Mechjeb Maneuver node editor?

[strider3]

I am trying to find information on using MechJeb's maneuver node editor to automatically start and stop my required burn using an existing Astrogator node. I've searched youtube but haven't found anything dealing with this specific MechJeb tool. When I open the editor, it has the information for the next node for the craft I'm flying...that's all good. What I don't know is what all the other buttons and entries do. I can get Mechjeb to do the burn but it seems to start a little late and stop a little early. I'm wondering if some of the settings might help with that. Specifically...what is "Set delta to:", "Conics mode" and "Tolerance" used for? In truth, when it comes to conics, all I know is how to spell it , to this day I have no idea what they are.

If anyone knows of a tutorial for this specific MechJeb tool, that would be awesome, rather than going over the whole thing here.

 

 

Thanks for any help.

[DerekL1963]

4 hours ago, strider3 said:

I am trying to find information on using MechJeb's maneuver node editor to automatically start and stop my required burn using an existing Astrogator node.

If there's an existing node, no matter how it was created, the "Execute next node" button will execute that node.

 

4 hours ago, strider3 said:

Specifically...what is "Set delta to:", "Conics mode" and "Tolerance" used for? In truth, when it comes to conics, all I know is how to spell it , to this day I have no idea what they are.

"Set delta", sets the amount the + and - buttons will change the burn (value in the left hand entry box) by.  (Or you can enter a custom value in the right hand entry box.)
"Conics mode", I have no idea.
"Tolerance" sets how close MechJeb will try to get to your desired velocity.  I would highly recommend not messing with it.  Lower values can cause MJ to execute some very weird burns.  Higher values will result in you invariably missing your target.

[strider3]

5 hours ago, DerekL1963 said:

If there's an existing node, no matter how it was created, the "Execute next node" button will execute that node.

 

"Set delta", sets the amount the + and - buttons will change the burn (value in the left hand entry box) by.  (Or you can enter a custom value in the right hand entry box.)
"Conics mode", I have no idea.
"Tolerance" sets how close MechJeb will try to get to your desired velocity.  I would highly recommend not messing with it.  Lower values can cause MJ to execute some very weird burns.  Higher values will result in you invariably missing your target.

OK..."if it ain't broke...don't fix it" . Without messing with any of these, MechJeb gets me..."close"...a very subjective term when dealing with Dres's inclined orbit. I end up having to create a third node to correct the MechJeb miss on the correction burn from Astrogator. But, it's still better than trying to hit the "thrust off" at the exact time needed on such a high anti-normal burn. What would be ideal is a mod which does not "overthink" an auto-burn...start on the created nodes timer, stop when burn Dv is 0. Even that could be a problem, though...PreciseManeuver and KER sometimes disagree on burn times.

For now it works well enough.

Thanks!

[DerekL1963]

5 hours ago, strider3 said:

What would be ideal is a mod which does not "overthink" an auto-burn...start on the created nodes timer, stop when burn Dv is 0.

That would invariably end up in you wildly missing the target.  A node is a precise instant in time, but it's a mathematical fiction because thrust is applied over time, not instantaneously.  So in reality you have to 'split the node', I.E. start burning before you actually hit the node and burn after passing through the node.  Where exactly to make the split (when to start burning) is a complex calculation (and partly magic) that's sensitive to both your spacecraft's performance characteristics and your orbital parameters.  (And subject to the limitations of the game itself - for example, throttles aren't instantaneous.)

 

5 hours ago, strider3 said:

Without messing with any of these, MechJeb gets me..."close"...a very subjective term when dealing with Dres's inclined orbit. I end up having to create a third node to correct the MechJeb miss on the correction burn from Astrogator.

Something very weird is going on then because using MechJeb alone I can almost always (99%+) hit Dres with no more than two burns.  One transfer burn from Kerbin orbit, and one mid-course correction burn.  (The latter is rarely very large nowadays because the transfer calculator now combines the ejection and plane change burns into a single burn.)

Try using MechJeb alone and see what happens.

Edited September 10, 2020 by DerekL1963

[strider3]

On 9/10/2020 at 12:24 AM, DerekL1963 said:

That would invariably end up in you wildly missing the target.  A node is a precise instant in time, but it's a mathematical fiction because thrust is applied over time, not instantaneously.  So in reality you have to 'split the node', I.E. start burning before you actually hit the node and burn after passing through the node.  Where exactly to make the split (when to start burning) is a complex calculation (and partly magic) that's sensitive to both your spacecraft's performance characteristics and your orbital parameters.  (And subject to the limitations of the game itself - for example, throttles aren't instantaneous.)

 

Something very weird is going on then because using MechJeb alone I can almost always (99%+) hit Dres with no more than two burns.  One transfer burn from Kerbin orbit, and one mid-course correction burn.  (The latter is rarely very large nowadays because the transfer calculator now combines the ejection and plane change burns into a single burn.)

Try using MechJeb alone and see what happens.

I will give MJ a "solo" shot at it next time I leave Kerbin. You'll notice in the screenshot...this is my mid-course as given to me by Astrogator. It has a huge amount of anti-normal which makes it very hard to arrive at Dres without a large inclination that must be corrected. Even allowing MJ to "do" the burn, I end up with a Dres encounter (so that's good ), but I spend DV getting to an equatorial (0 inclination) orbit...even the slightest error by MJ's burn results in being way off of the "planned" trajectory. I usually do well enough with a manual burn to celestials with a low inclination orbit...Dres is just...Dres . Maybe I'm demanding too much and should live with Dres being a bit difficult? Just getting an encounter with Dres is a "win", in my book.

