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Engine gimbal without thrust


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Is there a reason why engines gimbal during attitude correction even though they have no thrust? All the work is done by reaction wheels and thrusters. What is worse, the engines moving around induce oscillations which make it hard to maneuver and waste a lot of fuel from reaction thrusters.

I would think the default was to shut down gimbaling when the engine has no thrust. I know I can shut it down manually, and even have an action group set for that. However it seems like micromanaging something that should have been built in?

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Like talking and transforming, gimbaling is a free action. Yeah, it could have been made default... but given the nature of software I'm settle with an aesthetics issue over having to write one of the letters to the boss explaining why I just splashed a few million worth of hardware across a few square kilometers of landscape.

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@steuben I agree with the general sentiment but ..

This is something free and detrimental. I had  maneuvers delayed and/or botched because the engine gimbaling got in the way of attitude control. Ideally I was hoping someone would point me to a VAB option I missed which says 'smartify engine', and then links gimbaling to the presence of thrust.

If I understand correctly, are you saying that fixing this might have potentially introduced bugs that affect normal gimbaling, resulting in splashing 'a few million worth of hardware'?

 

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Just now, splashboom said:

I had  maneuvers delayed and/or botched because the engine gimbaling got in the way of attitude control.

Are you sure? Try disabling your reaction wheels and RCS thrusters and then see if you can tilt your craft with just the engine gimballing.

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7 minutes ago, HebaruSan said:

tilt your craft with just the engine gimballing

No, that would not work of course. However I blame the gimbaling for the oscillations that sometimes get really out of hand. Are you saying I cannot get rid of those?

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2 hours ago, splashboom said:

What is worse, the engines moving around induce oscillations which make it hard to maneuver and waste a lot of fuel from reaction thrusters.

Engine gimbaling has no effect whatsoever on a ship's velocity or attitude.  Put a Probodobodyne OCTO2 probe core (which has no reaction wheels) in orbit, with a gimbaling engine (but no fuel).  

See if you can make the ship alter velocity or attitude in any way by gimbaling the engine.  You can't.  It's purely visual, and has no effect on the velocity or attitude of the vessel whatsoever- unless the engine is running and producing thrust of course.

 

1 minute ago, splashboom said:

No, that would not work of course. However I blame the gimbaling for the oscillations that sometimes get really out of hand. Are you saying I cannot get rid of those?

Engine gimbaling is not the cause of ship oscillations.  There is something else going on.  A screenshot might help, but it's definitely not engine gimbaling causing the oscillations.  Unless the engines are running, of course.

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1 minute ago, splashboom said:

No, that would not work of course. However I blame the gimbaling for the oscillations that sometimes get really out of hand. Are you saying I cannot get rid of those?

Yes and no. This is one of the reasons why rocket science is hard. 

There is a whole bunch of Math! there that I will skim over. Because, I half understand it, and couldn't explain it. But if you want to you can look up PID controllers. There are better ones... but the math is very much heavier. IIRC, this is what the KSP stability control system use, and NASA in the Canadarms.

In KSP, most oscillations are caused by "ship flop". I'm guessing on the code, but when the SAS tried to do its job, based on your settings, it looks at the shape of ship, mass, available force vectors, and distance to setting, all of which is relative to the "Point of Control", at that slice of time.  It does it mathamagics and spits out the controls to the various devices. Now with a perfectly rigid spherical cow in a frictionless vacuum, it isn't a problem. The math can be solved over an espresso.  But, in KSP we've only got one of those things, a frictionless vacuum. The ships have varying degrees of floppiness. This, means that in every time slice the ship has a different shape, which changes the force vectors, which changes the torques they generate. These changes can either increase the available torques or reduce them. Which then changes what the SAS has to work with. Depending on Factors! this will usually cause the SAS to drive the ship through it's desired setting, and then starts the oscillations that you see. 

I think there's something as well with regards to there being too much control force available... But that'll require a deep dive into the literature and something I don't want to do standing at the kitchen counter making lunch.

 

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On 1/3/2022 at 8:45 AM, splashboom said:

waste a lot of fuel from reaction thrusters

This is kind of a digression from your main concern about the gimbal, but... is there a reason you've got your RCS thrusters turned on for attitude control?

i.e. there are two ways to control the attitude of a ship in orbit:  RCS thrusters, and reaction wheels.  Reaction wheels are "free" (just electricity), whereas RCS uses consumable fuel.  So, since it's not free... any reason why you're using it?  (If you like to have it turned on for realism, then never mind.)  ;)

RCS thrusters let you configure which actuators are enabled.  I like to always set all my RCS thrusters to be enabled for translation only, and have them disabled for pitch / yaw / roll.  Saves on monopropellant quite a bit.  ;)

(Of course, remembering to always turn off the rotation actuators every time I place an RCS thruster in the VAB is a hassle.  Fortunately, it's easy to make rotation "off by default" with a simple ModuleManager config patch.  Here's config that does that, if you're interested.)

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26 minutes ago, Snark said:

is there a reason you've got your RCS thrusters turned on for attitude control?

 

Very good question. By default they should be off, but for heavy ships approaching a maneuver quickly you need the extra speed in attitude correction.

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3 minutes ago, splashboom said:

By default they should be off, but for heavy ships approaching a maneuver quickly you need the extra speed in attitude correction

Fair 'nuff.  My own solution to that problem is "never approach a maneuver quickly", i.e. get the ship pointed correctly well in advance.

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1 hour ago, splashboom said:

Here is the wobble - it happens to other ships too, and to this one only when fully loaded. I suspect the solutions involves struts, and I may have been unfair to engine gimbals

After seeing the video of your ship I have a few suggestions.

  • Turn on Rigid Attachment for some parts.  Particularly the Pylons which the side tanks are attached to, as well as the tanks themselves (the center tank and the side tanks which attach to the pylons.
  • Turn on rigid attachment for the ISRU Converter and the Ore Tank on top of it.  Not sure why, but I've always found the ISRU converter to be a huge source of wobbliness.
  • Also turn on AutoStruts for the same parts just mentioned.  I like to lock to heaviest part.
  • For a time, AutoStruts were buggy and caused unexpected behavior.  I now find that in the current version of the game AutoStruts work extremely well.  So well that I rarely use actual struts anymore.

Rigid Attachment and AutoStruts for the parts mentioned should knock that wobble down considerably.  Again, I've found the ISRU Converter to be especially prone to wobble issues.  Also, the mass that those pylons are supporting is a lot.  Again, try Rigid Attachment attachment and AutoStruts for the pylons and the neighboring tanks on each side of them.

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4 hours ago, Snark said:

RCS thrusters let you configure which actuators are enabled.  I like to always set all my RCS thrusters to be enabled for translation only, and have them disabled for pitch / yaw / roll.  Saves on monopropellant quite a bit.

That's a great tip, I didn't realize you could do that.  I'm sure you can imagine how I've been doing it:

  • Turn RCS on when I need to translate, but make no attitude adjustments when RCS is on.
  • Turn RCS off when I need to make an attitude adjustment.  Then turn it back on.

That's a lot of wear on my 'R' button.  Can't wait to try that tip out!

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