Ferram aerospace Rocket Toppling Question

[AirComrade]

hey guys,

This is yet another FAR-rocket-toppling question. I've read many such questions and I think I understand what needs to be done in order to make a proper gravity turn. For some reason, I am unable to make my current rocket fly in a stable manner. The center of lift is far below the center of mass. I use fins in the back and procedural fairings for the payload. I adjusted the TWR to different values ranging from 1.1 to 2.0, and I start my gravity turn (5 degrees) over once I hit about 100m/s.

My problem is that as soon as I launch my rocket, it starts leaning in a certain direction (not the 90 degree direction). To correct it (even with fine controls) I try and steer it the other way; inevitably the rocket overshoots, which starts a death wobble. I tried using SAS, no SAS, extra reaction wheels, balancing the payload (it seems balanced when I check the center of mass in the VAB), fins or no fins..nothing seems to work in the way people are saying a gravity turn using FAR should work. The rocket requires constant adjustments in flight, which only increase the wobble.

What do you guys think I should do? Is there something I am missing?

Thanks for your help!

[blizzy78]

I found that the lighter your rockets are, the more straight up you need to go with FAR before starting any gravity turn. This way there will be less atmosphere inducing drag on the vessel so that it would tip over. Once you are high enough in the atmosphere, you can lean over quite far and not lose control. Oh, and I don't usually use any control surfaces because I don't need them.

[ferram4]

Without a picture of the rocket (including payload) there's not much we can do to help you. What it sounds like is happening is that you've got some imbalance starting (perhaps the rocket is flexing or fuel isn't draining symmetrically) and that's messing things up. If it's going off course as soon as you launch (right after it goes off the pad when velocity is very low) then it's not FAR, since aerodynamic forces will be very small compared to thrust and gravity.

[Vigelius]

Sound familiar?:

- the standard advice says rockets fly best if they are weighted like a dart - shift the CoM as high as possible

- the rocket lifts off ok, but then makes a concerted effort to arc over towards the ground, in a random direction

- masses of SAS can bully a small rocket into obedience, but big ones start bending. I had one flex so far, the top was at a ninety degree angle to the engines!

- failure.

I suspect the problem is that, whilst the CoL is happy to follow the CoM, the rocket is so top-heavy that it essentially falls over as soon as it goes anything other than straight up. Fins at the back won't help because it's like holding a big bendy stick with a lump on top, from the bottom end - hold on as tight as you like, it still won't go where you point it. Usually I find it actually helps to drop the CoM back down again - it only needs to be a little above the CoL to be stable. Failing that, a few control surfaces above the CoM might help but starts to smell of over-engineering.

[linkxsc]

Sounds to me like a bit of a wobbling problem. Usually thats all that causes failures on my launches unless the thing is just built wrong.

[AirComrade]

It does sound like a bit of a payload imbalance, simply because the rocket always starts to lean in the same direction. If you guys think this is the case, how can I be sure that the payload is truly balanced? Sometimes it is a bit hard to tell with the VAB CoM indicator. Either way, I'm including the picture of the rocket.

http://imgur.com/GjTmvqI

http://imgur.com/K33oTS5

[Bobnova]

It's your goo container. You need to have a matching pair, or something else that weighs exactly the same on the far side of the Materials bay from said goo container.

It only takes a tiny imbalance to cause big problems, and goo containers aren't all that light.

[ferram4]

It's never going to be balanced with that single Goo canister on the top. And trying to use an RCS tank to balance it is a bad idea, it'll always be off by a bit. The trick when balancing payloads is to remove anything from the payload that is definitely balanced, including symmetry-attached parts and anything that isn't needed to keep the parts that need balancing together. Then balance that, then add all the other parts back.

Just double up on the Goo. It'll solve your problem much faster than what you're trying to do.

[AirComrade]

Folks thank you so much for your help. It looks like after I completely stripped the payload, the rocket became much more stable. I didn't realize how sensitive rockets are to such small weight imbalances.

Thanks again guys!

[Mipe]

Once I tried putting (radial) rockets to the top and payload to bottom. Put horse in front of the cart, so to speak.

The results... weren't quite what I expected.

[linkxsc]

Once I tried putting (radial) rockets to the top and payload to bottom. Put horse in front of the cart, so to speak.

The results... weren't quite what I expected.

A little off topic to the OP.

However on craft that spend a lot of time in deep space with low TWRs. Especially of ones made out of docked together modules. Its often more effective to build them in a pulling config because there will be less wobble than if the engines were pushing.