My Rockets are tipping

[Ocie]

First of all, I understand that Aerodynamic forces can cause a rocket to lose control, and also that changes in the center of mass can, but now that I have eliminated those two possibilites I know of, I'm stuck. The rocket I am working on is a probe version using a different upper stage than the original, but the rest is the same. The center of mass in every stage is also almost completely the same. 

Of course the probe version didn't work as planned. It will go through it's first booster stage and raise it to around 12k meters. Then that stage is ejected and immediatly afterwards, then the second stage (which is supposed to be stable, it has wings and an okay center of mass) starts flipping around.

Observations -

It seems as if SAS makes it worse, but turning it off dosen't fix the problem

If I start the rocket on it's second stage (It will fly to the same place it flipped at) it dosen't become unstable even though it is at the same inclination and altitude that it would begin it's burn. 

I turned off the wing's yaw and roll control on the whole rocket, the same kind of unstability occured to the rocket directly after launch.

 

I really am unsure of what is causing the unstability, I will answer any questions about it if anyone asks! 

[Geonovast]

It's hard to even guess without at least pictures.  Ideally both in flight and in the VAB with CoM and CoL markers on, if you can.

The craft file itself would also be helpful.

Welcome aboard, @Ocie!  We'll get you squared away.

Even if the second stage appears stable launching by itself, if your nose is pointing far off prograde when you separate, it can still cause issues.  12km isn't that high.  I usually try to make my first stage last until at least 20km.

[Ocie]

@Geonovast Thanks for replying, the reason that my first stage only goes to around 12k is because i recently began my modpack game so I don't have access to great stuff yet. I will post a video of the spacecraft in flight, and of the structural details of the craft, that will be when i get home today.

[Aegolius13]

Welcome aboard!  I agree with @Geonovast, it's hard to know for sure without pics or a craft file or whatnot.  That said, from your description I have two main suspects:

1. Center of mass vs. center of drag issues.  First, if you've got wings, this is almost always something to deal with.  Second, you mention that this happens when you use a probe core instead of a command pod.  The command pod likely puts more mass at the front of the craft, which promotes stability.  This is also consistent with the idea that when you launch the second stage by itself, this problem does not occur -- that be because the fuel you've burned to get to that point has removed mass from the bottom of the craft.  If this is the case, solutions are:

• Move the center of drag further back by moving back the wings, along with any other draggy parts you can move (landing legs, etc).
• Move the center of mass further forward by swapping the order of your parts. 
• Keep drag at the front of the craft to an absolute minimum. (see next possibility).  

2.  Added drag at the front of the craft.  It's also possible your substitution of parts has increased drag somehow.  If you have any form factor changes (e.g., going from a .625m to a 1.25m part size with no adapter), those can impose a lot of drag, and hence instability.  A similar thing can happen if you have blunt parts at the front of your stack (instead of a nice pointy nose cone), or equipment attached radially.  It's not clear why this would not be happening when you launch the second stage by itself, but perhaps you're going slower at that point in your ascent, so the issue is just smaller in magnitude?   In any event, it's always good practice to avoid unnecessary sources of drag at the front of your rocket, since doing so promotes both delta-v efficiency and aerodynamic stability.  

 

 

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[Snark]

On 1/15/2019 at 5:13 AM, Ocie said:

I really am unsure of what is causing the unstability, I will answer any questions about it if anyone asks!

Really, really need a screenshot of your ship.  Virtually impossible to provide specific advice without seeing how you have things set up, since things like the exact placement of wings, fins, etc. matter a lot.

My guess is that your problem will be easily solved if we can see a screenshot; very difficult to help without one.

[CeeBod]

On a related note, does anyone have any advice for how to get the Mobile Lab into orbit in (relatively) early career mode?  I've not had trouble with my other rockets tipping over so far, butwhen I try to design something to lift the MPL into orbit, I find it's always tipping over.  A long and thin design I tried made sense to me as to why it was tipping, but I've also tried building out, even putting my main fuel tanks either side of the MPL and having them feed across to a rocket underneath the MPL, and even that didn't work!

I only just unlocked the science node with the MPL, so I'm missing lots of parts on the same tier, but as my current 'explore' contract is for docking on or around the Mun, I really wanted to try putting a basic Lab in orbit around the Mun as the start of a space station - which I could then dock to with a lander, to help maximise my science

[bewing]

5 hours ago, CeeBod said:

when I try to design something to lift the MPL into orbit, I find it's always tipping over.

Do you have kickback SRBs yet? They can get almost anything into orbit.

[MaverickSawyer]

7 hours ago, CeeBod said:

On a related note, does anyone have any advice for how to get the Mobile Lab into orbit in (relatively) early career mode?  I've not had trouble with my other rockets tipping over so far, butwhen I try to design something to lift the MPL into orbit, I find it's always tipping over.  A long and thin design I tried made sense to me as to why it was tipping, but I've also tried building out, even putting my main fuel tanks either side of the MPL and having them feed across to a rocket underneath the MPL, and even that didn't work!

I only just unlocked the science node with the MPL, so I'm missing lots of parts on the same tier, but as my current 'explore' contract is for docking on or around the Mun, I really wanted to try putting a basic Lab in orbit around the Mun as the start of a space station - which I could then dock to with a lander, to help maximise my science

If you have the Jumbo 64 tank and Skipper unlocked, you can get pretty much ANYTHING to orbit. Think "Delta IV Heavy", and you're on the right track.

 

Also, HIGHLY recommend using CorrectCOL and RCSBuildAid. They make designing anything much easier.

[CeeBod]

Thanks for the answers, but I actually managed to get it done after seeing someone else use have a rocket with the payload that wasn't at the top. I don't know why I didn't think of it before, but moving the MPL underneath a couple of fuel tanks used for the final stage made a huge difference to how stable it was, and as a result I just successfully docked a ship to a station in orbit around the Mun for the first time ever! :o)

I had to transfer fuel to the docked ship because I messed up its design and completely failed to connect up two of three small tanks, but I'm actually happy that it gave me an additional reason for needing to actually get my head around docking - or at least I was when I finally succeeded!

[Geonovast]

Glad you got it working, @CeeBod!

I've done exactly what you did to get it working.  However, while there's nothing wrong with it, it's more of a band-aid that may not work with everything.  I don't know about the other folks in here, but I'm still curious to see what you were originally working with, and what eventually worked.  Learning how to fix your original design, I think, would still help you design stuff in the future.

[CeeBod]

Thanks @Geonovast

I didn't save the ship, but I did save the final stage in its original form, so if I can actually figure out how to add a screenshot here, then here's a pic of that -

the one I got into orbit has the hitch-hiker module removed (I didn't realise how heavy that thing was!) and the MPL was moved below the 2 fuel tanks.  I think to get it into orbit I just strapped on some boosters!