Rocket Instability after Launch

[Kersplattle]

I have a biggish rocket with 6 large engines as a first stage, and about 15,000 metres up or more the rocket just starts to tilt and gyrate out of control. Having RCS on makes no difference, I have 3 ASAS modules, lots of fins (which maybe why it\'s stable while the atmosphere is 'thick'). Once it starts to tumble nothing stops it, basically it\'s abort or end flight.

Any ideas why it might suddenly go astray ? The craft seems pretty symmetrical. It has a second, third, and lander stages above all using small (1 m) parts, and the connection is 3 large decouplers connected to the second stage. The second and above stages have all flown successfully before in 0.15 and 0.16 on other first stages.

This is on the unmodded 0.16 Mac version and this rocket/model has serious lag issues at take-off.

[leopardenthusiast]

All I can think of is that your capsule isn\'t staying stable. ASAS uses the same information that you can get from the navball, and the navball\'s information is based on the capsule\'s position. Can you run some struts from the lowest point you can directly to the capsule?

Also, you only EVER need one ASAS. Replace some of them with normal SAS or more RCS fuel.

[amore555]

ASAS utilises gimballed engines, wings, surfaces and SAS modules to keep your ship on course. Think of it as the brain.

What you need is more SAS modules

[bsalis]

Does it wobble and or bend during ascent?

What happens if you take all the fins off?

... and whats with the RCS tanks on the first stage and SRBs!

[Kersplattle]

It doesn\'t wobble excessively, but it does a bit. The RCS tanks are just to give me enough RCS fuel to start lift off with RCS on, so many to remain symmetrical, may not be necessary with this version of the craft.

An earlier version had a smaller lander stage at the top, less height. It might be that it does get unstable at the top as that version was more stable, but had a smaller first stage. It enabled a Minimus landing, take-off, traverse to Mun, land, and take off with the final stage and make it home, but it was quite marginal fuelwise on the Mun landing, hence the attempt to lift a further few tanks into orbit.

I\'ll experiment a bit with the suggestions, thanks. Bit late now though, more tomorrow.

[bsalis]

To me it just seems that the rocket is tall (ie. bendy), and your using fins, and gimbling engines, and RCS. My instinct tells me that the ASAS is mebe getting a bit confused, and the parts are trying to go in all directions.

Also note that air friction seems more an issue now with big ships. Try throttling down to keep your speed slower to see if it\'s a threshold of friction causing the loss of control.

[Flowz0r]

If you need RCS for stabilization, there is something fundamentally wrong with your rocket design.

Can you try launching it without RCS and the wings and see if that fixes anything? It almost seems like those are overcorrecting a bunch, which is making the rocket wobble.

[yyri]

Try building smaller rocket to lift the lander/plane, it really doesn\'t need to be that huge. With smaller rocket you need less fuel-> size becomes smaller.

[Kersplattle]

Well, could be a little unsymmetrical at the bottom stage, I\'ve found that it helps with larger designs to avoid instability at lift off to use RCS until you get to,say, 40 m/s. That does tend to imply that the fins are needed for directional stability and that the thrust is not quite as symmetric as I would like.

I can build smaller craft, I want to lift more into orbit though. Might try using more large parts and reduce the staging. There\'s a fairly fine line I\'ve found between a reasonable stable craft and something with just a bit more power. Can be frustrating trying to get the balance just right.

I have a 0.15 craft that did about 20 Mun and/or Minimus landings quite easily, and want to get someone larger to enable more flexibility. Doing a Mun then Minimus, which seems a harder mission than Minimus then Mun.

Thanks again though for the feedback.

[Ziff]

I have a biggish rocket...

Yeah, you can say that again, louder, so the Kerbals at the top can hear you. Holy kerbals that\'s big. It looks to me like you\'re missing some struts between your top stage and the next one down. Those decouplers may very well be causing a wobble because of all the thrust and pressure below it. 15km? Sounds like you have excessive gforces working on the ship at that point. Are those 6? main 2m engines still firing at that point? Cause that could definitely be part of the problem.

Craft file so I can play with it and see?

-Ziff

[UmbralRaptor]

Are you still using the big LV-T30s when you have the instability issues, or or do they happen after you\'ve staged? (If the former, I suspect a certain amount of drag-related wackiness. If the later, it\'s lack of control authority in the upper stages, probably fixable with some vectored thrust.)

[Phoenix Aerospace]

Needs moar struts.