Unwanted crashing debris

[antipro]

Hi,
I'm usually able to solve these kind of things by myself but this time it's driving me out of my mind, so please give me some suggestions.
if there is one thing that bothers me during a launch, is to see separated empty tanks collide with each other.
I've already tried to move sepratrons and change their solid fuel amount, but every time I launch,
the two tanks on the east side of the ship, always collide.

So can you please give me some advices in order to avoid that collision?

Collision video: https://www.dropbox.com/s/v469qy9fkoak9st/2021-09-28 02-06-13.mkv?dl=0

Edited September 28, 2021 by antipro

[18Watt]

I sometimes find it helpful to reduce thrust greatly while dropping boosters.  Personally I am not concerned if the boosters hit each other, I’m more concerned about them hitting the remaining vessel.  But you might give that a try- reducing thrust, or going to zero-thrust when you separate.

Another thought- stage the dropped boosters about a second apart.

Also, you might try adjusting the location on the booster where it is attached to the decoupler.  That could introduce yaw moments which are causing the boosters to rotate inwards after separation.

[antipro]

10 hours ago, 18Watt said:

I sometimes find it helpful to reduce thrust greatly while dropping boosters.

I can't, is a GT managed launch.

 

10 hours ago, 18Watt said:

Personally I am not concerned if the boosters hit each other, I’m more concerned about them hitting the remaining vessel.

Of course, if they hit the main vessel the mission instantly fails. And have you ever seen 2 boosters collide IRL?

 

10 hours ago, 18Watt said:

But you might give that a try- reducing thrust, or going to zero-thrust when you separate.

As I said it's a GT handled launch, also if I reduce thrust I'll lose efficiency and by the way I'm not sure that will help cause the real problem here is that
those "boosters" have an angle of attack in the moment I separate them.

 

10 hours ago, 18Watt said:

stage the dropped boosters about a second apart.

Not sure I've understand, are you saying to separate them 2 or 3 at time?
If so, how do I do it? all the separators are linked, I would have to rebuild them by removing their symmetry.
This could be a good idea, thx I'll try it.

 

10 hours ago, 18Watt said:

Also, you might try adjusting the location on the booster where it is attached to the decoupler.

I've not so much gap to do it, anyway already tried. I think the problem is that when I detach them, they have an angle of attack.

 

10 hours ago, 18Watt said:

That could introduce yaw moments which are causing the boosters to rotate inwards after separation.

This sentence made me reflect and come up with another idea: add a separator between their engines and the tanks.
Or maybe I just replace the slanted cones with symmetrical cones.

I'll try soon. Thanks

Edited September 28, 2021 by antipro

[AHHans]

As @18Watt already mentioned: I usually don't care if the dropped boosters collide with each other or are only destroyed when they impact the ground. But that's not what you asked.

I noticed two things: one is that the rocket is not pointing straight into the wind (== pointing prograde in surface mode). That means that that wind could push the discarded boosters into the remaining rocket. (I.e. if you would keep the rocket pointing prograde while dropping the boosters then you might not need the sepratons. But, jaja, that's off-topic again...) The other - and probably relevant - issue is that the sepratons spend most of their effort rotating the dropped boosters and not pushing them apart. Actually: once the boosters rotated more than 90 degrees they actually push the boosters back towards each other again. So my suggestion is to place the sepratons around the center-of-mass of the empty boosters (probably far to the bottom because the engine is heavy), so that they don't rotate the boosters much. At least make sure they burn out before the boosters rotated more than 90 deg.

P.S. My guess(!) as to why it's always the same boosters collide is that it's some "strange" aerodynamic effect of the sideways wind on the boosters while they rotate.

[jimmymcgoochie]

Remove 80% of the Sepratron fuel, they only need to give a brief burst of thrust to push the boosters away from the core. You should also try aligning the thrust of the sepratrons to avoid inducing a roll once they separate as that combined with the longer burn time is pushing them back together again.

[antipro]

uhm.. ok one moment, I have to intensively use the translator.
 

8 hours ago, AHHans said:

I usually don't care if the dropped boosters collide with each other or are only destroyed when they impact the ground.

So you don't care about the high sea's pollution due to many micro-fragments? or even at the risk of destroying buildings or killing kerbals when the debris hits the ground? eheh.. no come on I'm just joking.

 

8 hours ago, AHHans said:

I noticed two things: one is that the rocket is not pointing straight into the wind (== pointing prograde in surface mode).
That means that that wind could push the discarded boosters into the remaining rocket.

Yes is correct, already mentioned 2 times, at the separation time the rocket has a certain AoA.
And yes, that's the reason why I have to use 3 sepra instead of 1 only.

 

 

8 hours ago, AHHans said:

(I.e. if you would keep the rocket pointing prograde while dropping the boosters then you might not need the sepratons. But, jaja, that's off-topic again...)

I'm quite pretty sure that even if I went perfectly Surface-Prograde, those slanted cones would be a problem at that low altitude/high pressure atmo,
usually I solve by adding a single vertical slanted sepra.
This mission involves three hi-mass vessels, and normally I always point as much prograde as possible, but this time I just can't.


 

8 hours ago, AHHans said:

The other - and probably relevant - issue is that the sepratons spend most of their effort rotating the dropped boosters and not pushing them apart.

How? All 3 sepra are pointing externally.

 

8 hours ago, AHHans said:

So my suggestion is to place the sepratons around the center-of-mass of the empty boosters (probably far to the bottom because the engine is heavy), so that they don't rotate the boosters much.

Ok, I'll try to match their CoM, but it is probably close to and not "far" from the bottom.

 

 

8 hours ago, AHHans said:

At least make sure they burn out before the boosters rotated more than 90 deg.

yep, that's what I'm trying to solve.
That east-sided couple of boosters, that rotate so much cause of AoA, independently by what I do with sepratrons, is the main problem.

Thanks, you made me think about another solution.

 

Edited September 28, 2021 by antipro

[antipro]

9 minutes ago, jimmymcgoochie said:

Remove 80% of the Sepratron fuel, they only need to give a brief burst of thrust to push the boosters away from the core.

Already tried yesterday before posting: nothing changes. The screenshots was only my last assembly try.
Anyway those 2 west-sided boosters need more push power or they will crash into the 2nd stage

 

14 minutes ago, jimmymcgoochie said:

You should also try aligning the thrust of the sepratrons to avoid inducing a roll once they separate as that combined with the longer burn time is pushing them back together again.

If I've understood well, you're talking about to change the 2 symmetrical sepra direction in order to reduce the roll.
Uhm.. yes good idea. I'll try this too.

Thanks
 

[antipro]

here's just few screens proving I'm working on it.
https://www.dropbox.com/sh/c0qgngjyb5l3s96/AAD9RrACa8lrBlv4ypFYM61Ka?dl=0

Edited September 28, 2021 by antipro

[antipro]

ok I think I solved.
In few words:
- change sepra direction = not possible if not using two sets of sepra = eyeballing symmetry.
- sepra match or close to CoM = not working cause west-sided tanks need more push on the top.
- add separators between engines and tanks = works.

"no longer crashing debris" video: https://www.dropbox.com/s/2ozv68jlidzk2ps/added sepa.mkv?dl=0

So, I'm now fine, no more debris collisions.
Thanks all of you dudes.

Edited September 28, 2021 by antipro