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Asparagus-staged rockets spin uncontrollably with FAR


condraj2

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Hey all,

I'm just getting used to the Ferram Aerospace mod and I'm struggling to deal with a major problem with asparagus-staging rockets after launch. I realize that I need to be gentle with my gravity turn.. I'd initially be sent into a tailspin around 10 KM because I think I was tipping over too drastically. Now I'm having the issue of being entirely out of the atmosphere, and after ejecting 2 of the 6 tanks/engines, I spin uncontrollably whenever the engines are active, causing it to be impossible to achieve orbit, or really do much of anything. Is this a bug with FAR, do asparagus-rockets not work with FAR, or do I just fundamentally misunderstand something here?

Any help is much appreciated!

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I use asparagus staged rockets with FAR all the time, and I've never seen this problem once I got out of the atmosphere. They're touchy below about 20-25km, but once that far up, it's easy to forget that FAR is installed. My guess is there's something else going on, though it could be a bug with the latest version of FAR, I haven't upgraded yet, still one version behind.

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This isn't a FAR-caused issue; this is an issue that happens anytime in KSP when you have insufficiently secured boosters.

What's happening is that your parallel stages are flexing at the radial decoupler under thrust, and are not simply flexing inwards; they are also flexing such that each booster's thrust produces a slight rolling moment. Likely what happened was you redesigned your asparagus-staged rocket for use with FAR and weren't as liberal with the struts as you were in previous designs or you struted the boosters in such a way that they were more prone to flex in one way than another. Another possibility is that you used fins for roll control in the atmosphere, and now that you're out of the atmosphere you don't have any roll control.

Short answer is, no, it's not FAR; your design flexing at the joints is what's causing that. If you put it in space in stock KSP it would do the same thing.

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This isn't a FAR-caused issue; this is an issue that happens anytime in KSP when you have insufficiently secured boosters.

What's happening is that your parallel stages are flexing at the radial decoupler under thrust, and are not simply flexing inwards; they are also flexing such that each booster's thrust produces a slight rolling moment. Likely what happened was you redesigned your asparagus-staged rocket for use with FAR and weren't as liberal with the struts as you were in previous designs or you struted the boosters in such a way that they were more prone to flex in one way than another. Another possibility is that you used fins for roll control in the atmosphere, and now that you're out of the atmosphere you don't have any roll control.

Short answer is, no, it's not FAR; your design flexing at the joints is what's causing that. If you put it in space in stock KSP it would do the same thing.

Interesting. I didn't do much differently in terms of design, though, so I'll have to mess around with placement of the decoupler, struts, etc. Thanks for the advice, ferram!

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There are a few issues at work here. As someone else mentioned, it's likely that your boosters need struts, and you should disable gimballing to be safe. That said, FAR adds in a more dynamic drag model that includes cylinder drag. Furthermore, as fuel burns the com of the rocket moves up and makes it more unstable.

I've found adding 3 or 4 canard type wings (the ones with control surfaces) to the bottom of the core rocket makes a huge difference. Don't put wings on the side boosters, if they flex it will really mess things up and it makes separation events dicey. Also, use the FAR setting to disable roll control. The wings will keep the rocket stable and minimize roll while providing pitch/yaw control. Adding reaction wheels helps too.

If the rocket is still too twitchy reduce the allowable control deflection in the FAR panel.

Before launch open the FAR info window. During flight monitor the dynamic pressure (Q) term. Its driven by speed and air density, so it goes up as you go faster, down as you gain altitude. Whenever launch broadcasts mention Max-Q this is what they are talking about. When it's big (depends on your rocket) make gentle movements.

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