What does "max" Isp of Jet engines mean?

[Cesrate]

http://wiki.kerbalspaceprogram.com/wiki/Parts#Jet_Engines

7282 max

8192 max

Even higher than PB-ion, is that even achievable?

[ThePsuedoMonkey]

Maybe that's the elevation where they have their highest Isp on Kerbin.

[wilt57]

Is that really what it means? It's in conjunction with a value for Stall. I wonder if it doesn't mean at terminal velocity. The faster your jet is flying, the more air compression you are receiving from ram air effects, the less the compressor section of the engine has to work, the more the power the engine is generating can go towards thrust. At stall, there is less ram air compression, the more the compressor section has to work, the less the engine's power is converted to thrust.

[shand]

either way, there is a fundamental issue of if oxidiser is included in the ISP - since the oxidiser is the intake air, but i beleive the calculation just uses the fuel used? i may be wrong there, but that would explain the insane ISP.

[Anton P. Nym]

As I figure it, you're correct in that the free oxidiser from IntakeAir is what gives you the high I(sp). I(sp) measures how much total momentum the onboard fuel can transfer to the craft per unit of mass; IntakeAir isn't an onboard resource, so it doesn't factor into the equation.

This would be game-breaking without the jet's natural limitation; there is no IntakeAir in space. So it works in KSP as it does in real life; jet engines rule the skies, but it takes rockets to go above them.

-- Steve

[numerobis]

http://wiki.kerbalspaceprogram.com/wiki/Parts#Jet_Engines

7282 max

8192 max

Even higher than PB-ion, is that even achievable?

Those stats refer to 0.17 ; as of 0.18 there's no stalling, and no intake.

Jet Isp is a curve that goes from 800 on the surface to slightly over 2500 then down to 1200 at the edge of space. But you only carry the fuel, so you can multiply by 16 to find the effective Isp. But wait, it's more complicated yet: the thrust depends on your speed, so the numbers I gave are only accurate at 1000 m/s; at zero or 2km/s, you should halve the numbers, and it goes down to zero at 2.4 km/s.

Still, you're getting the equivalent of better than 10,000s for your launch.

Edited August 8, 2013 by numerobis

[arq]

As others have stated above, Jet ISP depends on altitude and thrust depends on speed. ISP includes the intake air so it is actually much, much higher (16x) if you only consider the fuel mass.

Taken from part.cfg files:

The basic jet has 2000isp at 1atm (0km), 1800isp at 0.3atm (6.02km), and 1000isp in a vacuum. It has 100% thrust at 0m/s, 20% at 850m/s, and 0% at 1000m/s.

The tubojet has 800isp at 1atm (0km), 2500isp at 0.3atm (6.02km), and 1200isp in a vacuum. It has 50% thrust at 0m/s, 100% at 1000m/s, 50% at 2000m/s, and 0% at 2400m/s.

The ISP values scale linearly (in pressure) between the marks I listed. I'm not certain, but I expect the thrust scales linearly in velocity between the values I listed.

[numerobis]

It is not actually linear, though you're not too likely to notice the error in between everything else going on. Right-click on a turbojet and you'll notice the Isp exceeds 2500 around 5km or so for a little while. Both curves are derived an AnimationCurve from Unity, which doesn't quite document how it computes its values. One presumes it's some kind of spline.