Change trajectory in space

[Philae & Rosetta]

Hi all,

 

I  have an issue I can't explain. The problem happens only sometimes, and I don't know why. Maybe someone can help me to understand it.

When I am in space, engines switched off, I often want to prepare a change of vessel trajectory before switching them on again.

For this, of course I use the keyboard and the navBall.

Sometimes it works, and sometimes,...not really .

When it doesn't work there are two ways:

1/ (The most frequent way) The ship doesn't respond at all...Nothing happens and i need to switch on engines in order to be able to change trajectory.

2/ (Less frequent but it happens sometimes) The ship responds...too much! I mean, I change the trajectory and the ship continue to rotate on itself after I finish, indefinitively until I switch the engines on again.

 

 In both cases, I need to switch on the engines before changing the trajectory as I would like and it is problematic because I can't change it precisely and as fast as I would like.

Do you know why it is acting like this and how I can avoid it?

Thanks

[Draalo]

55 minutes ago, Philae & Rosetta said:

1/ (The most frequent way) The ship doesn't respond at all...Nothing happens and i need to switch on engines in order to be able to change trajectory.

Sounds like none or a weak reaction wheel is present

56 minutes ago, Philae & Rosetta said:

2/ (Less frequent but it happens sometimes) The ship responds...too much! I mean, I change the trajectory and the ship continue to rotate on itself after I finish, indefinitively until I switch the engines on again.

Activate SAS by pressing "T" - note that the Staypitnik probe core has no SAS stabiliser. A capsule without a pilot has also no stability hold.

[Spricigo]

I think you mean you want to change ship attitude. (since the trajectory, the orbit, will only change with some force being applied(e.g. engine thrust))

The same way you need some force to translate your ship you need torque to rotate it.

In KSP we have reaction wheels that provide a fixed amount of torque regardless of where in the ship you put it (also regardless of some real life physics, but that is beside the point here).

Another way to have torque is with thrusters not aligned with the CoM of the vessel. In that case the amount of torque will depend on how much thrust you have and by how much it 'miss' the CoM.

OTOH the vessel resistance to rotation* depends on how much mass it have and how it is distributed around it's rotation axis (CoM) . More 'tightly packed' mass offer less resistance to rotation.

So, how much your craft respond to your attempts to change its attitude depends on those things. 

*more correct, resistance to change of how fast it rotates

[Philae & Rosetta]

I'm so stupid. I just oticed that reaction wheels needed electricity...

The fact it worked or not was surely linked with batteries remaining or not...

Thanks to Spricigo for talking about reaction wheels, which give me the idea to look at it again...

[Starchaser]

I was going to mention electrical charge, as I've had it happen that after I removed some mods, when I restarted the game all my solar panels were no longer extended, then a couple hours later, I experience what you're describing.

Another time something similar has happened to me is that when I can't change ship attitude and realize I never extended it's antennae

[Oiff]

Yes solar panels closing on their own drives me nuts sometimes.

 

A trick to stop the ship spinning once it is pointing the right way is to engage the time warp, this stops the physics and hence the rotation.  Very handy for docking.