How do I stop SRBs from hitting the rocket on the pad

[Sepani]

So on my *thing* I have 4 SRBs on the side of the main rocket, and everytime I launch, about 5 seconds after liftoff, the SRBs rotate into the rocket, hit it, and explode. They rotate on the decouplers??? help?

[Wobbly Av8r]

On each SRB's PAW (Part Action Window, accessed by "Right-Clicking" each SRB) is the option to "Autostrut: Heaviest Part" as well as "Rigid Attachment". Selecting either or both of those should solve that problem.

[Entropian]

Adding struts at the top and bottom of the SRBs also helps.  Autostruts help as well.

[Spricigo]

4 minutes ago, Wobbly Av8r said:

 "Autostrut: Heaviest Part" 

I'd rather go with "grandparent". since the heaviset part can change as you burn fuel.

Also, the option "allow advanced tweakables need to be checked in the setting to autostrut be available

[DeadJohn]

2 hours ago, Sepani said:

So on my *thing* I have 4 SRBs on the side of the main rocket, and everytime I launch, about 5 seconds after liftoff, the SRBs rotate into the rocket, hit it, and explode. They rotate on the decouplers??? help?

The struts and autostruts already mentioned are good suggestions. Several other alternatives:

Mount the decouplers higher up on your SRB so the SRB nose is less likely to pivot inwards and hit the main rocket. Use "Tool: Move" (press the 2 key on PC) to drag the decouplers upwards on the main tank, then use the same tool to drag the SRBs downwards. The lower end of the SRB will have more opportunity to move, though, so you might still need struts.

Which radial decoupler are you using? Changing to a larger decoupler might hold your SRBs more strongly, and give more room between the main tank. Stock KSP has 3 radial decouplers.

If you use mods, Kerbal Joint Reinforcement Next can fix a lot of flexing issues.

Your rocket might be overpowered. What TWR do you see in the VAB on your first SRB stage? Lowering the Thrust Limiter to get 1.5 TWR at takeoff is a good starting point.

 

[HvP]

2 hours ago, DeadJohn said:

Stock KSP has 3 radial decouplers.

There are also two decouplers in the structural category. The Small Hardpoint and the Structural Pylon can be very useful for putting a little distance between boosters and separate cleanly.

[jimmymcgoochie]

Auto strut the boosters to their grandparent part and the nosecones on top to the root part- the extra struts on the nosecones will hold them steady. If you’re using fuel tanks on top of your boosters (which you should- set crossfeed on on the decouplers and you get more main engine fuel with tanks that are discarded with the SRBs) then auto strut the tanks to their grandparent part too, that will hold the boosters still relative to each other too.

[Superfluous J]

17 hours ago, Spricigo said:

I'd rather go with "grandparent". since the heaviset part can change as you burn fuel.

Autostruts don't recalculate, though, until staging. As SRBs tend to be the first thing staged, this is rarely an issue.

However I agree with the suggestion. "Grandparent" is the better choice here and in many instances.

6 minutes ago, jimmymcgoochie said:

If you’re using fuel tanks on top of your boosters (which you should- set crossfeed on on the decouplers and you get more main engine fuel with tanks that are discarded with the SRBs) then auto strut the tanks to their grandparent part too

I prefer to only autostrut the top fuel tank on the SRB to either the root or the heaviest part, depending on which one makes a more perpendicular connection to the SRB/fuel stack. Never had an issue with just that autostrut plus the grandparent autostrut on the SRB itself.

Edited August 26, 2020 by Superfluous J

[Klapaucius]

In addition to autostruts, rigid attachment works well  with rockets (though never used it for planes).