Jet Engine Thrust and Atmospheric Drag

[Old Foxboy]

Hello and welcome to some Physics discussion.

As you all know, Thrust decreases with Altitude, and Drag should decrease, too.

However, Kerbal Space Program obviously doesn't make any differences between True Airspeed and Indicated Airspeed (TAS / IAS). So, no matter how high you fly, you'll always hit the drag wall at the same speed (for basic jet engines, this is about the speed of sound or 330 m/s).

However, since in lower density atmosphere, 330m/s Airspeed means you are actually flying faster than 330 m/s above the ground.

Now i don't have figured out yet if the Navball indicator really displays IAS when you set it to surface mode or if it indicates TAS. "Surface" means it should indicate TAS.

Anyone?

Edited May 5, 2015 by Old Foxboy

[BruceKnowles]

Darn fine question... It may well be a safe bet to presume that if IAS was calculated at all somebody would already have designed a pitot tube for use in atmospheric environments.

[panzer1b]

it defenetely compensates for decreasing drag at altitude, as i can always pull more speed with any given craft in level flight by going higher up.

[Red Iron Crown]

Thrust decreases with Altitude

That is not so for all real life jet engines, and I'm not sure it's so for KSP jets (haven't played with them enough).

However, Kerbal Space Program obviously doesn't make any differences between True Airspeed and Indicated Airspeed (TAS / IAS). So, no matter how high you fly, you'll always hit the drag wall at the same speed (for basic jet engines, this is about the speed of sound or 330 m/s).

However, since in lower density atmosphere, 330m/s Airspeed means you are actually flying faster than 330 m/s above the ground.

Now i don't have figured out yet if the Navball indicator really displays IAS when you set it to surface mode or if it indicates TAS. "Surface" means it should indicate TAS.

Anyone?

As I understand it, TAS = IAS in KSP, there is no wind. Unless you mean TAS as speed at the surface and IAS as speed through the atmosphere, in which case the speed readout in KSP is IAS.

[Old Foxboy]

it defenetely compensates for decreasing drag at altitude, as i can always pull more speed with any given craft in level flight by going higher up.

Sure you can? You can "pull" more TAS, but you can't raise your Airspeed beyond the drag limit. That's why craft go faster higher up in the atmosphere.

The "Surface" speed is TAS, since no matter how high you go, the respective Orbital Speed in level flight (same heading) is always the same. You can check it: fly 100 m/s at sea level. ~275 m/s Orbital speed when heading east from KSC. And it's the same at 5000 metres or at 10.000. However, you won't be able to maintain 100m/s in most craft at 20.000 because the Airspeed is WAAAY lower.

Now here's the real issue in KSP: With a standard jet engine, you get nearly exactly the same TAS at sea level as you are getting at 5000 metres. But that's not real. Blimey, it's not even real for propeller driven airplanes of WWII. Even with a propeller airplane you will reach higher TAS than at sea level. With any commercial jet aircraft in the world, you will easily reach higher TAS up to 10.000 metres. While this is earth atmosphere, Kerbin resembles it at a certain scale.

There are different limits to a flight profile of speed / altitude.

1. On the low end, it's the lift of the wings. Go too slow, and you are out of the game. This low end curve will start at stall speed at sea level and increase in TAS (not IAS!) the higher you go. You IAS stall speed stays the same, but the higher you go, the higher the TAS gets.

2. The second limit is the airframe integrity limit. If you pass beyond a certain dynamic pressure, it's game over. KSP models this in a neglectable way. The only thing you have to care about are high G-forces or heating. However, it's a major factor in aviation. Go beyond the red line, and you won't walk away from this landing. Now dynamic pressure is nothing other than an IAS issue. The higher you go the less dynamic pressure you will get at a given TAS, which means you can speed like a bullet and pull high AOA at high altitudes without breaking your craft.

3. So what is the limiting factor then? It's the third limit, which is the thrust vs. drag ratio. To go fast, you want to go up high. But somewhere there's the equilibrium between your engine's thrust and the remaining drag. That's your service ceiling.

