Rockets veering off-course in Kerbin's atmosphere?

[Armament]

I am playing career mode, so don't have access to much yet. I am trying to make a mun lander (made several in sandbox) but they always seem to veer off-course during launch, usually uncontrollable by around 30km. I have SAS on but no modules (not unlocked) & some winglets on the sides, but it is still impossible to control. Everything was built in symmetry mode, & it is not the same direction each time, it will just start tilting off somewhere, every launch for the last 30+ attempts I have made. Anyone know why?

[Tex]

It is nearly impossible to tell without a picture of the vessel. Could you post one?

In general, though, it sounds as though you might be making the rocket too top-heavy. When you are performing the gravity turn, the weight at the front of the rocket might be enough to overpower your reaction torque and pull the whole thing over. Try making a bigger, or at least heavier, booster underneath the payload during the ascent.

[viktor19]

Couldn't agree more with Maximus97. Try to lighten lander or, if possible, make Apollo style mission.

[Red Iron Crown]

Are your winglets toward the bottom of the rocket? If they are below the center of mass they help stabilize it, if they are above the CoM they may destabilize it.

And post pictures++

[K77LostNSpace]

I have noticed this type of issue if my lifting stage has no thrust vectoring. example would be the LV-T45 vs LV-T30.

Early on if all my lifting stage thrust came from LV-T30 and solid rock boosters, the wings only help until i left the influence of the atmosphere. Once in space the only control came from the SAS.

[GoatRider]

I've noticed this problem if my craft isn't rigid, usually because of boosters on radial decouplers. The solution is moar struts.

[LethalDose]

Also, are you using any mods? FAR in particular has a big impact on how rockets fly in the atmo. I've noticed a lot of my rockets will get a little twitchy in the trans/supersonic range.

[D04S02B04]

My personal experience with FAR is that ever since installing it, my rockets have been insanely easy to design because it is simply intuitive for me to design something aerodynamic.

To solve your problems, consider the following:

1. Shaking/Rotating Radial Decouplers - This was a common problem I encountered when using a lot of Solid Fuel Boosters, regardless of RT-10 or BACC or any other model. A high thrust to weight ratio results in those joints shaking and you usually need 1, 2 or 3 strut per booster (RT-10, BACC, S1 SRB KD25k respectively) to secure to the main rocket you're boosting up.

2. Centre of boost significantly below top heavy designs. If your rocket has a huge amount of mass towards the top and very powerful boost towards the bottom, your rocket will become unstable immediately upon launch/stage activation.

3. Over compensation by RCS + SAS combination. Sometimes I completely disable RCS during launch and build in more SAS controls. RCS tends to over compensate and results in small twitchy movement. It continues to try to compensate that twitchy movement and you get even more twitch before it breaks apart. This is really a compounding effect of a rocket that has many parts and has weak joints/linkages or radial decouplers etc.

4. Weak structural joints/linkage - Certain joints in the rockets are weak. Often this occurs in large multi-stage rockets where the sections for orbit or interstellar travel starts shaking because it has a significant amount of mass and the joints are stressed. Placing struts as suggested will help this issue but you have to identify which parts are the one which are disconnecting etc.

My advice in general is plan your mission top down.

a. Bring only what is absolutely needed at your final point (e.g. Science Lab? Goo? Thermometer? etc. ONLY BARE MINIMUM). Fit the smallest possible engine and fuel.

b. Build a redundancy of 10-25% (depending on your competence) to account for human/piloting error throughout your stages working backwards from your final stage.

c. Use Solid Fuel Boosters in the early stages as much as possible. I found that S1 SRB KD25k great for lifting things into higher atmosphere and using RT-10 to assist in achieving stable orbit.

If you follow that advice your rocket will be really paired down and have less moving/shaking parts.

[MarvinKitFox]

My crystal ball says:

Your rocket is a teensy bit wobbly.

and

You have wing surfaces toward the front of the vehicle.

Your rocket is wobbling, presenting ever-changing angle-of-attack to the wings, worsening the wobble.

just move the wings to the extreme rear, and try again.

[Armament]

Thanks K77, my rocket was probably a little too top-heavy (2-stage lander) but using T-45s over T-30s meant I was able to keep stable

[K77LostNSpace]

Sweet, glad I could help.