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Odd Insertion Burn (Jool) - Better to do Manual or use Maneuver Node Marker?


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This relates to a couple of past threads where I learned about using Tylo or Laythe to get a Joolian orbit without massive expenditure of fuel.  The clock has been ticking, and I'm getting ready to do my burn to achieve a Tylo encounter (for the second try).

 

I've placed a maneuver node - and to get the proper encounter I need (using a NERV) a 25 second (73 m/s) burn - and the burn requires mostly Radial Out, a middling amount of Retrograde and a touch of Normal.  This places the dark blue maneuver marker about 5 degrees off the Radial Out marker.  On my first attempt I lined up on that marker and fired the engine for the required time... and completely lost the encounter.  Now - what I did not do was follow the marker when it started to wander - just kept my orientation where it was originally.

 

So - the reason behind the question - I've seen some guys manually 'feather in' their burns - firing aimed at the Radial Out for a while, then rolling to Retro for a squirt or two, then Normal and back to Radial Out to get the alignment they want, all the while zoomed in on the planetary system to see the effects of their burns.  Seems fiddly - but it obviously works.

 

The main question is - for a guy with a passing understanding of what is going on - is that a better method than 'aiming at the dark blue maneuver marker and hoping the computer lined me up correctly.  And a final question: should I try to follow the blue marker when it wanders?

 

 

Edited by JoeSchmuckatelli
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Following the maneuver marker is the best plan.  When you get close to the end of the burn throttle way back.  You can then follow the marker as it begins to wander.

The marker will try and compensate during the burn to give you the most accurate result compared to the original prediction.

Happy landings!

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1 hour ago, JoeSchmuckatelli said:

The main question is - for a guy with a passing understanding of what is going on - is that a better method than 'aiming at the dark blue maneuver marker and hoping the computer lined me up correctly.  And a final question: should I try to follow the blue marker when it wanders?

No, it's not better.  Just follow the blue marker.

The reason why the blue marker wanders is a simple one:  As the remaining burn amount gets smaller, then any minute errors in the original positioning get magnified.  Imagine that you're shooting an arrow at a target that's far away:  hitting the bull's eye is really hard, because even a microscopic aiming error would make you miss the target from that distance.  Imagine that you have a little camera mounted on the front of the arrow; what would it see?  As it leaves your bow, it's pointed almost perfectly at the target (you're a good shot, any error is invisibly tiny), so the target appears to be perfectly centered in the camera's view.  But as the arrow gets closer and closer to the target, any errors become more and more apparent, and pretty soon the bull's eye is sliding off to the side.

That's the "wandering" of the blue maneuver marker.  Think of it as the bull's eye, and your burn is the shooting of the arrow.  If you don't make some sort of adjustment, you're just about guaranteed to miss the target by some amount.

So if you make corrections as it goes, to keep the maneuver centered:  that's the equivalent of having a guided arrow that is tracking the bull's eye and making minor steering corrections to keep it on course.  That's the thing to do.

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4 hours ago, JoeSchmuckatelli said:

So - the reason behind the question - I've seen some guys manually 'feather in' their burns - firing aimed at the Radial Out for a while, then rolling to Retro for a squirt or two, then Normal and back to Radial Out to get the alignment they want, all the while zoomed in on the planetary system to see the effects of their burns.  Seems fiddly - but it obviously works.

This method of firing a bit on each of the axes (radial, normal, pro/retrograde) is less efficient than burning directly along the maneuver node (ignoring for the moment, corrections at the end of a burn when the maneuver node marker wanders because you did not exactly point at the maneuver node for the entire burn duration).

Sometimes you have fuel to spare and efficiency doesn't matter, and it's just easiest to manually 'feather' in a burn.

So it's a judgement call.  If you need every last bit of delta-v, or prefer not to be grossly inefficient, drop a maneuver node and follow it.  If you want to fine-tune an encounter, and have fuel to spare, and feel like eyeballing it, burn on the axes as needed.

Let's say you need to burn N m/s normal, R m/s radial, and P m/s prograde. If you do it one axis at a time, the delta-v cost is N + R + P. If you burn along the maneuver node the delta-v cost is sqrt(N2 + R2 + P2), which is always less than or equal to N + R + P.

For example, if N = R = P:    sqrt(N2 + R2 + P2) = sqrt(3)*N  <  N + R + P = 3N.  Burning along the axes burns up to sqrt(3) times as much fuel as burning along the maneuver node.

Edited by Yasmy
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