About air intakes in general and the Panther in particular

[Hotel26]

I down-loaded and flew @TheFlyingKerman 's Kerbus A380-400 this evening and, following the instructions, made an awesome discovery about the Panther.  (Incidentally, I love the Kerbal Express Airlines -- Regional Jet Challenge.  Always something interesting, educational and/or useful!)

Whereas heretofore, I've always used the after-burner to climb to altitude where the dry mode won't work, I had concluded that the altitude limit for dry mode was somewhat less than 10km.  However, dry mode is able to run as high as 15km (although probably better at about 13km[*]).  The thing is that dry mode has a speed limit (somewhere in the 630-700 m/s bracket(?)).  If you're going super-fast on the afterburner, you'll flame out switching mode to dry unless you first slow to a speed at which dry mode can operate...

(Makes me wonder if there are some configurations in which wet mode is required to get to an altitude and speed at which dry mode can then sustain propulsion but could not have achieved alone...?)

OK, so here is my first question: what else I don't I know about this marvelous, marvelous engine???  Please fill me in on all there is to know about the delightful Panther (dry mode)!!  How high?  How fast?  Etc, etc?

Second question is about air intakes as they perform in KSP: why would you want to close one?  Is this only a drag-reduction measure (when the engine has been shut down)?  Or does deeper magic lurk within?

[*] Depending upon the aircraft, of course, but "as high as"...(?)

 

Edited January 2, 2019 by Hotel26

[Foxster]

6 hours ago, Hotel26 said:

Second question is about air intakes as they perform in KSP: why would you want to close one?  Is this only a drag-reduction measure (when the engine has been shut down)?  Or does deeper magic lurk within?

AFAIK the ability to close intakes is a left-over from previous functionality. I don't believe that it now does anything for drag and just chokes the intakes, which I'd say has limited usefulness. 

[bewing]

Foxster is correct, closing intakes does nothing positive whatsoever, and has not done for many versions.

And you have the main points of dry mode. It provides significantly less thrust, somewhat better efficiency at low speeds and low altitudes, chokes in the mid-600 to low 700 m/s range (depending on altitude), flames out at about 17.2km, it can supercruise many designs at up to 15km, and can take a very low-drag airplane supersonic quickly and efficiently.

So for small planes it's a really good mode for takeoff. You get up into the 700 m/s range and a decent altitude and then kick it with the afterburner. At max speed and altitude, full afterburner will still give you better fuel efficiency for a long-range trip -- but the afterburner will eat a huge amount of fuel to get you up to that speed and altitude if you don't/can't use dry mode.

 

 

[FleshJeb]

The pressure and velocity curves for the jet engines are really educational: https://wiki.kerbalspaceprogram.com/wiki/J-404_"Panther"_Afterburning_Turbofan

All of them side-by-side in the expando box: https://wiki.kerbalspaceprogram.com/wiki/Jet_engine

Good question.

[TheFlyingKerman]

11 hours ago, bewing said:

So for small planes it's a really good mode for takeoff. You get up into the 700 m/s range and a decent altitude and then kick it with the afterburner. At max speed and altitude, full afterburner will still give you better fuel efficiency for a long-range trip -- but the afterburner will eat a huge amount of fuel to get you up to that speed and altitude if you don't/can't use dry mode

Dry mode should still be more efficient at 9000s ISP vs. 4000s ISP in wet, provided your plane is low drag enough that you can cruise in dry mode at 600+ m/s at 9000m+...
FYI for 4000s ISP the Whiplash is better at everything.

 

19 hours ago, Hotel26 said:

(Makes me wonder if there are some configurations in which wet mode is required to get to an altitude and speed at which dry mode can then sustain propulsion but could not have achieved alone...?)

The Kerbus K-380-400 is a typical example. There is not enough TWR to go above mach 1 in dry and climb in dry mode.

[Laie]

20 hours ago, Hotel26 said:

(Makes me wonder if there are some configurations in which wet mode is required to get to an altitude and speed at which dry mode can then sustain propulsion but could not have achieved alone...?)

Of the Panther-planes I built, every one that could sustain a supersonic cruise in dry mode was also capable of breaking the sound barrier in dry mode. Depending on factors, this could take some time and involve a shallow descent, or sometimes even a not-so-shallow dive -- but each and every one could be brought across. Somehow. Eventually.

[Hotel26]

42 minutes ago, Laie said:

Depending on factors,

Pilots might actually (cerebrally) prefer airplanes that require some loving understanding to perform to the best of their abilities -- as long as they are not just plain treacherous.

"Flying is 99% boredom punctuated by 1% terror" -- Jebediah Kerman

Back in the old days, before ATC assigned hard altitudes, pilots climbed out to an initial altitude and then adopted a very gradual climb to a higher altitude over the cruise that accounted for the lightening fuel load the airplane had to bear.  I bet they looked forward to the next "shift" to "tune their game".  Now pilots just "request higher" when they're ready but have to execute it as an incremental step.

I imagine that those old dogs back in those days compared notes, route by route, load by load.  They were the astronauts (thinking pilots) before the space age and before the electronic autopilot, programmed in Toulouse.  They were the airborne Casey Jones of the railways and their employers must have highly valued them -- even before OPEC oil embargoes, 9/11 and increasing airline costs.

Maybe KSP isn't this exacting, but such surprises are the inspiration to continue to love flying.  The challenge here, anyway, is clear:

download the Kerbus 380-400 and fly it to your most efficient altitude/speed -- without using the turbo-chargers...

(it's all gonna depend on the curves in those graphs)

 

Edited January 3, 2019 by Hotel26

[bewing]

2 hours ago, TheFlyingKerman said:

Dry mode should still be more efficient at 9000s ISP vs. 4000s ISP in wet, provided your plane is low drag enough that you can cruise in dry mode at 600+ m/s at 9000m+...

Well, not really. The benefits of flying higher and faster counteract the efficiency boost. In the image -- flying 30% faster, so you reach your destination 30% sooner, while spending about 30% more fuel per second. So it comes out about the same in this case.

 

[TheFlyingKerman]

I flew the K-380-400 under question in both wet and dry mode. Flying east over the equator with prograde lock, the plane automatically levels at 15000m and 12000m respectively, from the panel the AoA is below 0.02 degrees in both cases, and the thrust and drag balance out.

In wet mode, cruising speed is 872.8m/s, burning 0.34 units a second.

In dry mode cruising speed is 627.4m/s, burning 0.17 units a second.

So wet mode only flies 3000m higher, with about 10% lower drag, and burns 2x the fuel for 1.4x the speed.

Tried another plane and the result is similar

Moreover the engines flames out at around 13200m at 922m/s so it is not very feasible to further save fuel by flying higher.

[bewing]

You've gotta fly a lot higher than that to get the fuel benefits of full afterburner -- at least 20km. You need the engine to be on the edge of choking.

 

[bewing]

Ah, one other benefit of dry mode that I forgot to mention. When you are screaming along at mach 2.5 and you realize that now you have to turn -- trying to turn at 20km altitude with full afterburners is a PITA.

So, you shut down your engine, start a slow descent, and start a very slow turn. After just a short time, you will be in thicker air (so you actually have some control authority), and dry mode will work. So you switch to dry, and go full throttle. Now it's easy to make your turn, and you will still be high and going fast when you are done. Then you hit the afterburner again. Works nice!