I will try your suggestion of using MechJeb alone, next time, and see how that goes.

Thanks!!

[DerekL1963]

5 hours ago, strider3 said:

Even allowing MJ to "do" the burn, I end up with a Dres encounter (so that's good ), but I spend DV getting to an equatorial (0 inclination) orbit...even the slightest error by MJ's burn results in being way off of the "planned" trajectory. I usually do well enough with a manual burn to celestials with a low inclination orbit...Dres is just...Dres . Maybe I'm demanding too much and should live with Dres being a bit difficult? Just getting an encounter with Dres is a "win", in my book.

It can happen with any planetary encounter (even flying to the Mun) because of the inevitable +/- normal errors in your burn, but yeah.  Much of that is just Dres being Dres.  (Though you'll see it at Moho too.)  If your target planet's orbit is inclined with relation to your departure planet's orbit, you're going to generally end up in an inclined orbit at your target.  That's just orbital mechanics at work.

[strider3]

@DerekL1963...I tried MechJeb to create nodes for a return to Kerbin from Dres. It tells me, in yellow text, that my initial orbit must not be hyperbolic...my eccentricity for my Dres initial orbit is 0.0000...can't get any less "hyperbolic" than that! Thinking it's just a warning, I continued on and MechJeb created a node to depart Dres...but just the one node (that I can see in PreciseManeuver). I'm including a screenshot of what I am seeing.

 

 

Edited September 13, 2020 by strider3

[linuxgurugamer]

I'm pretty sure that is just a general message; you can still create and execute a node as normal

[DerekL1963]

2 hours ago, strider3 said:

It tells me, in yellow text, that my initial orbit must not be hyperbolic...my eccentricity for my Dres initial orbit is 0.0000...can't get any less "hyperbolic" than that!

Yeah, that's an ongoing bug in MJ.  Usually I delete the node and try it again....  Either way, if it creates the node and the results look good on the map screen, you should be good to fly that node.
 

2 hours ago, strider3 said:

I continued on and MechJeb created a node to depart Dres...but just the one node (that I can see in PreciseManeuver). I'm including a screenshot of what I am seeing.

If you're departing Dres (actually most situations), one node is all you want, one node is all you need.  (Setting up more than one node can cause problems, because subsequent nodes need to take into account any errors in previous nodes/burns.)  Once you're clear of your departure SOI, you can set up a "Fine Tune Closest Approach" node.

But looking at all the information on the screen, I think I see the real source of your problems, and it's not MechJeb.  It's your TWR, .06 is insanely low.  Trying to fly that lengthy burn time directly from low orbit is invariably going to lead to gross errors and significant correction burns.  My rule of thumb (honed over many years) is that your burn should be no greater than 1/4-1/3 of your orbital period, and preferably closer to 1/6-1/8.  (And even with that method, a TWR of .06 is still insanely low....  Long burns and accurate burns are all but mutually exclusive.)

With a TWR that low, if you want to depart from low orbit, you need to be departing Dres using a series of periapsis kicks - which MJ doesn't do.

Edited September 13, 2020 by DerekL1963

[strider3]

6 hours ago, DerekL1963 said:

With a TWR that low, if you want to depart from low orbit, you need to be departing Dres using a series of periapsis kicks - which MJ doesn't do.

@DerekL1963 I may not have increased the lander's throttle limit to 100% after doing an orbital correction. Let me try again...

Yep, forgot to undo the throttle limit. I now have a TWR of 12.35.

So, if I understand you, no mid-course correction is necessary? Because with the one new node MJ created (with throttle at 100%), I'm not seeing anything near a Kerbin encounter in map view. To the contrary, my nearest approach is over 28,000,000 km. As you can see in the screenshot, MJ says that my total Dv to insertion of Kerbin orbit is 3,659 km/s, but the only node I have is a 1,603 burn? There must be a second, mid-course node?

I think it important to point out that I'm not doubting MechJeb's abilities...only my understanding of how to use MechJeb correctly? This has been an ongoing issue, YouTube is great...when the poster's primary language is English. However (and no offense to YouTube posters of other nationalities and their attempts to help), the English language posts I have found are very hard to understand due to the language barrier.

 

Edited September 13, 2020 by strider3

[DerekL1963]

18 hours ago, strider3 said:

So, if I understand you, no mid-course correction is necessary?

No, you do not understand me...  You need only one node to depart Dres.  Once you're out of Dres' SOI, then you set up a "Fine Tune Closest Approach" node for a mid-course correction to correct for any errors in the departure burn.  It's unusual to miss that far, but you can correct with "Fine Tune".
 

18 hours ago, strider3 said:

As you can see in the screenshot, MJ says that my total Dv to insertion of Kerbin orbit is 3,659 km/s, but the only node I have is a 1,603 burn? There must be a second, mid-course node?

That (estimated) total d/v covers departing from Dres, and capturing (circularizing) in Kerbin orbit, you can't set up the latter burn until you're actually in Kerbin's SOI.  MechJeb does not account for mid-course corrections, it has no way of estimating the required size of the correction because that's unknowable until after the transfer burn.
 

18 hours ago, strider3 said:

I think it important to point out that I'm not doubting MechJeb's abilities...only my understanding of how to use MechJeb correctly?

MechJeb is a tool, and the best way to understand it it to actually use it.