Now in KSP, the jet engine's thrust/drag ratio seems to have the same value at all altitudes from sea level almost to it's ceiling. While it is perfectly clear you can't break the sound barrier at sea level because IAS = TAS at sea level, the IAS at 10.000 metres is way lower, and the drag is not much of a deal. But the jet engine in KSP loses his power too quickly. It behaves more like a prop engine.

My suggestion to make it feel more like a real jet would be to reduce it's overall power, but to make the power loss up to 10.000 less dramatic, so you could slowly accelerate to a maximum of 300 at sea level, and fly about 400-450 at 10.000 (TAS). Maybe you would even be able to make parabolic flights then up to those "report above xxxxx m at yyyyy" contracts.

[Azimech]

Totally support this.

But the jet engine in KSP loses his power too quickly. It behaves more like a prop engine.

A piston prop engine. One without a supercharger or turbocharger.

[Old Foxboy]

Well, i wouldn't say this. The problem is that it has too much excess power at sea level, but behaves correctly at higher altitudes. But i'd love to see the IAS / TAS.

Now seriously: While it is true a modern subsonic Jet engine (like on small subsonic Fighter Aircraft or a civil Jet) have nearly the same top speeds and ceilings as a Kerbal Jet, the structural limits are not correctly simulated in KSP by what i "feel". Maybe some aircraft nerds can tell us more.

[Radam]

If designing spaceplanes, you have to breach the sound barrier either by high twr or dropping down from 11km. When you are faster that that, drag is going down and engine thrust is going up (for rapier it maxes out at mach 3.7 at 1180kN...).

There is a drag curve vs mach inside shift f12 aero section. Its flat to 0.85 mach (0.5), then quadruples to mach 1.1 (2), and about halves by mach 2 (1.2).

[Old Foxboy]

That is not so for all real life jet engines, and I'm not sure it's so for KSP jets (haven't played with them enough).

As I understand it, TAS = IAS in KSP, there is no wind. Unless you mean TAS as speed at the surface and IAS as speed through the atmosphere, in which case the speed readout in KSP is IAS.

If there's no wind, TAS is equivalent to the speed above ground, not equivalent to IAS. IAS is the indicated airspeed, as "seen" by the pivot tube. Though there are many factors that influence IAS which cannot be taken into account in a game like KSP, IAS would be directly related to the dynamic pressure. So, using Mechjeb or something like that, you can see how this pressure differs dramatically with altitude while flying the same speed above ground (="surface" speed). It's as great as 2% per 1000ft, so at 33000 ft (10.000 m equivalent), you'll get 66% difference in speed readouts (oooops!!).

For flying, it's very important to have an IAS (or CAS) airspeed readout, so you know if you are near stall speed or near a critical speed limit. So, to simulate spaceplanes better which pass through very thin atmosphere, an IAS gauge would be imperative!

As for the thrust: Believe me: It decreases with altitude for a simple reason: missing speed (and that's IAS, not TAS, that's why i put up this topic)

- - - Updated - - -

If designing spaceplanes, you have to breach the sound barrier either by high twr or dropping down from 11km. When you are faster that that, drag is going down and engine thrust is going up (for rapier it maxes out at mach 3.7 at 1180kN...).

There is a drag curve vs mach inside shift f12 aero section. Its flat to 0.85 mach (0.5), then quadruples to mach 1.1 (2), and about halves by mach 2 (1.2).

To do this, the basic Kerbal Jet would need an afterburner. You can go beyond mach 1 in a dive, but it's not enough to break the drag barrier and accelerate further.

Edited May 2, 2015 by Old Foxboy

[NathanKell]

Check the atmCurve entries for jets. You'll find the decrease in thrust for decrease in density is not at all linear, indeed while all curves start at 1,1 (density is normalized to 1.0 = 1.225kg/m^3) up high the thrust multiplier at a given density may be 3x that normalized density or more. So jet thrust certainly does increase in relative terms, going higher.

