Flight Question Can't move while docked with other ship

[Bodrius]

I'm traying to make a mission to the moon, Apollo style, travelling in a Delta Glider XR1 with an attached Lunar Module. So after a lot of time and headaches I managed to load the LM and attach de DGXR1 to a rocket, make it to orbit, release both ships and docked them. But now I realized that I can barely move the DGXR1 with the LM attached.
Is there a way to make it easier to maneuvre? Or am I just wasting my time? I guess it has something to do with the uneven center of gravity of the docked ships...

Thx in advance.

 

[ar81]

If you dock a 5 tons fuel tank to the nose of a DGIV, you may find it is a bit hard to maneuver.

[ame="http://www.orbithangar.com/searchid.php?ID=2732"]External fuel tank v0.4[/ame]


I presume it must be hard to maneuver when docked to other vessel.

 

[2552]

Is your orbit changing at all in Orbit MFD while you burn?

 

[willy88]

Are you using ROT or LAN? It might be easier if you use LAN.

 

[Bodrius]

Is your orbit changing at all in Orbit MFD while you burn?

I can't even burn. When I manage to turn prograde (manually, the autopilot keeps turning the ship around and never reaches prograde or whatever) and try to burn, the ships slowly start to pitch down, although prograde button is on.

Are you using ROT or LAN? It might be easier if you use LAN.

I tried both. The ship moves slowly and is very hard to control. With some practice I think I could control it, but the problem is I can't keep it straight when I burn, it always pitches down.

 

[tblaxland]

The ship moves slowly and is very hard to control. With some practice I think I could control it, but the problem is I can't keep it straight when I burn, it always pitches down.

When the two vessels are docked, you can consider them as one vessel as far as the dynamics are concerned. Because the engines of your XR1 do not thrust directly through the centre of mass of the combined stack, a torque about is produced about that centre of mass, leading to the pitching that you are describing. Similarly when you try to use the rotational thrusters (including with the autopilots) they will not be symmetrical about the centre of mass of the stack. This is expected behaviour. You may be able to reduce the pitch tendency by gimballing the main engines so that their thrust vector goes through the stack's centre of mass.

 

[Tommy]

Pitching down may be caused by offset COG. The docking port on many DG type craft is located "below" the COG. This means that having a vehicle docked lowers the COG, as well as moves it forward. This means the thrust vector no longer passes through the COG, it passes above it - resulting in the "nose down" rotation imparted by the thrusters. You can adjust the engine gimble to compensate, but this means your thrust vector will no longer quite line up when using autopilots.

 

[TSPenguin]

I can't even burn. When I manage to turn prograde (manually, the autopilot keeps turning the ship around and never reaches prograde or whatever) and try to burn, the ships slowly start to pitch down, although prograde button is on.


I tried both. The ship moves slowly and is very hard to control. With some practice I think I could control it, but the problem is I can't keep it straight when I burn, it always pitches down.

The built in autopilots only take your own vessel's weight distribution and thruster positions in account. If you change the weight distribution by docking a mass anywhere on the ship it will not be able to know what thrusters achieve what effect on your rotation.
Your docked mass is most likely not centered on your center of gravity. So when you burn your main engines the thrust is not centered and you induce rotation.
You can compare it to balancing a platter on a stick. Works very well, but if you put a weight on one side of the platter you need to compensate.
In planetarium mode you can see the actual forces involved.

Some (unmanned) sattelites/probes actualy rotate rather fast when using their main engine to compensate for an uneven weight distribution. This is rather hard to achieve manualy. Those vessels have carefully placed thrusters or SRBs to induce the rotation exactly along the desired axis.

I recommend you manualy control your attitude and gimbal your main engines to compensate for the uneven CoG during your TLI burn.

 

[Bodrius]

I did what you suggested, I gimballed the main engines and now it works more than perfect.

Thank you, thank you to everybody, you are great! :thumbup: