Still Having problems With Rolling Rockets.

[The Flying Kerbal]

Sigh...  yet again I'm back on this tired old subject of rockets rolling during the flight through the atmosphere on their way to space.  I was absolutely certain I'd cracked it a few nights ago, but alas no... it's back again with a vengeance.  I'm really really stuck with this one, I have  absolutely no idea what to try next; possibly I've been trying to solve it for so long I'm missing something obvious, but anyway, maybe some of you can see what I'm doing wrong?

Image 1

In the VAB the KER readout said the torque for this rocket is 0.00 right across the board.  On the readout in this image It says the Thrust Torque is currently 0.1.  Not sure what that term meant, I googled it and discovered it refers to the gimbaling on a rocket engine.  I'm using a Swivel engine for the main stage but it doesn't fire up until the two SRB's are discarded, so I can't see how that could be having an affect during the initial minute or so of the flight.

Image 2

Assuming this Thrust Torque is some how responsible, and in Image 2 it has already went up to 0.51 only a small number of seconds after lift off.  Although it's very slight, and really not an issue at this stage, but if you look closely at the Navball you'll already see it has rolled very very slightly clockwise.  However I could live with this amount of roll.

Image 3

By the time the rocket reaches 2,100 meters and the roll is much more noticeable.  It would be much worse if I wasn't fighting to correct it.  The problem is it's extremely difficult to try to make a reasonable gravity turn while having to counter this, although as you will see shortly I did a pretty good job of it on this occasion (a little bit of unabashed blowing my own trumpet there! ).

When I launch this rocket without the side SRB's, this Thrust Torque remains in such small fractions as to be inconsequential, so the problem must be related to them in some way, however I've no idea why.  They seem to be on straight, and as I've already said, in the VAB KER says there is no torque in any part of the rocket, so what is my problem here?

It's a pity I'm having this problem, indeed in five attempts at a launch, four of them had ended up totally out of control because of this.  The fifth one I sent much higher than normal before pitching over, and it did help to lessen this rolling somewhat, but it was more like a sticking plaster rather than a proper cure.  The result was one of the best orbits I've ever established without tweaking it, the readings on KER in this image shows the Ap and Pe I achieved with my insertion burn!  Not too bad, even if I do say so myself!  However this was a fluke, I couldn't hit an orbit like that with the best rocket ever produced by any KSP player, let alone one rolling as bad as this one.

Image 4: That'll Do Nicely!

This rocket is pretty cheap and has the potential to put a small probe into LKO with sufficient Delta V to travel to quite a number of destinations with a few simple science experiments on board.  However the rolling makes it such a pain to fly it really is hardly worth the effort.

Do any of you have ideas why I'm having this problem?  I'm beat on this one, I just can't think what else it could be, so any and all suggestions will be very much appreciated.

Thanks everyone.

Edited December 4, 2017 by The Flying Kerbal

[Harry Rhodan]

SRBs tend to bend on their decouplers which will introduce torque and slightly offset drag if they aren't attached rigidly enough.

[The Flying Kerbal]

Well I have Autostrut enabled and regular struts on the bottom of both SRB's as backup.  However I'll go back into the VAB and add some up bear the top of each if them to see if that helps.

Many thanks for the suggestion! 

[Plusck]

You don't have controllable fins and you don't seem to have any item with significant reaction wheels. The reaction wheels in the probe core are very weak.

Therefore there is absolutely no way to control roll. The Swivel rocket gimbal cannot affect roll since it is in the centre of the craft.

I'm not sure that autostrut is really going to help either. I've actually never used it, because I find the usual real struts are perfectly adequate and if things wobble, it's telling me that my structural engineering sucks.

@Harry Rhodan is definitely right about the SRBs creating the roll. It does look like they are not actually centred on the decouplers, but even if they are they can shift and, if they both shift the same way radially, they'll make you roll.

So in your place, I'd drop those fins and add the smallest elevons or a set of controllable fins. If you'd rather not do that, you need reaction wheels.

3 hours ago, The Flying Kerbal said:

Well I have Autostrut enabled and regular struts on the bottom of both SRB's as backup.  However I'll go back into the VAB and add some up bear the top of each if them to see if that helps.

Many thanks for the suggestion! 

That size SRB shouldn't normally need struts. At most, one at the bottom (it's always a better idea to put the decoupler nearer the top, so that the SRB is pushed out the way on separation and the airflow will make it peel away cleanly). Just zoom well in between the core and booster in the VAB to place the strut centrally (starting on the SRB with snap on).

The only time more than one strut is needed is when your side boosters are really massive and/or made up of a number of fuel tanks.

[The Flying Kerbal]

Thanks very much for taking the time to respond Plusck.

Originally I had no fins on the rocket, then I put those on to see if they would help counter the rolling.  I do have small inline reaction wheels on board, two of them in fact, but you won't see them as I hide such things as batteries, reaction wheels, etc. by offsetting them inside the body of the rocket.

