Spaceplane losing control when air intake falls below 100%

[Desperado34]

Hi everybody,

For quite some time Ive been watching these forums with great interest and learned many things. Rockets hold no secrets for me anymore so I started building planes which are lots of fun. However Im currently stuck with a problem:

When building a spaceplane an issue that keeps comming back is that as soon as my turbojet engines start lacking air (air supply goes below the required 100%) my planes lose control.

Mods used:

- FAR

- Deadly Re-entry

- Pilot Assistent (doesnt seem to work well, so have made tries with and without it)

- Lots of other stuff that isnt related (TAC, Remote Tech, MJ, Flight indicators)

Planes used:

All kinds of and their FAR statistics seem ok. To eliminate the issue of me building bad planes Ive started flying with de Kerbodyne Initiale craft from Wanderfound: http://forum.kerbalspaceprogram.com/threads/90747-Kerbodyne-SSTO-Division-Omnibus-Thread?p=1626507&viewfull=1#post1626507

First I thought the sudden control loss was related to reaching a certain height. After lots of modifications to my own planes and different ascendance tactics I found out that the control loss happens only when engine intake air gets below 100%. In the many tutorials I read this problem doesnt get mentioned at all and people fly their planes up to regions of 30% intake air availability. What am I doing different/wrong?

Ascendance profile:

1) take off, steep climb to 10km

2) accelleration and further climb at a shallow angle to roughly 24km @ 1200-1300m/s

3) putting my prograde vector almost on the horizon to speed up while not losing height

4) not long after step 3 the air gets too thin. Somewhere between 26-28km the plane gets unstable: it drifts left/right or tries to roll. I see control options trying to counteract this, but even with counter steering from myself it quickly becomes impossible to hold the plane and I get into a stall.

For giggles a screenshot from two flight attempts when my plane is starting to lose control. The first with Pilot Assistent for stability control, the second one with FAR assist. Any tips are greatly appreciated.

Edited January 8, 2015 by Desperado34
Solved!

[Rodyle]

Right-click the engines when it is happening. Does either of them say "flamed out"?

[Dr-Drunk]

Turbojets need air to work obviously, so they won't work beyond 26.000M (the altitude your screenshot is at).

If you want to fly any higher and possibly even go into space, your plane is going to need non air breathing engines. You can either use rocket engines for this or engines that can switch modes such as the FAR Sabre engines or the stock Rapier. You could also use the Aerospike but it's highly inneficient in space.

So yea ... move over to engines that work on oxidiser. Ofcourse also make sure your plane actually has some of that LFO on board!

[Rodyle]

Turbojets need air to work obviously, so they won't work beyond 26.000M (the altitude your screenshot is at).

If you want to fly any higher and possibly even go into space, your plane is going to need non air breathing engines. You can either use rocket engines for this or engines that can switch modes such as the FAR Sabre engines or the stock Rapier. You could also use the Aerospike but it's highly inneficient in space.

So yea ... move over to engines that work on oxidiser. Ofcourse also make sure your plane actually has some of that LFO on board!

Not true. Given a decent amount of air intakes and speed, you can make them work (theoretically) up to the edge of space.

[RAINCRAFTER]

Not true. Given a decent amount of air intakes and speed, you can make them work (theoretically) up to the edge of space.

Yep, but that would require... AIRHOGING. And that's bad!

[Dr-Drunk]

Not true. Given a decent amount of air intakes and speed, you can make them work (theoretically) up to the edge of space.

Yea Air hogging used to be pretty good and would allow you to in theory fly up to nearly 69Km, practically an orbit. Thing is though, OP uses FAR wich has drastically nerfed Airhogging wich makes it impossible.

[Claw]

Also, engines will start to roll back when experiencing air starvation. They will enter a roll back state before completely flaming out. Given the way the current IntakeAir system works, one engine (most often the second one placed via symmetry) will roll back first, causing the aircraft to yaw. This can be insidious because you don't get the obvious "POP" and sparks. If your craft doesn't have enough control (or has a high TWR) to deal with this, then it will yaw and go out of control.

So based on your pictures, I would guess that the right engine is either placed second (if done manually) or is the mirrored partner (if placed with symmetry).

Check out this thread for more info about IntakeAir resourcing: http://forum.kerbalspaceprogram.com/threads/64362-Fuel-Flow-Rules-%280-24-2%29

And here is a thread trying to capitalize on that information, and reorganize intakes in an attempt to prevent asymmetric flameout: http://forum.kerbalspaceprogram.com/threads/100782-0-90-WIP-Intake-Build-Aid-0-4

Cheers,

~Claw

[steve_v]

You may also want to consider a bit more tail / vertical stabiliser since in the thin air at those altitudes aerodynamic stability can get a little dicey, especially with those draggy intakes (presumably) foward of the COM.

But yeah, asymetric thrust. From looking at your pics I would say your starboard engine is loosing thrust - throttle back more, move the jets closer to the midline (bicoupler?) or add yaw authority.

OP uses FAR wich has drastically nerfed Airhogging wich makes it impossible.

Not impossible, just counterproductive since with FAR the added drag from more intakes quickly overwhelms the decreasing thrust from jets at speeds >mach 3.5 or so. One ram intake per engine ought to be enough for most designs.

Edited January 8, 2015 by steve_v

[Maelstrom Vortex]

You have a thrust imbalance caused by dropping oxygen intake to one of the engines. Dropping your overall throttle causes both engines to consume less and brings them back in balance. Any jet which has 2 engines or more will have this problem even when very closely stacked. The only solution is to throttle back, or use one engine. You can delay this from happening until higher altitudes by using more air intakes, but only ram intakes and shock cones are very effective at above about 25K meters. Personally, I find this choking mechanism a good reason to justify only ever using one turbojet. The single turbojet will slowly drop off on its own without throwing you into the spin because the thrust isn't imbalanced. However, to max performance of a single turbojet you can still start cutting back on the throttle after around 30k feet to keep it from flaming out and prolong its ability to burn, just at a lower rate.

Other solutions are to use a mix of jets and rockets and once the jets start chocking turn off all but the centerline jet until your last remaining engine cuts out and then kick in the rockets to punch you out of atmosphere.

[Wanderfound]

As above: it's asymmetric thrust due to one jet flaming out before the other. If you want to run a jet in extreme low-oxygen conditions, it needs to be a single jet mounted on the centreline so the flameouts don't unbalance you.

For two jets, throttle back so as to keep the available air above 100%, and shut the jets down as soon as that is no longer possible.

[Desperado34]

Thank you for all the helpful replies! I didnt think of asymmetric thrust issues because the engines didnt suffer a flameout (they produce roughly 60-70KN of thrust each at the moment of control loss). Tonight I will try the tips to counter it and add some more vertical stability too. I will also attempt an flight with a single turbofan engine plane to double check if it's due to the assymetric thrust.

Just to be sure: Do I need to avoid dropping under 100% air intake with a multiple-turbofan to prevent assymetric thrust? Because I have seen pictures in tutorials where people (using FAR) went to very low air intake levels. For example Ive seen @wanderfound going to 54% (although assisted by a rocket engine) with a dual turbofan plane in his tutorial: http://i1378.photobucket.com/albums/ah120/craigmotbey/Kerbal/Tutorials/Hangar%20to%20Landing/screenshot334_zpseb061602.jpg

How do you manage that?

Once again thanks for the help, I will post the results tonight.

[Wanderfound]

Yes, you need to keep it above 100%. A single engine won't flame out until 30% or less, but they start to sputter and lose thrust as soon as it gets below 100%.

RAPIERs self-throttle to compensate, Turbojets don't...which explains the picture. To fly that plane at that altitude if you replaced the RAPIERs with Turbos, you'd need to throttle back so that the air stayed above 100%.

BTW, the Initiale was built specifically as an exercise in minimum-tech (nothing past Turbojets), and is somewhat compromised in performance due to that. If you've got access to RAPIERs or even just a better variety of wings and fins, there are some shinier options in the Kerbodyne range.

[sdj64]

In order to avoid asymmetric flameouts with two (or more) engines, don't use symmetry. Instead place them like this:

All of intakes on left side,

left engine,

all of intakes on right side,

right engine

Or with more engines, make sure you place an equal amount of air for each one, and place air, engine, air, engine, etc. in that order until you're done. If your intakes are placed correctly all of your engines should flame out all at once. Turbojets do auto-throttle to make up for lack of air at least in stock aero. Pretty sure FAR doesn't change the intake placement rule so you should be good to go.

[LordFjord]

The easy solution:

- click link in my sig

- download and install the mod

- load your craft in SPH

- hit F7

- try to fly your plane again and watch what's different. Chances are good that both engines flame out at the same time (or at least very close together).

Above, Claw linked to the thread where it is explained in detail why the flameouts happen this way. It's all about internal mechanics of KSP and how resource flows are handled - and how to make use of smarter part placing order.

You have no idea how many planes I have lost due to flameouts and that's the reason why this mod was created.

Good luck with going to space! Your plane looks as if it can make it.

[Hodo]

Yep, but that would require... AIRHOGING. And that's bad!

And airhogging doesn't work in FAR.

To the OP, your engines are suffering from asymmetrical airflow starvation problems. One engine is getting more air then the other, this is causing the one that is lacking air to create less power. This is most noticeable when your required air drops below 90%. As a general rule of thumb for my flights I dont let it drop below 110% before I switch over to rocket power.

EDIT- after looking at the picture closer, I also noticed you are going a bit slow for that altitude. Usually when I am up to 25km or higher I try to be going over mach 4.5 at that point. Because at 26km I am about to start my orbital burn which means I should be going at least Mach 5.

Edited January 8, 2015 by Hodo

[Desperado34]

Problem solved! The issue was indeed caused by assymetric thrust. When I switched to rockets at 105% intake air there was no problem at all.

Stubborn as I am I wanted to see if I could push the engines harder with proper balancing so I installed LordFjord's mod and added 2 vertical stabilizers to reduce the risk of unwanted yawing. The result is that I could push them to 75% intake air. Rather pointless as I started losing speed and altitude after 80% due to lack of thrust, but point is proven

As a double check I wanted to test a single turbofan design and as you guys predicted there was no control loss at all!

Thanks a lot for all the help. I just got a bit step closer to using spaceplanes in carreer games!

[Kryxal]

It's worth slowing your climb beyond 20k and continuing to accelerate. The higher your speed, the more air you'll get, and the higher you can go.