Space planes wings are shredded off

[Ker-boom]

Hello all,

Long time lurker, first time poster. So far, I could resolve all my questions myself.

I am trying to build a plane at the moment, which I have done often in the past. The twist is, I now have FAR installed (which everyone seems to think is great).

The weird thing is, it just shreds off the wings and the two side tanks when I pull up at 100 m/s (360 km/h, which is not a bad speed for a plane).

Looking at the indicators, I first get a "Large AOA", which means I am about to stall, but since I am fast enough, I am not stalling, and then I go into aerodynamic failure and everything false apart. I know for a fact that w/o FAR, this plane would "fly", it could even lift of vertically. But why is FAR eating it up?

From what I can tell, the main problem is that while I pull up, the plane continues horizontally, and therefore the wind pressure on the wings is too big. But that makes no sense, the wings should redirect the vector of the plane upwards.

Is there a bug in FAR that when you use gimballed engines instead of jet engines, it behaves more like a rocket and therefore the wings do not work properly?

Any help is much appreciated.

I also noticed now that, even without deadly re-entry, my rockets fall apart in the atmosphere when I come in unrealistically hot. The pressure is just too much then. I am kind of okay with that, since coming in with 2000 m/s or more, one would expect to be torn apart, although it was nice to be able to do that.

Thanks,

Ralph

Edited June 24, 2014 by Ker-boom

[Master Tao]

Since 13.2, FAR includes structural failures due to aerodynamic stress. You'll usually have a chance to see the "High Dynamic Pressure" warning before your plane disintegrates. To limit these you can:

• Go slower (less dynamic pressure)
  
• Go higher (less atmospheric pressure)
  
• Go easy on the inputs (Dynamic Control Adjustment [DCA] can help)
  
• All of the above
  

It's still possible to do ridiculous things like push a jet to Mach 2 at sea level.

Also, Kerbal Flight Data provides a direct readout of the dynamic pressure (Q), which has helped me a lot. When I pitch up too fast, though, nothing helps.

[TechnicalK3rbal]

-snip-

This.

Tenchars

[TotalNewb]

I don't use FAR, but I think that you get the "large AoA" alert, then for a split second you stall and the drag tears off your wings. I'm most

certainly wrong, though. Well, nevermind.

Edited June 23, 2014 by TotalNewb
Ninja'd

[Taki117]

First off, what is the purpose of your craft? If it's supposed to be an SSTO i suggest using a combination of both Jets and Rockets, and not just rockets. Master Tao hit the nail on the head though. I assume that this particular aircraft is a tad unstable and won't lift off of the runway (Judging from both your proximity to the ground in the first picture, and your extreme pitch angle in the second) I would check your CoM and CoL locations and adjust them as needed, also you really don't need a whole lot of thrust for an aircraft, 3 rocket engines is a tad overkill in atmosphere. A plane is...well...a plane, not a rocket, so don't try getting airborne and then immediately pulling up to 90 degrees, you will rip things apart every time. Instead pitch up slowly.

[Hodo]

I see several problems with that craft.

The first is everything Master Tao said.

Second you weren't going that fast but you "yanked" up on the nose in around a second. You went from just about 20deg nose up, to almost 90deg nose up. That is a change in pitch of 70+ degrees in less than a second, of course your wings and your craft is going to fly apart. You were traveling at 150m/s when you did it, which doesn't sound fast till you convert that to mph, and realize you made the change in direction at over 170mph!

I suggest turning on flight data in FAR so you can see what the atmospheric density is so you can plan your menuevers accordingly. It will be the number that gets larger the faster you go and will end in "pa" for units of pascal pressure.

One last thing, for those tank setups, you have to add a LOT more struts for them to be able to withstand any type of lateral G forces. On something that size I would have at least 5 sets of B9 struts along the top and 5 sets along the bottom. Why B9 struts? Because they are stronger then the stock ones.

[Xavven]

As others have said, your AoA (angle of attack) is too high. Your AoA is the difference between your prograde marker and your nose marker, which looks to be over 45 degrees in your second screenshot just before structural failure. For comparison, a lot of aircraft have a critical angle of attack (above which you are stalling) of around 25 degrees. I fired up DCS A-10C Warthog to see what it can do, and I get stall warnings at about 23 degrees at 200 KIAS, which is only about 100 m/s. The faster you go, the less AoA it takes to induce a high speed stall.

[QPDO]

I suggest that you use the flight data button in FAR. FAR also has a nice help button that explains all the AoA, Cl, Cd etc. You can also use Kerbal Engineer Redux to check the TWR. If I can spot it right, you use three K30 or K45 that might push your smaller craft to a TWR of over 3 ?

