Tilting the rocket

[Scarredclown]

I'm quite new in this game, i can make a fairly good rocket, but a problem that i frequently got is that my rocket start tilting a few moment after launching returning to crash... quite annoying , not so much for me but the pilot )) I'm guessing it have something to do with the gravity center...but how to correct that ?

Thanks ! (sorry for my bad English)

[sal_vager]

We'd really like to see your rocket Scarredclown, can you get a few screenshots with F1 and put them on imgur.com for us please?

[Northstar1989]

Posting a picture of the rocket might help.

If the rocket is built in perfect left/right symmetry (or 3-way or 4-way or whatever symmetry) using the VAB tools, then the Center of Mass should lie directly over the Center of Thrust. So your problem is more likely that your rocket has too large or too small of a gap between the Center of Mass and Center of Thrust.

You might also try adding more SAS force and control surfaces, and making sure your existing SAS wheels and winglets are as far from the center of Mass as possible to maximize their potential torque...

Regards,

Northstar

[BudgetHedgehog]

Press 'T' before you press Space.

[SRV Ron]

Press 'T' before you press Space.

A properly balanced rocket design will fly straight up without the need for SAS stabilization. Other designs will fly straight, then swap ends when you start a gravity turn. Rockets containing the pod with all the viewports is notorious for causing stability issues in the lower atmosphere. Still others go out of control due to a linkage or fuel flow failure which throws them out of balance.

Example, the one on the left is stable while the one on the right needed fins to keep it from swapping ends during an orbital turn after staging.

Posting a photo may help us sort out the problem you are experiencing.

[Scarredclown]

Thx for your help, i'll try to get a couple of pics soon, right now i'm doing trial and error.

[Sirrobert]

There are 3 big buttons in the VAB, below the list of parts.

These will show you your center of mass, thrust, and lift. You can use those to check if everything is correctly alligned

[AmpsterMan]

I think the new parts have different aerodynamic properties than the old ones do, therfore your craft is spinning because it wants to point the most aerodynamic part up. The reason your other craft doesn't spin could be because the Command Pod has more torque than the Cupuloa. Try putting an inline reaction wheel in the second one. I may be comepltely wrong thoguh.

[SRV Ron]

I think the new parts have different aerodynamic properties than the old ones do, therfore your craft is spinning because it wants to point the most aerodynamic part up. The reason your other craft doesn't spin could be because the Command Pod has more torque than the Cupuloa. Try putting an inline reaction wheel in the second one. I may be comepltely wrong thoguh.

The aerodynamics on the cupola, when flying in Kerbal's atmosphere is the culprit. While the torque is less, it is not by that much. It is just once it hits a critical point, the Skipper engine cannot gimbals enough to correct from the instability without fins. Once in space, there is no issue. It seems to be an issue in 0.23.5 as I have not encountered that problem in the past.

[Northstar1989]

A properly balanced rocket design will fly straight up without the need for SAS stabilization.

That's actually a common misconception, and not true. Lumping fins and thrust vectoring in with SAS stabilization (assuming you meant that a properly balanced rocket will just fly straight up without any guidance), if you have none of these, and your rocket is perfectly symmetrical, your rocket will *ALWAYS* end up tipping over...

Why, you might ask? Because of Kerbin's rotation.

As a rocket ascends *straight up*, its effective period of rotation about Kerbin's center increases- giving it velocity relative to the surface. As drag appears to be calculated relative to surface speed rather than orbital speed (at least at low altitudes- a rocket with no horizontal speed just after liftoff experiences no horizontal drag, despite moving at an orbital horizontal speed equal to Kerbin's rotational velocity), this means that the rocket will start to experience drag pushing it in the direction of Kerbin's rotation, despite continuing to fly straight up.

Assuming the rocket's Center of Mass isn't *exactly* halfway up the rocket, and that the drag coefficients of the parts above the Center of Mass aren't exactly equal to the drag coefficients of the parts below the center of mass (weighted for part mass, as stock aerodynamics uses something along the lines of Drag = Mass * Drag Coefficient * Velocity^2), this will generate torque in the rocket- either towards the direction of Kerbin's rotation, or against it. So a rocket with *absolutely NO* stabilization will ALWAYS tip over, even flying straight up...

Whether this occurs quickly enough to prevent the rocket escaping the atmosphere is another thing- based partly on the rocket's TWR...

Normally, this effect is trivial- and can easily be compensated for by thrust vectoring or the smallest probe core. But, in a rocket lacking any SAS, control surfaces or aerodynamic stabilization, or thrust vectoring whatsoever; a certain amount of torque on the rocket is almost inevitable, and could potentially lead to a crash...

Regards,

Northstar

[KerbMav]

.

Are you describing stock or FAR aerodynamics?

[Northstar1989]

Are you describing stock or FAR aerodynamics?

Stock aerodynamics.