Air Intake requirements change?

[-RanZ-]

I haven't played much since 1.0, did air intake requirements change? I had a VTOL jet that had 2 wheesley engines and 2 radial ramp air intakes that used to work, now it won't even power 1 engine with 2 radial intakes. I even did a test w/ just a fuel tank, 2 radial intakes and those 2 radial intakes won't even provide enough intake for 1 wheesley engine... is this normal behavior now or am I bugged?

[Rath]

Those are radial ram intakes, they barely do anything when still.  Shove on the scoops.

[Sharpy]

Air intake efficiency is directly proportional to the intake area. It's listed in parameters of the part in VAB/SPH part list. AFAIK Radial Structural Intakes are especially lousy. Circular intakes have the best area but also a high drag coefficient; standard (node mounted) ram air intakes provide a little less air but are quite aerodynamic.

Although two structural intakes is quite enough for Wheesley.

Also, one more thing. Didn't you mount your intakes backwards? Because at higher power, engines require more intake air, and intakes that are not "facing the flow" are pulling next to nothing. (one reason if your plane flips backwards, your engines are likely to flame out).

[-RanZ-]

It works when I am moving, looks like when you move you get more intake air which makes sense. That pretty much makes VTOLs using a jet engine useless then doesn't it? My 1 VTOL engine runs without flameout but only produces about 40kN of thrust which won't list much

Edited November 28, 2016 by -RanZ-

[Starhawk]

3 minutes ago, -RanZ- said:

That pretty much makes VTOLs using a jet engine useless then doesn't it?

You probably just need to change which intake you're using.  Some work much better at slow speeds than others.

Happy landings!

[KerikBalm]

If you want to VTOL, you're going to need more intakes.

When you look at the intakes descriptions, they'll mention if they are optimized for subsonic or supersonic flight. There is also some mention of static suction. I believe that the Engine nacelle has the best "static suction"... although I think the precooler does pretty well too if I recall. Anyway, try putting an engine nacelle or two on your VTOL.

[AeroGav]

I am not sure the Wheesley is the best choice, it consumes more air for a given thrust level because it is a high bypass civil turbofan.  Good for economy not so good if intake limited.

I suspect that's the reason most VTOLs seem to use Panthers.   Best TWR of all jet engines below mach 1.5,  in afterburner they produce twice as much power for the same airflow, though of course economy takes a nosedive.

[moogoob]

As said, you're gonna need more intakes:

 

[KerikBalm]

Check the "intake speed"

My understanding of it is that an intake speed of 5, has a static airflow equivalent to moving at 5m/s. An intake speed of 40 would be as if its moving at 40m/s

So going 40 m/s with an intake of a speed of 40, just doubles the effective speed of the air for the intake (80), but going 40 with an intake that has a speed of 5 is 5x faster (45 vs 5).... so intakes with low intake speeds need to get going faster before they work well enough.

http://wiki.kerbalspaceprogram.com/wiki/Structural_Intake speed: 10

http://wiki.kerbalspaceprogram.com/wiki/Ram_Air_Intake speed: 10

shock cone: 5

Circular intake: 15

XM-G50 radial: 15

diverterless supersonic intake: 12

Engine pre-cooler: 30

Engine Nacelle: 40

These last two are clearly the best for static air flow/supplying air while hovering.

[sardia]

Wait, how does this affect space planes? I thought the shock cones were the best for getting high speed at high altitudes. 

Low intake speed means they work better at higher speeds then say, an engine precooler at high altitude high speed. Is that right?

[KerikBalm]

yes, they work best at high speeds. They produce very little intake air from a standstill. At high speeds, they produce more than enough air for the jets, and they'll keep jets supplied even at high altitude, right to each jet's absolute altitude limits.

Only you get going much more than 50 m/s it doesn't make much difference. They simply aren't very good for supplying air when you need a lot of static thrust... ie they aren't good for VTOLs.

Also note differences in intake area.... 2x the intake area makes up for half the effective airspeed (intake speed + airspeed = effective airspeed for intakes)

Also note intake air requirements for various engines:

Wheesely: 127:1 Intake air:Liquid fuel

Goliath: 227:1

Panther Dry: 40

Panther Wet: 12

Turboramjet: 8

Rapier: 6

Juno: 22

Isp also factors into this. Using 100 air per LF at 8000 Isp would use the same amount of air per unit thrust as something using 50 air per LF at 4000 Isp.

So... normalizing to the turboramjet (4000 Isp, 8 intake per LF)... the relative intake air consumed per unit thrust for each engine:

Wheesley: 6.05x as much intake air per unit thrust

Goliath: 9.01x

Panther Dry: 2.22x

Panther Wet: 1.5x

Turboramjet: 1x

Rapier: 0.9375x

Juno: 1.72x

While the wheesley and Goliath may have good static TWR (only the Afterburning panther beats it at sea level), they really use a lot more intake air.

Afterburning panthers are what you want for VTOLs... best static TWR (rapier static TWR is the worst), Relatively low intake demands, excellent thrust vectoring.

 

Edited December 4, 2016 by KerikBalm

[-RanZ-]

thanks for the info, very helpful. I will have to change my vtol up a bit I guess.

[Sharpy]

On 3.12.2016 at 9:16 PM, sardia said:

Wait, how does this affect space planes? I thought the shock cones were the best for getting high speed at high altitudes. 

Low intake speed means they work better at higher speeds then say, an engine precooler at high altitude high speed. Is that right?

Primarily, they have the lowest atmospheric drag. Engine performance doesn't depend on amount of intake air, as long as it's greater than zero - it depends on airspeed though. And you can't reach high speeds with draggy profile, so shock cone - despite being somewhat mediocre in terms of amount of air provided  - is superior for high speeds because it doesn't slow your plane down!

[KerikBalm]

The shock cone is not mediocre in terms of amount of air provided,  except at very low speed. Its low drag, and provides a lot of intake air at higher speeds.

I rarely have any flame out problems once my craft is rolling aster than 50 m/s. Also note that turboramjets and rapiers don't need nearly as much intake air to function. So for spaceplanes the combination of good high speed intake performance and low intake requirements means that its rarely a problem.

Shock cones may not perform well at low speed, but you should only be at low speed when at low altitude... where the air is thicker and its easier to supply a craft with intake air. Also the static thrust of rapiers and turboramjets is quite low at low speed (their thrust multipliers for high speed get well over 3x), so at low speed when the shock cones aren't providing so much intake air, the engines aren't operating at maximum thrust, and aren't consuming as much intake air as they could.

So, for SSTO spaceplanes and other high speed designs, slap on shock cones and rapiers/turboramjets, and that's all you need to know.