You could replace the outer rocket engines with jet engines and add some air intakes. the jets will also feed from the liquid fuel of the rocket tanks. The jets will give enough thrust and allow for a controlled thrust at start and low altitudes.

[Ker-boom]

Since 13.2, FAR includes structural failures due to aerodynamic stress. You'll usually have a chance to see the "High Dynamic Pressure" warning before your plane disintegrates. To limit these you can:

• Go slower (less dynamic pressure)
  
• Go higher (less atmospheric pressure)
  
• Go easy on the inputs (Dynamic Control Adjustment [DCA] can help)
  
• All of the above
  

It's still possible to do ridiculous things like push a jet to Mach 2 at sea level.

Also, Kerbal Flight Data provides a direct readout of the dynamic pressure (Q), which has helped me a lot. When I pitch up too fast, though, nothing helps.

Thanks for your input. I have a couple of follow-up questions:

1. Go slower: I do not want to, the question is, how can I change the plane to go faster - different wings? I fear I learned the original KSP "aerodynamics" too well, where delta wings are not an issue.

2. Go higher: Trying to. :-)

3. Go easy on the inputs: I think this may be the real problem. I actually am not trying to pull up THAT fast, but it does that anyway.

Some more remarks:

a) The plane is stable. I put the rear gears too far back so I cannot accidentally break the T-45 engines. I think I will replace them though with the 909s. This plane is supposed to be able to fly in non-ox environments, hence why the rocket engines instead of jet engines. I am just test driving it here.

It might become an SSTO, I am just experimenting with rocket driven planes.

c) Do I have too many control areas that I can change the orientation of the plane too quickly? Are the wings unusable for what I want? What would anyone guess is the lift-off speed for such a plane?

I suggest that you use the flight data button in FAR. FAR also has a nice help button that explains all the AoA, Cl, Cd etc. You can also use Kerbal Engineer Redux to check the TWR. If I can spot it right, you use three K30 or K45 that might push your smaller craft to a TWR of over 3 ?

You could replace the outer rocket engines with jet engines and add some air intakes. the jets will also feed from the liquid fuel of the rocket tanks. The jets will give enough thrust and allow for a controlled thrust at start and low altitudes.

Can you explain to me how the jets give a more controlled thrust at low altitudes? I was under the impression the main difference between jet and rocket engines is: ISP, the need for oxidizer, possibility of flame-outs.

As others have said, your AoA (angle of attack) is too high. Your AoA is the difference between your prograde marker and your nose marker, which looks to be over 45 degrees in your second screenshot just before structural failure. For comparison, a lot of aircraft have a critical angle of attack (above which you are stalling) of around 25 degrees. I fired up DCS A-10C Warthog to see what it can do, and I get stall warnings at about 23 degrees at 200 KIAS, which is only about 100 m/s. The faster you go, the less AoA it takes to induce a high speed stall.

Thanks, that is interesting. I am not trying to pull up very fast, and unfortunately, a search just now on DCA or dynamic control adjustment in Google came up empty - search only finds time controls.

Edited June 24, 2014 by Specialist290
Merging sequential posts

[Roastduck]

Can you explain to me how the jets give a more controlled thrust at low altitudes? I was under the impression the main difference between jet and rocket engines is: ISP, the need for oxidizer, possibility of flame-outs.

I've only dabbled in spaceplanes, but what I can work through is this: you want to be as fuel efficient as possible, which includes using a fuel optimized for whatever environment you're in. When you're in the atmosphere, you don't want to be burning LOX, because there's plenty of oxygen all around you. much more efficient to scoop that up in an intake and burn it with jet fuel, then switch over to LF/LOX when the air gets too thin

[DeMatt]

Thanks for your input. I have a couple of follow-up questions:

1. Go slower: I do not want to, the question is, how can I change the plane to go faster - different wings? I fear I learned the original KSP "aerodynamics" too well, where delta wings are not an issue.

2. Go higher: Trying to. :-)

3. Go easy on the inputs: I think this may be the real problem. I actually am not trying to pull up THAT fast, but it does that anyway.

You're trying to go too fast at too low an altitude and pull too tight a turn. Fix one (preferably two) of these factors and you won't tear your plane apart. My suggestion would be to make it a tail-sitter, so you take off and land vertically.

For comparison, the MiG-25 has a takeoff speed of <92 m/s, while the F-104 lifts off at <98 m/s.

