• 0
jpinard

I guess I don't understand optimal RCS thruster placement

Question

I keep screwing up where I put RCS Thrusters on my latest Mun lander. It is filled with science and my last attempt saw me not able to rotate properly in space, it would always slant a bit when I was trying to rotate clockwise/counterclockwise while trying to dock around the Mun (I dump the grabber before I dock as it's for a mission on the way).  If someone could just tell me which thrusters to put where I would be eternally grateful. I'm also wondering how those built in RCS thrusters on my capsule affect things and if you think I should get rid of the bottom slanty fuel tank if that makes it more difficult too.

I really appreciate the help. Thanks! :)

Y6QFYgd.jpg

Share this post


Link to post
Share on other sites

9 answers to this question

Recommended Posts

  • 1

Whoa... I just had a Eureka moment.  So I removed the RCS thrusters up top and discovered the amount of bad torque generated was cut way down.  WHat I didn't realize is that the capsule thrusters were adding issues with the places thrusters.  So things will rotate a lot slower but now I won't be knocked off kilter translation so badly:

EHItYhR.jpg

Share this post


Link to post
Share on other sites
  • 0
Posted (edited)

In the vehicle assembly building there is a button down the bottom left that lets you see where the center of mass is for your craft, if you know this you can make sure all the thrusters are equal distant from it and so supply the same amount of torque, if one is further away it will have more leverage and twerk your craft more than the others making RCS uneven.

maybe that helps? :3 

(your center of mass can change throughout the mission as fuel is expended too so it's a moving target >_<)

Edited by Dale Christopher

Share this post


Link to post
Share on other sites
  • 0
Posted (edited)
27 minutes ago, Dale Christopher said:

In the vehicle assembly building there is a button down the bottom left that lets you see where the center of mass is for your craft, if you know this you can make sure all the thrusters are equal distant from it and so supply the same amount of torque, if one is further away it will have more leverage and twerk your craft more than the others making RCS uneven.

maybe that helps? :3

It does, but since I have tubes of different diameters wouldn't that change things?  I installed RCS Build aid, but I'm not sure how to interpret its findings and use it.  But an interesting thing I found in these images.  You can see I started with thrusters at the very bottom and at the very top in #1.  Then I move them down to what I thought weas equidistant in #2.  Then in #3 I put the top thrusters right on the CoM and the red circle disappeared.  I wish I didn't have to go all the way to the Mun to test this as that's where my space station is, so testing these would take a long time (hoping I get it right before next trip).1K1m3QH.jpg

 

So apparently, the red circle might just be rotation speed so large red circle is fine.  What is bad is red arrows, and when I change the view I see these all have a large red arrow which represents torque and I can't seem to find a way to eliminate it:

 

z4CkzQ5.jpg

Edited by jpinard

Share this post


Link to post
Share on other sites
  • 0
15 minutes ago, jpinard said:

Whoa... I just had a Eureka moment.  So I removed the RCS thrusters up top and discovered the amount of bad torque generated was cut way down.  WHat I didn't realize is that the capsule thrusters were adding issues with the places thrusters.  So things will rotate a lot slower but now I won't be knocked off kilter translation so badly

You can disable thrusters , and RCSBuildAid will work properly

Share this post


Link to post
Share on other sites
  • 0
4 hours ago, jpinard said:

If someone could just tell me which thrusters to put where I would be eternally grateful.

I know a lot of people love RCSBuildAid, but I've never bothered with it, because I don't find it necessary.  A very simple technique works 100% of the time for me and requires no finicky futzing around, and is completely stock:

  1. One 4-way ring of thrusters at one end of my long skinny ship
  2. Another 4-way ring of thrusters at the other end of my long skinny ship
  3. Only one of those two rings needs fore-and-aft capability.  i.e. use the 4-way thrusters at one end, and the linear ones at the other (or none, if I have the Mk1-2 command pod which provides them for free).

That's it.  That's all there is.  That guarantees that the CoM will be somewhere in between the two, and as long as it's between them, it doesn't matter at all where it is.

