There seems to be a bug with center of lift

[thereaverofdarkness2]

I am frequently having my rockets flip over in flight as if their center of lift is ahead of the center of mass, however the rocket reads stable in the VAB both with full and empty fuel tanks. Furthermore I have lots of fins to help stabilize it but it seems like I can't possibly get enough. What is wrong here? I can't figure out how this could possibly be realistic.

This is one of the strongest examples, but it's been happening on some very basic rockets that should fly very stable.

Spoiler

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[Plusck]

It's quite normal, since the top of that rocket is very light (according to the VAB).

The CoM being so far down the rocket, it's bound to want to flip. The top of the rocket is also very draggy, so it will tend to be hard to keep it heading prograde.

And since those are Reliants on the bottom, and non-controllable fins, you actually have zero control over the whole of your rocket other than the puny reaction wheels in the command pod. There is simply no way that rocket can do anything but flip, and no way to stop it from doing so once it starts.

P.S. are those empty fuel tanks at the top? Or the structural parts? More importantly, why on earth are you doing that? If you want to radially attach crew cabins, stick a full small fuel tank near the top with a nosecone on it, and lock it off to keep mass high until you're out of the atmosphere.

[thereaverofdarkness2]

That would work, but I still don't see how my design doesn't work. Regardless of how far back the center of mass is, the center of lift is considerably further back.

I've been experiencing this with rockets that have only an aerodynamic profile and a midway center of mass.

[Harry Rhodan]

3 minutes ago, thereaverofdarkness2 said:

That would work, but I still don't see how my design doesn't work. Regardless of how far back the center of mass is, the center of lift is considerably further back.

That's because it is not about lift but about drag/pressure at the top and the CoL indicator doesn't indicate any of that. Just ignore the CoL indicator on rockets and don't add multiple blunt surfaces to the top of the rocket.

[Plusck]

1 hour ago, thereaverofdarkness2 said:

I am frequently having my rockets flip over in flight as if their center of lift is ahead of the center of mass, however the rocket reads stable in the VAB both with full and empty fuel tanks. Furthermore I have lots of fins to help stabilize it but it seems like I can't possibly get enough. What is wrong here? I can't figure out how this could possibly be realistic.

This is one of the strongest examples, but it's been happening on some very basic rockets that should fly very stable.

The bit about "centre of lift behind centre of mass" only really applies to the longitudinal stability of craft which are aerodynamically stable in all other respects. In other words, if you have a pointy front, not much drag along the sides, and zero thrust, then having a centre of lift behind the centre of mass will ensure that if flies like an arrow through the air. It can't do anything else.

However, as soon as you start adding drag and thrust into the equation that changes. An arrow with a sponge stuck on the front, a lead weight near the back and flights behind the lead weight will have a CoL behind CoM, but will most definitely flip and fly back-to-front.

And your rocket here, with an extremely draggy front, is behaving like a sponge-tipped arrow.

I tested a variant of this design just now. Just adding nosecones made the rocket fly straight, even if those are empty tanks or structural fluselages at the top (which is still a bad idea, btw).

 

edit: actually that changes as soon as you add parachutes and high  thrust. It is stable without parachutes, but adding parachutes adds just a bit too much drag to stay prograde on its own, and this is worse if you exceed about 2/3 thrust. Which actually goes to show, again, that it is drag which is the problem. Your centre of mass is far, far, far behind centre of drag.

Edited May 1, 2016 by Plusck

[Pecan]

3 hours ago, thereaverofdarkness2 said:

...Regardless of how far back the center of mass is, the center of lift is considerably further back....

The back wants to lift more than the front.  The back seems to be above the front when it's falling out of the sky.  What's the problem?

Centre of lift and centre of mass tell you nothing whatsoever about the centre of drag and that's the great big sideways shove that'll make vehicles fail.

[thereaverofdarkness2]

Blunt surfaces may be an issue, but this is happening on rockets that are much more streamlined, as I already stated before.

When the center of lift is behind the center of mass, the rocket should tend to remain pointing forward. A blunt rocket may be buffeted and shake around, but it will still tend toward a forward orientation.

My rockets are tending toward a reverse orientation, as in, when they flip completely around, they become stable despite having a blunt surface on the rear. This can only happen if the center of lift is behind the center of mass. Now I know it's not a display bug in the VAB, there's no way the actual center of lift would be that far forward. It has to be a bug in the physics engine. A real rocket like mine would fly reasonably straight.

On 5/1/2016 at 8:58 AM, Pecan said:

The back wants to lift more than the front.  The back seems to be above the front when it's falling out of the sky.  What's the problem?

The back wants to "lift" more than the front. This means the back will orient toward the direction of airflow, and the front will orient against it. This is one of the most basic tenets of aviation science.

[FullMetalMachinist]

8 hours ago, thereaverofdarkness2 said:

When the center of lift is behind the center of mass, the rocket should tend to remain pointing forward.

As others have said, to an extent you have to ignore the CoL marker because it doesn't show the whole story. It only shows you the average center point for all lift forces. The problem is that lift is not the only aerodynamic force that's acting on your rocket. There's also drag, and lots of it. Unfortunately there is no stock way to visually represent where those drag forces will be centered, because it's very dependant on speed, atmo density, and angle of attack. 

8 hours ago, thereaverofdarkness2 said:

My rockets are tending toward a reverse orientation, as in, when they flip completely around, they become stable despite having a blunt surface on the rear. This can only happen if the center of lift is behind the center of mass.

(bold mine) This is not true, you're not taking into account drag, which acts on a point called Center of Pressure. The game doesn't show you this point (see above) so you have to infer it's location by looking at your craft. Even if you have lots of lifting devices at the back (and a corresponding CoL marker in the back) the CoP could still be far forward if you have lots of lightweight stuff at the front. This is exaggerated if that light stuff is really far in front of the CoM, because that gives the drag forces a longer lever arm to act on. 

8 hours ago, thereaverofdarkness2 said:

It has to be a bug in the physics engine.

No, there's not. 

[Plusck]

8 hours ago, thereaverofdarkness2 said:

 A real rocket like mine would fly reasonably straight.

No it wouldn't.

The VAB doesn't show drag. It also doesn't show body lift very well.

Your rocket isn't stable - and wouldn't be stable in reality either - because the mass is too far back. It might be stable as long as it stays perfectly prograde, but as soon as anything pushes it off prograde, you have drag forces pushing hard on - and easily moving - that light draggy (and body-lift-giving) front, while the momentum of the heavy rear keeps it going straight ahead. The combined forces are too great for the fins to compensate, so it flips.

Your rockets are stable flying backwards because the mass is now at the front. Heavy front end = stable.

Another way of thinking about this: try throwing a hammer. Guess which end has more drag? Guess which end stays resolutely facing prograde? You could put a ton of fins on the head of the hammer, and try to throw it handle first, it will always flip.

[thereaverofdarkness2]

8 minutes ago, Plusck said:

Your rockets are stable flying backwards because the mass is now at the front. Heavy front end = stable.

What about when it's empty? Then the mass is evenly distributed, yet the fins are at the back which will cause a high change in the center of lift, the center of drag, and the center of pressure. And how many times do I have to remind you, this happens in rockets that don't have a blunt tip? There is absolutely no explanation as to why this rocket should be stable flying backwards with low to no fuel.

 

I should add that KSP doesn't even model blunt vs pointed tips very well. It doesn't completely ignore them, but putting a nose cone on top of a rocket stack doesn't do much to stabilize it. If you don't believe me, try it yourself.

Edited May 6, 2016 by thereaverofdarkness2

[HebaruSan]

9 hours ago, thereaverofdarkness2 said:

Blunt surfaces may be an issue, but this is happening on rockets that are much more streamlined, as I already stated before.

 

2 minutes ago, thereaverofdarkness2 said:

And how many times do I have to remind you, this happens in rockets that don't have a blunt tip?

We can only react to the screenshots you show us. Right now, that's a blunt-tipped rocket with a rearwards center of mass.

[FullMetalMachinist]

9 minutes ago, thereaverofdarkness2 said:

What about when it's empty? Then the mass is evenly distributed,

Not quite, that will actually make things worse. The reason being that all the empty fuel tanks will now be light, and the engine still weighs the same. You actually can see that in the VAB. Just have the CoM marker showing, then right click on all the fuel tanks and take the fuel out. I'm sure you'll see the CoM move further and further down, which is bad for aero stability. 

