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My SSTO is spinning out of control.


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So I built this:

Album https://imgur.com/buClQbA will appear when post is submitted

The "S-95 "Decent" Torodial Nuclear Interplanetary Transport":

9mDYNTC.png

And I launched it:

keeXguL.png

And tried to get out of Kerbin atmosphere:

uOB67HR.png

But it spun out of control:

EEBLvMa.png

And I dunno what happened:

Sv1UO20.png

Help me?

Edited by Thorn_Ike
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Your engines are all thrusting below the center of mass, trying to impart a nose-up rotation. Down where the air is thick, the aerodynamic parts are keeping that from happening, but when you get up high where the air is thin, that force wins out over attitude control. Try adjusting the engine positions. 

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2 minutes ago, Vanamonde said:

Your engines are all thrusting below the center of mass, trying to impart a nose-up rotation. Down where the air is thick, the aerodynamic parts are keeping that from happening, but when you get up high where the air is thin, that force wins out over attitude control. Try adjusting the engine positions. 

Like front, or back?

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They're very useful for avoiding this sort of thing, center of mass should be VERY close to in line with center of thrust.  One way to do that might be to make the wings higher, so "below the wing" is closer to being on the same level as CoM

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1 hour ago, Kryxal said:

They're very useful for avoiding this sort of thing, center of mass should be VERY close to in line with center of thrust.  One way to do that might be to make the wings higher, so "below the wing" is closer to being on the same level as CoM

Unfortunately, the thing flipped out again, despite CoM and CoT being perfectly aligned.

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You also have about 14 tons of engines at the back,  and 2 tons of cockpit up front.     I am guessing that fuel is helping to weigh the nose down a bit, but as it gets used, there is a risk of the plane becoming tial heavy.

RCS build aid is a great spaceplane design tool, it shows you a red ball in SPH/VAB which indicates where your CoM will be when the tanks are empty.  It also shows the torque from underslung engines etc.

CorrectCoL is another good mod, it makes the blue CoL indicator more accurate by taking account of drag from fuselage pieces.

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10 hours ago, AeroGav said:

You also have about 14 tons of engines at the back,  and 2 tons of cockpit up front.     I am guessing that fuel is helping to weigh the nose down a bit, but as it gets used, there is a risk of the plane becoming tial heavy.

RCS build aid is a great spaceplane design tool, it shows you a red ball in SPH/VAB which indicates where your CoM will be when the tanks are empty.  It also shows the torque from underslung engines etc.

CorrectCoL is another good mod, it makes the blue CoL indicator more accurate by taking account of drag from fuselage pieces.

But I have infinite fuel on, and the problem still occurs.

14 hours ago, bewing said:

Canard wings have a lot more leverage to help with attitude control than elevons do -- since the elevons are usually relatively close to the CoM.

Does that mean I should redo the wings? They took a loong time.

Also, I can't use the elevons to stop the spinning. It's completely involuntary.

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2 hours ago, Thorn_Ike said:

Does that mean I should redo the wings? They took a loong time.

No, canard wings are the little contol winglets up near the nose of a plane, that do the same job as an elevon.

... And I'm sure those delta wings you built did take a long time.

 

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1 hour ago, bewing said:

No, canard wings are the little contol winglets up near the nose of a plane, that do the same job as an elevon.

... And I'm sure those delta wings you built did take a long time.

 

So that means the canards will try to stop the SSTO from flipping ? 

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38 minutes ago, Thorn_Ike said:

So that means the canards will try to stop the SSTO from flipping ? 

Yes. But it may also make things worse. :/

Thing moving in the air want to go heavy part in front, draggy part behing.  Your may force the heavy part to stay behing (reaction wheels, RCS, control surfaces)  but it become more and more difficult as drag increase.

You may use a cannard to try to stop the flpping, but, without adjusting other thing to make the craft more ballanced,  it increase the drag in the front even more. So, use it if you want, but move the main wing further behind and some of those heave engines further ahead.

 

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50 minutes ago, Spricigo said:

Yes. But it may also make things worse. :/

Thing moving in the air want to go heavy part in front, draggy part behing.  Your may force the heavy part to stay behing (reaction wheels, RCS, control surfaces)  but it become more and more difficult as drag increase.

