What is the reason why some rockes are tipping over

[WeirdCulture]

Hi Fellow Kerbonauts,

what is the reason why some rockets stays on course when the gravity turn begins, and some rockets, or better.. the prograde during gravity turn, start to tip over in horizontal direction?

Some of my rockets are going mostry straight in the direction where the nose is pointing, some of them seems to have a heavy head.

Is this TWR Related?

Cheers

WeirdCulture

[Streetwind]

Your center of mass and center of thrust are probably not aligned. This can happen if you place parts on one side of the rocket and not the other.

[cantab]

Maybe centre of mass far forward and insufficient control. Solution is add more reaction wheels, fins, or even RCS/vernors.

Maybe centre of lift far forward. Fins belong at the bottom of the rocket.

Maybe too much flex in the rocket is causing it to be difficult to control. Solution is to add a probe core further down, right-click it and choose "control from here" before launch. Make sure it points the right way, and be ready to switch ships after staging drops that core.

[Wanderfound]

Give us screenshots from the VAB with the CoM/CoL/CoT indicators on if you want a proper diagnosis.

[Tex_NL]

Apologies for stating a no-brainer but are you sure you've switched the SAS on every time? Without SAS fins, reaction wheels and RCS are useless as there is nothing even TRYING to keep things stable.

[SRV Ron]

Some payloads, the cupola as an example, are aerodynamically unstable during launch. They can cause your rocket to swap ends at certain stages of flight. Others can cause a swap if you execute an orbital turn too quickly.

Example, the rocket on the left flies stable to orbit. The one on the right will swap ends early in second stage flight without the stabilizing fins.

[Murph]

There's another snag to watch for, if you have radial-mounted stacks/boosters. If they are not strutted, the flex in their attachment can allow them to move slightly off alignment under thrust. If that happens, the mis-aligned thrust can cause a loss of control. I believe that it's worst when you have side-mounted stacks which use a vectored thrust engine.

[WeirdCulture]

Maybe centre of mass far forward and insufficient control. Solution is add more reaction wheels, fins, or even RCS/vernors.

Maybe centre of lift far forward. Fins belong at the bottom of the rocket.

could be that the finanace-department was to harsh with the cut down of the funding of the launch vehicle, no fins allowed, gimbals should work I should consider a management meeting for a budged rise

[impyre]

Assuming your center of lift, com, and structural integrity are good, (and especially if using "natural" gravity turns with aero mods like NEAR or FAR installed) it's probably thrust-related. When aerodynamics are working properly, essentially gravity pulls the nose down... but the effect is based upon the ratio between your vertical acceleration component and the acceleration due to gravity (obviously if you keep the nose straight up, it won't tip at all if it's aerodynamically stable). Basically, gravity pulls the same on every rocket... but the TWR of the rocket in question will describe its ability to fight back. I try to keep TWR around 2 during launch, but have had very successful launches with TWR as low as 1.5-1.6... if you go much lower than that, you'll need to wait until higher up before starting gravity turn (so as to build up more vertical velocity to compensate for the lack of thrust). This is a less efficient launch profile, and should be avoided. Clever staging can help manage TWR.

[NecroBones]

Pretty well covered already, but as a summary:

1. Center of Thrust and Center of Mass not aligned. This is easy to avoid by using symmetry for everything.

2. (most common for new players) Center of Drag is in front of Center of Mass. This is aerodynamically unstable, and the rocket will want to fly backward.

Using the icons in the lower left, turn on all three of the markers. Make sure the Center of Thrust lines up perfectly with the other two, in a line going straight up the middle of your rocket. Turn off the thrust marker, and compare the other two. Center of Drag must be behind center of Mass, for EVERY STAGE that will operate in the atmosphere. You can momentarily pull off the lower stages, to see if this is true at each stage. Remember, as fuel is expended, center of mass may change, so it's best to have some distance between the markers.

[biohazard15]

Assuming your center of lift, com, and structural integrity are good, (and especially if using "natural" gravity turns with aero mods like NEAR or FAR installed) it's probably thrust-related. When aerodynamics are working properly, essentially gravity pulls the nose down... but the effect is based upon the ratio between your vertical acceleration component and the acceleration due to gravity (obviously if you keep the nose straight up, it won't tip at all if it's aerodynamically stable). Basically, gravity pulls the same on every rocket... but the TWR of the rocket in question will describe its ability to fight back. I try to keep TWR around 2 during launch, but have had very successful launches with TWR as low as 1.5-1.6... if you go much lower than that, you'll need to wait until higher up before starting gravity turn (so as to build up more vertical velocity to compensate for the lack of thrust). This is a less efficient launch profile, and should be avoided. Clever staging can help manage TWR.

You do not want (and do not need) high TWR with FAR\NEAR. Especially if you also have DRE. TWR 1.5-1.8 is ideal, 2 is maximum. Anything higher than 2 means that your rocket is too powerful - which means it consumes too much fuel - which in turn means that its CoM will shift very quickly, and rocket will eventually lose control and tumble.

The "ideal" FAR\NEAR rocket is either:

- powerful first stage (surface TWR 1.5-1.8) with great dV (around 3000), or

- SLS-style: low-thrust first stage (TWR 1-1.2, maybe even less than 1) with SRBs, with total surface TWR 1.5-1.8.

[SRV Ron]

Something as simple as a broken strut can cause this;

When the problem was located, this design flew with no spin or other control issues;