Rocket tumbling in ascent

[jospanner]

Hi, just a quick question...

When performing an ascent and starting my gravity turn (~10km depending), the rockets I build tend to want to go end-over-end. I'm running FAR, but this happens with the stock aerodynamics too. I'm not exactly brilliant at the physics behind this, so I was wondering what I could do to avoid this?

-Jo

[Pecan]

1) Check the CoM in the VAB and make it lower.

2) Add SAS/RCS/Control-Surfaces to help steer the ship.

3) Make sure you haven't got too much thrust and aren't going or turning too fast, causing large amounts of drag.

4) Stop using FAR if that's a problem.

[Red Iron Crown]

In addition to the above, you could add more control surfaces to the bottom of the rocket. Also, with FAR it's essential that you turn gradually, and you would likely be better served by starting your turn much earlier ~1500m or so.

[Frannington]

Could this be as a simple as the upper stages being oversized for the boost stage they're sitting on?

[Zylark]

Screenshot(s) of your rockets would help to figure out the problem.

A few general tips:

- Always activate SAS before launch (press T)

- Wobble is trouble - Strut it up.

- Lower Center of Mass is better.

- If you got it, put a reaction-wheel as close to CoM as you can get away with.

- Gimballed engines helps a lot with stability - so don't overdo it with the SRBs...

[crparris]

reaction wheels help as well.

[Borogove]

1) Check the CoM in the VAB and make it lower.

This is completely backwards. In FAR (and reality), you need the center of mass forward, and the center of lift/drag lower.

The easy way to do this is to put fins/winglets at the base of the rocket (adding drag). This will likely fix OP's problem.

[Pecan]

This is completely backwards. In FAR (and reality), you need the center of mass forward, and the center of lift/drag lower.

The easy way to do this is to put fins/winglets at the base of the rocket (adding drag). This will likely fix OP's problem.

Well, yes and no. The big question is whether it's drag causing the thing to flip (which I mentioned in point 3) or the whole thing is so top/front heavy that it's being a lawn dart. In most rocket designs it's more likely to be the latter, I think, which is why I mention it first. In either case more control will help - which is why I mention control surfaces in point 2.

Meanwhile, Zylark made the perenial essential point - without pictures we're guessing.

[problemecium]

My guess (because this also happened to me) is that this is early in Career Mode when your rocket is comprised of a tall stack of small tanks. The top tanks are draining first, which moves the center of mass back and makes the craft aerodynamically unstable. The solution is either to add tons of fins to the back end, or if that doesn't help, turn off the top few tanks until the bottom ones start to run out so as to keep the center of mass at the front.

[Guest]

Well, yes and no. The big question is whether it's drag causing the thing to flip (which I mentioned in point 3) or the whole thing is so top/front heavy that it's being a lawn dart. In most rocket designs it's more likely to be the latter, I think, which is why I mention it first. In either case more control will help - which is why I mention control surfaces in point 2.

No, you're wrong. A top-heavy "lawn dart" is exactly what you want! Ever seen an actual dart? Weighed on the front, fins at the bottom... Notice that it flies tip first. That's how a rocket needs to be set up, too. A rocket needs to be stable, first and foremost (unlike an airplane, where there's such thing as "too stable"). IRL, the gravity turn is done exactly by tipping the rocket off balance and letting it tip over at the right rate. This can be controlled with gimbals, while to fight aerodynamic forces, you need a lot more torque (usually meaning a Shuttle-style high-range gimbal and enormous mass to go with it) and you risk ripping your ship apart in the process.

And yes, an oversized upper stage does tend to cause this kind of problems. Add fins at the bottom, the problem should go away.

[LethalDose]

In addition to the above, you could add more control surfaces to the bottom of the rocket. Also, with FAR it's essential that you turn gradually, and you would likely be better served by starting your turn much earlier ~1500m or so.

I can almost guarantee that this is the issue. In stock, you can climb to 10 km, then do a fast 45 degree gravity turn. In FAR, this will bone your assent. Start the turn (10-15 degrees) almost right off the pad, and work your way towards 45 degrees as you go up. This takes some practice.

Also I agree with everyone else who's saying Pecan is wrong about the CoM issue. You want your CoM forward, not back. You also want low drag at the top of the rocket, but if you're using fairings at the top and fins at the back, drag probably shouldn't be an issue.

Also pics help.

[tsotha]

I have this problem when I use stacked small tanks, engines without gimbals, and don't add SAS modules. You pretty much have to do one or the other (or both) unless your rocket is so small the capsule has enough juice to keep it straight.

[SRV Ron]

A picture of the rocket would help. Also, some designs will swap ends if going fast in the lower atmosphere, or if they are turned too fast in the lower atmosphere.

Example, the one on the left flies stable. The one on the right will swap ends after being staged without the fins. Note the payload that causes this issue.