why isn't this rocket stable?

[king of nowhere]

I am an experienced player, i have launched hundreds of rockets, i have completed several challenges. i know about aerodinamic stability. and now i'm completely baffled.

as far as everyone says, for a rocket to be stable, the center of mass must be in front of the center of drag. heavy part forward, draggy part backwards. and this is the case here, the vab indicators clearly show that i have a center of drag in the back.

Except, somehow i don't. this rockets flips. fast. at around 350 m/s, it tumbles. i tried to increase the number of fins, didn't help. didn't even make a difference in how fast this thing was flipping. i put swivel engines to help stability. helped moderately. i tried to launch straight up, which helps for an unstable rocket. no good.

and in all this, i have no idea why the rocket is misbehaving. I mean, judging by the controls, i could swear that it has a center of drag in front. Except that the vab is telling me otherwise. but ok, the vab can be wrong for complex shapes. that's why i added way more fins than necessary. but still it makes no improvement. at this point, even assuming i really have aerodinamic instability, i wouldn't even know what's causing it or how to fix it. i mean, look at that rocket, it's a perfectly common asparagus stack. even without seeing any indicator, this shape should be stable.

i assumed some of the problems were caused by the open cargo bay (i need that for some kerbalism experiments). but i tried closing it, and no improvement.

at this point i want to know what i'm missing.

[linuxgurugamer]

Craft file?

[Vanamonde]

Looks like it's a lack of control force.  You've selected fins, engines, and capsule variants which do not contribute to steering. Try Swivel engines rather than Reliants, pivoting fins, and put a reaction wheel module on it. 

[king of nowhere]

19 minutes ago, Vanamonde said:

 Try Swivel engines rather than Reliants,

i already have 3 swivels. as for the rest, not yet unlocked

[bewing]

The vostok pods have no reaction wheels. Once your swivels decouple, you will have no direction control whatsoever, so it's gonna crash at some point, no matter what.

But the thing you are probably getting messed up by this time is drag. If your rocket flips, it means the front end is too draggy for your control surfaces/devices to overcome.  I don't know which part is causing the worst of the drag. It used to be that the vostok pods had extremely low drag, but they got modified by hand -- so I don't know how bad they are now.

Unlocking basic aerodynamics would get you tailfins, which are some of the best control surfaces. But they only work up to maybe 25km (while there is enough air for aerodynamics to have an effect).

[king of nowhere]

1 minute ago, bewing said:

The vostok pods have no reaction wheels. Once your swivels decouple, you will have no direction control whatsoever, so it's gonna crash at some point, no matter what.

 

yes, i realized that later. when i was already in orbit. good thing i managed to use the terrier engine to turn the ship around, so i could salvage the mission; i was surprised by how efficiently you can use a gimbaling engine for stability.  unfortunately, those crew pod are the only ones with a decent habitability yet in the kerbalism mod, and i haven't unlocked most techs that would let me go around the problem. i never wanted to use those command pods in the normal games because i know they are draggy,

i did manage to launch, by the way, by making sure that i never got above 300 m/s while in the low atmosphere. of course i had huge gravity losses this way, so the mission, who was supposed to be a mun orbiter, had to stop at mun intercept and come back.

[Spricigo]

5 hours ago, king of nowhere said:

at this point i want to know what i'm missing.

CoM too far behind. Drag bits on top of the rocket have a  much longer level arm than the drag bits on the bottom, thus producing a bigger torque. 

 

[JoeSchmuckatelli]

Bigger fins at the back will help.  But - if they are attached to your SRBs - and you detach them while still in the atmosphere - you lose the drag you were hoping for.

 

If you have ANY reaction wheels - use them; hugely beneficial for Neandertal Spaceship Makers  -- I put mine near the pod so I don't lose them

[BigFatStupidHead]

That command pod is a big problem. The Onion series have massive drag and cause all sorts of rocket flipping. If you were to put any MkX pod or cockpit on there, I'd expect your rocket would fly straight.

[Kerbal Productions]

Try adding struts. It decreases instability dramatically.

[Superfluous J]

In addition to all of the above, just glancing at the rocket makes me think your TWR is too high. That plus lack of control plus the draggiest capsule in the game right at the front is asking for trouble.

If it went through the lower atmosphere slower, it'd probably be okay.
If it was less draggy up front, it'd probably be okay.
If you had more control authority, it'd probably be okay.
With all 3 of those missing, it's not okay

[king of nowhere]

everyone is telling how draggy is the onion module (i know it is), but the game still shows the center of drag much behind the center of mass. is it an error of the game, you think?

because i've been launching long rockets before, i've launched rockets that were draggy in front before. heck, i even launched rockets with the CoD forward, and it was fine as long as i had a gimbaling engine. and i have three of them here. and still i never had such a bad rocket since my earliest launches.

3 minutes ago, Superfluous J said:

In addition to all of the above, just glancing at the rocket makes me think your TWR is too high.

