Why does my rocket start spinning around and does not respond to turns well?

[adamgerd]

So I have built a rocket and it appears to be and should be aerodynamically but it rotates around its vertical axis (wobbling slightly but the wobbling gets worse), and starts turning randomly which prevents me from launching it into an equatorial orbit or controlling the inclination.

Edited September 17, 2018 by adamgerd

[Geonovast]

I don't see any struts.  Are you using autostruts?

[adamgerd]

2 minutes ago, Geonovast said:

I don't see any struts.  Are you using autostruts?

I didn't think it needed struts, so yeah no I am not

[Geonovast]

3 minutes ago, adamgerd said:

I didn't think it needed struts, so yeah no I am not

Pretty much everything needs struts, especially with radially mounted parts like boosters.

A few other things:

No nosecone on the top probably isn't hurting you too bad, but I would put one on top of the Command Module.

Your radial decouplers appear crooked.  Are you building with snap on?  (It defaults to off, for some reason)

Do you have inline decouplers under the engines?  I don't see any, but it may be the angle of the picture.

I'm also not sure what's going on on the bottom of the rocket.  Can you upload a few more pictures at different angles?

 

 

Edited September 17, 2018 by Geonovast

[Dark Lion]

Looks like a center of mass issue to me. It's certainly heavy, but there's not enough drag around the business end to counter rotation. Slap some fins on down there, strut up your wobbliest bits and give it another go!

[adamgerd]

8 minutes ago, Geonovast said:

No nosecone on the top probably isn't hurting you too bad, but I would put one on top of the Command Module.

 

Yeah, I am not really used to the new aerodynamic model, that the nosecone helps, probably because for a time I pretty much stopped playing it

9 minutes ago, Geonovast said:

 Your radial decouplers appear crooked.  Are you building with snap on?  (It defaults to off, for some reason)

 

They might be slightly but I don't think so, I'll check to be sure

5 minutes ago, Dark Lion said:

Looks like a center of mass issue to me. It's certainly heavy, but there's not enough drag around the business end to counter rotation. Slap some fins on down there, strut up your wobbliest bits and give it another go!

Yeah, will try that

[Geonovast]

5 minutes ago, adamgerd said:

They might be slightly but I don't think so, I'll check to be sure

 

28 minutes ago, adamgerd said:

You can see in this picture that the decoupler to the right of the orange vein is significantly further away from the vein than the one on the left, so that's probably causing issues.

You also have the fins on the twinboar clipped into the boosters.  This won't be a problem... until you decouple them.  Once they're decoupled, they become different ships, and being clipped inside will cause weird forces to act on your rocket.

Same issue with the fins clipped into the launch stabilizers.  You can hit the launch stabilizers once you detach on launch.

And you want to strut to the parent.  So the outer boosters should be strutted to the one they're coupled to, and the inners should be strutted to the rocket's core.

 

Edited September 17, 2018 by Geonovast

[adamgerd]

4 minutes ago, Geonovast said:

You can see in this picture that the decoupler to the right of the orange vein is significantly further away from the vein than the one on the left, so that's probably causing issues.

 

Yeah

4 minutes ago, Geonovast said:

 You also have the fins on the twinboar clipped into the boosters.  This won't be a problem... until you decouple them.  Once they're decoupled, they become different ships, and being clipped inside will cause weird forces to act on your rocket.

 

Well that helps explain why the instability always significantly increased when I decoupled the boosters

[adamgerd]

I added struts and made it more balanced and while it is less, the boosters push into the main engine when flying and then back out in a sort of X shape, where they go outwards and then inwards, not sure what causes it. Also the struts that I added don't seem visible in the middle while their ends are

[Tyko]

Some great suggestions...I'd add this -

You could attach all 8 boosters to your core rocket and just have 4 decouple at a time - assuming they clear the core engine pods. For example, the up/down/right/left boosters would be a group and the ones set at a 45 degree angle would be your other group. You'd still have the functionality of having 2 sets of 4 boosters, but by pulling those outer boosters tighter to the core, rather than hanging way out there on struts, you'll reduce wobble.

