What is the most efficient jet engine thrust level?

[Kerboman]

I built a jet plane, and I want to go as far as I can without going high enough to flame out. What throttle level should I use? 5%? 10? 80? 100?

[numerobis]

What exactly do you mean by "high enough to flame out"? High enough *at that throttle level*? Then the answer is just barely over 0%, and you want to fly at the edge of the atmosphere at circular orbit speed.

But if you mean you want an altitude where you wouldn't flame out at full throttle, yet you want to improve your efficiency, that's going to be more complicated to work out.

[Cashen]

I think you should go full throttle to maximize speed, at the highest altitude possible without flaming out. Just keep an eye on your intake air and when it's around 0.20, that's the altitude you want to be at. But the engine should be at 100% to maximize speed. Remember that, at least for the ram air intakes, the faster you go the more air you get at a given altitude.

[NoMrBond]

As you go higher, the amount of thrust you can exert without a flame-out decreases due to the diminishing amount of intakeair available, but the more that diminished thrust does for you due to the reduced drag

In the lower atmosphere, exceeding terminal velocity is a waste of fuel due to friction losses

So most efficient is really relative to your altitude

You're going to want just enough thrust to stay as close as possible to terminal velocity as you ascend (to get up to the more efficient higher altitudes as fast as possible, without undue friction losses), and then slowly back off the throttle as you get higher due to intakeair limitations

Edited July 18, 2013 by NoMrBond

[Kerboman]

Thanks for the help!

[Spyritdragon]

Try loading the stock Ravenspear mk. 1, but replace the intakes with RAM intakes. I find that it's great to get a feeling for RAM intakes and jet engines.

It can also comfortably hold a 900 m/s surface velocity, given a bit of attention, which i find to be very useful if i want to get somewhere on Kerbin quickly, since it only allows me 4x timewarp.

Somewhat hard to land on non-smooth surfaces though.

[Skorpychan]

I've found that it's viable to go up like a rocket with an early gravity turn, go full throttle to get as much speed as possible, then dump engines progressively as the air runs out, before dropping the jet stage entirely and switching to rockets instead.

But that system had the habit of exploding when firing the first rockets, because leftover jet fuel.

Viable, but not very practical...

Edited July 18, 2013 by Skorpychan

[zerotwo]

Kerboman.

There's no set answer to this question (without breaking down KSP's "physics" from programming), there are two guidelines:

At a certain altitude, there is no atmosphere thick enough for the engines to run regardless of intake quantity, "space".

At a certain altitude, drag is too high to make acceleration worth while. (so climb)

Two things keep a jet in the air: Lift generated, and thrust/air keeping the engines flowing.

For example, I had a 50t jet with 2 turbojets and two regular intakes. This aircraft could operate to approximately 19,500m and could accelerate to Mach 4.1 (KSP does not handle jets well).

I modified that jet up to 60t, with 4 turbojets, a ram intake, a radial air intake, and I increased the lift by about 20-35%. This jet could operate to 24,000m and accelerated to Mach 4.4, it also demonstrated significantly better climbing power, maintaining a 10 degree incline from T+5 until 18,000 m.

It was likely a combination of the additional air intake and lift that kept it operating at a higher altitude. One thing the SPH doesn't show you is quantity of lift. If the CoM and CoL match up, but the lift generated isn't enough to get it off the ground, you'll never know!

You'll fortunately (yay more ksp!) have to experiment to find out what combination of air and intakes works for you, but there are a lot of examples out there to go by. Realistically (just a guess), I don't see how anything other than an exploit-based jet could operate above 26 km, you'd have to start doing some crazy things to get it working up there, at that height you need to be looking at an orbital or sub-orbital swap i.e. changing from jets to rocket power.

From a thread by CaptainArbitrary:

Kerbin's atmoshpere:

sea level to 8.4km troposphere

8.4km to 14000km tropopause

14km to 35km stratosphere <- atmospheric density 0.1~ or lower starts at 14 km~

35km to 39km stratopause

39km to 56km mesosphere

56km to 63km mesopause

63km to 70km+ thermosphere

[numerobis]

For distance, you want to be flying high and fast -- so lift surfaces are irrelevant (unless you plan on landing again).

Realistically, with 2 ram intakes one turbojet you're at full throttle at 25 km, and near orbital speeds. This means you can be at almost the same speed at 30 km at throttle 1/e (about 1/3), at 35 km at throttle 1/e^2, and so on. Two intakes might not be quite enough to iterate this all the way to 69 km, you might need three or four.

Before complaining too loudly about 'sploits, note that the Mirage 2000 can be described as 2 intakes 1 jet.

Ram intakes are by far the best; all others have much more drag per unit of airflow they provide. The flatter intake has 25% less air for the same drag, the radial has 2.5 times less air for 10x the drag, and let's not talk about the nacelle.