You may use a cannard to try to stop the flpping, but, without adjusting other thing to make the craft more ballanced,  it increase the drag in the front even more. So, use it if you want, but move the main wing further behind and some of those heave engines further ahead.

 

However, the main wing is pretty far behind. And I want those Nerv engines to stay at the back. 

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If the problem is underslung engines,  then the plane should immediately come under control again as soon as the OP cuts the power.

@bewing  canards are fine but you are recommending more control authority to fight instability. This never works.  Same with RCS.   This will fail - aerodynamic forces easily overwhelm any RCS system you can fit.  By putting canards at the front, you are adding more lift to the front end, shifting CoL further forward when it is probably already too far forward. 

You can see this airplane is unstable even from this shot -

uOB67HR.png

The nose is above prograde, but you can see the pitch control has a fair bit of nose down input applied already, trying to stop the nose rising further.    On a completely stable aircraft SAS would be having to push the nose UP ,  to keep the nose from dropping back onto prograde.


Shows a red dot in the SPH/VAB which indicates where your CoM will move to when the tanks are empty.  Makes it much , much easier to build planes that don't flip out on re-entry.


 

CorrectCoL does two things.  Makes the blue CoL indicator in the SPH more accurate by taking into account aero forces acting on fuselage parts.  

 

Assuming that cutting the engines doesn't make the OP's plane controllable again,  it's got to boil down to either CG shifting rearward as fuel burns off or basic aerodynamic instability (CoL too far forward).  OP says he still gets the same problem when using infinite fuel cheat so that means it's just aerodynamic instability.   Maybe when he gets too high up for the Whiplash to make much power, their thrust vectoring is no longer able to stop the plane flipping.

I recommend sliding the wing backwards, cutting down the front part of the wing, or extending trailing edge/increasing sweep angle to move CoL aft.

When the CoL is well aft of CoM,  it can become difficult and draggy to hold the nose up with trailing edge elevons as they are close to CoM, and don't have much lever arm to work with.   Also,  they get the nose up by pushing the tail down, which is not good for efficiency,   canards contribute to overall lift when they pull the nose up.     Also,  trailing edge elevons are probably directly over the main landing gear legs, or close to them.   This can make it hard to take off, since it becomes impossible to push the tail down to get the nose up.    Again , canards still enable you to take off in this situation, though you could just reposition the landing gear.

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28 minutes ago, Thorn_Ike said:

However, the main wing is pretty far behind. And I want those Nerv engines to stay at the back. 

The point is:  the wing is not far behind the CoM. Maybe slight behind, but in no way  behind enough to compensate all the drag in the nose. Where is the geometric middle of the craft is irrelevant for aerodynamic stability.

You cant/don't want to change the nervs and the main wing? Ok. How about those other engines? How about changing the fuel tanks? If nothing can be changed the 'solution' become more drastic, we need to add things, things your craft don't need and will make it heavier and draggier  (IOW: things that make your craft worse. )

 

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3 minutes ago, AeroGav said:

 

  Also,  trailing edge elevons are probably directly over the main landing gear legs, or close to them.   This can make it hard to take off, since it becomes impossible to push the tail down to get the nose up.

That remark always annoys me*. I know that it is correct but the solution is so simple: place the landing gear so the plane is already with the nose a bit up when landed. Really, I don't even remember when was the last time I had a plane that needed pitch to take off.

*mildly, but it annoys me.

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38 minutes ago, Spricigo said:

That remark always annoys me*. I know that it is correct but the solution is so simple: place the landing gear so the plane is already with the nose a bit up when landed. Really, I don't even remember when was the last time I had a plane that needed pitch to take off.

*mildly, but it annoys me.

That's already done.

55 minutes ago, Spricigo said:

The point is:  the wing is not far behind the CoM. Maybe slight behind, but in no way  behind enough to compensate all the drag in the nose. Where is the geometric middle of the craft is irrelevant for aerodynamic stability.