 

Not really. it would be about 2 if i went all in, but i keep the swivel between 50 and 20% while the boosters are on, so actual twr is around 1.5

[Streetwind]

1 hour ago, king of nowhere said:

but the game still shows the center of drag much behind the center of mass

Actually a common misconception because the game doesn't explain this properly to the player:

The blue marker shows center of lift, not center of drag. This is great for building planes in the SPH, but unfortunately entirely useless for rocket construction in the VAB. You have fins at the bottom, and fins can generate lift, so you get a center of lift near the fins; but that doesn't mean that your combined center of drag is in the same spot.

There is no method I am aware of to actually visualize where your center of drag sits.

[BigFatStupidHead]

3 hours ago, king of nowhere said:

everyone is telling how draggy is the onion module (i know it is), but the game still shows the center of drag much behind the center of mass. is it an error of the game, you think?

Not really? It -seems- wrong, but it isn't. The HUGE dragginess of the onion module is compounded by how far away it is from the center of mass - the long neck will work as a lever arm the moment your rocket isn't flying true. The only luck I've had with those pods is burying them in fairings or putting them at or behind the center of mass. Some interesting designs have resulted!

Edited December 16, 2020 by BigFatStupidHead

[ConArt70]

I've always had trouble with those capsules so just never bothered with them. 

[Spricigo]

4 hours ago, king of nowhere said:

everyone is telling how draggy is the onion module (i know it is), but the game still shows the center of drag much behind the center of mass. is it an error of the game, you think?

 

 

[vv3k70r]

Draggy capsule - put control surfaces on top. And some nose cone.

It will even help on way back for this capsue - it will orient it corectly for descent.

 

It lloks like You have too much force in these engines. Meybe some staging? Or less?

4 hours ago, king of nowhere said:

everyone is telling how draggy is the onion module (i know it is), but the game still shows the center of drag much behind the center of mass. is it an error of the game, you think?

Draggy parts do not count to lift surface in VAB.

 

 

[AstroG]

In addition to all of the above, the centre of mass is going to move down towards the engines as fuel and oxidiser is consumed making it more flippy as you climb higher.

If you have advanced tweakables enabled, you can modify the fuel priority of the lower tanks higher so they are the ones emptied first to keep the mass higher up the rocket.

[AHHans]

[Getting the elephant out of the room first: I assume you close the service and science bay doors before launch.]

12 hours ago, bewing said:

The vostok pods have no reaction wheels.

Indeed. I usually put a small reaction wheel between the pod and the parachute on my designs.

12 hours ago, king of nowhere said:

i did manage to launch, by the way, by making sure that i never got above 300 m/s while in the low atmosphere.

You could also try to do a gravity turn where you always point surface -> prograde during the ascent through the atmosphere. That way the wind comes straight from the front, which on a craft with rotational symmetry - which your craft seems to be - minimizes the torque from aerodynamic forces. And thus reduces the need for control forces.

[Zhetaan]

13 hours ago, king of nowhere said:

i did manage to launch, by the way, by making sure that i never got above 300 m/s while in the low atmosphere. of course i had huge gravity losses this way, so the mission, who was supposed to be a mun orbiter, had to stop at mun intercept and come back.

For me, that settles it:  you have a drag problem.

3 hours ago, AstroG said:

In addition to all of the above, the centre of mass is going to move down towards the engines as fuel and oxidiser is consumed making it more flippy as you climb higher.

If you have advanced tweakables enabled, you can modify the fuel priority of the lower tanks higher so they are the ones emptied first to keep the mass higher up the rocket.

This is a good point to raise; you may want to set your LFO tanks to drain from the bottom-most to the top-most, since you have so many in the stack.  However, I don't think it's central to your problem:  if you're flipping at 350 m/s, then you're not draining enough of the propellant for its mass distribution to contribute much to the instability.

Edited December 16, 2020 by Zhetaan

[Wobbly Av8r]

My two cents (because most of the issues have already been hashed above):

There is not just one issue causing your problem - its a little bit of many issues which contribute to your instability problem;you have the unfortunate accumulation of destabilizing problems... death by 1,000 paper cuts, as it were. It's confusing to the experienced player because, each taken by itself, we know how to compensate for each issue, but the "perfect storm" of so many contributory factors can seem perplexing.

The good news is that if you only eliminate one or two of them adequately, you probably don't need to deal with them all.

Edited December 16, 2020 by Wobbly Av8r

[king of nowhere]

10 hours ago, Streetwind said:

Actually a common misconception because the game doesn't explain this properly to the player:

The blue marker shows center of lift, not center of drag. This is great for building planes in the SPH, but unfortunately entirely useless for rocket construction in the VAB. You have fins at the bottom, and fins can generate lift, so you get a center of lift near the fins; but that doesn't mean that your combined center of drag is in the same spot.

There is no method I am aware of to actually visualize where your center of drag sits.

oh, that's it!

i wanted to figure out why the vab was lying to me, now i'm happy

[vv3k70r]

2 hours ago, king of nowhere said:

i wanted to figure out why the vab was lying to me, now i'm happy

For sure Chief Engineer in this VAB do not have any dark spots on his previous career and he is most thrustworthy person experrienced in recruitment that cut salaries beyond any measure^^