KSP2 Manoeuvre Nodes - A love/hate relationship

[Cailean_556]

Manoeuvre nodes (MN). A system designed to assist you in planning an intended orbit or trajectory. The node, once calculated, provides a burn time and a count down to said burn time as well as a visual representation of your intended orbit or trajectory. You orient toward the trajectory/target marker and wait. When the timer reaches zero, you burn along that trajectory until the desired velocity and time is achieved then cut engines. You should now be in an orbit more or less matching your desired altitude or on your way to wherever you were travelling to, along a trajectory that more or less matches what you intended.

Yeah, you might be anywhere from a few hundred, to a few (single digit) thousand metres out in an orbit - depending on how you managed thrust and how quick you were in cutting thrust - but it was in the ballpark. With interplanetary (including transit to the Mun and Minmus) intercepts, your periapsis could be tens of thousands of metres out if you timed it wrong or under/overflew the trajectory target marker. But you had plenty of time to turn retrograde (or prograde, depending on what you had intended) and make a slight correction burn, either manually or using another MN.

That's the way it worked in KSP1.

Maybe I'm approaching MNs in KSP2 wrong, but when I make a MN at suborbital apoapsis and drag prograde, I'm expecting an at least roughly circular orbit. Instead I get a wildly elliptical orbit. Tweaking the radial/normal/pro/retro has no where near the desired effect, dragging the entire node results in wildly differing orbital patterns. I know how to fix it manually, but the point is I shouldn't have to - that's what MNs are supposed to do. 

Previously, I make a MN from 90km perapsis, that is essentially the "centre" of the node. My 90x90 desired orbit wouldn't be exact - even if I didn't play around with moving the node - but it'd be ballpark and I could work with it. In KSP2, I make an MN from 90km perapsis, to have my desired periapsis in orbit at 90km or anywhere close to it, my apoapsis is now well over 200km.  Way worse than anything KSP1 would do, but fixable once I'm actually in orbit. Burn says 37 seconds. Wait to burn, do so when the timer hits zero. Don't look at the map, just focus on the burn time and make I'm point toward my intended trajectory. 37 seconds is up, cut thrust when it reaches zero. Periapsis is under 90km. Apoapsis is now 480ish km... I shake my head, mutter under my breath some things that I know I couldn't do better than the devs, and then try to burn for the Mun - using a MN. My MN projected path is put my periapsis at 2,234km. Can't be bothered tweaking it, already over it. Fast forward to burn and burn. Watch the timer run to zero, cut thrust. My periapsis is...interplanetary.  Now if I was following this in map view, I'd have seen this coming and cut thrust early. I could (did) also see the Apo/Peri readout on the main screen go beyond my established parameters, but I still have like 7 seconds of burning to do and I wanted to see just how and why the MNs are so wrong almost all the time. If I'm aimed at the right spot (which I was - according to the nav ball), burning for the right time (which I did, according to the timer), whether I'm looking at a map or not I should be pretty bang on where I intended to be.

Now I will admit, and concede, that perhaps I am approaching MNs in KSP2 wrong and there could be a better way of getting circularised orbits (using MNs, doing it manuually isn't that much of an issue but that's not the point) and interplanetary trajectories with the current system. But Mk1 Eyeball Trial and Error Pro/Retrograde burns seems to yield far more reliable results and be far more efficient than using a node. That shouldn't be a thing.

In a physics-based simulation game about travelling to other planets why such inaccuracy like this even a thing? Especially when the previous iteration of the game seemingly has a better algorithm for calculating burn times and is relatively accurate with not only those times but effecting the desired orbit or trajectory you were seeking.  Sequels are meant to be better, not be actively worse.

What am I doing wrong?

[Entroper]

The difference is that in KSP 1, a planned maneuver is assumed to be an instantaneous change in velocity.  However much delta-V you plan for, it just calculates a new orbit starting from the same point, with that much velocity added.

So the way you execute maneuvers in KSP 1 is to look at the burn time, subtract half of it, and start burning that much time before the node, so that you split your burn evenly.  This gets you reasonably close, as long as your burn times are under a minute or two.  But the longer your burn, the less accurate the result.

KSP 2 calculates maneuvers differently.  When you plan a maneuver, you tell it where to start, and it actually calculates your entire burn, taking into account your ship's movement during the process.  So if you plan a burn at apoapsis, your burn will execute mostly after apoapsis, leaving you with an ellipse when you were trying to circularize.  If you want to circularize, you have to plan a burn that starts a little before apoapsis.  The current UI doesn't give you an easy way to know exactly how far in advance you should start the burn.  You can try planning one at apoapsis, and then sliding it backwards, but it's still kinda difficult to fine-tune.

Theoretically, the new system is much more powerful, allowing you to plan very long burns and see their effects much more accurately than in KSP 1.  I'm hoping we get some better maneuver planning tools in the future.

Edited March 17, 2023 by Entroper
wording

[Cailean_556]

35 minutes ago, Entroper said:

The difference is that in KSP 1, a planned maneuver is assumed to be an instantaneous change in velocity.  However much delta-V you plan for, it just calculates a new orbit starting from the same point, with that much velocity added.

So the way you execute maneuvers in KSP 1 is to look at the burn time, subtract half of it, and start burning that much time before the node, so that you split your burn evenly.  This gets you reasonably close, as long as your burn times are under a minute or two.  But the longer your burn, the less accurate the result.

KSP 2 calculates maneuvers differently.  When you plan a maneuver, you tell it where to start, and it actually calculates your entire burn, taking into account your ship's movement during the process.  So if you plan a burn at apoapsis, your burn will execute mostly after apoapsis, leaving you with an ellipse when you were trying to circularize.  If you want to circularize, you have to plan a burn that starts a little before apoapsis.  The current UI doesn't give you an easy way to know exactly how far in advance you should start the burn.  You can try planning one at apoapsis, and then sliding it backwards, but it's still kinda difficult to fine-tune.

Theoretically, the new system is much more powerful, allowing you to plan very long burns and see their effects much more accurately than in KSP 1.  I'm hoping we get some better maneuver planning tools in the future.

I'll take on board your advice and give that a try next time I play some KSP2.

Perhaps a hyrbid of KSP1 and the current system could work better - where you create the node is considered the "centre" (for lack of a better term, but not the literal centre) of that manoeuvre. The node takes your intended information, calculates it then applies a formula to it so your starting point isn't at the node itself but further ahead/behind as required, the node is just the interactive point at which you tweak your data. It needs to be somewhere along your projected trajectory (rails) because that's how the node is able to be created, but your start point is calculated based on what you want the thing to do, rather than where you want to do it. Works in my head, but there's probably a ridiculous amount of complex calculations occurring in the background to make that a reality. 

Edited March 17, 2023 by Cailean_556

[Sea_Kerman]

Issue is for many of the use cases of the new maneuver node system, the "center" of the node can be in an entirely different SOI from the start of the node.

That said, perhaps there could be 2 node modes, one for precision high-thrust short burns where it splits the burn and generates a sort of secant to your trajectory, and one where it works like it does currently.