Two other things for the above to work well with minimal hassle (these are whole-game settings, not something I do to individual ships):

  • I don't want to use RCS for ship rotation, ever.  I prefer to use reaction wheels for that, since that doesn't spend propellant.  It's tedious (and easy to forget) to turn off the rotation actuators on every RCS thruster all the time, so I just use a very simple ModuleManager patch that sets pitch / yaw / roll actuators to off by default for all RCS thrusters all the time.
  • Very important, make sure to always turn "fine maneuvering" on when RCS is active (caps lock key is the default binding for this, at least on PC).  This is what activates the magic stock behavior of "automatically adjust the power of all RCS thrusters to compensate for CoM so you don't have to care".  If you don't turn that on, then you'll get torque if your thrusters aren't perfectly symmetrically placed around CoM, which of course is a pain.  So just turn it on and it's not a thing anymore.

 

Share this post


Link to post
Share on other sites
  • 0

@jpinard:

15 hours ago, Snark said:
  • Very important, make sure to always turn "fine maneuvering" on when RCS is active (caps lock key is the default binding for this, at least on PC).  This is what activates the magic stock behavior of "automatically adjust the power of all RCS thrusters to compensate for CoM so you don't have to care".  If you don't turn that on, then you'll get torque if your thrusters aren't perfectly symmetrically placed around CoM, which of course is a pain.  So just turn it on and it's not a thing anymore.

I can't stress Snark's point here enough.  Don't forget to press CapsLock for all of your orbital docking manoeuvres.  If you're a brute about translating (because you're impatient) then you can still overpower it, but this feature alone makes it much easier to pretend that your translation and rotation axes are independent.

Anyway, to answer some of your earlier questions, yes, the large red arrow is torque, which you do not want to have when translating because it means that you have asymmetric thrust.  The circular red arrow is the direction of rotation and the size of the circle indicates the magnitude of that torque; it's not fine at all.  I see that you discovered the reason for your torque was that you essentially had two sets of thrusters at the fore of the vessel and only one set aft.  In your progression of three images, the third had no torque because the fore (capsule) and aft thrusters balanced one another, and the middle set of thrusters, being centred on the centre of mass, had no asymmetry to balance in the first place.

If you purpose to continue using RCS Build Aid, then the important value for setting up your vessel for translation is the numeric torque display, not the graphical display.  Move thrusters as needed to minimise that number and you won't need to pay attention to the coloured arrows.  Also, the different COM markers mean different things:  the yellow is the wet centre of mass as in stock, the red is the dry centre of mass, and the orange is the average of the two.  You seem to be referencing the average for your torque minimisation; that's okay but be mindful that doing so does not always minimise problems.

 

Share this post


Link to post
Share on other sites
  • 0
20 hours ago, Snark said:

I know a lot of people love RCSBuildAid, but I've never bothered with it, because I don't find it necessary.  A very simple technique works 100% of the time for me and requires no finicky futzing around, and is completely stock:

  1. One 4-way ring of thrusters at one end of my long skinny ship
  2. Another 4-way ring of thrusters at the other end of my long skinny ship
  3. Only one of those two rings needs fore-and-aft capability.  i.e. use the 4-way thrusters at one end, and the linear ones at the other (or none, if I have the Mk1-2 command pod which provides them for free).

That's it.  That's all there is.  That guarantees that the CoM will be somewhere in between the two, and as long as it's between them, it doesn't matter at all where it is.

Two other things for the above to work well with minimal hassle (these are whole-game settings, not something I do to individual ships):

  • I don't want to use RCS for ship rotation, ever.  I prefer to use reaction wheels for that, since that doesn't spend propellant.  It's tedious (and easy to forget) to turn off the rotation actuators on every RCS thruster all the time, so I just use a very simple ModuleManager patch that sets pitch / yaw / roll actuators to off by default for all RCS thrusters all the time.
  • Very important, make sure to always turn "fine maneuvering" on when RCS is active (caps lock key is the default binding for this, at least on PC).  This is what activates the magic stock behavior of "automatically adjust the power of all RCS thrusters to compensate for CoM so you don't have to care".  If you don't turn that on, then you'll get torque if your thrusters aren't perfectly symmetrically placed around CoM, which of course is a pain.  So just turn it on and it's not a thing anymore.

