boosters not coupling correctly with hydraulic detachment manifold

[king of nowhere]

i have made a rocket, and i used the hydraulic detachment manifold to fix the boosters, because i needed more strenght. and since i didn't want them to wobble, i put two of them per booster instead of one.

but the boosters still wobble. i investigated the problem, and i discovered that the lower hdf was not connected to the booster.

https://imgur.com/a/kNKP9ya

here is a detail of the rocket, in green the hdf properly coupled, in red the one who is not. tested on the launchpad, detaching the lower hdf has no effect, while detaching the higher one alone causes the booster to separate.

i noticed this now, but i'm sure it happened other times, with other decouplers.

no amount of fiddling solved the problem. i could not figure a way to properly attach the hdf and the booster.

any advice?

[Linkageless]

As far as I know, there is still no stock way of having decouplers or these manifolds providing two connections to the rest of the vessel, we are limited to one.  I believe this is a architectural limitation.

The only way I know of kind of overcoming this is to use docking ports as separators, they will now cling and effectively dock to each other when put on the launch pad.

The real solution to your problem, however, is to use struts or even better (in my opinion) autostruts.  Turn on the "advanced tweakables" option in the settings, if you don't see them yet in the part action menus.

[king of nowhere]

38 minutes ago, Linkageless said:

As far as I know, there is still no stock way of having decouplers or these manifolds providing two connections to the rest of the vessel, we are limited to one.  I believe this is a architectural limitation.

 

so.

all this time i tried to fix the wobble problem by adding more and more decouplers. and i noticed that occasionally it didn't work, but i assumed it to be an occasional failure.

now i learn that i've been throwing money down the drain all this time??? i just removed all the extra decouplers and saved 10% of the cost on the rocket i was trying to launch

i use struts to reinforce, but i'd rather not rely on them too much because they increase the chances of the various boosters not separating correctly and exploding/hitting the mothership after detachment.

Edited June 12, 2020 by king of nowhere

[micha]

44 minutes ago, king of nowhere said:

i use struts to reinforce, but i'd rather not rely on them too much because they increase the chances of the various boosters not separating correctly and exploding/hitting the mothership after detachment.

Struts should auto-break on decoupling.  Use the little solid-rocket engines to push your boosters away.

[Grogs]

Don't worry, I spent a lot of time doing exactly the same thing before I figured this out. The way parts work in KSP, each one is connected to the root part in exactly one chain of parts, for example, Capsule-decoupler-Fuel Tank-Decoupler-Booster. It's not possible to connect things in two places, although maybe there are some mods to do this.

 

To make radial boosters stable, enable 'advanced tweakables' in the menu. Then when you right-click on a booster, you should see an option labeled 'autostrut'. For radial boosters, autostrut to grandparent part usually makes for a solid attachment that won't bend. This prevents you from having to add physical struts that increase the part count and also add drag. I think using the autostruts is 'fair' because in a real rocket there would be multiple attachment points.

 

BTW, I usually position my radial decouplers so they're about 2/3 - 3/4 of the way up the booster. I usually attach them and then use the 'Move' tool to get them down where I want them. When they decouple this will kick the top portion out and away from the center stage. If you put the bottom of the booster slightly below the engines of the center stage this makes it less likely they will collide when being ejected. It also helps to be near prograde when releasing the decouplers otherwise one side will get blown back towards the central stack.

[micha]

4 minutes ago, Grogs said:

It's not possible to connect things in two places, although maybe there are some mods to do this.

No, it's literally not possible due to the way the core of the physics engine works in this game.

It would require a major rewrite. Maybe KSP2 will fix this, but I doubt it as it would make the logic a LOT more difficult as you'd need to figure out ways to stop physics effects from propagating around in circles in self-amplifying loops.

[king of nowhere]

10 minutes ago, micha said:

Struts should auto-break on decoupling.  Use the little solid-rocket engines to push your boosters away.

they do? i always attach struts to decouplers. the first and only time i did not insert the decouplers, the rocket exploded, so i figured it was necessary...

then again, it's far from the only time my rocket exploded, so in retrospect it did not prove anything....

well, great! more savings.

[Linkageless]

I almost always use Autostrut to Grandparent part for everything, even decoupled parts.  The main reason for having a decoupler in the first place is to provide a way to detach a stage in flight.  The main reason for having struts (and autostruts) is to provide structural integrity.  The two do different things but work together well.

[micha]

Build your core rocket. Put on radial decouplers. Put on your boosters. Strut your boosters to the rocket.

When you stage the radial decouplers, the struts auto-break.

EDIT: 1 radial decoupler per booster. As you discovered, more than one won't connect anyway.

Edited June 12, 2020 by micha

[Linkageless]

In other words, continue to use decouplers for your boosters, assuming you wish to discard them once spent.  You can stabilise them with struts and/or autostruts and they will automatically break when the decoupler fires.  This allows you to shed that dead weight, maximising your delta-v.

Be aware that Autostruts set to 'root' or 'heaviest part' can be Kraken bait.  I rarely see the need for anything other than 'Grandparent part', and rarely need to use visible struts.

I might also add... I autostrut to 'Grandparent' on everything I consider structural, as a matter of course.  This gives a vessel integrity without requiring it to be completely rigid, which I've always assumed gives it some resilience compared to rigid attachment or even the more severe forms of autostrut.

[AHHans]

1 hour ago, micha said:

Struts should auto-break on decoupling.

Let me correct that for you.
(O.K. You can argue that they don't "break" but "vanish" as soon as the two parts that they connect are no longer part of the same craft.)

[micha]

10 minutes ago, AHHans said:

Let me correct that for you.

I said "should" because that's the way they are expected to work. But there's always the chance of bugs... and @king of nowhere claimed that struts caused his boosters not to separate properly. There's, of course, the greater chance of PEBCAK, but it never hurts to err on the side of caution

[AHHans]

1 minute ago, micha said:

But there's always the chance of bugs...

I agree, it isn't like KSP is lacking in bugs. And I also prefer to use defensive language when I'm not 100% sure.(*) But this is one of the cases where I'm sure enough that I don't feel the need to use qualifiers in my statements.

[king of nowhere]

perhaps the reason my boosters exploded is that i tried to link the boosters to each other to keep them still, not to the main craft.

it would have made no sense to link a booster to the main craft with a strut, since i already connected them with plenty of decouplers

[Spricigo]

7 hours ago, micha said:

No, it's literally not possible due to the way the core of the physics engine works in this game.

Not the physics, just the craft structure. Each part have one, and only one, parent part. (except the root part of course) 

The mod just need to make a connection to a part that is not a parent part (like an strut) that have a similar strength. And actually there is a mod: