jets, and as I go faster at the same altitude I'm using a greater percentage. Which indicates that velocity factors directly into the equation of how much air+thrust your jets will use.

I used to think it was an indirect relationship: My engines increased thrust as I accelerated because my intakes were in turn capturing more air. Not true though. If that were the case, then you'd see 100% air intake usage at takeoff, and then you could attribute the increasing thrust due to increasing speed capturing more air. For example my air intake demand goes from around 0.2 up to 1.08, but the entire time my air intake supply is always greater. So at no time is air supply a limiting factor. The only thing changing is my velocity.

If you use Kerbal Engineer and add the air intake demand and air intake supply fields from the Vessel section, you'll be able to determine if at any point during your flight you don't have enough air intakes to meet demand. Consider though sometimes the small amount of demand an extra air intake will meet, will not provide enough thrust to counter act the drag.

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Another example: On re-entry I'll get below 32k altitude and have .3 air intake, and the engines won't cut on until around 25k. Now you might say there wasn't enough air to overcome flameout, however when they cut on around 25k there is .6 air intake supply, and the engine's demand is only .2. So this indicates there was more than enough air intake for the engines demand at 32k altitude. It simply was that the engine has a direct relationship with altitude+velocity rather than air intake. Air intake doesn't determine thrust, it can only limit thrust when there's not enough air. It's very non-intuitive in my opinion.

[SAI Peregrinus]

I've done tests like this with nose cones and it's straightforward, but when you introduce air breathing engines it becomes alot more complicated as some profiles only really shine on a lower angle of ascent where they have time to accelerate in thick air. Because of the way air breathing engines behave, I think it's alot harder to come up with a definitive test.

Personally I've found inline nacel intakes to be nice, because you can still put a nose cone on the end of the stack. I usually use the tapered tank that holds 80 liquid fuel with a small nose cone on the tip as that provides a very aerodynamic tip.

You don't change the engine to test drag. You stick the part on an SRB with a probe core.

[GoSlash27]

I just stuck one out on opposite sides of an SRB with RCS tanks to balance the weight. Whichever side it veers to is draggier.

Best,

-Slashy

[AaronLS]

You don't change the engine to test drag. You stick the part on an SRB with a probe core.

I didn't say anything about changing engines.

That's good for testing drag alone, but not for air intake supply performance.

[GoSlash27]

I didn't say anything about changing engines.

That's good for testing drag alone, but not for air intake supply performance.

Ahh. That's true, but it's something that doesn't really need tested. You either have enough intake to feed the engine where it still produces thrust or you don't. In that sense, there is no improvement to be had in intake performance.

You can gauge that by the intake area in the .cfg.

Best,

-Slashy

[Sharpy]

Yep, as long as you have any intake air below 20,000 you're good. Above that, it doesn't really matter. So, yes, with tiny intakes like the Structural ones this might be a concern - or if you try to reduce their number, say, giving one intake per two engines.