Every rocket I build likes to do flips: stability non-existent in 1.0

[Sacheverell]

Title summarizes the problem: I've built at least 7 different rocket designs - some from old templates that I know worked before, some brand new - and every single one of them has a massive stability issue. Right around 10k meters up from launch, the rocket will tumble end-over-end, no matter what I do, or change. I've checked everything I can think of: weight distribution, where thrust is actually being applied, RCS, no RCS, fins, no fins, solid boosters, liquid engines, every possible combo of thing I could think of.

I did have ONE rocket that mostly flew stable, but I can't figure out why it does, or why all the others do not. At first I thought maybe the rockets I was building with the AI pilot were wonky (and they still may be), but even manually piloted rockets have this problem. Any ideas?

Edited May 1, 2015 by Sacheverell

[quietghost]

Try adding some fins to the bottom of the rocket. If the center of lift (or center of pressure) is above the center of mass, then the rocket will be unstable. Also remember that the old method of ascending to orbit no longer works. If you try to pitch over to 45 degrees at 10km, you will lose control of the rocket no matter where the Center of pressure is. My method of ascending to orbit is as follows:

1. Vertical until rocket hits 65m/s

2. SLOWLY start pitching over the rocket, gradually adjusting towards horizontal.

3. During this process, manage the speed to be slowly increasing the entire time.

4. By an altitude of 17-20km, you should be 45 degrees off the vertical and going about 500m/s

5. Keep adjusting towards horizontal. Use your horizontal velocity to push your apoapsis up, rather than vertical velocity.

Hopefully this helps!

[Anquietas314]

Post screenshots of your rockets, but remember that 1.0 included an overhaul of aerodynamics; you need nosecones and such now. If you have fins they need to be behind the center of mass or they'll cause instability.

[WafflesToo]

What's your TWR on liftoff? If you're using the old 2:1 standard odds are you're overspeeding in the lower atmosphere.

What flight-profile are you flying? The NuStock aero doesn't like violent maneuvering so if you're doing the old "straight-up to 10km, pitch-over to 45" maneuver you'll tend to pitch-pole the rocket. You need to follow the prograde marker which means starting your turn earlier and doing it more gradually.

Do you have a bunch of greebles located high up on the rocket? Goo cannisters and such? That's going to push your center-of-drag in front of your center-of-mass and that could cause instability.

Without more details thats all I can think of as common mistakes. Good luck with your future flights.

[KSK]

Sounds like you're going too fast. Are you getting a lot of Mach effects (white blurry lines) at 10K? The turbulence is probably knocking you into a tumble.

Also - a quick example of a rocket that will get you to Munar orbit and back, although it's nowhere near enough for a landing.

Flies pretty well. LV-T45 plus fins for stability in the atmosphere. SRBs thrust limited to 75%. Launch using SRBs only, start your gravity turn once you hit about 200m/s and make it a very gradual turn - no more flying up to 10K and pitching over 45 degrees.

Edited May 1, 2015 by KSK

[Sacheverell]

Answering as many of these as possible:

Yea, I was doing the ol' "10k then turn" routine, and realized after a bit of looking around that the old way isn't flying anymore (pardon the pun). TWR, honestly not sure, but given what I'm hearing, likely way too high. Will look into that! Center of mass is pretty much centered, but I'm betting this has more to do with the new atmos not being a lake anymore, and actually being atmos. Will do a few more tests with this, and see what I come up with. Thanks for the feedback!

[KSK]

No worries - good luck!

[Sacheverell]

Right, I basically had to re-learn the entire game from scratch. Must've done something right though, because I finally put a station in orbit for the first time ever, even since beta! Has a lab, extra fuel, a bunch of crew space, and of course, docking. Thanks for the help, everyone.

