Why does this rocket insist on flying backwards?

[zarakon]

The root part is the service bay, and inside that is an OKTO probe and four small battmans. The upper stage engine is a 48-7S and the first stage engine is an LV-T45

Around 10-15km it suddenly decides to flip backwards, and I can't do anything about it. I have SAS on stabilize, and I haven't locked the LV-T45 gimbal

Replacing the 48-7S with an LV-909 seems to help a tiny bit, but not much.

If I replace the service bay, probe, and nosecone all with an Mk1 Command Pod, it works flawlessly. Is there some kind of issue with the service bay or the nosecone that's causing me grief?

[Logris]

It will be because there isn't enough mass at the top of the rocket.

I imagine by about 15km you have emptied the top 1 maybe 2 fuel tanks.

This means your center of mass gets lower and lower until it causes the rocket to flip over.

This is why changing to the command pod fixes it, because that adds enough mass right at the top to compensate.

Perhaps you could add another stage, or add more fuel to the payload.

Also bigger/more fins as low as you can get them will help.

[Starwhip]

The aero forces around the 48-7S are not helping, I assure you. Remember, aerodynamics now plays a huge part in the game!

Are you staying inside your prograde marker on ascent? If not you will be generating unnecessary drag, and most likely flip over.

[zarakon]

It will be because there isn't enough mass at the top of the rocket.

I imagine by about 15km you have emptied the top 1 maybe 2 fuel tanks.

This means your center of mass gets lower and lower until it causes the rocket to flip over.

This is why changing to the command pod fixes it, because that adds enough mass right at the top to compensate.

Perhaps you could add another stage, or add more fuel to the payload.

Also bigger/more fins as low as you can get them will help.

Yeah, this seems to be spot-on

I doubled the fins and slid them farther down, and also moved the upper stage fuel tank to be above the service bay. Now it flies like an arrow!