 

This is all super helpful.  But what if you build a lopsided design like mine, or something much worse?  Wouldn't that create some kind of weird 

 

Statement #3 I don't understand.  Is fore/aft the "H and N" key?  If so, why do I want to change how many will do this?

Share this post


Link to post
Share on other sites
  • 0
1 hour ago, jpinard said:

This is all super helpful.  But what if you build a lopsided design like mine, or something much worse?  Wouldn't that create some kind of weird

No.  It's fine.  That's my point.  That's why fine control mode is such a valuable tool, and why you should always have it turned on when you're maneuvering with RCS.

As long as your CoM is somewhere in between the ring of thrusters at the top and the ring of thrusters at the bottom, then it will work.  It's fine if it's "lopsided" in the sense that the CoM is much closer to one set of thrusters than the other.  If your CoM is, way closer to one end than the other, then the game will just apply a lot more thrust to the near ones than the far ones in order to cancel out the torque.  It just works.

I'm literally telling you that the craft you pictured above is already just fine.  You literally only have to hit the CAPS LOCK key when you turn on RCS.  That's it.  One key press, and then it'll just work for you.

1 hour ago, jpinard said:

Is fore/aft the "H and N" key?

Yes.

1 hour ago, jpinard said:

If so, why do I want to change how many will do this?

If you want to put 4-way thrusters on all of them, fine, that's your lookout-- all I was saying is that you don't need it.  As long as one of your two rings of thrusters has fore/aft capability, that's enough.

So, for example, on my own craft, I only use four of the 4-way thrusters, and at the other end I use four of the linear ones instead of another four 4-ways.  I do this because the linear ones are lighter than the 4-ways, so I might as well use 'em if I don't need the heavier ones.  Or I may not need any at all, if there's RCS functionality built into something else.  For example, if I've got a craft that has a Mk 1-2 command pod at one end... it's already got lateral thrusters, just not fore/aft.  So for a craft like that, all I need to do is to put a ring of 4-way thrusters at the opposite end of the vessel and I'm done.

Share this post


Link to post
Share on other sites
  • 0
On 7/30/2019 at 8:20 PM, Snark said:

No.  It's fine.  That's my point.  That's why fine control mode is such a valuable tool, and why you should always have it turned on when you're maneuvering with RCS.

As long as your CoM is somewhere in between the ring of thrusters at the top and the ring of thrusters at the bottom, then it will work.  It's fine if it's "lopsided" in the sense that the CoM is much closer to one set of thrusters than the other.  If your CoM is, way closer to one end than the other, then the game will just apply a lot more thrust to the near ones than the far ones in order to cancel out the torque.  It just works.

I'm literally telling you that the craft you pictured above is already just fine.  You literally only have to hit the CAPS LOCK key when you turn on RCS.  That's it.  One key press, and then it'll just work for you.

Yes.

If you want to put 4-way thrusters on all of them, fine, that's your lookout-- all I was saying is that you don't need it.  As long as one of your two rings of thrusters has fore/aft capability, that's enough.

So, for example, on my own craft, I only use four of the 4-way thrusters, and at the other end I use four of the linear ones instead of another four 4-ways.  I do this because the linear ones are lighter than the 4-ways, so I might as well use 'em if I don't need the heavier ones.  Or I may not need any at all, if there's RCS functionality built into something else.  For example, if I've got a craft that has a Mk 1-2 command pod at one end... it's already got lateral thrusters, just not fore/aft.  So for a craft like that, all I need to do is to put a ring of 4-way thrusters at the opposite end of the vessel and I'm done.

I see now.  Thanks!

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Answer this question...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.