mewp

I've made a video that might actually help a bit here: ..but perhaps you already know what's going on with the COM and COL. First off, it's ok to put control surfaces ahead of your COM, it is only the static aerodynamic surfaces (wings, fins, etc..) that should be aft of your COM. That being said, control surfaces ahead of your COM should be strictly controlled using ASAS (or some form of control system) as without that they're just the same as a static wing (and thus detract from your stability if they're ahead of the COM). My first thought about your setup is this: you're using aerospikes, which have no gimbaling ability. As your rocket fly's through the atmosphere your control surfaces slowly become less and less effective - what I suspect is happening is at 10000m or so you're hitting a point at which your forward control surfaces can no longer keep you stable (they just can't grab enough air to keep the rocket stable) and your aerospikes, unlike other engines that can gimbal, aren't doing anything to help keep your rocket in the right direction. My recommendation: get some more control surfaces aft of your COM, and one thing that should help is having RCS for when you reach higher altitudes. For rockets without gimbaling nozzles there is no form of control when you're in the vacuum of space without RCS and SAS torque (which isn't a whole lot to begin with). Hope this helps!

Thanks! And you bring up another good point about stability of things while they're flying through air. The placement of the COM and the COL laterally along the plane (in the direction of intended travel) matters significantly in how well the plane fly's (really how well the plane controls its pitch), however the COM and COL placement along the height of the plane does matter as well. The height-wise placement of the COM and the COL dictate your roll stability - having the COM above your COL would act in the reverse pendulum effect, and the reverse for having the COL above the COM. The physics behind a plane being more stable in roll with the COL above the COM is a bit complicated, as well, the height-wise placement of the two points affects other stability parameters. That being said there are some interested examples to note - significantly is what is called "wing dihedral", which is how much the wings are rotated up or down with respect to the long axis of the plane. Planes with a positive dihedral are trying to bring the COL up, and the planes with negative dihedral are trying to bring the COL down. A good illustration of a positive dihedral plane, as well as a quick preview of how the physics make the plane more stable in roll can be summed up simply by this image http://history.nasa.gov/SP-367/fig144.jpg Hope this hasn't put you to sleep!

If you're looking to fly on Duna, it should be relatively similar to Kerbin. The handling characteristics of your plane should be similar to when you're in the upper atmosphere of Kerbin - you will have a little less dynamic control authority from your flaps / canards / movable wings (that is to say if you wanted to up and turn your plane around using your controls, it would take a little longer), and you should have nearly identical static stability of the plane (the natural tendency to your plane - does it flip over itself when you let go of the controls? or does it glide nicely?). The one thing to keep in mind is if you're burning fuel from your plane to get to Duna, of course your center of gravity can shift significantly. So although you've tested your plane out and it fly's fine on Kerbin on a full tank of gas, it may not be stable or as stable on Duna with a half a tank of gas. If you're interested I did a quick video on aircraft stability (and rocket stability - they're similar topics):

As mentioned, I have done a quick tutorial on aircraft stability as well - here a look I'll be doing a few more videos on some more complex flight concepts - let me know if these are useful. Thanks!

Hi Jason, I can't quite give you a step by step response on building an aircraft (although at some point I'd like to make a video along those lines), however I did make a quick video on aircraft stability The moral of the story here is to generally keep your center of lift (the blue ball) behind your center of gravity (the yellow ball).Hope this helps, and feel free to PM me or respond here if you have any further questions.

How many stages are you using? Remember appropriate staging can drastically increase the fuel efficiency of your rocket.

Did you remember to use capslock to turn on fine control? Generally that helps quite a bit.

Hi everyone, I understand this may be understood by some of you, but I figured it was an essential enough topic that anyone who makes a rocket in KSP could benefit from understanding it. So I made a quick video tutorial on rocket dynamics and stability, located here. I wanted to post it to this forum to have a place to record feedback - if you have any suggestion, questions, or comments, please feel free to let me know and I'll try to respond as quickly as possible. I will also soon be doing a brief tutorial on more advanced rocket/aircraft dynamics and control theory (in a nutshell, how to make the most out of your SAS/ASAS). Hope this helps!