Edit: I just realized, it seems a little like we're ganging up on you just to tell you all the ways that you're wrong. Not the case (at least for me). We really are trying to help you, but part of that is that you need to let go of some pre-conceived thoughts, because some of this stuff is pretty counter intuitive. 

Edited May 6, 2016 by FullMetalMachinist

[thereaverofdarkness2]

28 minutes ago, HebaruSan said:

 

We can only react to the screenshots you show us. Right now, that's a blunt-tipped rocket with a rearwards center of mass.

 

 

This rocket becomes unstable after separating the second SRB. The nose cone at the top makes zero difference. What matters most is speed: if I get it going fast enough, it flips over. If it's not going fast enough, it remains stable.

Edited May 6, 2016 by thereaverofdarkness2

[Pecan]

8 hours ago, thereaverofdarkness2 said:

...The back wants to "lift" more than the front. This means the back will orient toward the direction of airflow, and the front will orient against it. This is one of the most basic tenets of aviation science.

Ok, I'm completely at a loss now.
You posted this gameplay questions and posed it as a question.
You completely reject all the answers people have given you because you know different.
Good luck with that.

[HebaruSan]

7 minutes ago, thereaverofdarkness2 said:

 

 

This rocket becomes unstable after separating the second SRB. The nose cone at the top makes zero difference. What matters most is speed: if I get it going fast enough, it flips over. If it's not going fast enough, it remains stable.

Thanks, that helps. Several notes about that one.

First, again you've got very little control. Engine without gimbal, fins without control surfaces. The only thing that can bring the nose back to prograde if it diverges is passive aerodynamic forces (which aren't fully represented in game since it only shows center of lift, not drag) and the pod's reaction wheel.

Second, the dependence on speed is consistent with drag being the key factor, since drag depends on speed.

Third, that stack of fuel tanks is going to guarantee that your center of mass moves backwards more than it otherwise would in flight, as the top tanks will empty first while the bottom ones are still full. Using one big tank would keep the center of mass more stable while the fuel is partway expended.

Fourth, it's only got three of the smallest fins. The force they contribute is more in line with what you'd need to stabilize a smaller rocket.

Finally, this craft also has a notable source of drag: the flat edge of the 1.25m service bay attached to the 0.625m end of the pod. Both front and rear nodes are checked for smoothness, and some measurements have indicated that the drag penalty is actually worse at the back. Try moving the pod to where the nosecone is and keeping the middle parts of the rocket all the same radius.

At this point I agree with other commenters that a bug has not been demonstrated here.

[Plusck]

37 minutes ago, thereaverofdarkness2 said:

 

 

This rocket becomes unstable after separating the second SRB. The nose cone at the top makes zero difference. What matters most is speed: if I get it going fast enough, it flips over. If it's not going fast enough, it remains stable.

It's understandable what you're trying to do here - recover the materials science bay Jr. - but it's a notorious problem. Everyone who's ever played KSP has been there.

Going up, it's extremely light and it's adding a huge ball of fluff to the top of your rocket. The trouble is that if you put it under the command pod, it'll pose a huge problem for re-entry. The service bay is the same.

This is why the astronaut complex upgrade to be able to do EVAs is so vital: you don't need to recover the part itself, just the science in it, and that can be done on EVA by any Kerbal (you only get one report if it isn't a scientist, but then again you only get one report if you bring it home anyway).

Still, you seem to have perfectly understood the problem with re-entry stability: keep mass by the heatshield with light and draggy things above that. So it's really just exacly the same question on the way up: mass at the front.

One thing you could do to help here is to lock off the fuel and oxidiser on the top two fuel tanks until you get most of the way out of the atmosphere (say about 45-50km). That keeps the mass up front.

[Kerbart]

10 hours ago, thereaverofdarkness2 said:

Blunt surfaces may be an issue, but this is happening on rockets that are much more streamlined, as I already stated before.

When the center of lift is behind the center of mass, the rocket should tend to remain pointing forward. A blunt rocket may be buffeted and shake around, but it will still tend toward a forward orientation.

When the air hits straight on, those blunt surfaces to not generate any lift. So they're not counted towards the center of lift.

Once air starts hitting those surfaces off-axis, there will be a sideways aerodynamic component. If anything as a reaction to redirecting the air sideways, but I'm sure there's aerodynamically more to it. But that's moot: the most important thing is that there's a significant sideways f0rce coming from those blunt tops once your angle of attack is no longer 0.

