HELP Steering with Atomic Engines

[DaveBowman]

This feels like a dumb question but I'm having a lot of trouble steering any craft I make, large or small with standard LV-N Atomic Engines. Since they have no vectoring my craft tends to wobble off course -- I can keep it online using RCS but I'm ending up carrying ridiculous amounts of Monoprop to do so. 
 

I know there must be a better way! All help much appreciated! 

[pandaman]

You can use SAS by adding the modules when building the ships, you will need to make sure you have enough electricity though, so don't forget batteries and solar panels.

[Vanamonde]

Pictures of the ship would help pinpoint your trouble. Could you post some? 

Also, moving this thread to the how-to section. 

[Geschosskopf]

1 hour ago, DaveBowman said:

This feels like a dumb question but I'm having a lot of trouble steering any craft I make, large or small with standard LV-N Atomic Engines. Since they have no vectoring my craft tends to wobble off course -- I can keep it online using RCS but I'm ending up carrying ridiculous amounts of Monoprop to do so. 
 

I know there must be a better way! All help much appreciated! 

Welcome to the forum!

Anyway, with all rockets, hit the T key to turn on SAS.  This will use whatever control authority the rocket has to attempt to maintain a constant orientation of the ship.  NOTE:  SAS is basically the software that attempts to hold the rocket steady, the control authority comes from actual parts.  Reaction wheels, aerodynamic control surfaces (in air only), and RCS.  With reaction wheels, you need a source electrical power (which you need anyway).  With RCS you need monopropellant tanks.

Now, I myself don't like engine gimbal at all and almost always disable it because I find it more a cause of wobbles than a cure.  Thus, the lack of gimbal in the LV-N isn't causing your problem.  There are 3 main possibilities for the root cause of your wobbles:

1.  You're not using SAS

2.  The engine's line of thrust doesn't pass through the rocket's center of mass.  If you're only using 1 engine, this can happen if you put the engine off-center or not straight.  It can also be from using radially mounted fuel tanks that aren't all draining at the same rate, so the rocket's center of mass moves to the heavy side over time.  If you've got multiple engines, it can be from not arranging them symmetrically, imbalanced fuel flow, or accidentally putting a slight thrust limitation on 1 engine but not the others.

3.  Your rocket is long and structurally weak, so its joints flex.  This causes the line of thrust to swing back and forth or revolve around the rocket's center of mass.  Fix this by redesigning the rocket and/or using the Kerbal Joint Reinforcement mod.

[Sovek]

It sounds like the CoM is not in line with your CoT.

[Snark]

Welcome to the forums!

+1 to Geschosskopf's excellent explanation.

Just as a sanity check-- do you have a reasonable amount of reaction torque on your ship?  i.e. do you have reaction wheels?  Generally speaking, you should never need RCS to rotate your ship (since that expends monopropellant when you don't need to).

I only ask because you specifically mention LV-Ns as a problem because they don't have gimbal.  Geschosskopf mentions some good potential problems for steering rockets in general, but if those were your problems, I'd expect you to have it with everything, not just specifically LV-N, because none of those are specifically involving engine gimbal per se.