There ought to be a mod: Background rocket burns

[Loren Pechtel]

KAC is invaluable but it would be a lot better of we didn't have to use it so much.

What I'm picturing is being able to mark a maneuver node as background.  The rocket retains it's current orientation relative to whatever body's SOI it is in, or any orientation that can be set by SAS.  (Thus if it's pointing 5 degrees radial from prograde it will still be 5 degrees radial from prograde when the rocket lights.)  Cooperation with MechJeb would be ideal but in most cases simply prograde or retrograde would be pretty close.

Skip the physics of the parts calculations as the rocket burns, treat it as a single body so you can do the calculations a lot faster.  It can't be done on a maneuver node in atmosphere and the burn shuts down if it touches atmosphere.

[DStaal]

This is one of those things that sounds simple, until you start thinking about it.  How much propellant have you used?  What happens if you ran out for this stage?  How about if the rocket's thrust isn't centered, or you don't have enough attitude control to keep it on-point?  These all actually require you to be running physics on the whole vessel over the burn to answer correctly.

And of course all of this is complicated by the fact that rockets more than 2km from your current viewpoint *don't exist.*  They are just markers saying 'if we switch to this, load in these resources to create the rocket'.

So, it's one of those things that sounds easy and would be nice - but is actually very complicated and done the way it's done in KSP for a reason.

[Loren Pechtel]

26 minutes ago, DStaal said:

This is one of those things that sounds simple, until you start thinking about it.  How much propellant have you used?  What happens if you ran out for this stage?  How about if the rocket's thrust isn't centered, or you don't have enough attitude control to keep it on-point?  These all actually require you to be running physics on the whole vessel over the burn to answer correctly.

And of course all of this is complicated by the fact that rockets more than 2km from your current viewpoint *don't exist.*  They are just markers saying 'if we switch to this, load in these resources to create the rocket'.

So, it's one of those things that sounds easy and would be nice - but is actually very complicated and done the way it's done in KSP for a reason.

I'm only suggesting this for use in space.  That requires a circularization burn--the rocket worked while in space.  The only way I see to make a non-flyable rocket is to dock something lopsided, or to rebuild it with KIS/KAS.  Totally ignoring the physics would permit a certain amount of cheating along these lines but I don't think it would be a big deal.

As for the fuel--if the planned burn would run out of fuel you don't allow it to be done in the background in the first place.  It's not a hard calculation.

As for manipulating things in the background--I suggest looking at the mod Bon Voyage.  It simulates rover driving in the background.  The simulation leaves something to be desired because it's terrain mapping is imperfect (thus sometimes putting your rover in a place it couldn't have gotten to and sometimes refusing to go where it could actually reach) but it does work.

No need to unpack the rocket, you simply adjust the velocity and remaining fuel.

[DStaal]

I've launched lopsided rockets in one launch, easily.  (Try sending a Karibou rover with some Duna modules to the Mun - 3.75m launcher stage is far enough back to compensate, but a 2.5m transfer stage will need you to run RCS to stay on point.)  And really the attitude control for *semi* lopsided is where this is more likely to cause issues: You end up burning more fuel to compensate as your engine wags all over the place.  I've had to redesign rockets repeatedly to compensate, either by giving more fuel or some other way.

Or you can have the opposite: Have a burn that takes advantage of the Oberath effect, and uses less fuel than a naive calculation would account for.

Bon Voyage simulates driving a rover - what it does is check if you have 'enough' EC generation to drive, then moves the rover's location directly.  There's no fuel use to compensate for - and yet getting that 'enough' EC generation calculation right has been an issue that's been tweaked several times since the release, and still has people asking for adjustments.

There are lots of corner cases.  There's been at least one mod out there that tried it, but nothing's done a great job.

[Loren Pechtel]

3 hours ago, DStaal said:

I've launched lopsided rockets in one launch, easily.  (Try sending a Karibou rover with some Duna modules to the Mun - 3.75m launcher stage is far enough back to compensate, but a 2.5m transfer stage will need you to run RCS to stay on point.)  And really the attitude control for *semi* lopsided is where this is more likely to cause issues: You end up burning more fuel to compensate as your engine wags all over the place.  I've had to redesign rockets repeatedly to compensate, either by giving more fuel or some other way.

Or you can have the opposite: Have a burn that takes advantage of the Oberath effect, and uses less fuel than a naive calculation would account for.

Bon Voyage simulates driving a rover - what it does is check if you have 'enough' EC generation to drive, then moves the rover's location directly.  There's no fuel use to compensate for - and yet getting that 'enough' EC generation calculation right has been an issue that's been tweaked several times since the release, and still has people asking for adjustments.

There are lots of corner cases.  There's been at least one mod out there that tried it, but nothing's done a great job.

Your first one sounds like a legitimate edge case.  However, I'm not asking for perfection, I would accept a rocket like that flying when it shouldn't.  You'll just be in for a rude awakening at the destination.

Oberth doesn't need compensating for as it's inherent in the physics.  The simulated burn would take place at the right place, Oberth comes along for free.

I do agree Bon Voyage doesn't have to worry about fuel but that doesn't seem that complex.  It's just a number in the data.  The correct tank(s) to burn from can be noted when you enable it for a background burn, no need to look at the structure of the rocket when you're doing it.

I have caught an edge case on Bon Voyage myself--and I consider it a very minor deal.  Yeah, my rover moved with the panels folded but so what?  My only real gripe with Bon Voyage is terrain handling--and that doesn't happen in space.