SSTO tumbles when leaving atmosphere

[Bojac6]

Hi, so I'm just getting into SSTO building and I'm hitting a snag. I have FAR installed and used some of their planes, and it went pretty smoothly, more or less. But then I get to my designs and things fall apart. I understand the basics of getting to about 20k, and can tweak my designs to do that reliably. However, once I get to the cut over from jets to rockets, my own designs always start tumbling. It's uncontrollable and not recoverable every time. There's usually quite a bit of rocking to start, and then a full tumble.

I've tried SAS, ASAS with RCS, and the avionics package. I've tried cutting off the jets earlier (around .14 air intake) to make sure I'm not having flame outs. I've tried cutting the jets and coasting before engaging the rocket and I've tried running them simultaneously. Either way, the same result every time.

Here's a picture of my latest design. Generic, sure, but it's mine and I want it to work

Any tips or help is greatly appreciated.

[stupid_chris]

Well first off, the engine you're bringing along can't gimbal, so for the force it applies to your rather small craft, it's understandable that RCS can't keep it straight.

Second, those huge "tail fins" are probably moving your center of mass upwards, making it even worse for the problem mentioned above. You should check in the SPH where your CoM is compared to the CoT with the engine. It's likely above it.

Changing this LV-T30 for an LV-45 should do the trick. If it doesn't and your CoM really is slightly up because of those wings on the top, switch them for actual tail fins or winglets.

[Bojac6]

I thought the T30 was the one without gimbals and the 45 was?

In any event, I'll test out your changes now, thanks!

[Aragosnat]

Yeah. I had that problem as well. The tail wings are most likely the colpreate at high speeds and high altitudes. I have stopped using tail fins and the conrads (little wings in front) altogether now as you can see in this pic.

[stupid_chris]

I thought the T30 was the one without gimbals and the 45 was?

In any event, I'll test out your changes now, thanks!

Exactly. It seems to me you have a T30 on your plane. I might be wrong, it's hard to tell from that angle, but if you do have a T45, ignore that comment. Chances are that it'S the CoM that's the problem then.

[Bojac6]

Ok, your suggestions did not work, but I don't think it's because you were wrong. I tried a rocket launch and when I started the gravity turn, it completely flipped out and could not be controlled at about 20k, so I'm thinking there may be some kind of mod clash going on or something

Thanks for the design tips and I'll take them into account after I reinstall

[Turd]

I had the same issue on my craft. The likely fixes:

Install thrust vectoring engines

Make sure center of thrust is PERFECTLY aligned with center of mass

Make sure center of mass is in a position that wont shift very much due to fuel use

Keep center of pressure/lift slightly behind your center of mass to keep it slightly more stable.

Position RCS on the far ends of craft to help with any remaining stability issues

When you craft reaches extreme altitude it stops behaving like a plane and becomes a rocket, aerodynamics will no longer serve to stabilize your plane, and control surfaces can no longer be used to keep control. Instead the little atmosphere only serves to make your craft more difficult to stabilize after it flips out.