SSTO keeps losing control

[Ikaneko]

I built an SSTO on Sunday. It functions perfectly in atmo but when I fire up the rocket engine (LV-T30) in space it pitches upwards almost uncontrollably. However once I deorbit and enter into atmo again the craft rolls and spins whenever I attempt to control it. Is this a usual problem?

Edited April 21, 2014 by Ikaneko
Question Answered

[Ikaneko]

Will get sceenshots in a bit.

[aesthetics]

It sounds like a balancing issue. Is the thrust centered behind the COM?

[Sirrobert]

Sounds like the rocket engine isn't in line with the COM

[Ikaneko]

O.K. Will check next time I get on KSP.

[Claw]

It sounds like a balancing issue. Is the thrust centered behind the COM?

Sounds like the rocket engine isn't in line with the COM

Pictures will help a lot. Try to include a side shot with the CoM, CoT, and CoL markers on.

What is probably happening when you fire the rocket is what Sirrobert said. The LV-T30 will be pretty powerful if your ship is small and will cause it to go out of control. You want the Center of Thrust (CoT) pink marker to line up with the Center of Mass (CoM) yellow marker.

As for the out of control on reentry, thr CoM is likely shifting back during flight because you burn fuel. The Center of Lift (CoL) blue marker needs to stay behind the CoM during atmospheric flight. You can test the CoM's location by right clicking on the fuel tanks while in the VAB. When you right click, there is a slider that lets you adjust the fuel load. Set up the fuel configuration to match your reentry fuel condition and make sure the CoM is in front of the CoL. (Remember to put the fuel load back when you're done.)

Also make sure your plane is pointed mostly at the prograde marker during reentry (or slightly nose up) and not sideways, top first, or backwards.

[Ikaneko]

Screenshots can be found here: http://imgur.com/a/zH6wd#0

[Taki117]

Screenshots can be found here: http://imgur.com/a/zH6wd#0

You're immediate issue is that your CoM (Center of Mass) is too far behind your CoL (Center of Lift) This causes instability, and even more so as you burn fuel (And your CoM shifts) A good way to fix this is to remove the canards on the front of the craft,

[Claw]

Also, the landing gear causes the SPH to lie about the CoM (they are massless in flight). So you may want to remove your landing gear if you are dealing with very small CoM/CoT margins.

[Ikaneko]

How do I change the prefix to answered now? (Sorry, joined earlier today)

[lammatt]

your thrust doesnt line up with your center of mass.

[Taki117]

How do I change the prefix to answered now? (Sorry, joined earlier today)

You need to edit the first post and then click "Go Advanced"