another nub question

[Conarr]

If you have a launch vehicle flipping ass over tea kettle (end over end) is that an indication that the rocket is too tall and the center of mass is too high?

[DMagic]

It could be. Does this happen right away? or during the later stages of ascent. If you have a lot of fuel at the top you can sometimes manually transfer it down to your lower stages to make it less top heavy. This can be a little tricky though, and that fuel is usually there for a reason.

This could also mean there is an imbalance somewhere that is pulling the rocket off center. Being too wobbly can also make things tip over sometimes, too.

[RoboRay]

No such thing as "too tall" really. In fact, a longer rocket tends to be easier to stabilize because you can put control surfaces or thrusters further away from the CoM.

Which flies without tumbling, an arrow or a ball?

In any case, you need to ensure that most of the craft's drag is behind the CoM. If the drag is mostly in front of the CoM, then yes, it will want to weather-vane around backwards. You also need to ensure the CoM doesn't shift laterally during flight from asymmetrical fuel burn-off.

Edited May 15, 2013 by RoboRay

[Conarr]

It could be. Does this happen right away? or during the later stages of ascent.

The loss of control event starts around 1500m. I don't think there is an imbalance, at least the CoM seems to be centered when looking down on the crafts from above. As for wobble, it's rock steady.

[Conarr]

If the drag is mostly in front of the CoM, then yes, it will want to weather-vane around backwards. You also need to ensure the CoM doesn't shift laterally during flight from asymmetrical fuel burn-off.

I'm not sure how to tell if the drag is behind the CoM. How do you tell? I think you're on to something with the asymmetric fuel consumption. I wonder if I might have accidentally knocked off a fuel line while mucking about. I'll triple check that

EDIT: You nailed it RoboRay. There is definitely a fuel imbalance. I nuked all the fuel lines and ran a new line from the outer tanks to the inner and it went straight as an arrow. I'll setup the lines properly now and continue with my testing. Thank you for your help.

Edited May 15, 2013 by Conarr

[Specialist290]

EDIT: You nailed it RoboRay. There is definitely a fuel imbalance. I nuked all the fuel lines and ran a new line from the outer tanks to the inner and it went straight as an arrow. I'll setup the lines properly now and continue with my testing. Thank you for your help.

Improperly-routed fuel lines are always "fun" to deal with. One of these days I'm going to have to share the story of my one lopsided Mun lander in full.

[RoboRay]

I'm not sure how to tell if the drag is behind the CoM. How do you tell? I think you're on to something with the asymmetric fuel consumption. I wonder if I might have accidentally knocked off a fuel line while mucking about. I'll triple check that

KSP doesn't show you the center of drag, but on a spaceplane it's usually pretty close to the center of lift (lift and drag are directly proportional). For a vertical rocket, just try not to put draggy stuff up high. So, no control fins up there for sure. Your aero control surfaces should all be attached as low as you can possibly get them on the vehicle... fins in the middle are useless and fins up toward the nose destabilize you. If you simply must lift a high-drag payload, you can compensate for it with huge fins on back. You'll have to burn more fuel to overcome all that drag, but at least you can fly straight.

EDIT: You nailed it RoboRay. There is definitely a fuel imbalance. I nuked all the fuel lines and ran a new line from the outer tanks to the inner and it went straight as an arrow. I'll setup the lines properly now and continue with my testing. Thank you for your help.

Make sure you don't have any loops in the fuel system. Each engine needs to see a branching tree of linear fuel tank connections, with no loops or multiple connections to the same tank. If you split the flow and then bring it back together, or let fuel run out of a tank then back into that same tank, you can confuse the logic. When that happens, the processing gets stuck in a loop and eventually terminates in a silent error. The system then allows the engine to draw fuel from whatever tank was being checked when it terminated, often leading to unexpected fuel-drain results.

Edited May 15, 2013 by RoboRay

[Conarr]

KSP doesn't show you the center of drag, but on a spaceplane it's usually pretty close to the center of lift (lift and drag are directly proportional). For a vertical rocket, just try not to put draggy stuff up high. So, no control fins up there for sure.

snip

Make sure you don't have any loops in the fuel system. Each engine needs to see a branching tree of linear fuel tank connections, with no loops or multiple connections to the same tank. If you split the flow and then bring it back together, or let fuel run out of a tank then back into that same tank, you can confuse the logic. When that happens, the processing gets stuck in a loop and eventually terminates in a silent error. The system then allows the engine to draw fuel from whatever tank was being checked when it terminated, often leading to unexpected fuel-drain results.

Hey thanks. I always used advance canards about halfway up the rocket if things got a little wonky. It seemed to help, but what you're saying actually makes sense. I'll give it some testing on my existing lifters - well lifter. I only have one I trust right now for the 50 ton (unit) payload I want in orbit.

Conarr

[RoboRay]

Aero controls in the middle of the craft can affect roll, but they can't really do anything to pitch or yaw. So, if you need more fins, just keep adding to the back. That's where they're the most effective.