To make things worse, those blunt tops are about as far away as you can get from the center of mass; so it will generate a lot of torque. Easily more than the fins (that would counteract said torque) that are extremely close to the COM (and thus cannot generate a lot of torque)

There's a lot of weight at the bottom; them rocket engines are heavy! Furthermore, tanks empty top to bottom (even SRB's, unrealistic as that is), moving the COM down as you get higher. 

That makes three factors:

• Angle of attack tends to increases as you get deeper into the pitch maneuver, resulting in greater aerodynamic forces
• Velocity increases, resulting in greater aerodynamic forces
• COM moves down, increasing torque from aerodynamic forces where you don't want it (the top) and decreasing torque where you do want it (the bottom)

As a result, the torque that will flip your rocket increases more and more into the flight, to a point where SAS and gimbal can no longer correct for it. At that point a Flight-induced Local Inversion Procedure (FLIP) takes place.

Solutions:

• Nose cones are your friend. Not even to reduce drag, but also to keep your design aerodynamically stable
• The basic fin and T1 winglet don't do a lot; don't expect miracles. If you don't have access to the AV-R8 winglet then there's only so much your designs will allow for
• Move your COM upward. Easier said than done, but that's really what it comes down to.

[NathanKell]

Lift is not the only aerodynamic force that provides torque. Simple as that.

[pincushionman]

Grab the root part and start rotating it in small increments. It still won't tell the whole story, but I bet the CoL indicator will start to shoot upards when you do that and the editor starts to show some of the body lift being applied.

[thereaverofdarkness2]

On 5/6/2016 at 10:08 AM, FullMetalMachinist said:

Not quite, that will actually make things worse. The reason being that all the empty fuel tanks will now be light, and the engine still weighs the same. You actually can see that in the VAB. Just have the CoM marker showing, then right click on all the fuel tanks and take the fuel out. I'm sure you'll see the CoM move further and further down, which is bad for aero stability. 

Edit: I just realized, it seems a little like we're ganging up on you just to tell you all the ways that you're wrong. Not the case (at least for me). We really are trying to help you, but part of that is that you need to let go of some pre-conceived thoughts, because some of this stuff is pretty counter intuitive. 

On the first rocket, depleting the fuel moves the center of mass forward. On the second rocket, it moves backward a tiny bit (not nearly far enough to reach the center of lift) before going forward again. Don't forget that the capsule at the top is also heavy, almost as heavy as the engine, and the fuel tanks are all toward the back.

I understand this stuff is counter-intuitive. I'm a veteran KSP rocketeer going back to before they re-textured the alpha game, and I play very nearly at Scott Manley's level. I understand how all the aerodynamic forces are supposed to work in the game. I also understand that it has bugs that create some very unrealistic situations.

On 5/6/2016 at 10:32 AM, Pecan said:

Ok, I'm completely at a loss now.
You posted this gameplay questions and posed it as a question.
You completely reject all the answers people have given you because you know different.
Good luck with that.

I'm simply pointing out that the answers I am being given don't match up with what I'm talking about. Excuse me for having some background knowledge on the subject, but I'm trying to get some good answers. I'm not being argumentative, I'm clarifying.

[thereaverofdarkness2]

On 5/6/2016 at 10:44 AM, HebaruSan said:

Thanks, that helps. Several notes about that one.

First, again you've got very little control. Engine without gimbal, fins without control surfaces. The only thing that can bring the nose back to prograde if it diverges is passive aerodynamic forces (which aren't fully represented in game since it only shows center of lift, not drag) and the pod's reaction wheel.

Second, the dependence on speed is consistent with drag being the key factor, since drag depends on speed.

Third, that stack of fuel tanks is going to guarantee that your center of mass moves backwards more than it otherwise would in flight, as the top tanks will empty first while the bottom ones are still full. Using one big tank would keep the center of mass more stable while the fuel is partway expended.

Fourth, it's only got three of the smallest fins. The force they contribute is more in line with what you'd need to stabilize a smaller rocket.

Finally, this craft also has a notable source of drag: the flat edge of the 1.25m service bay attached to the 0.625m end of the pod. Both front and rear nodes are checked for smoothness, and some measurements have indicated that the drag penalty is actually worse at the back. Try moving the pod to where the nosecone is and keeping the middle parts of the rocket all the same radius.