[VegiPaddy]

I'm running into the same issue. I can't figure out how I have make a rocket that will fly straight. I've tried having the weight at the back (with the force), but then it just uses the light part up from like one "fin" and flips backwards. I've tried having the weight upfront (again with the force), but it's just not stable. And I can't get the weight evenly distributed, because the weight is dynamic, I can't have anything that will be even the whole flight. I only have a couple of sciences, so my options are limited, but I should still be able to get something into orbit. I've also tried fast, and slow. And basing a design off of KSK's Example will almost immediately flip over on me. I've even stripped all non vital components, so no unnecessary drag. Any help on the correct equation would be appreciated.

[MaxxPower]

Finally my method of ascending, that i used forever is the correct method!

[Wcmille]

Tumbling is caused by too much body lift above your center of mass.

Your choices are to:

1. Reduce that unwanted lift

2. Counteract that lift.

3. Move the center of mass higher.

1. Lift above the center can be caused by wide payloads. Not much to do here other than making the payloads smaller.

2. You can counteract the lift by moving your center of lift below the center of mass with fins at the bottom. The farther below the center of mass and stronger the lift, the more stable the flight.

3. Finally, rockets with long individual stages can start out stable but become unstable. This is because fuel drains from the top first, lowering your center of mass below your center of lift. You can pump fuel in flight (from the bottom of the active stage to the top of the same stage) to keep your center as high as possible.

[Clipperride]

Having the CoM too far back can cause problems too. Think of a dart or javelin, both have a weight at the front. As the rocket will rotate around the CoM (the old analogy of thinking of a model hanging from a piece of string at the balance point/CoM is helpful) if it is too far back any slight movement at the back from gimbeling engines, will cause a large movement at the nose. If your still deep in the atmosphere (say at 10km) the resistance hitting the edge of the rocket from this large nose movement can send you tumbling.

I'm rarely travelling at much more than 300m/s when reaching 10km. Try a lower TWR for the first stage, with fuel to take you a little above 10km. When you stage you can set your second stage up to really start to rocket along (sorry couldn't resist that one!). I also generally find my rockets behave better with the RCS turned off till 15-20km.

I'm lucky in a way as I have only played since KSP 1.xx came along and so haven't had to relearn anything with the new realism.

[nigelvn]

I'm lucky in a way as I have only played since KSP 1.xx came along and so haven't had to relearn anything with the new realism.

I do not completely agree upon that... I started playing in 0.23 and it took me about 100 to learn every aspect of the game. The relearning only involved in the athmosphere, outside stayed the same. 'relearning' only took me 2 hours

my advise to the OP is: keep trying, watching some youtube tutorials doesn't hurt either

[Wcmille]

if it is too far back any slight movement at the back from gimbeling engines, will cause a large movement at the nose.

This is actually what causes a problem when the engines are too *close* to the center of mass. The closer the engines are to the center of mass, the less they have to move to change the angle of the craft.

Try this simple test to prove it to yourself: build a simple, fairly tall rocket with 3 long fuel tanks of equal size. Drain all the tanks except the top tank for one rocket and the bottom tank for the second rocket. Put a nose cone on both rockets. The rocket with the high center of mass will fly nicely, with smooth, authoritative turns. The rocket with low center of mass will spin like a pinwheel as soon as you try turning.

[Empiro]

Too much mass at the front can also cause issues if your rocket is very long and starts to flex. It gives you a long lever arm to create torque, and may cause your rocket to spin out of control or break.

It's usually much less of an issue than if the rocket is too bottom heavy, however.

[xebx]

The rocket with the high center of mass will fly nicely, with smooth, authoritative turns. The rocket with low center of mass will spin like a pinwheel as soon as you try turning.

Very true

And use high vectoring engines (mammoth/mainsail)

add small wings or fins in the bottom to lower center of lift

add reaction wheels near the center of mass

AND heavy nose once more, fr ex lock upper tanks

(and don't launch a rocket with an active upside down probecore@wtfisdoingmyrocket)

This way impossible for a rocket to flip, do the contrary and it's 100% flip garanteed.

Edited September 23, 2015 by xebx