Why won't my rocket stay straight?

[Deseoso]

I don't understand this. I have built a rocket that as far as I can tell is symmetrical but it twists as it goes up. Below is a short video of my problem. What am I doing wrong?

[Hodo]

Simple, you dont have struts holding the SRBs straight. They are twisting due to the torque of the boosters.

[The_Rocketeer]

Ok, here goes:

The rocket is twisting because the amount of thrust and mass are high compared with the rigidity of the design. You can improve the rigidity by using struts to hold the tops of your radial SRBs straight and parallel to the inside stack. Basically, every join between parts is a place where your rocket can bend slightly, so the more places that each part joins to other parts the more rigid it is - hence, struts.

If you haven't unlocked struts through the tech tree, there are a few other things you can do.

Use a gimballed engine or winglets near the bottom of the rocket, or add a reaction wheel or two. These will allow you to make corrective steering when you start to stray off course. However, if the design is too floppy, you could end up making the problem worse by adding to the torque between parts and making them twist further.

Another option is to reduce the thrust limiter of the boosters. This will reduce the amount of force being pushed through the joints and decrease the amount that they twist.

On the same tack, drain all the solid fuel from your payload SRB - you don't need it to actually burn fuel to complete the test contract, it just needs to activate. Reducing this mass will again reduce the force transmitted through the joints and reduce the amount they twist.

Hope this helps

[Zylark]

Off the top of my head, I got two hypothesis - I realize this is early career so not all goodie parts are available to you:

1: The radial SRBs are not strutted up, so their thrust vector veers off a bit as they are not held firmly in place.

2: The LFO engine, is that the LV-T30 that do not have Thrust vectoring or is it the LV-T45 that do? If the former, you have no way of controlling the craft as you got no engines that vector thrust, and the little reaction wheel torque on the capsule simply can't counter the force of the engines to adjust direction. If the latter, then I don't know. The LV-T45 should be able to keep three SRBs under control.

[Deseoso]

Thanks. This is a big help.

[SRV Ron]

1. Use the other radical decouplers that is first unlocked on the tech tree.

2. Mount the three boosters as centered as possible. If they are off to one side or the other, you will have turning issues.

3. Leave just enough fuel in the test booster so you can deorbit to bring Jeb back home when you run the test.

If properly balanced, you won't have the tipping issue as well as you won't need the decouplers and struts. See Jebs Orbiter based upon a similar design as yours.

[cantab]

Yet another suggestion: If you lack struts to stiffen things up, don't build in 3-way or 6-way symmetry, but in 2-way or 4-way. It's been found through playing, mainly from people doing big starting tech rockets, that the 3-way symmetry results in rotations like you see.

A wild guess as to why 3-way symmetry gets phantom rotational forces while 2-way and 4-way don't is that the game's physics work in rectangular co-ordinates. So with 2 or 4 way symmetry the situation truly is symmetric, but with 3 or 6 way there are small rounding errors that magnify.

[purpletarget]

Your video did not include your resources panel. One of the things with early SRB + Pod rockets is that the Pod torque can somewhat handle some of the forces to keep things straight, but once you throttle down the VT, there's no power generation and your torque wheels will loose effectiveness when it runs out of charge.

As was already suggested though, tweaking down the thrust level of the initial boosters will probably do wonders to get you higher and further without as much bending.