COL, COM questions and stability

[V3xatious]

Okay, so to start off. Hi! First time on the forum. Played for a couple years, took a huge break, and recently started going nuts again. I play career mode on hard. I feel that's the only way to play. (Reverting flight? Pfffftt)

Anyways, I've looked on this forum, as well as googled, and read many tutorials and videos on this subject.

My main problem is, even when I'm watching my Center of lift and my Center of mass during each stage, I keep having issues keeping the rocket going up while trying to escape the atmosphere. I read a guide where the COL should be below the COM, but even with that I've had issues. I've tried putting it inside the COM. Still issues.

Randomly, I'll have ships that are perfectly stable. Others aren't for unknown reason.

I'm wondering if someone can better describe why these things happen? I keep wasting untolds amount of money in career mode having to restart and do it again, only to get stuck as soon as I get to the contracts involving orbit/mun pass by's.

I can splice some videos together if need be showing the COL and COM, and then the post flight. I haven't been able to find a pattern based on those numbers as to why my stability feels so random.

Oh! Before you ask. I've seen this elsewhere. SAS or no, this still happens.

Thanks!

[Kelderek]

Unless you're using a spaceplane with wings (lifting surfaces) then you can safely ignore the CoL. For rockets, you want the CoM as high as possible. If you are having trouble with your rocket flipping then that usually means you are straying too far from your prograde marker while you turn. Your rocket needs to be streamlined, so use nose cones or fairings and make sure any turns you do in the atmosphere (like the main gravity turn) are very slow and gradual.

If you do more searches you should be able to find a ton of threads talking about this exact problem and how to fix it in 1.0.2.

[cybersol]

Spinning out of control essentially comes down to a contest between drag and control. When the drag overcomes your control authority, you go for a spin. You can decrease drag by staying closer to prograde and building a more aerodynamic rocket. You can increase control using engine gimbals, stabalizer wheels, and control surfaces.

Also note that drag increases significantly right around mach 1 (340 m/s). So you should stay pointed as prograde as possible at speeds around 280-400 m/s.

[V3xatious]

Oh. I've been using CoL this whole time...it moved around when I placed fins. I just realized that the guide I read was for planes.

This is going straight up. I'm not trying to perform my turn yet. Before the new physics this was no issue. Orbit was the easiest thing. Though I do like the challenge.

I think it might have to do with Mach 1. Usually after my first solid fuel booster i'm 400m/s+. As soon as I activate my normal engine, it spins out. It's a single pipe with a few fins, so I'm not sure why it wouldn't be aerodynamic. I'll have to mess around with it more and see if I can find some patterns. I haven't been able to research stabilizers or any of those yet, so it comes down to the Pilot and myself to keep it flying correctly.

[cybersol]

Post a pic of the stage that spins.

On a rocket control fins should be as low and outside as possible. If you are using control fins on more than one stage, the CoL needs to stay below the CoM at all times, even as the tanks empty.

[V3xatious]

Wait, so CoL does matter? And that's what I had been doing, keeping it below CoM. I've tried it at varying lengths between the two and often get the same result. Let me work on those photos.

[cybersol]

Wait, so CoL does matter? And that's what I had been doing, keeping it below CoM. I've tried it at varying lengths between the two and often get the same result. Let me work on those photos.

CoL does matter if you using fins, but on a rocket the standard advise really should be to keep it as low as possible.

[V3xatious]

CoL does matter if you using fins, but on a rocket the standard advise really should be to keep it as low as possible.

That's what I originally thought, but it's at the lowest before I put any fins on it. Putting fins on it moves the CoL around the CoM. I'm about to do another flight, I'll see what happens and take some screenshots.

[Snark]

What really matters for stability (for rockets) is center of drag-- that's the thing you have to make sure stays behind the CoM.

Unfortunately, there's no handy CoD indicator in the VAB, since drag is situational. Main thing is to keep drag in mind. In particular, make sure your payload up front is as streamlined as possible, ideally a smooth cylinder with a pointy front end. Service bays are great for this, and are available early in the tech tree. Fairings also help.

If you're having trouble debugging why a particular rocket has problems, the aero overlay helps while in flight. Toggle with F12, though you may need to remap the Steam screenshot hotkey to make it usable.

[Zablonski]

One trick I found for keeping weight forward was to have one of the uppermost tanks (in the first stage if you have multiple stages) set to not flow (right click on tank in vab or on pad and click little arrow in top right hand corner - should turn into circle with diagonal line) this will keep the fuel in it. Then, during your ascent you can turn on the flow again.

[Zeratul_r]

That's what I originally thought, but it's at the lowest before I put any fins on it.

Initially Col is at the centre of the floor. That means that there is nothing on your rocket that creates a lift. As soon as you put some aerodynamic elements on, Col will move to its actual position. The general rule that I found to be true on my rockets is that more bigger fins and winglets at the very bottom of the rocket equals more stability.

[Good_Cat]

CoL and CoM both matter. The further the two are apart the more stable the rocket it but its harder to turn due to it stability. You want you CoL as low as possible and your CoM high enough, to low closer to CoL and you rocket is too sensitive to imputs, to high and the opposite the rocket will not want to turn.

Think of you CoM as a pivot point about which your rocket will move around and CoL is where the turning,yaw,pitch force is applied.

Bigger wings and gimballing engines give you more control, bigger wings also mean more drag.

If you have a bit of a dogdey stage try draining the fuel of of the stage while in the VAB and watch where the CoM moves. Short fat rockets the COM will move a lot

And around Mach 1 you want the craft as stable as possible due to the extra aerodynamic forces in effect.

And struts always struts the more rigid your craft the more precise your imputs can be, as control imputs wont be soaked up by wobble and flex.

[Janos1986]

Slap some fins on the bottom of the stage that gets you right out the atmosphere (usually the second stage) and you should be golden.

If you have a non-gimbal engine, use fins with control surfaces, if you have gimbal, use the fixed ones and reduce the gimbal angle a bit if the rocket has too much TWR.

[Cebi]

To get simple answer to your qustion we need to see craft itself. Without it we are only guesing.

Fins are not mandatory for stable rocket. With fins you gain stability but also you gain more drag. I think that if you are placing fins in 4 or 3 way symmetry total lift force will be zero. It is drag of those fins that will make your rocket point in prograde direction.