At this point I agree with other commenters that a bug has not been demonstrated here.

All good points, however there is little I can do to fix any of them at the stage in the game the rocket is built for. I have no control surfaces, no larger fuel tanks, I don't have a high enough part limit to put on more fins, and the only capsule I have is a cone. I can't bring the science home unless it is behind the capsule on re-entry because it is so light relative to the capsule that the capsule's weight will flip it around and take the heat shield away from the direction of movement.

I'm trying to find a solution that doesn't involve getting farther into the game. I wish to become good at getting to orbit on my second flight, not my fifteenth flight after I have spent a few hours picking up science in several of Kerbin's biomes. I've got to work with what I have.

 

Still, regardless of whether or not I could make the craft fly by rearranging the parts or adding higher tech parts, it doesn't change the fact that it should be stable as it is, albeit less stable than it should be with a higher tech build. The uneven surface will create a bit of drag and I could believe the rocket might not be able to remain steady, but it would not cause it to flip around and become stable pointing its engine into the wind. For starters, the back of either rocket will have a lot more drag than the front. Yet they are in fact doing exactly this. And that's why I can't accept any of the answers I have gotten thus far. You guys are ignoring this small fact.

On 5/6/2016 at 11:17 AM, Plusck said:

One thing you could do to help here is to lock off the fuel and oxidiser on the top two fuel tanks until you get most of the way out of the atmosphere (say about 45-50km). That keeps the mass up front.

Thanks for understanding what I'm trying to do here. Also good suggestion. I'm still not convinced my rocket should be unstable, but that should help make it work with KSP's buggy aerodynamics, anyway.

On 5/6/2016 at 11:23 AM, Kerbart said:

To make things worse, those blunt tops are about as far away as you can get from the center of mass; so it will generate a lot of torque. Easily more than the fins (that would counteract said torque) that are extremely close to the COM (and thus cannot generate a lot of torque)

There's a lot of weight at the bottom; them rocket engines are heavy! Furthermore, tanks empty top to bottom (even SRB's, unrealistic as that is), moving the COM down as you get higher. 

That makes three factors:

• Angle of attack tends to increases as you get deeper into the pitch maneuver, resulting in greater aerodynamic forces
• Velocity increases, resulting in greater aerodynamic forces
• COM moves down, increasing torque from aerodynamic forces where you don't want it (the top) and decreasing torque where you do want it (the bottom)

As a result, the torque that will flip your rocket increases more and more into the flight, to a point where SAS and gimbal can no longer correct for it. At that point a Flight-induced Local Inversion Procedure (FLIP) takes place.

The CoM is NOT that close to the engines, it is much more toward the center especially when the fuel tanks are partly or mostly empty.

If it's just what you say, then the exact same thing would happen once the rocket is pointed the other way: it would also be unstable, and as soon as it turns a bit off center, it'll be swept sideways. End result: it would flip around uncontrollably.

But that's not what's happening, as I have stated plenty of times already. Once it has flipped around with the engine at front, it becomes fully stable and refuses to turn very far away from that orientation. It doesn't buffet in the wind at all. It's like the rocket engines are a nosecone.

Edited May 10, 2016 by thereaverofdarkness2

[HebaruSan]

5 minutes ago, thereaverofdarkness2 said:

All good points, however there is little I can do to fix any of them at the stage in the game the rocket is built for. I have no control surfaces, no larger fuel tanks, I don't have a high enough part limit to put on more fins, and the only capsule I have is a cone. I can't bring the science home unless it is behind the capsule on re-entry because it is so light relative to the capsule that the capsule's weight will flip it around and take the heat shield away from the direction of movement.

Understood, but limited parts or a choice to design for re-entry of extra parts does not mean there's something wrong with the aerodynamics. When claiming there's a bug, it's important to have a simple, clear example to rule out alternative explanations. Do you still flip if you fix the problems that people have been posting, maybe in sandbox mode?

