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montferrer

Why does this rocket always flips?

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I'm designing an Eve launcher but I'm having problems while testing it on Kerbin. I'm using asparagus staging, with 3 booster groups. 2 with aerospikes and the last one with "LV-909 "Terrier" Liquid Fuel Engine".

But when the half of the first tank gets consumed, the rocket always flips. I don't know what I'm doing wrong, since the COM is always below de middle. Here are some pics that hopefully will help you to understand the problem, as you can see I have tried with diferend pods to make sure it was not a drag problem: http://imgur.com/a/opUs4

 

Thank you very much, :D I need to rescue some Kerbals! 

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Among other reasons you're having trouble, the aerospike engine does not gimbal to assist with steering and attitude control. 

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Eve ascent vehicles are quite harder to deal with flipping than other rockets.

dV is much higher than on Kerbin, so you need more staging. That mean your rocket frame will change quite rapidly. On Kerbin, you usually dump you lower stage when the atmosphere don't affect your ship much so lack of fins on your second stage aren't an issue.

But on Eve, you dump your first stage while still in heavy soup. If your second stage doesn't have fins, it'll probably flip. but if it has fin, your first stage may flip because you moved your COL forward (so your rocket become more unstable).

Then you can still add bigger fins on first stage which will solve the whole issue... until you remember you have to land this thing on Eve. Big fins can make the whole ship flip head first on reentry (which is baaad)

Eve landers/ascent vehicles are kind of a puzzle.

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32 minutes ago, montferrer said:

I don't know what I'm doing wrong, since the COM is always below de middle.

...and that's exactly what you're doing wrong. Your COM should be as close to the front as possible (which, when going up, is at the top).

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1 minute ago, Kerbart said:

...and that's exactly what you're doing wrong. Your COM should be as close to the front as possible (which, when going up, is at the top).

Well, "doing it wrong" is maybe a touch harsh, since it's extremely difficult to make a rocket that is top-heavy in the early stages of its ascent.

But certainly "understanding it wrong", yes.

 

In addition to the obvious need to add fins, one thing that might help is to switch the fuel tanks around vertically. As it is, the small fuel tanks stay full at the bottom of the rocket as the bigger ones empty at the top. Switching them around would leave CoM higher up the rocket at all times.

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7 hours ago, Plusck said:

Well, "doing it wrong" is maybe a touch harsh

Eve is harsh, though, and (especially below 3-5km) a low COM is a death sentence. It's more like swimming than flying down there. You NEED a sleek craft that is front heavy, at the exact same time it's hard to have a sleek craft that's front heavy. Flipping was the last problem I solved on my Eve ascent vehicle and it was the reason I GAVE UP trying to make for for several weeks.

To the OP, you need fins at the back and a sleeker craft. You may even need delta wings to keep things pointed straight. There are some great ideas in this thread:

 

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Long skinny rigs are happiest on Eve, and hardest to land in one piece. Eve is a harsh mistress.

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1 hour ago, 5thHorseman said:

Eve is harsh, though, and (especially below 3-5km) a low COM is a death sentence. It's more like swimming than flying down there. You NEED a sleek craft that is front heavy, at the exact same time it's hard to have a sleek craft that's front heavy. Flipping was the last problem I solved on my Eve ascent vehicle and it was the reason I GAVE UP trying to make for for several weeks.

To the OP, you need fins at the back and a sleeker craft. You may even need delta wings to keep things pointed straight. There are some great ideas in this thread:

 

I've only managed one Eve ascent, in 1.0.5, purposefully "old-school" style (i.e. without using hyperedit, which was actually a very painful and pointless restriction since I brought three landers to Eve the slow way before I got one able to survive the landing).

Still, my solution was like some of the lighter ones in that thread, for control: the smallest and lightest ailerons rather than actual fins. Oddly I never encountered flipping problems on Eve, those control surfaces on their own made the actual ascent very easy. Maybe I was just lucky...

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20 hours ago, Vanamonde said:

Among other reasons you're having trouble, the aerospike engine does not gimbal to assist with steering and attitude control. 

Best answer. You need gimbals and/or fins for stability.

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