Are External Command Seats Unstable?

[youbar]

I designed a Kerbal Dropship made for dropping off 8 Kerbals into a specific area in the Kerbin atmosphere. As soon as I load any Kerbals, the VTOL engines become highly unstable and the thing can barely take off without flipping over. When replacing the seats with ladders (which leaves me far less space), the dropship is completely stable. This also occurs with another craft (although slightly less balanced), despite the extra engines and ASAS modules to counteract this. (If you attempt to use this, keep engines at 2/3 thrust).

Why is this?

I've done a thorough search, so excuse me if this has been mentioned before.

[Kasuha]

I don't know about ladders but Kerbal in a seat adds about 90 kg weight to the ship so it is possible that as you seated your Kerbals this little change moved the ship's CoM enough for it to become unstable.

[magnemoe]

This is an known problem for miniature landers like this one http://imgur.com/RMDl14P

For it I solved it by placing the seat in center to get center of mass right.

best practice might be to use 7 or 9 seat and put the odd kerbal in the center.

A bit of warning kerbals can fall out of seats, this is an major problem with parachutes on landers with just seats and as you don't have iva you don't know then you are 500 meter over surface.

[Geschosskopf]

A seat weighs 0.005 tons. An EVA Kerbal weighs 0.095. Thus, a Kerbal in a chair weighs 0.1 tons but you can't have full seats in the SPH. To get around this, during construction, you can put some other part that weighs 0.1 tons where the chair will go eventually.

HOWEVER, this only works if all the Kerbals will get on and off at the same place. If you have an airplane with individual Kerbals getting off here and there as it flies, and the seats are lined up along the fuselage, then the CoM will change as they leave and you're back in the same boat. To avoid this happening, line the seats up along the wingspan so all are in line with the CoM. Thus will prevent the CoM from changing longitudinally as individual Kerbals get on and off. It will, though, make the CoM shift from side to side unless you drop the Kerbals in pairs. However, this lateral CoM movement is usually no big deal--torque can keep the thing level anyway. In any case, this is a LOT better than a longitudinal CoM shift that screws up where the CoM is compared to the CoL.

[Claw]

I didn't load your craft in KSP (which I might shortly), but I've designed a VTOL with 8 seats on it. It's all symmetric and completely stable. So my guess (before looking at your design) is that it isn't the seat itself.

[Vanamonde]

Yes, Kerbals in external seats can cause phantom forces which can even be strong enough to destroy space stations.

[Claw]

I designed a Kerbal Dropship made for dropping off 8 Kerbals into a specific area in the Kerbin atmosphere. As soon as I load any Kerbals, the VTOL engines become highly unstable and the thing can barely take off without flipping over. When replacing the seats with ladders (which leaves me far less space), the dropship is completely stable. This also occurs with another craft (although slightly less balanced), despite the extra engines and ASAS modules to counteract this. (If you attempt to use this, keep engines at 2/3 thrust).

Why is this?

I've done a thorough search, so excuse me if this has been mentioned before.

Okay, so I will start off with a new question because I want to be clear.

You said your craft goes unstable as soon as you load any Kerbals. When you say that, do you mean even after you load the first passenger kerbal? And no matter how many kerbals you add, it goes crazy?

I ask because I flew your VTOL and it was rock solid. I then loaded it up with 8 kerbals and proceeded to land on top of the VAB, and managed to do so on the first try. Your craft has 10 jump seats so I tried various configs of 2, 4, 6, and 8 Kerbals. I flew with the front most seats empty, and the back most seats empty. (I landed on the VAB with the back seats empty.)

The one gotcha to that though (and I'm hoping your problem is this), is that when you load a kerbal into the command seat, his position becomes the new orientation for your navball. So, for instance, if you load a kerbal in the left side seats, take off, and try to roll left you might end up pitching forward. So I can see if the craft starts to sway, and you counteract but it's not in the intended direction, the whole craft will quickly flip over.

If it is this, (after you've loaded your passengers) right click on your MK-1 cockpit at the front and select "Control From Here". That should fix your navball/control issues.

If that's not your problem, then you might be having some other issue such as clipping or just a plain bug. Your kerbals at the very back clip a little into the pods you have hanging off the back, and they clip into the kerbals in the seats right behind them. It's such a small amount of clipping that I don't suspect this is the problem.

BTW: I'm running completely stock KSP except for MechJeb and KER. Not sure if you're running any mods.

Edited March 1, 2014 by Claw