Please help me identify source of loss of control

[Shimmergloom]

Hey guys!

Got KSP for a week now and I think I learned the basics pretty well - got satellites orbiting a lot of planetary bodies, got a basic space station up and running, had my first Munar landing. And now I am at my wit's end concerning a specific rocket design.

You see, this is my Munar lander - it launches like a dream, easily controllable, rock solid:

https://dl.dropboxusercontent.com/u/100318/ksp/munar_lander.png

Now I wanted to add fuel tanks to my space station to have it work as a refuelling station - nothing fancy, using the same basic rocket design:

https://dl.dropboxusercontent.com/u/100318/ksp/fuel_pod.png

And this thing doesn't work - it feels wobbly from the start and starts running out of control once I try to maneuver into my gravity turn. And I don't get it! The payload is actually less than on my Munar lander design (23.37t vs 27.56t)!

I figured that it might be due to the remote control part not developing as much torque as the 3-kerbal-pod, but adding SAS modules still doesn't help. So: what am I missing?

Thanks a lot!!

P.S.: Craft files for those who are interested - got fuel balancer, mechjeb and kerbal engineer parts running, the rest is stock

https://dl.dropboxusercontent.com/u/100318/ksp/KerbStation%20FuelPod%20Mk%20I.craft

https://dl.dropboxusercontent.com/u/100318/ksp/Mun%20Lander%20MkII.craft

Edited June 6, 2013 by Shimmergloom
Question answered

[Richy teh space man]

You could try adding more struts to your design so it's a bit more solid. Other then that, tall builds tend to be a little less stable then ones that build outwards. You could try redoing your design along those lines instead.

[Shimmergloom]

The design is actually not wobbly in itself (at least not that I could see it), since the Mun lander design, which is highly similar, works like a dream... only the fuel pod thingy is spinning out of control wildly...

[DerekL1963]

The design is actually not wobbly in itself (at least not that I could see it), since the Mun lander design, which is highly similar, works like a dream... only the fuel pod thingy is spinning out of control wildly...

The Mun lander has a lot more torque from the command pod than the fuel tanker does from the remote control unit...

[Maxwell Fern]

It seems like your junction between the launcher and the fuel tanker is a small decoupler attached to a regular docking port, am I right ?

If so, this is highly fragile, as if the docking port and the decoupler had no point of contact. You're basically trying to push a fully loaded tank without actually being attached to it, if not for those 4 struts. No wonder this is wobbly.

Consider using a large docking port and a large decoupler, or a LOT more struts, or no docking port on that end of your tanker. You also might wanna try using two docking ports already connected, above the decoupler.

[Uberick]

Note that, the probe bodies give much less torque, and SAS modules do NOT provide any torque(common misconception). The only way to get torque is from LOTS of probes, or command pods. Sometimes, adding a dummy command pod can be worth it just for the torque it provides(either that or RCS).

[Vanamonde]

Fins with control surfaces low on the stage help a great deal while you're still in atmosphere.

[Cranium]

Note that, the probe bodies give much less torque, and SAS modules do NOT provide any torque(common misconception). The only way to get torque is from LOTS of probes, or command pods. Sometimes, adding a dummy command pod can be worth it just for the torque it provides(either that or RCS).

Uhh, what? Since when does SAS not apply any torque? I can see in the parts list clearly that SAS modules have a max torque rating of 20. You might be thinking of ASAS which does NOT have torque whatsoever.

[Aragosnat]

Now I wanted to add fuel tanks to my space station to have it work as a refuelling station - nothing fancy, using the same basic rocket design:

https://dl.dropboxusercontent.com/u/100318/ksp/fuel_pod.png

And this thing doesn't work - it feels wobbly from the start and starts running out of control once I try to maneuver into my gravity turn. And I don't get it! The payload is actually less than on my Munar lander design (23.37t vs 27.56t)!

I figured that it might be due to the remote control part not developing as much torque as the 3-kerbal-pod, but adding SAS modules still doesn't help. So: what am I missing?

The area with the dockig port. The last decolper you fire is way to narrow. You can try adding more struts there as docking ports when being launched like to wiggle unlesss struted heavily in place.

[Anglave]

Uhh, what? Since when does SAS not apply any torque? I can see in the parts list clearly that SAS modules have a max torque rating of 20. You might be thinking of ASAS which does NOT have torque whatsoever.

This changed sometime after 0.17. SAS used to provide RotPower just like a command pod. It no longer does.

It provides torque only to fight axial rotation of your craft, you cannot "steer" with it.

To the OP, I recommend adding some RCS. Add one or two small RCS tanks somewhere on your design, and put four of the multi-port RCS thrusters symmetrically around the "nose" of your craft. Turn on RCS (with ASAS already on) just before the point in your ascent where you have control issues.

Don't bother putting the RCS near your center of mass, you won't be needing linear control, just use it to keep the nose pointed where you want.

[Shimmergloom]

Thanks for the replies!

Note that, the probe bodies give much less torque, and SAS modules do NOT provide any torque(common misconception). The only way to get torque is from LOTS of probes, or command pods. Sometimes, adding a dummy command pod can be worth it just for the torque it provides(either that or RCS).

THAT was exactly it! And damn, that's some pretty bad in-game documentation - after all, all pods (including the remote control drone pod thingies) have a "Maximum Torque = 5" listed, with the SAS module having a rating of 20!!

So I tried mounting a lander can as a dummy pod and lo and behold: rock solid flight! Thanks so much, Uberick and Anglave - that was the reason.

@Rest: it may look fragile, but apparantly isn't

Actually even with my fragile-looking construction of rockomax adapter -> small decoupler -> docking port jr. -> rockomax adapter -> fuel pod payload: 4 struts were enough to keep it fully solid! Not a wee bit of wobbling!

So I just need to redesign the whole thing, apparantly my fuel pod is now brought to the station by some kind of manned tug and not by remote control... ah, why the hell not.

[Nao]

The design is actually not wobbly in itself (at least not that I could see it), since the Mun lander design, which is highly similar, works like a dream... only the fuel pod thingy is spinning out of control wildly...

It flies perfectly well for me :/. The only thing is that since i don't have FuelBalancer mod so i the part from the fuel pod. And also your Lander design doesn't have one so maybe its the balancer's fault.

edit: uhh ok just refreshed your replay, welp its great you the problem is resolved Although i didn't see a problem flying the old design huehue

Edited June 6, 2013 by Nao

[Shimmergloom]

According to the wiki, the value that differs between manned pods and remote control units is "rotpower": http://wiki.kerbalspaceprogram.com/wiki/Parts#Pods

That fits, given that I couldn't recover from rolling, whereas a few SAS pods helped with basic directional steering. Man, this is so weird

[Shimmergloom]

And this is the final version of my fuel pod:

https://dl.dropboxusercontent.com/u/100318/ksp/fuel_pod_final.png

Works flawlessly, no wobbling whatsoever despite only a dockingport jr and 4 struts to hold it together - go minimal!

Docking:

Back to Kerbin:

Thanks again!!