Air intake and wings

[xriz00]

How many air intakes is optimum for one jet engine, just one? Does wanting to fly higher then say 7k mean two or more intake is better? And at what height do wings start to lose their effectiveness and at what height are they pointless?

I've flown a bunch of flights but can't quite figure out the proper best use of air intakes and wings, thanks for you help.

Edited January 11, 2015 by xriz00

[GoSlash27]

xriz00,

The optimum intake type and number depends on what you're trying to do. If you're just bombing around at low altitude, you want minimal drag, so a single structural intake is fine. For a spaceplane, you want at least .18m^2 of intake area per engine.

Wings will begin to lose their effectiveness at around 25km and become useless at around 32km, depending on wing loading.

I have some tutorials and comparison data on intakes, wings, and spaceplane construction tips in the tutorial section.

Best,

-Slashy

Edited January 10, 2015 by GoSlash27

[Starhawk]

In my experience, it greatly depends which intake you are using. Generally, more of the cheaper, lower tech intakes are needed than the fancier, higher tech ones.

You definitely want more intakes if you are going above the thick part of the atmo.

Air intakes become very ineffective above about 36 km and jet/turbojet engines will flame out unless the throttle is pushed way down. Wings also become less effective at this altitude, but it seems as though they are still providing some lift.

I don't know enough about the aerodynamic model in KSP to give you any exact values, but I have a spaceplane with one turbojet, and I can easily make orbital velocity using only two shock cone intakes.

I did a lot of trial-and-error design when I first started making planes/spaceplanes.

Hope this helps. Good luck!

[thereaverofdarkness]

Wings will really get a lot less lift as the air thins, and this effect can be noticed as low as 10km if your velocity is too low. If you intend to fly high and fast, you actually want very large wings. This is not realistic but is due to some inaccuracy in the aerodynamics model in-game. As you get into the upper part of your flight, where intakes start having difficulty, you also need extra wing surface and control surface to generate lift or else the atmosphere may start to pitch your ship toward your heading and you won't be able to get the high angle of attack you need to continue raising your altitude.

If you have enough air intakes and wing surface (bigger emphasis on intakes) then you will be able to pass above 20km and into the range where the more the atmosphere thins the faster you can go. If you raise your altitude slow enough, you can simply keep increasing your speed as you go up, and your intake air will decrease a lot slower. Also, as drag evaporates into nothingness, you need less and less engine power to keep going. For instance, a plane with 3 jet engines can switch off the outer two when the air thins too far to run them, and keep the center engine running to a significantly higher altitude, and still have enough thrust to increase your velocity. You'll want a TWR greater than 2:1 for passing above 20km but around 25-30km when even multiple intakes begin to suffer, you can lower your TWR even below 1:1 and likely still have your velocity increasing. A big part of this is that you are already going a significant chunk of orbital velocity.

Once down to a single engine, you don't have to worry about your craft spinning out of control when the engine starts to sputter from low oxygen. If it goes out, just throttle down until it turns on again, and nose down a bit to level out your trajectory until you reach a higher velocity. As you burn fuel, your craft will get lighter and you will be able to go even higher and faster with some patience. Using this method with a somewhat reasonable number of air intakes I was able to take my craft higher than orbital velocity at around 28km altitude. It gave me an apoapsis greater than the diameter of Kerbin, and had I known about air-canning, I could have made an orbit with no rocket engines. That's when you close intakes at an altitude and velocity in which you were still able to run engines, and it holds a tiny chunk of air inside them. Then at apoapsis you throttle up your engines and point prograde, then open the intakes. The engines burn for an instant, and increase your velocity just a tad.

Edited January 10, 2015 by thereaverofdarkness

[Heagar]

It depends on what your plane should be capable off.

For a plane with a basic jet engine one intake is enough, since this engine starts to loose power at about 15 km altitude anyways.

With the advanced jet engine and one intake you can go up to 26 km.

But in case you are planning to build a STTO:

• with three intakes per engine you are on the safe side without problems
  
• with two intakes per engine you should hit at least 1800 m/s between 30-36 km, then it is not important, if your air-intake reading is zero. You can achieve altitudes up to 56 km with working jet engines with sutch a plane.
  

BTW:

Very important is to mount your jet engines seperatly, not in symmetry mode. The same with the air intakes. That reduces the ammount of uneven thrust/flameout for a multiengine plane.

That means, that you mount your first engine. Then mout the air intakes for that. Then repeteatly the same procedure for the rest of the engines/intakes.

[xriz00]

Thanks for the help and tips guys, just what I was looking for.