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Asymmetrical Lift with Symmetry


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So, this morning I decided I'd make a nice Parker Solar Probe recreation, and have a little fun.   Made the probe, stuck it on a DeltaIV Heavy, and launched that badboy!

To my dismay, I kept having lots of trouble at launch; the thing just wouldn't fly straight.  It's not my first launch here, I know how to build a nice KSP rocket, or at least I thought I did.

Turning on the Aerodynamics display, I was taken aback to see some tanks having lift displayed, and some not... on parts that are identical, mirrored symmetrically.

Take a look at the two side boosters here, and the problem is obvious.  This feels like the kind of thing that should not happen in a post-release environment.  As much as I love KSP, having crap like this happen years and years after the game's launch just sours me, man. :/  Just look at the randomness of what tanks have decided to have lift for whatever reason, and which don't.   I've built rocket like this a bajillion times.  What the heck, how am I supposed to fly this?

Turns out fuel flow priority didn't apply symmetrically. -> solved.

ManyH2P.jpg

 

So, I just closed KSP and didn't have fun.  Yay?

Edited by klesh
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So at 3300m you're doing 140m/s with 1.5g. That's pretty slow. If it was me, I would tend to take that particular rocket much higher before I turned it. And try to keep it pointed a bit closer to prograde for as long as I could. Things might become a bit more controllable then.

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27 minutes ago, mystifeid said:

So at 3300m you're doing 140m/s with 1.5g. That's pretty slow. If it was me, I would tend to take that particular rocket much higher before I turned it. And try to keep it pointed a bit closer to prograde for as long as I could. Things might become a bit more controllable then.

 

Those values are not typical of the regular ascent, its because I was trying to keep the thing straight and having it wobble all over, checking the various displays etc.  Speed, alt, everything is wildly off from nominal.  Everytime I'd control+f12 to toggle displays, it lowers the throttle, etc.

 

23 minutes ago, OHara said:

You might be noticing the old problem with overlay arrows., which you can ignore   
If those are structural tubes you might be noticing the new problem with drag on variant parts, which you can patch as shown on the bug-tracker if you can use module manager.

 

It has inexplicable pull to one side though, so there are some manner of forces acting on the ship.  There are no structural tubes at all, but the center stack does have an engine plate on stage 2.

 

Edited by klesh
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16 minutes ago, klesh said:

Everytime I'd control+f12 to toggle displays, it lowers the throttle, etc

Why Ctrl+F12 ? Enabling aerodynamic force overlay by default is just done by using f12 by itself.

With the central stack try setting autostrut for all parts to root. For the joining booster tanks set autostrut to grandparent. That should help to get rid of some of the wobble.

And you are full throttle and only hitting 1.5g - take it higher before you turn.

 

Edited by mystifeid
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5 hours ago, mystifeid said:

Why Ctrl+F12 ? Enabling aerodynamic force overlay by default is just done by using f12 by itself.

With the central stack try setting autostrut for all parts to root. For the joining booster tanks set autostrut to grandparent. That should help to get rid of some of the wobble.

  

 

Because steam uses f12 for something else.

Not that kind of wobble, the stacks themselves are rock solid.  The wobble is in the entire ship, it literally wont fly straight because (I think) lift is being applied randomly in a non-uniform way.

Here is the craft if you want to try it, MH required:

It certainly use more TWR, but I hadn't even gotten to that phase yet.  If you need some more, take out some fuel from the side boosters.

[Giant browser-destroying .craft file content removed by moderator]

Edited by Snark
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42 minutes ago, klesh said:

The wobble is in the entire ship, it literally wont fly straight because (I think) lift is being applied randomly in a non-uniform way. 

Actually I'm pretty sure it's because the fuel flow priorities have been set asymmetrically - ie tanks on one side are emptying before tanks on the other side are used.

Here are the fuel flow priorities for just the bottom six tanks. The ones on the left and right need to be the same, not different.

6ANjAbK.png

Edited by mystifeid
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18 minutes ago, mystifeid said:

Actually I'm pretty sure it's because the fuel flow priorities have been set asymmetrically - ie tanks on one are emptying before tanks on the other side are used.

Here are the fuel flow priorities for just the bottom six tanks. The ones on the left and right need to be the same, not different.

 

Of course they should be the same... I'll fire it up again.  Doesn't that stuff apply via symmetry?  My recollection is that it should, no?

Edit:  quick mockup in vab shows it does respect symmetry. 

 

Hmm.

 

So, indeed, symmetric fuel flow priority didn't seem to get applied, or somehow the symmertical boosters don't seem to be on via symmetry.  THAT I have never seen before.  Flies as expected with manually symmetrical flow priority.  Odd. Very odd.

Edited by klesh
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2 minutes ago, klesh said:

Doesn't that stuff apply via symmetry?  My recollection is that it should, no?

I think manual overrides to fuel flow priorities might not be transferred to all members of a symmetry group. Anyway, I got your rocket into orbit with no problem although I didn't start turning until my speed reached 300m/s.

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6 minutes ago, mystifeid said:

I think manual overrides to fuel flow priorities might not be transferred to all members of a symmetry group. Anyway, I got your rocket into orbit with no problem although I didn't start turning until my speed reached 300m/s.

 

Yeah, I ended up swapping on some better engines for more TWR.  Its not meant to be some fantastic recreation, just a Saturday morning quick funtime launch.

Thanks for the keen eye on the fuel flow, I could not for the life of me figure out the problem, and I didn't even think of flow priority.  Now to figure out how it happens, or doesn't happen, and how to recreate.  I need another coffee.

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5 hours ago, klesh said:

Here is the craft if you want to try it, MH required

I'm sorry, but we've had to snip the content.  Please do not paste ginormous text content into the forums.  It causes technical problems with peoples browsers, especially for folks on mobile devices (may lock up the browser entirely).

If you want to share some giant content with someone, the best way to do so is to upload it to some 3rd party site, then paste a link to it here, rather than pasting the content directly into your post.

Thank you for your understanding.

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3 hours ago, Snark said:

I'm sorry, but we've had to snip the content.  Please do not paste ginormous text content into the forums.  It causes technical problems with peoples browsers, especially for folks on mobile devices (may lock up the browser entirely).

If you want to share some giant content with someone, the best way to do so is to upload it to some 3rd party site, then paste a link to it here, rather than pasting the content directly into your post.

Thank you for your understanding.

 

My mistake, I was unaware that was a problem. It doesn't cause any issue with my browser. I didn't figure a 95kb text file was considered "ginormous".

Why not prevent this with the forum software itself?  Character limit etc.

Edited by klesh
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15 hours ago, klesh said:

It doesn't cause any issue with my browser. I didn't figure a 95kb text file was considered "ginormous".

It can cause problems for a lot of folks on mobile browsers, which turn out to be especially sensitive to that sort of thing.

15 hours ago, klesh said:

Why not prevent this with the forum software itself?  Character limit etc.

Not a bad idea, but it can be difficult to pick a number that's high enough to allow people to do what they need, but low enough to prevent problems-- basically, any number we pick would be wrong.  Making changes to the forum settings themselves can be tricky and prone to unintended consequences, so we try to tread lightly around that sort of thing.

 

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