Question? What's the mission you're trying to perform. With that core alone, minus the 8 SRBs, you should already have more than enough DeltaV to orbit Kerbin. If you're not going further than Kerbin orbit you may want to try it without any SRBs

Edited September 17, 2018 by Tyko

[Geonovast]

18 minutes ago, adamgerd said:

I added struts and made it more balanced and while it is less, the boosters push into the main engine when flying and then back out in a sort of X shape, where they go outwards and then inwards, not sure what causes it. Also the struts that I added don't seem visible in the middle while their ends are

They're probably not actually connected, which is why everything's still floppy.

Curious though, what's the mission?  If all you're doing is needing to get the thing to orbit, that rocket is severe overkill.  If you did just the nosecone, command pod, decoupler, the two fuel tanks and the twinboar, you'd get to orbit with a significant amount of fuel to spare as an SSTO.  I'm assuming you don't want the CM back as there's no parachutes.

[adamgerd]

35 minutes ago, Tyko said:

Some great suggestions...I'd add this -

You could attach all 8 boosters to your core rocket and just have 4 decouple at a time - assuming they clear the core engine pods. For example, the up/down/right/left boosters would be a group and the ones set at a 45 degree angle would be your other group. You'd still have the functionality of having 2 sets of 4 boosters, but by pulling those outer boosters tighter to the core, rather than hanging way out there on struts, you'll reduce wobble.

 

Ah, thanks for the suggestion

28 minutes ago, Geonovast said:

They're probably not actually connected, which is why everything's still floppy.

 

Ah yeah that does make sense

35 minutes ago, Tyko said:

 Question? What's the mission you're trying to perform. With that core alone, minus the 8 SRBs, you should already have more than enough DeltaV to orbit Kerbin. If you're not going further than Kerbin orbit you may want to try it without any SRBs

 

28 minutes ago, Geonovast said:

 Curious though, what's the mission?  If all you're doing is needing to get the thing to orbit, that rocket is severe overkill.  If you did just the nosecone, command pod, decoupler, the two fuel tanks and the twinboar, you'd get to orbit with a significant amount of fuel to spare as an SSTO.  I'm assuming you don't want the CM back as there's no parachutes.

Yeah your assuming of the CM is correct, its not intended to return as for the mission, its to attempt a rendezvous and docking with another vehicle in LKO that is close to equatorial orbit and then transfer its excess fuel across

[Geonovast]

9 minutes ago, adamgerd said:

Yeah your assuming of the CM is correct, its not intended to return as for the mission, its to attempt a rendezvous and docking with another vehicle in LKO that is close to equatorial orbit and then transfer its excess fuel across

Well, if that's all you're doing.. ditch the command module entirely.  Just use the probe core.  The probe core does not need the CM present to work. 

Does the vehicle it's meant to dock with have RCS?  While docking is possible without it, it is much, much easier.  Although you will only need it on one vessel.

I was going to ask about the dual docking ports.  You don't need them.  You just need the one attached to the fuel tank.  Put the probe core on top the tank, then the docking port, then a large nosecone.  You can decouple the nosecone from the docking port in orbit.  You don't need a docking port on each side.

This is what I would build for your goal.  6 parts total.  You could knock that down to 5 if you use the Rockomax Jumbo 64 instead of the two Rockomax X-200 32s.

The only downside is it's not a whole lot of fuel to orbit.  So you may need to stick the boosters back on as @Tyko suggests.

Edited September 17, 2018 by Geonovast

[adamgerd]

14 hours ago, Geonovast said:

Well, if that's all you're doing.. ditch the command module entirely.  Just use the probe core.  The probe core does not need the CM present to work. 

Does the vehicle it's meant to dock with have RCS?  While docking is possible without it, it is much, much easier.  Although you will only need it on one vessel.

I was going to ask about the dual docking ports.  You don't need them.  You just need the one attached to the fuel tank.  Put the probe core on top the tank, then the docking port, then a large nosecone.  You can decouple the nosecone from the docking port in orbit.  You don't need a docking port on each side.

This is what I would build for your goal.  6 parts total.  You could knock that down to 5 if you use the Rockomax Jumbo 64 instead of the two Rockomax X-200 32s.

The only downside is it's not a whole lot of fuel to orbit.  So you may need to stick the boosters back on as @Tyko suggests.

Ah thanks for the help