Wobbling mini-ship

[Irvin]

Hi,

I have a light lander designed to put into orbit a very small ship made of a seat, a small xenon tank+engine, a 200 battery, a 3x2 panel and the smallest reaction wheel.

Once in orbit the small ship keeps wobbling when I fire the xenon engine, it is almost impossible to pilot it. Val is on the seat if that matters.

 

Any idea on what's going on ?

Thanks.

[Foxster]

Some thoughts...

Are you running out of power? You don't have enough power generation there to run an ion engine. 

It might be worth adding an OKTO2 and doing "Control from here" on it.

Try swinging the seat  through 90° so that the Kerbal faces the direction of travel. 

 

[swjr-swis]

Yes: the stock SAS kinda sucks for very tiny/light craft, and even the smallest reaction wheel is too overpowered for it to handle on such a small mass.

It's an ongoing issue, hopefully they'll find a solution at some point.

[Irvin]

No I'm not running out of power...

OK if it's a known bug it sucks but I'll have to live with it.

Thanks !

[Foxster]

Just for interest, here's a little ion lander I have used...

[Irvin]

Mine is barely smaller, is it wobbling too ? What are these things sitting on the ground btw ?

Edited April 23, 2016 by Irvin

[swjr-swis]

42 minutes ago, Irvin said:

What are these things sitting on the ground btw ?

Fuel cells. Ion engines draw a LOT of EC, so fuel cells tend to be the most efficient way of keeping up with that.

[Foxster]

No, mine doesn't wobble at all </smug>

I much prefer using fuel cells. They are more compact than solar panels, produce more charge and can't be occluded. 

I put the fuel cells on the bottom like that cos they make a handly platform to land on and so save the mass of legs. 

They need some LF+O to work, hence the little fuel tank. 

For information, you can calculate the amount of LF you need by multiplying the amount of Xenon by 0.02. On the craft above there is 700 units of Xenon so you need 700x0.02=14 LF (plus matching O).Nearest tank size to that being an Oscar-B, which you can take a bit out of.   

[Hodari]

Is your thrust properly aligned with your center of mass?  Not sure if Val has any mass which affects this, but if so, make sure you account for that also. 

[Snark]

If you have a very tiny ship, the smallest reaction wheel can be way overpowered, as @swjr-swis points out.  This causes jitter.

My usual solution to that is to put a HECS probe core on the ship, instead of a reaction wheel.  The HECS has built-in reaction wheels that are considerably weaker than the smallest-size standalone reaction wheel, and works just fine for very small craft.

If that solution doesn't appeal to you, another possibility would be to install the TweakableEverything mod, which will give you a slider that you can adjust to control the amount of torque your reaction wheel generates, instead of it being a simple on/off switch.

(By the way, could you post a screenshot of your ship?  Would help for diagnosing problems.)

[swjr-swis]

2 hours ago, Hodari said:

Not sure if Val has any mass which affects this

75kg, but don't you dare tell her I said that...

[Irvin]

Hi,

First many thanks for all your interesting answers.

The solution was quite obvious, Val was not in line with the COG and as the ship is really light it made it wobble !

 

Thanks again.