RCS and multi-part craft.

[Skorpychan]

I have plans for multi-part ships, that may well require docking them as multiple parts.

If each individual ship in the docked-together chain has RCS blocks on it's center of mass, will that work as effectively as RCS blocks in the common center of mass?

Edited December 9, 2013 by Skorpychan

[kurja]

yes.

How do you use ten characters to say that??

[Skorpychan]

Thank you.

Now to go design this stuff.

[Taki117]

As long as your ship is symmetrical and you have RCS balanced for each part, then the craft as a whole should have it's RCS balanced as well.

[Eric S]

I'm going to disagree. Having each part be individually balanced is probably a good starting point, but unless all the parts are the same mass, size, and RCS thrust, there will probably be imperfections in the RCS balance of the joined craft, and it's possible for the imbalance to be more than slight.

As an example, imagine a craft that is the size and mass of a Rockomax 200-16 fuel tank, and you've got four RCS quads mounted around its center of mass. Dock this end-to-end to another craft the same and mass of a Rockomax 200-32 fuel tank, also with four RCS quads mounted around its center of mass. You've now got two craft that individually are perfectly balanced but when docked, the center of mass and center of RCS thrust are no longer in the same place. The center of mass would be a meter towards the larger craft from the docking point, but the center of RCS thrust would be half a meter from the same point in the same direction.

Now, if the larger craft had twice the RCS thrust of the lighter one, then the center of mass and center of RCS thrust would stay together as the craft is joined. However, it will get harder and harder to maintain this as you connect multiple parts all with different masses, as RCS thrust isn't very flexible.

On the other hand, if you turn on fine control (caps-lock), there's a very simple RCS balancer that kicks in that should help the craft be closer to balanced.

[Skorpychan]

Also, while I have a thread up, does anyone know when the docking/rendezvous autopilots are unlocked on MechJeb?

[JumpsterG]

Had to think about it, but Eric S is right. Thrust/Mass ratio being different on various segments will cause them to move at different speeds when RCS fires, causing rotation when attempting to translate the combined vessel. Then again, keep in mind that you should be docking via control of the smaller vessel anyway, and that it is usually not important for the more massive end result vehicle to be able to translate.

When planning these combos and you need the result to be balanced, it's helpful to build the combined vessel in the VAB to simulate. Find the RCS balancer mod (a simple plugin), if you're interested, as it will give you a visual representation of the RCS balance right in the VAB. If you don't want to install a mod, you can use the cheat menu to disable gravity and test your combined vessel right off the launchpad for RCS balance.

A final note: beware of what happens when fuel and RCS tanks lose mass as fuel is burned. Balance can easily go out the window when fuel is draining only from one end of the ship.

[3mon]

Also, while I have a thread up, does anyone know when the docking/rendezvous autopilots are unlocked on MechJeb?

I think you have to unlock the "Advanced Unmanned Tech" node for that. Its Tier 8 and costs 550 science according to the wiki.

[kurja]

Had to think about it, but Eric S is right. Thrust/Mass ratio being different on various segments will cause them to move at different speeds when RCS fires, causing rotation when attempting to translate the combined vessel. Then again, keep in mind that you should be docking via control of the smaller vessel anyway, and that it is usually not important for the more massive end result vehicle to be able to translate.

When planning these combos and you need the result to be balanced, it's helpful to build the combined vessel in the VAB to simulate. Find the RCS balancer mod (a simple plugin), if you're interested, as it will give you a visual representation of the RCS balance right in the VAB. If you don't want to install a mod, you can use the cheat menu to disable gravity and test your combined vessel right off the launchpad for RCS balance.

A final note: beware of what happens when fuel and RCS tanks lose mass as fuel is burned. Balance can easily go out the window when fuel is draining only from one end of the ship.

dang, that's right.

so - answer is "yes", if all part are of similar size.

isn't rcs thrust automagically balanced if you switch to fine control mode?

[Murph]

so - answer is "yes", if all part are of similar size.

Size and mass.

isn't rcs thrust automagically balanced if you switch to fine control mode?

I'm not certain, but I find RCS control of with less than perfect balance to basically be ok with both SAS and fine-control active. If SAS isn't on, any imbalance in RCS thrust can quite easily add some rotational velocity, but SAS does a reasonable job of cancelling it out.

Or, to put it another way, I don't put any real effort into balancing RCS, and seem to manage just fine. I do use the RCS balance visualisation mod, however, just not for balancing RCS, but to see the dry CoM.