5 minutes ago, thereaverofdarkness2 said:

the fact that it should be stable as it is

it would not cause it to flip around and become stable pointing its engine into the wind

the back of either rocket will have a lot more drag than the front

It seems that your support for these claims is mainly your own intuition, which is not shared by those you're talking to. It's possible that it should not be stable as it is, or that it should be flipping around, or that the back of the rocket doesn't have as much drag as you think. The only way to know is to run tests to pinpoint the cause, and this is where fixing the issues people have pointed out comes in. Maybe the F12 aerodynamic arrows would also be helpful to see what's going on.

[Plusck]

46 minutes ago, thereaverofdarkness2 said:

Still, regardless of whether or not I could make the craft fly by rearranging the parts or adding higher tech parts, it doesn't change the fact that it should be stable as it is, albeit less stable than it should be with a higher tech build. The uneven surface will create a bit of drag and I could believe the rocket might not be able to remain steady, but it would not cause it to flip around and become stable pointing its engine into the wind. For starters, the back of either rocket will have a lot more drag than the front. Yet they are in fact doing exactly this. And that's why I can't accept any of the answers I have gotten thus far. You guys are ignoring this small fact.

Thanks for understanding what I'm trying to do here. Also good suggestion. I'm still not convinced my rocket should be unstable, but that should help make it work with KSP's buggy aerodynamics, anyway.

Thanks for that - and yes it should help.

However, it isn't an example of buggy aerodynamics (I think) because there is no doubt that they should not be stable as they are. The tops of your rockets have a lot of drag - in both cases. This isn't shown by the CoL marker. The centre of mass is far down the rocket. All torque forces will apply around that centre of mass. And you have a substantial lever arm between the draggy top and the centre of mass.

Throw an object - any object - into the air and see how it flies. Without exception, the heavier end will end up coming down first. This is a problem for rockets due to the weight of the engines, and in KSP this is only just countered by the weight of the command pod, SAS and fins at the back.

 

On the other hand, the problem shouldn't really be all that bad for the second rocket. I managed to get the first rocket to fly alright with nosecones and 1/2 thrust - so drag forces were never too great.

I just tried the second one. With 4x symmetry for the fins on the last stage, it was fine. With 3x symmetry, it wasn't quite enough to cope with full thrust for the final stage, when the heating effects were at their strongest.

However, I certainly didn't find it resolutely facing engine-first into the airflow. The fins are weak, but not that weak.

Edited May 10, 2016 by Plusck

[thereaverofdarkness2]

3 minutes ago, HebaruSan said:

Understood, but limited parts or a choice to design for re-entry of extra parts does not mean there's something wrong with the aerodynamics. When claiming there's a bug, it's important to have a simple, clear example to rule out alternative explanations. Do you still flip if you fix the problems that people have been posting, maybe in sandbox mode?

It seems that your support for these claims is mainly your own intuition, which is not shared by those you're talking to. It's possible that it should not be stable as it is, or that it should be flipping around, or that the back of the rocket doesn't have as much drag as you think. The only way to know is to run tests to pinpoint the cause, and this is where fixing the issues people have pointed out comes in. Maybe the F12 aerodynamic arrows would also be helpful to see what's going on.

Being able to fix it by changing the design doesn't mean that the current design isn't bugged. If it should work, and it doesn't work, then it's bugged.

I'm sorry people can't see what I'm talking about. I'm trying to make it as clear as I possibly can. If drag is what's making it unstable, it would be unstable when flipped around. However it is not unstable when flipped around.

I'll try using the aerodynamic indicator and see what I can figure out.

[HebaruSan]

9 minutes ago, thereaverofdarkness2 said:

Being able to fix it by changing the design doesn't mean that the current design isn't bugged.

By the same token, being able to demonstrate something that might be a problem with a design that has clear flaws that are known to cause that problem doesn't mean there's a bug.

9 minutes ago, thereaverofdarkness2 said:

If it should work, and it doesn't work, then it's bugged.

And that's the rub. How do we determine whether it should work? You're just asserting that it should. Others are explaining why it shouldn't (center of mass is too rearward in both cases).

9 minutes ago, thereaverofdarkness2 said:

I'm sorry people can't see what I'm talking about. I'm trying to make it as clear as I possibly can. If drag is what's making it unstable, it would be unstable when flipped around. However it is not unstable when flipped around.

If drag is what's making it unstable, then it would be unstable whenever the center of mass is toward the back. If flipping around moves the center of mass toward the front, then it could stabilize in that orientation. That's what appears to be happening in both of the examples so far.