Using FAR: tri-coupler spins my rocket

[Zwer.ch]

Hello all,

recently I started a new career game with FAR enabled, quite a different experience

Now I stumbled about a phenomenon where I'm a bit stuck (or just blind).

When adding a tri-coupler to a vessel, the VAB lift indicator shows a strong sidewards lift.

Consequently the vessel is starting to veer of its course soon after launch, which ends up in a wild spin and disassembly.

I put in some images into an album, maybe that helps a bit:

Javascript is disabled. View full album

Is this an intended behaviour? If so, what are the means to counteract this?

With respect to FAR, I don't understand why the tri-coupler posseses such a strong lift force in exactly one direction. Rotating the tri-coupler leads to the same lift indication

Any help appreciated

Edited September 11, 2014 by Zwer.ch

[Motokid600]

Hm.. the only thing I can think of is thar tri coupler is not an even triangle.

[Godot]

There can be several reasons

Do you try to change the attitude of the rocket so that it points out of the movement vector?

(the higher the angle of attack for the aerodynamic drag (which increases the more the rockets nose points away from the movement vector), the higher the chance that it will overcome the forces that try to keep your rocket stable, resulting in a spin out of control)

If yes, then try to be more careful with your attitude changes

Also:

Is the Triplet of rockets below the Tricoupler connected in regular intervals via Strut Connectors?

If not each of the 3 rockets may wobble (and therefore try to bring your rocket away from its course)

In this case use moar struts

Addendum:

2nd point answered as not being the case, by looking further at your pictures

That brings me to ... Point 1.

Also:

Put Tail fins to the lower end of your rockets (i.e. one for each of the 3 rockets of your Tricoupler) ... helps keep your rocket more stable.

Also, if this doesn´t help yet, put an Inline Reaction Wheel to your rocket

Edited September 11, 2014 by Godot

[dzikakulka]

First thing first, your vessel will not be stable if center of lift is ahead of center of mass. Change its geometry or compensate with surface below center of mass.

[Zwer.ch]

First thing first, your vessel will not be stable if center of lift is ahead of center of mass. Change its geometry or compensate with surface below center of mass.

Thanks for the heads up. I put additional winglets at the bottom so CoL is below CoM and I got this thing to orbit. It flies like a bucket of rusty nails, but it worked

I'm still wondering why the tri-coupler adds so much directional lift ...this does not feel right? The force of direction is always the same, no matter how I rotate the tri-coupler.

[Godot]

I guess the answer lies in the angle of attack ...

if your ships nose points direcly into your movement vector (and the air flows equally for all sides) your Tricouplers lift should be equal for all sides ..

but if your ships nose points into another direction, the strength of air flow along all sides of your rocket becomes unequal and the lift generalted by all sides of the Tricoupler uneven

[SRV Ron]

Far may also be affecting the struts which are not exactly parallel. That can cause a slow spin. Far or not, some designs have a habit of swapping ends at certain speeds due to the calculated center of pressure being ahead of the center of mass. The same swapping ends can happen with model rockets as well. Since we don't have a wind tunnel in KSP to test different designs, the only way to find out if it flies or not is the Kerbal School of Hard Knocks. Just put it on the pad and fly it.

[Hodo]

The other issue you are having is to much POWER!

In FAR with rockets ideally you want a TWR of 1.2-1.4:1 anything higher than that and you will accelerate way to fast.

[Taki117]

The other issue you are having is to much POWER!

In FAR with rockets ideally you want a TWR of 1.2-1.4:1 anything higher than that and you will accelerate way to fast.

In either case, even without FAR a TWR of >2 is horribly inefficient because you start running into Terminal Velocity which is just a waste of dV.

[dzikakulka]

Have you tried attaching stuff to tri-coupler with symmetry disabled? By the way, so we won't get confused - you shouldn't be thrown off by lenght/direction of CoL vector. It is usually (I mean unless you do fancy stuff with wings) horizontal and it is totally OK. The only unusual thing is that is off main vertical axis.

@Hodo

With FAR it is most efficient to aim at fastest ascent while retaining control and structural integrity Terminal velocity is reaaaaally high with sleek rockets, losses to gravity are much much bigger as long as you don't go supersonic in 10 seconds or something.

Edited September 11, 2014 by dzikakulka

[Hodo]

Have you tried attaching stuff to tri-coupler with symmetry disabled? By the way, so we won't get confused - you shouldn't be thrown off by lenght/direction of CoL vector. It is usually (I mean unless you do fancy stuff with wings) horizontal and it is totally OK. The only unusual thing is that is off main vertical axis.

@Hodo

With FAR it is most efficient to aim at fastest ascent while retaining control and structural integrity Terminal velocity is reaaaaally high with sleek rockets, losses to gravity are much much bigger as long as you don't go supersonic in 10 seconds or something.

Ideally you don't want to do that. A TWR of higher than 1.2-1.4:1 will lead to over acceleration at launch. My best rockets actually launch with a 1.25:1 TWR on launch and are pushing 2.2-2.8TWR by 12km.