Suggestions on why ship is uncontrollable

[SteveD80]

So I built a small RCS powered tug and it is horribly unstable.  It is constantly bouncing in flight and I would like to know how to fix it.

It is 3 1.25 mono tanks, the probe core that costs 650, 3 retractable solar panels, 2 clamp'o trons, 4 (I think) z-400 batteries, The 1.25 reaction wheel, 8 RCS blocks, the 1.25 storage bay (with the batteries and probe core), and an antenna.  The cubic octagonal strut is just for purposes of the craft file.

Craft File

[bewing]

Weeeeeelll, that reaction wheel is unbelievably overpowered for such a tiny little satellite. My guess is that it is the source of all your issues -- the basic construction looks sound. The OKTO probe core that you are using already has a little reaction wheel in it that is probably just right for what you need -- so I think you can probably ditch the other one safely. If you need that major reaction wheel during launch, then you might want to disable it before you decouple this little guy.

[Reactordrone]

You might also be better off with just a single set of RCS quads on the centre of gravity or they'll tend to fight each other when you're trying to use heading hold modes other than stability assist.

[Venusgate]

Your probecore also looks a little clipped into your battery, which 9 times out of ten doesn't do anything. But tempts the kraken...

Also, yeah, I'd take the 1.25 reaction wheel out. You have three SAS systems trying to compete for top dawg. I'd say this is a long standing bug, but honestly, I removed that build habit out of my method a long time ago.

[FullMetalMachinist]

To those saying take the reaction wheel out, remember that this is supposed to be a tug. Which means that it likely needs that wheel for when it's docked to other craft and doing its job. 

So instead, leave it there, but make an action group to toggle power to the reaction wheels. 

[bewing]

46 minutes ago, FullMetalMachinist said:

To those saying take the reaction wheel out, remember that this is supposed to be a tug. Which means that it likely needs that wheel for when it's docked to other craft and doing its job. 

It's a tug powered by 8 microscopic RCS jets. It can't create enough torque to out-turn my aunt Rae. The reaction wheel in the OKTO is 10 times more powerful than the jets for torque. With that setup, you cannot possibly need an Advanced Reaction Wheel for the tug to do its job.

[Plusck]

4 hours ago, bewing said:

It's a tug powered by 8 microscopic RCS jets. It can't create enough torque to out-turn my aunt Rae. The reaction wheel in the OKTO is 10 times more powerful than the jets for torque. With that setup, you cannot possibly need an Advanced Reaction Wheel for the tug to do its job.

If its job is to leave a space station, go out to meet an incoming ship, latch on and direct it to a station docking port, then the rcs and reaction wheels together would be both necessary and sufficient. :shrug:

[Claw]

Well, I wouldn't take the reaction wheel off, but you can disable it for times when it isn't needed. (i.e. transiting but not tugging)

I would agree, having too much control authority likely is the issue here. The SAS has a hard time with a craft when it has excess control authority compared to it's size.

[SteveD80]

Disabling the wheel was the first thing I tried and it did not help. I will try moving the batteries when I have time.

And I did make a similar tug without the wheel and it struggled to turn with its loads.

[Norcalplanner]

I've found that things work better when you have both a probe core and a reaction wheel to join them together directly. I'd lose the service bay and mount the core directly to the reaction wheel and see if that helps.

[SteveD80]

After moving the batteries it stopped bouncing around unless I had an SAS target, and even then it was controllable, it just wastes monopropellant.  Hopefully a heavy load will fix the SAS targeting issue as that is quite helpful when docking large items that have poor RCS balance.  

On a future generation I may try @Norcalplanner's suggestion, but I'd rather do that with the stack size core which I haven't unlocked.

Edited May 14, 2016 by SteveD80

[Plusck]

2 hours ago, SteveD80 said:

After moving the batteries it stopped bouncing around unless I had an SAS target, and even then it was controllable, it just wastes monopropellant.  Hopefully a heavy load will fix the SAS targeting issue as that is quite helpful when docking large items that have poor RCS balance.  

On a future generation I may try @Norcalplanner's suggestion, but I'd rather do that with the stack size core which I haven't unlocked.

All joints have a certain flexibility, and this may be compounding your problem (because the part that is controlling sees a change in direction a fraction of a second after the powerful reaction wheels (or RCS) have already aligned the body of the ship to that direction).

The stack-sized core is certainly better, but I'm really not sure it would be necessary here. If you mount the probe core directly to the reaction wheel, with battery above that then docking port on the top, then you'll be sure that "controlling from" the probe core means that the core of the ship corresponds to what the controlling probe thinks is the right direction.

When you attach that to a heavier ship, however, you really must make the "controlling" part something that is as closely bound to the core of the heavy ship as possible - which is likely to be its docking port. If you maintain "control" at the tug's level, it'll be bound to flutter around as the docking port connection is not a strong one.

[FullMetalMachinist]

6 hours ago, SteveD80 said:

After moving the batteries it stopped bouncing around unless I had an SAS target, and even then it was controllable, it just wastes monopropellant.

I'm glad you got it working better, but I have a solution for the part I put in bold. 

A new feature in 1.1.x is that you can set the RCS thrusters to respond only to certain control inputs, like aircraft control surfaces can. So just set your RCS thrusters to use translation only, and they won't do anything for rotation. You shouldn't need them to help rotate, that's what the reaction wheels are for. 

[SteveD80]

1 minute ago, FullMetalMachinist said:

I'm glad you got it working better, but I have a solution for the part I put in bold. 

A new feature in 1.1.x is that you can set the RCS thrusters to respond only to certain control inputs, like aircraft control surfaces can. So just set your RCS thrusters to use translation only, and they won't do anything for rotation. You shouldn't need them to help rotate, that's what the reaction wheels are for. 

Is there a mod that will make this the default setting?  I almost never want to turn with RCS.

[bewing]

15 hours ago, SteveD80 said:

Is there a mod that will make this the default setting?  I almost never want to turn with RCS.

Not yet. But disabling rotation for RCS is a brand-new feature, and Squad will certainly be making tweaks to make it even better in the future.