Just as you advised, I had indeed the decouplers at the very top of the SRB's, with a strut right down at the bottom.  Autostrut is really good, it basically does what normal struts do, except there's no additional parts (if the VAB hasn't been upgraded a big plus point), and to me it just looks extremely realistic.  This Rusty Star Rocket is heavily auto-strutted; without it it would have been like flying a piece of string, but with it it was as steady as a rock. 

The only normal struts were low down on the SRB's, and I'm not sure it needed them even there.

Anyway, I'm only home from work and have things to do, but I'll be trying the suggestions both you and Harry Rhoden made and see how things go.

Thanks again guys for the help.

[Plusck]

27 minutes ago, The Flying Kerbal said:

Thanks very much for taking the time to respond Plusck.

Originally I had no fins on the rocket, then I put those on to see if they would help counter the rolling.  I do have small inline reaction wheels on board, two of them in fact, but you won't see them as I hide such things as batteries, reaction wheels, etc. by offsetting them inside the body of the rocket.

Just as you advised, I had indeed the decouplers at the very top of the SRB's, with a strut right down at the bottom.  Autostrut is really good, it basically does what normal struts do, except there's no additional parts (if the VAB hasn't been upgraded a big plus point), and to me it just looks extremely realistic.  This Rusty Star Rocket is heavily auto-strutted; without it it would have been like flying a piece of string, but with it it was as steady as a rock. 

 

The only normal struts were low down on the SRB's, and I'm not sure it needed them even there.

Anyway, I'm only home from work and have things to do, but I'll be trying the suggestions both you and Harry Rhoden made and see how things go.

Thanks again guys for the help.

Ah well, looks like the only remaining bit of my answer is "elevons or controllable fins".
I know elevons don't look right, but they are cheap and they work perfectly.

 

One other thing though: I looked at your pic in orbit and I'm finding it hard to understand what I'm seeing. The payload and the upper stage appear to be connected, but they are connected by a very fine line.

Could it be that your roll is actually a phantom movement caused by your controlling probe core moving around inside the fairing? Just a thought...

[Corona688]

4 hours ago, The Flying Kerbal said:

I hide such things as batteries, reaction wheels, etc. by offsetting them inside the body of the rocket.

This Rusty Star Rocket is heavily auto-strutted; without it it would have been like flying a piece of string, but with it it was as steady as a rock.

Autostrut + parts offset inside other parts can cause phantom forces.

I suggest putting parts inside little bays instead.

Edited December 4, 2017 by Corona688

[Aegolius13]

In lieu of control surfaces,  you could try to mount a few Spiders at the bottom of your rocket and have them start with the SRBs.   They should be able to some some roll control.   I often use them instead of fins on rockets since they're cheaper and provide at least a little thrust. And they work better than reaction wheels in low atmo.

[A_name]

I second what others have said that the roll is probably being caused by the SRB's flexing and thus producing offset thrust. Sadly I don't have much in the way of tried solutions, but there are some things I can think of:

-Limit the thrust on the SRB's. Less thrust should mean less flexing. This will make you have to adjust your flight profile, but it should be manageable with a rocket this size.

-Activate rigid attachment in the advanced tweakables. I've never actually done this for any craft but who knows, it might work?

-Strut the decoupler as well as the SRB itself.

[The Flying Kerbal]

You guys who suggested the roll being caused by the SRB's flexing have been proven right.

As an experiment I put on six struts on each booster and the roll instantly was gone.  I then took on board A_name's suggestion and turned on Rigid Attachment, something I've never used before for some reason, remove all but the original strut at the bottom of each booster, and it seems to have kept the roll at bay.

On 04/12/2017 at 6:43 PM, Plusck said:

One other thing though: I looked at your pic in orbit and I'm finding it hard to understand what I'm seeing. The payload and the upper stage appear to be connected, but they are connected by a very fine line.

Could it be that your roll is actually a phantom movement caused by your controlling probe core moving around inside the fairing? Just a thought...

The line you see must be the little Ant engine which is attached to the satellite.

Do you think I should add struts up here too?  I do have Autostrut turned on.

There are other suggestions I haven't tried yet, putting parts in bays instead of offsetting them for example, I'll look at those tomorrow!.

Thanks to all of you for helping me with this.

[Spricigo]

Well, asuming the satellite is around 500-600kg, I'd try:

-Add an OscarB or Round8 fuel tank to the probe

-Increase the fuel capacity of the terrier stage to a full FL--T800. 

At this point you got about +1,8km/s, so your lifter can be smaller.

-use a Thumper as central lifter stage with a pair of Thud at the sides (fuel for those on top of the thumper)

-have no thrust limit on any engine and no roll authority on the thuds (often it cause problems to me) . Give the smallest tilt east you can on the editor and hold it with launch clamps. Add fins, if needed, to make it aerodynamic stable.

-launch with SAS hold position and at the key velocity (IME velocity is easier to watch than time) switch SAS to prograde hold. If you did everything correct just stage when necessary and the rocket will fly itself to orbit.

A few tries will be necessary to find the key velocity, if the trajectory is too steep start the turn at lower velocity, if too shallow start at higher velocity. When you find the ideal velocity write it in the craft description  so you know it next time.

Edited December 6, 2017 by Spricigo