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I am new and I have a problem with decouplers


GamingHalo975
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Whenever I use decouplers, they act really weird in a hard to explain way. Essentially, they act almost like loose hinges. When I have an SRB attached to it, for example, on the side of a rocket, instead of simply staying there, they will wobble around and the SRBs will even go inside the rocket. I have tried putting another decoupler on it, but the SRBs ignore the other decoupler. Additionally, I have figured out that I can use struts to prevent the SRBs from wobbling around, but whenever I detach the SRBs, that will usually just fall into my rocket and the whole thing goes to pieces. Anyone know if this is a rare glitch, or maybe just a rookie mistake?

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You don't need a mod to fix this.

If you attach two heavy things (like a rocket and an SRB) with a single point connector (which is all the game gives you as parts, it's the way the software works) then they will wobble. This occurs in real life as it does in the game.

In real life, they fix it by connecting SRBs to rockets with more than one thing.

In the game, you fix it by doing exactly the same thing. Either use the "Strut" part available in the parts list, or use auto-struts in the right-click menu of the srb to attach a special invisible strut to something. For radially attached parts, I prefer "Grandparent" but "Root" and "heaviest" are usually perfectly fine options.

Edited by Superfluous J
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A picture might help. This is my typical collision-safe and wobble-free design. The important parts:

  • Radial decouplers attached a bit above the center of mass of the big SRBs.
  • (Red oval) Bottom of the large SRBs is strutted to the core tank, this will eliminate any wobbling. A single strut can handle it but I tend to use 2, one on each side, to even out any drag effects.
  • (Green ovals) Sepratrons used to push the boosters away from the core on decoupling.

WUwcGMsw_o.jpg

Notice the staging, the Sepratrons have to be in the same stage as the radial decouplers. On detaching these will fire and push the boosters away from the core. I typically put 4 at the top (2 on each side of the SRB) and 2 at the bottom (1 on each side of the SRB), this will push them away with the nose outwards, creating that nice Korolev Cross effect.

When you decouple a SRB it will naturally move on in the prograde direction, which means that if your core is not aimed exactly prograde too, there's a big chance of collisions. The Sepratrons prevent this from happening. Make sure you point the Sepratrons in the right direction, the black exhaust nozzle needs to point towards your core rocket :)

 

 

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I mean, not precisely a glitch, just that things are wobbly. Struts do fix the wobble, but as for the way they drop it's a balance thing. The radial decouplers have a bit of force when they blow off the SRBs. If that force is too far below the SRB's center of gravity, it will cause it to tilt nose-first into your rocket. Anything attached to the booster could cause drag effects that make for weird behavior too.

It's only a mistake if you don't learn from it. :)

 

 

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In recent versions of KSP I’ve found I can fix the ‘floppiness’ of boosters fairly easily and effectively, playing 100% stock, no mods required.

I activate rigid attachment for radial booster sections, and also for the radial decouplers.  Rigid attachment is a toggle you’ll see in a part’s PAW (part action window) while in the VAB.

I also use auto struts, which is also available in the PAW in the VAB.  I usually auto strut to the heaviest part, but other auto strut options likely would also work.

If you are playing a Career or Science mode game, I don’t think the option to Autostrut appears until you have unlocked the tech node containing regular struts.

Auto Struts, combined with Rigid Attachment, works so well now that I rarely use actual struts at all anymore.

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At the risk of merely repeating what has already been said…

With a hefty solid booster like the Thumper producing a lot of thrust and a relatively small single point of connection to the core booster, the physics system will naturally make it wobble. You can solve this in several ways:

  • Put the decouplers near the top of the boosters- use the Move tool in the editors to reposition the boosters since they’ll attach to the middle when placing them. This should not only reduce the pivoting but will also mean that the boosters get pushed away from the core once decoupled, avoiding collisions.
  • Use struts. A carefully placed strut will hold any wayward booster in place by adding a second point of connection between the booster and the core.
  • Turn on “advanced tweakables” in the main menu settings. This enables a lot of extra features that are very useful, including autostruts: they’re like normal struts, but don’t add any extra parts or weight, however they are limited to connecting to either the root part of the vessel, the heaviest part on the vessel or the “grandparent” part- for any given part, the parent part is what they’re attached to (SRB > decoupler) and the grandparent part is the parent’s parent part (SRB > decoupler > fuel tank in the middle).
    As a general rule I use grandparent autostrut for the boosters, grandparent autostrut for any fuel tanks I’ve stuck on top of those boosters* and root autostrut for the nosecones, which keeps everything in place. Heaviest mode can be glitchy when staging as it recalculates the new heaviest part, root mode is great for engines to avoid compressing your rockets when under thrust with physics warp on, and grandparent mode works great for parts attached radially as they can form connections to lots of other parts and lock everything tight.
    I’m not a big fan of rigid attachment mode- yes, it keeps things very still but in my experience it also makes the connections brittle and prone to snapping under extreme forces.
  • Use a mod such as KJR or QuantumStruts to do it all for you instead, but that’s boring…
Spoiler

* Yes, I put fuel tanks on top of solid boosters. And so should you! With crossfeed enabled on the decouplers, those fuel tanks will drain first before using fuel from the rocket’s core meaning you get more delta-V; the solid boosters are there for pure power so they can handle the extra weight; and the tanks get dropped with the SRBs once drained so it’s more efficient than more fuel tanks in the core stage. It’s not so important at stock scale, but on larger scale planets it can make a big difference.

 

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At risk of stating the obvious, add more struts.

Also, try removing the engine shroud and replacing it with a fairing. This worked great in my bad stock Saturn 5 rebuild that landed on the Mun using only Vectors.

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In addition to all of the tips above for positioning the decouplers and using struts (auto and otherwise), I always set the first boosters to be dropped along the North/Side alignment, especially with heavy vehicles with low TWR. Since the whole mess is often moving relatively slowly  by the time the boosters run out of gas, I've found that if they're aligned East/West they can fall into the vehicle. Of course I'm not above using Sepratrons to counter this as well, but why use more parts when you can let gravity do the work for you?  :) 

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