You cant/don't want to change the nervs and the main wing? Ok. How about those other engines? How about changing the fuel tanks? If nothing can be changed the 'solution' become more drastic, we need to add things, things your craft don't need and will make it heavier and draggier  (IOW: things that make your craft worse. )

 

I've already added solar panels, a ladder, a drill, and many ore containers (thanks to Scott Manley's logic)

Edited by Thorn_Ike
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Adding more stuff will just make your craft heavier and therefore less efficient (but if you've got infinite fuel on, what's the point), the reasons for instability have already been covered:
Too much mass, (i.e. engines) at the rear, wings too far forward. Underslung engines and (draggy) wings above the centreline will tend to cause pitch up as well.
Where are the CoM and CoD markers in the hangar?
If you won't change the design to impart natural stability, the only other option is to brute-force it with gimbaled thrust and/or RCS. Aerospikes have no gimbal.
Sounds to me like you're more interested in aesthetics than function though...

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26 minutes ago, Thorn_Ike said:

That's already done.

I've already added solar panels, a ladder, a drill, and many ore containers (thanks to Scott Manley's logic)

 

Unfortunately, nothing of this solves your issue. Will you try to move some mass and drag or not?

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1 hour ago, AeroGav said:

@bewing  canards are fine but you are recommending more control authority to fight instability. This never works.

I disagree. Almost all airplanes have a small region of dynamic pseudostability around prograde, surrounded by a large region of instability. This is why flat spins happen in the real world -- the plane has gone beyond its stability zone into the instability zone. There are almost no planes in the real world that will recover from a flat spin all by themselves, which means that all of those real world planes are unstable to some degree. The only reason they maintain stability is by starting inside the stability zone and staying there through control inputs.

A plane with no control authority won't fly straight, and is therefore unstable. The only way you make any of your airplanes stable is by adding enough control authority to create that region of dynamic stability. Modern fighter jets are inherently unstable, and only fly because they have enough control authority, and fast enough automatic response, to keep the nose pointing straight. So it does work -- but only on a design that's close enough to being stable in the first place.

 

 

 

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I'm going to chime in because I experienced something similar to this with my heavy lifter.

FatStar2-1.png

My first incarnation had all of its engines mounted below the wing, similar to your design. This worked until it got to the thin upper atmosphere, and then the craft would flip upward. The centre of mass was slightly higher than the centre of thrust.

The third (or second-and-a-half) iteration, pictured here, has all of its engines and tanks attached with radial symmetry to the cargo bay and not to the wings. So there are engines above the wings and below them, and this aligned my centre of thrust with the centre of mass.

Interesting trick: You can attach tanks with 2 x radial symmetry to the bottom part of a cargo bay and the opposite tanks will 'attach' to the doors. You can use the move tool and then pretend they're attached to the wings I suppose; this craft abuses autostrut a lot. But that helped me line up the tanks, intakes and engines without needing to fiddle with the rotation tool.

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  • 2 weeks later...
On 10/23/2017 at 5:46 PM, Gordon Fecyk said:

I'm going to chime in because I experienced something similar to this with my heavy lifter.

FatStar2-1.png

My first incarnation had all of its engines mounted below the wing, similar to your design. This worked until it got to the thin upper atmosphere, and then the craft would flip upward. The centre of mass was slightly higher than the centre of thrust.

The third (or second-and-a-half) iteration, pictured here, has all of its engines and tanks attached with radial symmetry to the cargo bay and not to the wings. So there are engines above the wings and below them, and this aligned my centre of thrust with the centre of mass.

Interesting trick: You can attach tanks with 2 x radial symmetry to the bottom part of a cargo bay and the opposite tanks will 'attach' to the doors. You can use the move tool and then pretend they're attached to the wings I suppose; this craft abuses autostrut a lot. But that helped me line up the tanks, intakes and engines without needing to fiddle with the rotation tool.

Thank you SO MUCH, @Gordon Fecyk!

You have revived the craft!

Album https://imgur.com/71K8Oni will appear when post is submitted

Thanks also for your help, @Vanamonde, @Kryxal, @AeroGav, @bewing, @Spricigo, and @steve_v!

Here is the KerbalX link to fly it yourself!

https://kerbalx.com/Thorn_Ike/S-95-Decent-Interplanetary-Craft

Again, thank you!

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