1.0.1: pod,6x FL-100 fuel tanks, LV-T30 engine strictly vertical launch - 100% flip

[Mipe]

See title. I always get a flip at around 10-12k altitude, no matter what - with SAS or without, without tilting at all.

Alternatively, use 3 fuel tanks instead - it flips at 2k-3k elevation.

This was not the case in 1.0. What has changed in 1.0.1 that has rendered the basic stack rockets utterly, completely useless?

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It seems to flip over soon after mach effect appears.

[Red Iron Crown]

I suspect that your center of mass is getting behind the center of pressure as fuel drains from the top tanks first. As an experiment does it still do it if the top two fuel tanks are disabled?

[MalfunctionM1Ke]

Can we get a picture of the Rocket?

[LostOblivion]

Put some tail fins at the bottom of your rocket like IRL.

[Mipe]

As an experiment does it still do it if the top two fuel tanks are disabled?

That seemed to have delayed the flipping somewhat, however it still flips over soon after mach effect appears.

[Caelib]

I've noticed that a lot of my rockets early-on were very unstable if I started going too fast below 20km. Try throttle back when you start seeing vapor trails and see if that helps. And as LostOblivion stated, try adding some fins for stability.

[Mipe]

Put some tail fins at the bottom of your rocket like IRL.

I'm curious: why did such a rocket fly fine in 1.0, but always flips over in 1.0.1?

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Can we get a picture of the Rocket?

It's just a pod+parachute, the six small fuel tanks and a T30. Do you really need a picture?

[WhirlwindMonk]

I'm curious: why did such a rocket fly fine in 1.0, but always flips over in 1.0.1?

Probably because of this line from the patch notes:

* New values for physics global drag and lift multipliers.

[Thnd3r]

Sounds like your TWR is too high and you're accelerating too quickly through the lower atmosphere. Try lifting off at 50% or 75% thrust. You can speed up later once your at 20km.

[Mipe]

I suspect that your center of mass is getting behind the center of pressure as fuel drains from the top tanks first. As an experiment does it still do it if the top two fuel tanks are disabled?

Okay, I've experimented a little - with the 6 small fuel tanks, I disabled top three and let the rocket burn through bottom tanks first. It didn't flip after breaking the sound barrier. After I reenabled the tanks (with bottom dry), it still didn't flip over, as the white effect turned firey red.

Curious.

[Red Iron Crown]

I'm curious: why did such a rocket fly fine in 1.0, but always flips over in 1.0.1?

The aero model got some tweaks according to the 1.0.1 changelog, I bet that is the source of the difference. They also added some winglets to tier-0, so I guess those are supposed to be used to overcome it.

[Kerbol Macrosystems]

Aerodynamics change above mach 1 in the aero model. In the old one it used the same equation above and below mach 1. I like the change as it's more realistic. My fix has been to throttle back as soon as I start to see mach slip streams appear. As altitude increases I throttle back up to 100%, trying to stay just below mach 1 the whole time. This way I don't have to deal with the sharp drag increase in the supersonic regime. Just stay transsonic or subsonic.

[Mipe]

Sounds like your TWR is too high and you're accelerating too quickly through the lower atmosphere. Try lifting off at 50% or 75% thrust. You can speed up later once your at 20km.

So why was it working fine in 1.0?

[Sean Mirrsen]

It flips because of the center of mass shifting, and the rocket lacks any control mechanisms to prevent the turbulence from nudging it sideways and flipping it around.

Adding four Tier 0 basic fins to get some stability going fixed the problem completely. The test rocket (pod, parachute, 6xFL-100, LV-T30, 4xfins) fired at full throttle from the launchpad and went up straight like an intercontinental ballistic arrow. Reached 146km before falling back down.

(edit: to clarify, I did test the proposed craft first. Without the fins, it starts to drift as soon as the top two tanks are empty, and rapidly flips out afterwards.)

[LostOblivion]

As Caelib says, I've found with both (old) FAR and the new stock aero that going too fast in the thickest part of the atmosphere usually makes a rocket unstable.

[Mipe]

I see, so this is the normal behaviour and the 1.0 non-flippage was anomalous then?

[NoClass]

So why was it working fine in 1.0?

Because the aero model in 1.0 was radically simpler and less realistic. Most parts had a very simple drag model based on their mass, which made it much easier to control a rocket that wasn't pointed into the relative windline.

*edit* ninja'd

[rodion_herrera]

Even a little 5 year old boy knows that rockets don't fly straight without them fins...

When the OP first posted, this was my initial reaction--no fins? It was almost instinctive. Throw everything you know about earlier versions of KSP's virtually non-existent aerodynamics model--the new model "tries" to be realistic. Although it's not yet even perhaps 75% realistic, it would be a good suggestion to think more in real-world terms now, when designing stuff.

Also, MAX-Q and throt up/down. Should also be something instinctive.

Edited May 1, 2015 by rodion_herrera

[Mipe]

@rodion_herrera:

[Red Iron Crown]

Because the aero model in 1.0 was radically simpler and less realistic. Most parts had a very simple drag model based on their mass, which made it much easier to control a rocket that wasn't pointed into the relative windline.

*edit* ninja'd

Er, you're thinking of .90, not 1.0. 1.0 and 1.0.1 have the same aero model, just some of the values have been tweaked.

Edit: Mipe, that javelin would have stability issues going transsonic.

[Sean Mirrsen]

Because the aero model in 1.0 was radically simpler and less realistic. Most parts had a very simple drag model based on their mass, which made it much easier to control a rocket that wasn't pointed into the relative windline.

*edit* ninja'd

You... may be confusing the drag model in 0.90 and the drag model in 1.0. The drag models in 1.0 and 1.01 are almost identical.

[rodion_herrera]

Edit: Mipe, that javelin would have stability issues going transsonic.

You beat me to it. Shock waves.

[Melfice]

@rodion_herrera:

Javelin picture

And it takes considerable skill to not have it tumble arse over teakettle while in flight and still go far.

It also doesn't have to deal with the kinds of speeds and forces rockets have to deal with.

[Mipe]

Er, you're thinking of .90, not 1.0. 1.0 and 1.0.1 have the same aero model, just some of the values have been tweaked.

Edit: Mipe, that javelin would have stability issues going transsonic.

How would you know? Point taken, though. I realize now transsonic flight has different mechanism than subsonic.

[MalfunctionM1Ke]

Rebuilt your rocket, tested it and I think I got a Workaround.

Try to stay under 300 m/s until 7000m high up. After that you can go with full trhottle again.

I am guessing that it is the changes to the dragmodel they made with 1.01 that flips you over.