[Old Foxboy]

Check the atmCurve entries for jets. You'll find the decrease in thrust for decrease in density is not at all linear, indeed while all curves start at 1,1 (density is normalized to 1.0 = 1.225kg/m^3) up high the thrust multiplier at a given density may be 3x that normalized density or more. So jet thrust certainly does increase in relative terms, going higher.

"relative terms". Now practically, for the basic jet engine, it goes down, because you simply aren't able to get the same IAS airflow into the turbine - that's what i was trying to say. The turbojets on the other hand can maintain enough thrust to keep the game going a little longer - up to the structural or heat limit, or up to the point where you simply run out of intake air

I believe the real problem hasn't shown up yet because you "only" need ~Mach 6+ to get to a reasonable apoapsis on Kerbin. On Earth, you'll need ~Mach 20+

So, a good addition to the spaceplane series would be an actual hypersonic SCRAM jet engine that can boost the plane to orbital speeds above what Kerbin needs.

[Rickenbacker]

As I understand it, TAS = IAS in KSP, there is no wind. Unless you mean TAS as speed at the surface and IAS as speed through the atmosphere, in which case the speed readout in KSP is IAS.

Indicated Airspeed = what you see on your gauge in the cockpit. Useful to avoid stalling and get a general idea of how fast you're going. Not affected by wind, but gets lower as the pressure decreases with altitude.

True Airspeed = airspeed compensated for pressure altitude. The main use of this is to prevent overspeeding, causing flutter and ripping the wings off . Not affected by wind.

Ground Speed = Your speed across the ground. Not particularly useful for flying, but very useful for navigation. This value IS affected by wind, but since there is no wind in KSP it doesn't matter.

That said, I don't think the gauge in KSP shows IAS - it would show zero in space, for one thing!

[NathanKell]

Rickenbacker: it displays TAS.

Old Foxboy: Uh, I think we're talking past each other. Because dynamic pressure is quite literally rho * v^2, and you'll note the mach curves are pretty quadratic This means there's rough proportionality with IAS, although again it's not linear (in particular, the density curve is a curve, not a line, and each engine's compressor handles escalating mach differently).

[Old Foxboy]

Let me come back to my main problem, to make clear why there is so much confusion when talking about speeds and jet engines.

A jet engine's thrust is very nicely defined in this Article: http://en.wikipedia.org/wiki/Jet_engine#Thrust

Though true airspeed (the speed with which a craft moves through the air) ist part of the equation, a major portion is the mass rate of air flowing through the engine, which is directly related to the dynamic pressure or, so to say, roughly what the IAS gauge on an airplane will measure.

This means: Given a constant true airspeed (=surface speed in KSP),when climbing, the available mass rate of air flowing through the engine is being reduced the higher you fly, resulting in less thrust. Period.

That's one of the reasons why i'd love to see a IAS gauge in KSP for planes. You get a lot more information that's important for piloting: Correct stall speeds, engine effectiveness etc. For now, i help myself with Mechjeb and a custom display which shows me dynamic pressure.

[NathanKell]

I can add IAS to AeroGUI if you want; it's already got mach and EAS and a bunch of other stuff... http://forum.kerbalspaceprogram.com/threads/117141-1-0-AeroGUI-v1-0-28-Apr

[Old Foxboy]

Nah, it's okay. Nothing i couldn't see with Mechjeb, too. I would love to have it as another reference in the Navball (Orbit/Surface/Target/IAS). TAS would be nice, but it matches Surface, since there's no wind. Groundspeed would be great, too, but that's nothing you need very often, and you get it with Surface speed if you fly level, which is mostly the case when you need it to navigate around, anyway.

But if you are into creating mods, i'd LOVE to see one mod i didn't discover yet: A graphical display of the flight envelope, like that: http://www.newbyte.co.il/products.html or like that: http://www.christiancurry.com/primary-flight-display/

http://niquette.com/books/chapsky/glossfly.htm (look for "envelope, flight")

With such a display one could easily figure the correct speed/altitude to keep any craft in it's optimum performance window.

I bet the difficulty would be to create a pop up window that shows the correct information for each craft.