Unstable Rocket with Fins

[digger1213]

This rocket has been having stability problems on its 2nd stage.

I have checked everything that normally makes rockets unstable. The center of mass is way above the center of lift, I checked for clipping parts, I strutted everything together. It hasn't helped. Through making the winglets into an entire wing, I managed to make the 1st stage only just stable, however no matter what I've done, the 2nd stage is still unstable. I've moved the wing as far down as possible, but it still flips. I have no idea why, and would like some help. I don't know how to upload the craft file, so I cant do that, but I will if i figure out how to.

Thanks for any help. 

[Blaarkies]

Are you aiming dead prograde through the launch? Going 5degree off-center from the prograde marker causes flips with most rockets. (use the SAS-aim-prograde mode)
If you can shorten that fairing tower you should have less problems to begin with. A brute force fix is to install Vernor RCS ports at the bottom, pointing in cardinal directions. Launch with RCS enabled then this will help a lot

[Streetwind]

You are right, this is quite odd. It doesn't look like it should be unstable.

Could you do some screenshots with the aero forces overlay enabled (F11 I believe)? In a past version of KSP there was a bug where fairings would project their center of pressure well ahead of the actual model, resulting in a lever arm that could flip otherwise perfectly normal rockets. It should have been fixed ages ago... well, at minimum it will help you and us both see where the torque is coming from.

And on the topic of torque in general: are you sure it's aerodynamics that's flipping your rocket? And not, say, thrust torque? Your second stage engines look like they might possibly be susceptible to bending.

 

EDIT: another possible source of unexpected flip could be the payload inside the fairing either wobbling or clipping through the fairing wall, or otherwise being improperly shielded, causing it to catch the airstream and become a source of drag that cannot be seen from outside.

Edited April 19, 2017 by Streetwind

[Harry Rhodan]

Your payload is too long and light, your CoM way too far down. Never use the CoL indicator for rockets. It is telling you where the lift of the wings and winglets will be and not where aerodynamic pressure attacks.

[Spricigo]

Y@Harry Rhodan is correct.  With CoM this low is no surprise at all it flips. 

Also notice the CoL in VAB/SPH assumes perfect alignment with the airflow something you will never find in flight. 

[Streetwind]

...Oh, riiiight.  o__O  Pivot point. That was dumb of me. Remind me to not rocket science before I've had lunch...

[CSE]

There's some good advice here already.

If you don't mind a major redesign, you could try replacing the side-mounted boosters with a conventional in-line liquid first stage. Although there are some drawbacks - SRBs are cheap (in career) and a rapid ascent reduces gravity losses - there are also benefits. I've attempted to list these in order of importance.

• A larger fraction of thrust will be coming from gimballed engines, giving you more control effort.
• The gimballed engines, and any fins, will be further from the CoM, giving your control effort more leverage to turn the rocket.
• The ascent will be a little slower, costing some dV but reducing aerodynamic effects in the lower atmosphere.
• The rocket's moment of inertia will be greater, so all turning - including unwanted wobbles - will be slower.
• Stage separation will be straight backward so it's less likely to cause unwanted pitch movement.

[digger1213]

Thanks for the responses. I assumed that the center of lift and the massive wing fins at the bottom would be helping keep it stable, but I guess not . I've used the same fairing tower setup before for launching multiple relay probes at a time, and it used to always work, but I guess I extended this one too much. I modified the design, and no matter how ugly of a bulb it now has on the top, or how much more expensive, it's a lot shorter and therefore it works. Triple relay to Minmus ftw.

Thanks for the help!

[AeroGav]

11 hours ago, digger1213 said:

This rocket has been having stability problems on its 2nd stage.

I have checked everything that normally makes rockets unstable. The center of mass is way above the center of lift, I checked for clipping parts, I strutted everything together. It hasn't helped. Through making the winglets into an entire wing, I managed to make the 1st stage only just stable, however no matter what I've done, the 2nd stage is still unstable. I've moved the wing as far down as possible, but it still flips. I have no idea why, and would like some help. I don't know how to upload the craft file, so I cant do that, but I will if i figure out how to.

Thanks for any help. 

 

Digger, the problem is stock physics has two ways of calculating aerodynamic forces depending on whether the part is classified as a wing part or not.     Wings, fins , control surfaces have a lift rating and drag is calculated based on AoA, mach number, atmo density.    The blue indicator in the stock game only takes account of these parts.   If you make a rocket with no fins, there will not be any blue indicator at all.

Non wing parts are handled using the "drag cube" model, they can impart a lot of torque on your craft.

Fairings are much draggier than fuel tanks of the same diameter, especially 2.5m ones which are surprisingly slick. 

For the love of all things good,  please install this mod 

The blue indicator will take account of non-wing parts.   You can actually trust that if your blue indicator is behind your Centre of mass,  your vehicle will be aerodynamically stable, *even* if it is a rocket or an airplane with a long forward fuselage.

10 hours ago, Harry Rhodan said:

Your payload is too long and light, your CoM way too far down. Never use the CoL indicator for rockets. It is telling you where the lift of the wings and winglets will be and not where aerodynamic pressure attacks.

Harry, whilst this is true,  it would be much better for the OP to use CorrectCoL because it saves us having to invoke Voodoo and folklore.    Blue indicator behind Yellow indicator is the correct principle and people should trust in it, the problem is the unmodded blue indicator is faulty.  Get the blue indicator to take into account the aero properties of the non-wing parts of his rocket (especially the fairings, which behave like sails)  and he'll find out the centre of lift was much further forward than he thought it was.

 

 

[digger1213]

I already managed to fix my problem, but thanks for the ideas. That mod looks good, I might get it anyway

[Spricigo]

1 hour ago, AeroGav said:

Harry, whilst this is true,  it would be much better for the OP to use CorrectCoL because it saves us having to invoke Voodoo and folklore.    Blue indicator behind Yellow indicator is the correct principle and people should trust in it, the problem is the unmodded blue indicator is faulty.  Get the blue indicator to take into account the aero properties of the non-wing parts of his rocket (especially the fairings, which behave like sails)  and he'll find out the centre of lift was much further forward than he thought it was.

Mildly disagreement: Its our fault to take the CoL for the CoD (which is what we are really interested when building rockets). A blunt heatshield perpendicular to the airflow produces no lift but a lot of drag.

Granted, its nice to have a graph like this:

 

also