Why do rockets jiggle and settle off-vertical when placed on launchpad?

[rocketman768]

I'm new to this game and I wonder if I'm missing something.

I had a perfectly-symmetric rocket I was trying to launch yesterday. The first stage was 3 boosters placed with the symmetry tool around an empty body section. When I went to the launchpad, I could see the rocket go downwards a bit, jiggle, then settle off-vertical with one of the 3 boosters slightly below ground. It was slightly different each time I reverted to launch, and once the rocket just completely tipped over and exploded.

I tried putting launch stability enhancers in 3-symmetry around the first stage. That kept the rocket straight on the pad, but as soon as they let go, I took off at a 70 degree angle with some minor rolling.

I solved it accidentally by reverting to VAB and relaunching (after removing stabilizers). Somehow it settled and launched straight.

So, why did it settle off-vertical?

[Snark]

It's been that way a long time.  Basically, what happens is that when you launch a rocket, there's no physics at all for the first few moments; then they turn on the physics, and there's some collision-detection finickiness that can cause behavior like what you describe.  Would love to see a fix for it, but I assume it's not trivial to do or they would have done it already.  It's been on a lot of players' personal wish list for a long time.

Anecdotally,  it seems to me that rocket design can make a difference-- it's been my experience that rockets that sit on multiple points (e.g. radial boosters) are more prone to this, compared with rockets that sit on a single point (e.g. the central stack's engine, with radial boosters not quite touching the ground).  YMMV.

If you're having a lot of problems with this, one way to address it is with launch clamps-- you can use them to hold your whole rocket so it's not quite touching the ground.  Generally you'll be rock-solid stable if you use this technique.  I very rarely use them myself, mainly because I don't seem to get bitten by this much (maybe it's just something about how I tend to design my rockets).

[kBob]

Did you move the rocket while in the VAB?  This happens to me sometimes I move it over from the center of the circle marked on the floor for whatever reason and then forget to move it back then when you launch and it will be offset from the center of the launch pad by the same amount.

[eddiew]

11 minutes ago, Snark said:

Anecdotally,  it seems to me that rocket design can make a difference-- it's been my experience that rockets that sit on multiple points (e.g. radial boosters) are more prone to this, compared with rockets that sit on a single point (e.g. the central stack's engine, with radial boosters not quite touching the ground).  YMMV.

This is my experience too. It's particularly bad if your fins contact the ground, and the whole thing may simply amble half way across the pad under strange torsional forces.

Kerbal Joint Reinforcement is probably useful for mitigating this problem, since it dials up the forces slowly on load rather than dropping them on at full power. If nothing else, the rocket will be less inclined to wiggle along its length 

[bewing]

Also, the tier 1 and tier 2 launchpads are not flat. Try driving a little rover across them, and you will see. IIRC the tier 1 launchpad is slightly humped in the center. The tier 2 launchpad has all kinds of holes and edges all across its surface.

As kBob said, be very careful about grabbing your ship and moving it around in the VAB. These days, I restrict myself to only ever moving my ship straight up and down with the scroll wheel in the VAB -- if you move your ship off center, it will be very tilted on the VAB and may well fall over.