Some more remarks:

a) The plane is stable. I put the rear gears too far back so I cannot accidentally break the T-45 engines. I think I will replace them though with the 909s. This plane is supposed to be able to fly in non-ox environments, hence why the rocket engines instead of jet engines. I am just test driving it here.

It might become an SSTO, I am just experimenting with rocket driven planes.

c) Do I have too many control areas that I can change the orientation of the plane too quickly? Are the wings unusable for what I want? What would anyone guess is the lift-off speed for such a plane?

For a conventional plane, with that tankage, I would say you have too little wing surface to permit aerodynamic flight. As-is, you're flying a rocket with really large fins.

Can you explain to me how the jets give a more controlled thrust at low altitudes? I was under the impression the main difference between jet and rocket engines is: ISP, the need for oxidizer, possibility of flame-outs.

Turbojets have reduced performance at low altitudes and low speeds. The reduced performance means they're gentler on your craft at takeoff, while still giving a lot of power at high altitude and speed.

Thanks, that is interesting. I am not trying to pull up very fast, and unfortunately, a search just now on DCA or dynamic control adjustment in Google came up empty - search only finds time controls.

... Xavven's "DCS" stands for "Digital Combat Simulator". So that would be a different flight simulator.

[Khatharr]

DCA is a setting available in FAR's in-flight tool. It adjusts your control input according to the environment so you're less likely to turn too sharply. Put a FAR module on the plane and it will be in the FAR window during flight.

[Ker-boom]

Thanks DeMatt,

That is very helpful information. I do have some mods installed, I think there even is an engine that can switch between vertical take off and normal thrust. Need to check if it is an airbreather.

I still have not found the DCA for KSP - is that a mod or in the stock installation?

Well, it is back to the drawing board for me.

-- Edit:

I found DCA now, thanks for the help.

I changed a few things, and it can sort of fly. With AOA help, it will never take off. With DCA on, it works a bit better. The direction of flight is still different from the heading, even after staying there for a long time. I think the tanks are really to heavy for these wings.

Cool, thanks.

Ralph

Edited June 24, 2014 by Ker-boom
Adding some more info.

[Ker-boom]

Sorry, double post.

Edited June 24, 2014 by Ker-boom
Double post

[Hodo]

I am seeing one problem with your design and your control setup. You have not set your control surfaces to assigned jobs. You currently have your LARGE wing ailerons as a elevator and an aileron, which is ok if you have a true delta wing. And you have moving tail elevators, which is giving you TO much control. Which makes the craft over control and fly apart. You also have no yaw control, no rudders at all on that aircraft. Your engines and SAS module are doing all of that work, which is bad.

I suggest you look at a real aircraft design and mimic the control setups.

[Alshain]

Some more remarks:

a) The plane is stable. I put the rear gears too far back so I cannot accidentally break the T-45 engines. I think I will replace them though with the 909s. This plane is supposed to be able to fly in non-ox environments, hence why the rocket engines instead of jet engines. I am just test driving it here.

Most SSTOs will use both Jet engines and rocket engines. The idea is that Jet engines are significantly more fuel efficient so you need to carry less weight. You ascend to just a little shy of running out of intake air, level off to build speed, then when you finally switch from jets to rockets (use action groups or RAPIERS, don't let your engines flame out on their own) you will have so much speed that you can pitch up and get into suborbit using very little fuel.

[Ker-boom]

Hello,

Could someone try out the FAR Hypersonic Screamer (stock, comes with the mod) and see if it wall fall apart quite easily for you as well? If I power the engine to maximum, it gets ripped to shreds quite easily.

I would like to know if there is something wrong with my FAR mod, or if it is that easy to destroy a plane in FAR.

I have also noticed that rockets can now achieve stall conditions during launch, which is quite odd.

Edit:

Here is what I mean. This is a Stock FAR airplane. I take it off, with AOA and DCA enabled, and climb so that I do not exceed Mach 1. But as soon as I hit Mach 1, say at 5000m (which is 15000 feet or so), it breaks apart around Mach 1. Note that the weird angle of the plane is because it already spun out of control, but heading and velocity vector were identical before. Could someone tell me if the plane works that way with them as well?

Edited June 24, 2014 by Ker-boom
Providing example

[Alshain]

Hello,

Could someone try out the FAR Hypersonic Screamer (stock, comes with the mod) and see if it wall fall apart quite easily for you as well? If I power the engine to maximum, it gets ripped to shreds quite easily.

I would like to know if there is something wrong with my FAR mod, or if it is that easy to destroy a plane in FAR.

I have also noticed that rockets can now achieve stall conditions during launch, which is quite odd.

