CoM too high?

[CanOmer]

Why is Center of Lift ahead of the Center of Mass in my rocket?

You can see there are 4 small wings and 4 larger wings on it. But center of lift should be close to the larger wings which is behind the Center of Mass.

Edit: I found out that Mk1 Command Pod has 0.35 relative wing area. Is it vanilla setting or some mod caused it?

Edited December 28, 2017 by CanOmer

[Deddly]

Hi @CanOmer, I split your question off into its own topic so you can get the best answers. Hope someone can help you here

[Urses]

The CommandPod if i remember right, does not have lift in vanilla. Normally Ferram will add this ability. But i think your problem is more that the winglets are to small for such a back heavy design as you have. That is why the COL is so high up.

[Zhetaan]

@CanOmer:

The Mk. I Pod does have .35 'wing area' in stock.  More accurately, it has a lift deflection coefficient of .35 and a lifting surface module, which used to be only for wings.  It also has a slight offset for the centre of lift.

The reason your centre of lift is higher than your centre of mass is because your winglets are angled incorrectly to provide lift, but the command pod's lift is supposed to function when it is pointed relative-down (it's meant to give you some directional control during re-entry).  If you grab the root part and rotate the whole rocket so it points more horizontally, then you should see the centre of lift move back.

Or don't worry about it; the marker is for centre of lift, but of course you're really more interested in drag, which is not displayed.  This will not affect your launch--though I would caution you to be careful with those adapter-less diameter changes at the top of your rocket.  Abrupt size mismatch will contribute a lot of drag.

[Aegolius13]

1 hour ago, Zhetaan said:

  This will not affect your launch--though I would caution you to be careful with those adapter-less diameter changes at the top of your rocket.  Abrupt size mismatch will contribute a lot of drag.

Fairings are a good solution for this too.

[BadOaks]

12 hours ago, Zhetaan said:

@CanOmer:

The reason your centre of lift is higher than your centre of mass is because your winglets are angled incorrectly to provide lift, but the command pod's lift is supposed to function when it is pointed relative-down (it's meant to give you some directional control during re-entry).  If you grab the root part and rotate the whole rocket so it points more horizontally, then you should see the centre of lift move back.

I think that's only the case in the SPH, which shows center of lift, and the VAB shows center of drag (just of aero surfaces, not every part).  So the fins would be aligned right for the VAB's marker to take them into account.

[Plusck]

17 hours ago, BadOaks said:

... the VAB shows center of drag (just of aero surfaces, not every part)...

That seems a bit odd and pointless. Doesn't mean you're wrong, but still...

On 28/12/2017 at 1:45 PM, CanOmer said:

Why is Center of Lift ahead of the Center of Mass in my rocket?

You can see there are 4 small wings and 4 larger wings on it. But center of lift should be close to the larger wings which is behind the Center of Mass.

Edit: I found out that Mk1 Command Pod has 0.35 relative wing area. Is it vanilla setting or some mod caused it?

The CoL marker in the VAB is pretty useless, I think, unless you tilt your rocket to see what happens when it goes off prograde.

What is certain is that the VAB does not behave like the SPH when all aero surfaces are neutral.
If I had to take a guess, it's that the SPH "cheats" and adds a fractional angle above prograde when calculating CoL. It can't do that in the VAB since there is no particular direction that a rocket can rotate towards for the aero direction to be meaningful, so you have to do it yourself (click and grab your root part then do shift-WASD once).

So yes, the Mk1 pod definitely has "lift" in stock. No matter how many tanks you put under it, "lift" will stick resolutely to its underside:

Spoiler

And while facing prograde, the CoL marker is pretty meaningless. You have to tilt it in the VAB:

Spoiler

And once you do tilt it, the CoL marker should show up in the same place as it would facing directly prograde in the SPH:

Spoiler