FAR doesn't come with any parts.

EDIT: Oh it's a ship. Nevermind. I never install the ships, I like to build my own.

[Master Tao]

Looks like the Hypersonic Demon hasn't been updated since December 2012. It's also massively overpowered (TWR > 3). For me it broke up on descent from 45 km.

[Xavven]

Thanks DeMatt,

The direction of flight is still different from the heading, even after staying there for a long time. I think the tanks are really to heavy for these wings.

http://cloud-4.steampowered.com/ugc/558759332823355049/B238DDF2585B1FF5E8C9F92E1A14AD675A096ADE/

In order for a plane to maintain altitude, it does have to have some angle of attack. However, it could also mean that you need larger wings. As far as I know, the higher the AoA, the higher the lift, but also drag, until you reach a stall. So, wings that are too small will require you to have a high AoA until you gain more speed.

[ferram4]

@Ker-boom: Yeah, that's supposed to happen. The Hypersonic Demon is a lethal joke plane, really only there to show you what not-to-do. As for controlling it, as it goes faster it gets more and more pitch-up tendency due to drag from the wings which needs to be compensated with trim or hoping that SAS can handle it (strangely, this is one of the few planes that SAS can handle fairly well). If you do it right, the plane is capable of flying at Mach 2.3 at sea level, although even the slightest jerk will cause it to completely disintegrate.

Everything that you have been experiencing and showing is intended behavior, and FAR appears to be running quite accurately for you. The only issue you really have with your original rocketplane design is that it lacks sufficient wing area to take off at a reasonable speed given its mass; you'll either have to get rid of the second strip of tanks or add more wings to get that to take off at a low speed.

No issues here, only intended behavior.

[Ker-boom]

@Ker-boom: Yeah, that's supposed to happen. The Hypersonic Demon is a lethal joke plane, really only there to show you what not-to-do. As for controlling it, as it goes faster it gets more and more pitch-up tendency due to drag from the wings which needs to be compensated with trim or hoping that SAS can handle it (strangely, this is one of the few planes that SAS can handle fairly well). If you do it right, the plane is capable of flying at Mach 2.3 at sea level, although even the slightest jerk will cause it to completely disintegrate.

Everything that you have been experiencing and showing is intended behavior, and FAR appears to be running quite accurately for you. The only issue you really have with your original rocketplane design is that it lacks sufficient wing area to take off at a reasonable speed given its mass; you'll either have to get rid of the second strip of tanks or add more wings to get that to take off at a low speed.

No issues here, only intended behavior.

Ah, ok, good to know.

I did not realize it was a joke plane.

@Ferram4: is it possible to build a plane that can go to several Mach numbers, at least at higher altitudes? Would you have a plane design or do you know of one that can achieve that, that I can study?

I have to admit that I may have to remove the FAR mod. Yesterday, I was trying to land a rocket on Laythe, no wings or control surfaces, and as I was fine-tuning the landing spot at about 2km up, I achieved a STALL condition and the rocket tumbled out of control. It may be accurate, but it is a little too accurate at the moment. I did survive re-entry this time though. :-)

[ferram4]

Almost every example plane can get up to those speeds, so long as it uses turbojets, not the basic jet engine. You need to baby them around Mach 1, but past there they'll probably be fine.

And if your rocket ended up actually bringing up a "stall" warning in the GUI, you did have wings / control surfaces on it. High AoA will show up without wings, but if it actually told you something was stalling, you placed wings on there somewhere. And fine tuning a landing spot on an atmospheric body is not something you want to do in the lower atmosphere unless you've reduced your velocity significantly, since a very large number of landers and rockets become aerodynamically unstable at high angles of attack.

[LethalDose]

@Ker-boom: For reference, the turbojets and air-breathing RAPIERs on my SSTOs perform best between about 16 and 22 km. My speeds in this zone are usually in the range mach 2 to mach 5. I usually top out around 1500 m/s before kicking over to the LF-Ox engines, but air-breathing sub-orbital jets should behave the same up until that cut-point.

[Ker-boom]

So, I built a new plane today, and it worked a whole lot better. Oddly enough, it looks a lot like the FAR Hypersonic Screamer, but it handles a lot better. Biggest difficulty that I have right now is to align the center of thrust height wise with the center of gravity, otherwise it spins out of control at the edge of space. With a lot of trimming, I managed to keep it stable.

I honestly would not be able to tell you why this one works better now, no babying at Mach 1, no stalling, nothing. Yes, the wing area is increased, but I have a lot of fuel onboard again. Maybe those B9 parts have less drag than the stock parts or something.